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HomeMy WebLinkAbout2008-11-04; City Council; 19627 Part 2; Drainage Master Planfthronnlnnv if I Hyaruiugy Prepared by David Hauser 9/2-08 Chronology of Basin BJ Hydrology Studies No.Report Title Preparer Commissioned By Date Description Flooding Issues Related to Agua Hedionda Creek for Rancho Carlsbad Mobile Home Park (RCMHP) Howard Chang Consultants RCMHP Residents May 1996 Delineated the 100-year floodplain within RCMHP; evaluated channel capacity; identified channel deficiency; and characterized potential 100-year flooding conditions within the RCMHP B Flood Control Alternatives for Rancho Carlsbad Mobile Home Park Howard Chang Consultants RCMHP Residents June 1996 Evaluated several potential structural and non structural flood control measures to mitigate flooding impacts within RCMHP. Recommended a solution employing a non-structural flood control measure including advance warning and evacuation plan. Hydrology Study Update Howard Chang Consultants RCMHP Residents September 14, 1996 Letter report updating study to incorporate the then current conditions. Letter report addressing technical review of a 1992 hydrology study prepared by Ensign and Buckley.D Supplemental Hydrology Review Howard Chang Consultants RCMHP Residents November 10, 1996 Rancho Carlsbad Channel & Basin Project Rick Engineering Company City of Carlsbad June 30,1998 Hydrological analysis of full Agua Hedionda drainage basin and hydraulic analysis of the Calavera Creek and Agua Hedionda Creek channels through the RCMHP. Recommended construction of four detention basins, dredging and improvements to Calavera and Agua Hedionda Creek channels. Modeling included Calavera Dam retention without any valve outlet controls and a starting lake level elevation of 209. Rancho Carlsbad Mobile Home Park Preliminary Alternative Analysis For Agua Hedionda Channel Maintenance Rick Engineering Company City of Carlsbad January 8, 2003 Date of document stated as January 8, 2002. Analyzed potential alternative channel designs for Agua Hedionda channel improvement project. Deleted Calavera Creek channel widening and assumed diversion of some stream flow along north side of RCMHP wall. Slightly modified watershed boundaries to account for Robertson Ranch. No changes to Calavera Dam modeling from prior report. G Rancho Carlsbad Mobile Home Park Preliminary Alternative Analysis For Agua Hedionda Channel Maintenance Rick Engineering Company City of Carlsbad September 30, 2004 Hydrology analysis revised to incorporate proposed Lake Calavera Dam improvement project. Revised storage volume and included outflow through valves during storm event. Used the same water surface start elevation of 209 as in the prior reports. H Rancho Carlsbad Mobile Home Park Alternative Analysis For Agua Hedionda Channel Maintenance Rick Engineering Company City of Carlsbad December 13, 2004 Analyzed different Agua Hedionda Channel configurations. Included revisions to account for new regional water quality standards, Calavera Dam improvements and final design parameters for detention basins BJB, Faraday and Melrose. The modeling assumed opening of all valves at Calavera Dam but did not include lowering of lake level in advance of a forecasted storm. No analysis was done to determine what would result if Basin BJ was removed from the mix of proposed flood control improvements. Drainage Study for Robertson Ranch O'Day Consultants McMillin June 28,2005 Study prepared in support of the Robertson Ranch Master Plan project. Used rational method for project's onsite hydrology analysis. Included a hydraulic analysis prepared by Chang Consultants dated April 27, 2005 to analyze the proposed 84" storm drain pipe designed to divert flows away from the Calavera Creek channel. Robertson Ranch 84" Storm Drain Analysis Chang Consultants McMillin April 27, 2005 Study prepared in support of the Robertson Ranch Master Plan project's proposal for the 84" storm drain alternative. Study did not evaluate need for Basin BJ. K Rancho Carlsbad Mobile Home Park Alternative Analysis For Agua Hedionda Channel Maintenance Rick Engineering Company City of Carlsbad December 12, 2005 No changes in hydrology analysis. Included minor changes to floodplain mapping. Engineering Analysis of Detention Basin BJ Chang Consultants McMillin October 4, 2005 Used latest Rick Engineering modeling parameters as basis for the analysis. Modeling was revised to incorporate the proposed 84" storm drain pipe and diversion structure. An additional analysis was included that eliminated Basin BJ from the modeling. The analysis concluded that there would be no significant change in flood protection to the residents of the RCMHP with the removal of Basin BJ. Prepared by David Hauser September 8, 2008 M David Cannon EIR Comment Letter with City EIR Response David Cannon, PE Everest International Consultants Rancho Carlsbad Owners Association November 22,2005 Mr. Cannon submitted this letter in response to draft EIR for the Robertson Ranch Master Plan project (EIR 03-03). The letter included numerous opinions and comments challenging the validity of the assumptions and methodologies employed by the various engineering firms in the preparation of the hydrology and hydraulic studies used to support the Robertson Ranch project and the Rancho Carlsbad flood control program. Mr. Cannon did not undertake any independent studies of his own that contradicted the studies prepared by O'Day Engineering Consultants, Rick Engineering Company or Chang Consultants. As stated in his letter, Mr. Cannon concludes that the various studies are flawed and recommends that the hydrologic analysis for the Agua Hedionda drainage basin should completely and comprehensively redone. City staff disagrees with Mr. Cannon's opinions and conclusions. Staff is confident that the study methodologies are valid and that the conclusions presented in the various studies referenced in EIR 03-03 accurately and adequately represent the hydrologic and hydraulic conditions of Agua Hedionda and Calavera Creeks during the projected 100 year storm event. The City and its EIR consultants adequately responded to each of Mr. Cannon's concerns ia the response to comments section of EIR 03-03. N Final Conditional Letter of Map Revision Due to the Agua Hedionda & Calavera Creeks Dredging and Improvement Project Brown and Caldwell with included modeling by Chang Consultants City of Carlsbad June 2, 2008 A Conditional Letter of Map Revision (CLOMR) is a letter submitted to FEMA requesting conditional approval for a proposed alteration to the FEMA flood mapping. The specified CLOMR letter includes a completed application and supplemental hydrology and hydraulic analysis in support of the City's request to modify the 100-year floodplain boundaries within the RCMHP community once the Agua Hedionda and Calavera Creek Channel dredge and improvement project is complete. Included with CLOMR was the December 13, 2004 Rick Engineering Alternatives Analysis Report together with new supplemental modeling and analysis provided by Chang Consultants. The supplemental model analysis by Chang Consultants was commissioned by the City and included the Calavera Dam valve outlet control and lowering of the Calavera Lake level in advance of a major storm event to an elevation of 106. The supplemental Chang Consultants study did not include Basin BJ as the installation of Basin BJ is not contemplated to be part of the channel dredge and improvement project. The results of the analysis conclude that all but a few of the residences within RCMHP will be removed from the 100-year FEMA flood zone with the completion of the channel dredge and improvement project, assuming operation of Calavera Dam as a flood control facility and no construction of Basin BJ. O Hydrologic and Hydraulic Analyses Chang Consultants McMillin September 2,2004 Analysis of flood impact to Calavera Creek resulting from development of Robertson Ranch East project. Study concludes RR East Village will not adversely impact flooding within Rancho Carlsbad with assumed construction of the Master Planned facilities. Prepared by David Hauser September 8,2008 Flooding Issues Related to Agua Hedionda Creek for Rancho Carlsbad Mobile Home Park Prepared for Rancho Carlsbad Mobile Home Park Prepared by Howard H. Chang, Ph.D., P.E.' May, 1996 t Howard H. Chang Consultants Hydraulic and Hydrologic Engineering Erosion and Sedimentation P. 0. Box 9492 Rancho Santa Fe, CA 92067 TEL: (619) 756-9050 FAX: (619) 756-9460 c Flooding Issues Related to Agua Hedionda Creek for Rancho Carlsbad Mobile Home Park I ,:K,,s I. SUMMARY "^' ' Rancho Carlsbad Mobile Home Park is located along the banks of Agua Hedionda Creek in Carlsbad. This report was prepared for Rancho Carlsbad Mobile Home Park to address the flooding issues for the park. In order to assess the potential for flooding, a hydraulic study was made covering the following aspects: (1) Delineation of 100-yr floodplain for the park - A HEC-2 study was made for Agua Hedionda Creek through the park area. JMPPW&PW1*1^^ floliiftJf^W mlp! Certain areas outside• » ^i**» ». • % ^ the main channel are subject to shallow flooding. Manitou Engineering also determined the number of units subject to flooding. Flow velocities for the channel and floodplain area are provided in this report. Velocities outside the channel are too small to be hazardous. (2) Capacity for the existing channel - The existing channel for Agua Hedionda Creek through the park is insufficient to convey the 100-yr flood discharge. The maximum capacity of the channel was determined by HEC-2 computations to be about 6,500 cfs which is the 50-yr flood. (3) Duration of flooding for overbank areas of channel - The time duration of flooding of areas outside the channel was determined using the 100-yr hydrograph of the Agua Hedionda Creek. This duration is less than 2 hours. (4) Feasibility of floodwater detention: The golf course south of the park was considered as a possible floodwater detention basin in order to cut off the high flows of the channel. The volume of water storage provided by this area is insufficient for significant reduction of the flood peak. 1 H. HYDROLOGY FOR THE STREAM CHANNEL The hydrology for the stream flow of Agua Hedionda Creek was based upon the results of a hydrology study by Chang (1989). According to that study, the 100-yr flood discharges for Agua Hedionda Creek through Rancho Carlsbad Mobile Home Park are as follows: Upstream of the confluence with Calavera Creek: 8,080 cfs Downstream of the confluence with Calavera Creek: 9,850 cfs. The hydrographs for the 100-yr flood are shown in Fig. 1. The 10-yr flood discharges for Agua Hedionda Creek in this area are given below: Upstream of the confluence with Calavera Creek Downstream of the confluence with Calavera Creek Discharges for floods of other return periods were determined based on a statistical analysis. It was assumed that flood discharges for the creek follow a log-normal distribution as shown in Fig. 2. Discharges obtained from this distribution are tabulated below. Return Period Years 100 90 80 70 60 50 40 Discharge, cfs Above confluence 8,080 7,700 7,450 7,200 6,800 4. . 6,400 6,000 Discharge, cfs Below confluence 9,850 9,700 9,300 8,800 8,300 7,800 7,300 •C 30 20 10 5,300 4,700 3,470 6,400 5,400 3,900 m. THE 100-YR FLOODPLAIN A floodplain is the area subject to inundation by a 100-yr flood. Water-surface elevations for Agua Hedionda Creek were obtained using the HEC-2 computer program by the U. S. Army Corps of Engineers. Locations of cross sections used to define the channel and floodplain geometry are marked on the topographic map. Profiles of these cross sections near the main channel are shown in Fig. 3 . ^m^m&^min&x sttii&itt rt&&'"y&te; on the order of 2 to 3 fett/testd on * ^ The computed water-surface profile for the 100-yr flood together with the existing channel- bed profile are shown in Fig. 4. The bridge at Rancho Carlsbad Drive is subject to overtopping during the 100-yr flood, Ttejmtuf ti» ftiiiiil*iiiii mm iwnim IIMI Hinmgli iln bridge opening Wlff^ito ww *e ««a W>f. A part of the flow may also spread out toward the north to affect other areas. IffigKl^topigmiitteod^ racoanaendtid that an. wm be the flow. »T*ente*»» seeds. Jo -have, the, top,.?leyation of ,62 Engineering, Other computed results for the 100-yr year flood are summarized in the table below. The table contains the information on the flood discharge (Q, in cfs), computed water-surface elevation (CWSEL, in feet), depth of flow (DEPTH, in feet), discharge in the main channel (QCH, in cfs), velocity in the main channel (VCH, in feet per second), velocity in the left overbank area (LOB, in feet per second), and velocity in the right overbank area (ROB, in feet per second). It can be seen that overbank velocities are generally too small to present any hazard. SECNO CWSEL DEPTH QCH VCH LOB ROB .010 1.100 2.800 5.000 6.210 7.580 9.150 12.200 14.650 17.190 19.830 21.510 22.810 25.410 25.860 27.160 28.040 29.180 30.120 30.500 9850.00 9850.00 9850.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 8080.00 40.24 42.20 42.39 45.41 45.20 45.29 45.75 46.52 47.44 47.96 48.77 49.04 49.13 49.62 52.12 52.84 53.56 55.59 56.18 59.22 7.24 8.90 8.69 11.11 10.30 9.69 9.65 9.92 9.74 9.46 9.47 8.74 8.23 7.72 9.82 10.14 9.96 11.09 11.28 14.32 9850.00 9850.00 9820.21 8032.94 7928.59 7925.38 7948.00 7910.44 7819.05 8080.00 8065.18 8001 .81 7962.17 8080.00 8040.08 8080.00 8080.00 8080.00 8080.00 7303.20 13.07 10.55 15.07 9.00 10.93 11.80 11.63 11.58 10.62 10.58 9.76 9.88 10.74 11.73 14.61 15.20 14.88 11.21 19.00 13.50 .00 .00 2.05 1.41 1.40 1.36 1.60 1.00 2.03 .00 1.30 1.46 2.05 .00 1.58 .00 .00 .00 .00 1.48 .00 .00 .55 1.42 1.78 2.31 1.96 1.88 .00 .00 .54 .69 .00 .00 .00 .00 .00 .00 .00 1.73 IV. CAPACITY OF THE EXISTING CHANNEL Since the 100-yr floodplain extends beyond the banks of the main channel, the existing channel for Agua Hedionda Creek is insufficient to convey the 100-yr flood discharge. The maximum capacity of the channel was determined by HEC-2 computations as described below. Water-surface profiles of the channel were obtained using the HEC-2 program for several flood discharges lower than the 100-yr discharge. The computed results are summarized in the table below. The water-surface profiles are shown in Fig. 5. The water-surface elevations for each profile were compared with the bank height of the channel. From these comparisons, it may be concluded that the channel under its present configuration has the bankfull discharge of about 6,500 cfs. This discharge has the approximate return period of 50 years. SECNO CWSEL DEPTH QCH VCH LOB ROB .010 8300.00 .010 7200.00 39.39 38.88 6.39 8300.00 5.88 7200.00 12.56 11.88 .00 .00 .00 .00 .010 6000.00 38.33 S.33 6000.00 11.00 .00 .00 1.100 1.100 1.100 2.800 2.800 2.800 5.000 5.000 5.000 6.210 6.210 6.210 7.580 7.580 7.580 9.150 9.150 9.150 12.200 12.200 12.200 14.650 U.650 14.650 17.190 17.190 17.190 19.830 19.830 19.830 21.510 21.510 21.510 22.810 22.810 8300.00 7200.00 6000.00 8300.00 7200.00 6000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 41.24 40.52 39.69 41.53 40.92 40.22 44.33 43.58 42.67 44.11 43.41 42.57 44.25 43.55 42.73 44.97 44.29 43.55 45.80 45.16 44.45 46.70 45.99 45.21 47.26 46.59 45.85 48.07 47.39 46.62 48.33 47.64 46.87 48.43 47.74 7-94 7.22 6.39 7-83 7.22 6.52 10.03 9.28 8.37 9.21 8.51 7.67 8.65 7.95 7.13 8.87 8.19 7.45 9.20 8.56 7.85 9.00 8.29 7.51 8.76 8,09 7.35 8.77 8.09 7.32 8.03 7.34 6.57 7.53 6.84 8300.00 7200.00 6000.00 8291.49 7197.76 5999.95 6971.09 5983.95 4996.08 6958.36 5986.28 4998.66 6940.34 5973.48 4996.50 6941.17 5983.74 4995.51 6922.42 5975.10 4998.13 6849.79 5933.47 4985.33 rooo.oo 6000.00 5000.00 6992.88 5996.05 4998.89 6970.86 5991.05 4998.32 6933.26 5972.94 10.01 9.59 9.08 14.45 13.84 13.07 8.89 8.44 8.08 10.96 10.37 9.83 11.91 11.40 10.97 11.24 10.69 10.07 11.11 10.49 9.79 10.20 9.75 9.24 10.11 9.60 9.03 9.24 8.70 8.14 9.46 9.01 8.52 10.35 9.95 .00 .00 .00 1.46 1.06 .41 1.28 1.06 .65 .99 .93 .53 .35 .00 .00 1.09 .36 .00 1.12 .84 .45 1.67 1.26 .84 .00 .00 .00 1.17 .97 .64 1.11 .96 .64 1.67 1.19 .00 .00 .00 .00 .00 .00 1.29 1.08 .68 1.28 .96 .55 1.95 1.46 .76 1.58 1.23 1.07 1.45 .93 .46 .00 .00 .00 .00 .00 .00 .01 .00 .00 .33 .00 .00 .00 .00 22.810 5000.00 46.98 6.08 4994.05 9.50 .82 .00 * * # * * # « * * • * * • * • « * * • * * * * * 25.410 25.410 25.410 25.860 25.860 25.860 27.160 27.160 27.160 28.040 28.040 28.040 29.180 29.180 29.180 30.120 30.120 30.120 30.500 30.500 30.500 31.160 31.160 31.160 31.940 31.940 31.940 33.140 33.140 33.140 34.960 34.960 34.960 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 4a-97,^'«>-33 ,* 47.64 51-20 . _,., 50.48' 'J 49.72 *' 52.11 ^3 51 -3B -,* 50.59 52.85 52.15 51.38 54.79 54.00 53.13 55.12 54.15 53.12 55.12 54.36 55.36 59.33 57.39 56.88 59.94 59.38 58.20 60.34 59.95 58.90 61.47 61.02 59.31 7.07 6.43 5.74 8.90 8.18 7.42 9.41 8.68 7.89 9.25 8.55 7.78 10.29 9.50 8.63 10.22 9.25 8.22 10.22 9.46 10.46 12.03 10.09 9.58 11.24 10.68 9.50 11.14 10.75 9.70 11.57 11.12 9.41 7000.00 6000.00 5000.00 6991 .38 5999.22 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 7000.00 6000.00 5000.00 6656.11 5898.59 4657.58 5717.86 5203.97 3166.84 5646.88 5058.06 2922.02 6568.90 5771.43 3068.97 11.32 10.85 10.37 14.52 13.97 13.32 14.67 14.11 13.47 14.39 13.86 13.25 10.81 10.37 9.88 18.18 17.22 16.18 18.17 16.83 12.68 9.74 10.80 9.11 12.17 11.82 8.40 12.42 11.66 7.72 9.49 8.86 6.13 .00 .00 .00 1.26 .70 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 3.42 2.45 1.21 3.61 2.86 1.95 4.15 3.50 2.15 2.64 2.02 2.48 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 3.65 2.79 1.78 4.46 3.84 2.45 4.41 3.77 1.33 2.67 2.04 .00 t V. DURATION OF FLOOD FOR OVERBANK AREAS OF CHANNEL The time duration of flooding for areas outside the channel was determined using the 100- yr hydrograph for Agua Hedionda Creek shown in Fig. 1. Based on the hydrograph upstream of the confluence with Calavera Creek, the duration for discharges greater than 6,500 cfs is less than 2 hours. VI. FEASIBILITY OF FLOODWATER DETENTION The golf course east of the Mobile Home Park and south of the stream channel was considered as a possible site for a floodwater detention basin in order to cut off the high flows of the channel. The feasibility of this scheme was determined as described below. The golf course is a lower area or a basin. The floodwater detention basin may be created by a berm separating the basin from the main channel. Flows higher than 6,500 cfs would be spilled from the channel by overtopping the berm into the basin. In order to provide adequate floodwater storage, the basin must have a capacity equal or greater than the volume of water for the flood at discharges higher than 6,500 cfs. The volume of floodwater is given by the area under the hydrograph. For discharges higher than 6,500 cfs, the volume of floodwater is obtained from Fig. 1 to be 96 acre-feet. The maximum volume of storage that may be provided by the detention basin was measured from the topographic map of the golf course. The volume of storage below the elevation of 58 feet is about 25 acre-feet. Since the volume of storage is much less than the volume of floodwater, the floodwater detention scheme is not feasible in this case. REFERENCES 1. Chang, H. H., 1989, "Hydrological Study for Northeastern Carlsbad". LIST OF FIGURES Fig. 1. Hydrographs of 100-yr flood for Agua Hedionda Creek in the Park Fig. 2. Recurrence frequency versus flood discharge for Agua Hedionda Creek Fig. 3. Cross-sectional profiles of stream in the park Fig. 4. Computed water-surface profile of the 100-yr flood Fig. 5. Computed water-surface profiles for several flood discharges APPENDIX. Input/Output Listings of HEC-2 for the 100-yr flood «. - u. JUN 2 7 1995 *Flood Control Alternatives for Rancho Carlsbad Mobile Home Park Prepared for Rancho Carlsbad Mobile Home Park COPY Prepared by Howard H. Chang, Ph.D., P.E. June, 1996 ^^^^ Howard H. Chang Consultants Hydraulic and Hydrologic Engineering Erosion and Sedimentation P. O. Box 9492 Rancho Santa Fe, CA 92067 TEL: (619) 756-9050 FAX: (619) 756-9460 TABLE OF CONTENTS I. INTRODUCTION 1 II. ALTERNATIVE 1 - ON-SITE FLOOD WATER DETENTIONS 2 Requirements for Floodwater Detention Basins 2 Cost Estimates 4 III. ALTERNATIVE 2 - DIVERSION OF FLOODWATER FROM AGUA HEDIONDA CREEK 5 Preliminary Diversion Channel Geometry 6 Inlet Structure 8 / Redesign of Calavera Creek Channel 8 Cost Estimate 9 Flow Diversion to the South 10 Cost Estimate for this Option 11 IV. LEVEES FOR THE CHANNEL 13 Cost Estimate 14 V. CONCLUSIONS AND RECOMMENDATIONS 15 Recommendations 16 REFERENCES 17 LIST OF FIGURES _,_.. ,^_? , s . . ,^. ... 17 FIGURES FLOOD CONTROL ALTERNATIVES FOR RANCHO CARLSBAD MOBILE HOME PARK I. INTRODUCTION This report is in supplement to a previous report entitled "Flooding Issues Related to Agua Hedionda Creek for Rancho Carlsbad Mobile Home Park". The existing channel of Agua Hedionda Creek through the mobile home park has been found inadequate to handle the 100-yr flood. Certain overbank areas along the channel are within the floodplain and therefore subject to inundation. The capacity of the existing channel was found to be the 50-yr flood. In an effort to solve the flooding issues, numerous consultations were held with the City of Carlsbad. Flood control alternatives were considered and discussed in recent meetings. Pursuant to these recent meetings and discussions, certain flood control alternatives for the Rancho Carlsbad Mobile Home Park were identified. This report provides a study of the following flood control alternatives. Alternative 1. On-site floodwater detention Alternative 2. Diversion of flood flow from Agua Hedionda Creek Alternative 3. Levees for Agua Hedionda Creek Other measures including the advanced flood warning system and evacuation plan are being undertaken by other consultants. Flood hydrographs for Agua Hedionda Creek are shown in Fig. 1. The 100-yr flood has the peak discharge of 8,080 cfs through most of the park. The flood discharge was obtained for the ultimate development of the drainage basin for Agua Hedionda Creek. The maximum capacity of the channel was determined by HEC-2 computations in the previous study to be 6,500 cfs, which has the approximate return period of 50 years. Flooding duration of overbank areas for Agua Hedionda Creek in the park is found to be about 2 hours based on the hydrograph. The 100-yr flood hydrograph obtained from the previous hydrology study (Chang, 1989) for Calavera Lake Creek at the Park is shown in Fig. 2; it has the peak discharge of 1,740 cfs. It is 1 important to point out that the hydrographs shown in Figs. 1 and 2 for these two streams have almost the same time to peak, at the 11th hour. II. ALTERNATIVE 1 - ON-SITE FLOODWATER DETENTIONS Under this alternative, on-site floodwater detention basins will be constructed; they will be used to store floodwater for Agua Hedionda Creek such that the 100-yr peak flood will be reduced to be within the capacity of the existing channel in the park. The basins, of course, should be located east of the Park to capture the flow before it enters the Park. The feasibility of constructing the detention basin was investigated. The hydraulic requirements for such basins are provided in this report, together with a cost estimate for construction. The volume of floodwater is given by the area under a hydrograph. Based on the 100-yr flood hydrograph in Fig. 1, the volume of floodwater for discharges higher than 6,500 cfs is obtained to be 96 acre-feet. In order to reduce the downstream peak discharge down to 6,500 cfs, the volume of 96 acre-feet must be stored in the basins. In other words, the detention basins must have a combined storage capacity of at least 96 acre-feet. The golf course east of the Mobile Home Park is the logical site for such basins. It was assumed that the present golf course grounds both north and south of the stream channel and other undeveloped areas in the vicinity may be used as the detention basin sites. The golf course south of the stream channel is a presently a lower area or a basin. Higher areas may also be graded to a lower elevation. Each floodwater detention basin will be created by a berm separating the basin from the main channel. Flows lower than 6,500 cfs would stay in the channel, but those higher than 6,500 cfs would spill from the channel into the basin by overtopping a weir. Requirements and evaluation of the feasibility for this scheme are described below. Requirements for Floodwater Detention Basins - The basic requirements for the floodwater detention basins are summarized below: (1) They must have a combined storage capacity of at least 96 acre-feet. (2) Water storage in the basins should be maintained at the surface elevation of 58 feet or lower to avoid overtopping of Rancho Carlsbad Drive or flooding of adjacent areas. (3) The bottom elevation of a basin should be higher than the low flow water surface in the channel of Agua Hedionda Creek so that water stored in the basin may be drained into the creek by gravity. Draining of the basin by pumping is not considered an option. Storm water in the basin prior to the design flood may be drained if the basin level is higher than the low flow water surface in the main channel. The stream typically has a low flow prior to the 100-yr flood. Such a low flow may be assumed to have a water depth of 3 to 4 feet. Based on this consideration, the bottom elevation of the basin should be kept at 53 feet or higher. (4) Based on the requirements described above, the average depth for water storage in the basin is 5 feet. Since the total required storage volume is 96 acre-feet, the surface area of the basins needs to be 20 acres. This surface area will occupy all the low-land areas east of the Park to the edge of the equestrian practice area. Fig. 3 shows the possible sites for these basins. (5) A berm surrounding each basin is required to prevent local drainage from getting into the basin. Storm water runoff from adjacent hillsides should be drained into the stream directly bypassing the basin. The berm can be made of earth; its top elevation should be 2 feet above the adjacent ground outside the basin. (6) A berm is also needed to separate a basin from the adjacent stream channel. Because of limited available space, this berm will be small in width. The berm must be made to be impervious to prevent stream water from getting into the basin by seepage. Since the berm is quite small in thickness, its surface along the main channel must be protected with riprap to prevent scour. Bank protection for the channel must be designed by a registered civil engineer. Geotechnical engineer shall be consulted on the seepage protection of berm. (7) Whenever water level in the stream channel exceeds a certain value, the excess flow will be diverted to the basin by overflowing the weirs. Inlet structures are needed to control streamflow into the basin. Overflow weirs will be installed at the inlet to the design elevation. The hydraulic geometry of a weir must be designed based on detailed hydraulic computation, such that only flow greater than 6,500 cfs will be admitted to the basin. The weirs must be of adequate size to handle the excess discharge. (8) Outlet structures from the basin to the stream channel shall be designed and equipped with one-way control gates. Flap gates, such as those shown in Fig. 4, are typically used to allow basin water draining into the stream channel but not to permit stream flow into the basin. Cost Estimates: The cost for the requirements described above are itemized below. (1) Earth moving and disposal: Excavation of the basins requires earth moving of 180,000 cubic yards. The earth removed from the basins will be disposed at nearby sites to be selected. The unit cost depends on the location of disposal sites; it is assumed to be $ 1.20 per cubic yard for excavation and disposal. Cost for item $ 216,000 (2) Berms surrounding basins: The total cost is estimated to be $ 20,000. Cost for item $ 40,000 (3) Bank protection for berms adjacent to channel: These berms will have a total length of about 2,300 feet along both banks of the channel for Agua Hedionda Creek. Bank protection will have the length of 2,300 feet, 2 to 1 side slope, height of 14 feet including toe entrenchment, surface width of 28 feet, and surface area of 64,400 square feet. The unit cost is assumed to be $ 7 per square foot. Cost for item $ 450,800 (4) Inlet structure $ 60,000 (5) Outlet structure and flap gates $ 15,000 (6) Golf course reconstruction $ 1,400,000 (7) Loss of income during golf course reconstruction for 9 months $ 350,000 (8) Loss of residents activities for 9 months $ 80,000 (9) Permit fees: Permits for the project may be required from the City, County, Army Corps of Engineers, U. S. Fish and Wildlife, California Fish and Games, etc $ 150,000 Subtotal $ 2,761,800 Continency: 10 % of the combined cost $ 276,180 Total for this alternative $ 3,037,980 € III. ALTERNATIVE 2 - DIVERSION OF FLOODWATER FROM AGUA HEDIONDA CREEK Agua Hedionda Creek at the Park has the capacity only for the 50-yr flood. An idea suggested by the City of Carlsbad for flood control is to divert the part of flood discharge beyond the channel capacity of Agua Hedionda Creek to the north to Calavera Lake Creek or to the south across El Camino Real directly to the lagoon. These schemes are now investigated; their details are described below. It should be noted that the diversion to the north alternative is contingent upon the completion of the proposed Calavera Lake Creek channel improvement. A diversion channel from Agua Hedionda Creek to Calavera Lake Creek is expected to be on the east side of the mobile home park. A possible alignment for the diversion channel is shown in Fig. 5. In order to handle the required discharge of diversion, this channel must have sufficient width, depth, and slope. Since the channel must also fit in the existing physical constraints of topography and land use, it became necessary to investigate the feasibility as the first step. The purpose of the feasibility study is to determine if a diversion channel with adequate capacity can be constructed within the given physical constraints. If it is found feasible, then hydraulic geometries of the channel and a cost estimate shall be provided in the study. This diversion will increase the flow of Calavera Lake Creek, therefore this creek must be enlarged to handle the augmented flow. It is the assumption that sedimentation problems for Calavera Lake Creek will be considered as a part of the future channel design. Sediment investigation for Agua Hedionda Creek and Calavera Lake Creek is not a part of the present scope. It should be pointed out that a detailed hydraulic analysis for the channel system will involve the use of the HEC-2 program that is outside the present scope. In such an analysis for the channel system including Agua Hedionda Creek, the diversion channel, and Calavera Creek, different sets of channel geometries shall be used for the diversion channel, the intake, and Calavera Creek channel. Physical constraints shall be applied. These channel geometries shall be adjusted in order to arrive at the adequate geometries, it possible. The detailed analysis will also involve hydrologic simulations to determine the modified hydrographs. The diversion will change the flood discharges for Agua Hedionda Creek and Calavera Lake Creek. Hydrologic simulations shall be made for flood routing from Agua Hedionda Creek to Calavera Lake Creek. Since the times for floods to peak for these creeks are somewhat different due to the different drainage basin areas, the timing for these flood hydrographs shall be considered in flood routing in arriving at the modified hydrographs as a result of diversion. Preliminary Diversion Channel Geometry - For this feasibility study, the effort is to arrive at preliminary estimates of the hydraulic geometry such that a preliminary cost estimate can be made at this time. In the following steps, the hydraulic geometry of the diversion channel as shown in Fig. 6 is first obtained. (1) Channel length: For the channel alignment along the eastern boundary of the Park, the route' from Agua Hedionda Creek to Calavera Creek has a length of about 2,400 feet. This means that the diversion channel shall have the length of 2,400 feet. (2) Channel capacity; The diversion channel must have the minimum capacity of 1,600 cfs. This value is the 100-yr discharge of 8,080 cfs subtracted by 6,500 cfs which is the channel capacity for Agua Hedionda Creek. To allow a safety margin, the diversion channel should have the design capacity of 2,000 cfs. (3) Permissible velocity: An earth channel with natural vegetation is recommended for the diversion channel. This type of channel is considered more desirable environmentally. In order to maintain channel stability, the maximum velocity should be kept at 5 feet per second or lower to avoid significant potential scour. In other words, the velocity of 5 feet per second is the permissible velocity. 4fe (4) Channel cross-sectional area: The cross-sectional area is the channel capacity divided by the permissible velocity, i.e., A = 2,000/5 = 400 square feet. (5) Channel depth: The diversion channel should have a depth no greater than 10 feet in order to be compatible with Agua Hedionda Creek. With a freeboard of 2 feet, the flow depth is 8 feet in the channel. (6) Side slope: The channel should have a side slope of roughly 2.5 to 1 to maintain stability for the earthen bank material. (7) Channel roughness: With natural vegetation, the roughness coefficient in terms of Manning's n is estimated to be 0.04. (8) Bed width: Based on the conditions described above, the bed width of the channel is computed to be 30 feet. Its water-surface width is 70 feet, and the channel width is 80 feet at the top. These dimensions provides a cross-sectional area of flow of 400 square feet. The wetted perimeter of .the channel is 73.1 feet, and its hydraulic radius is 5.47 feet. (9) channel slope: The channel slope is computed using the Manning formula based on the conditions given above. S = nV/(2.21xR4D) = 0.042 x 52/(2.21x5.474'3) - 0.00188 (10) Required energy head: The required energy head includes the drop in water surface elevation along the diversion channel from Agua Hedionda Creek to its outlet at Calavera Lake Creek. The head difference is the channel slope times the channel length. Additional head is required at the inlet, estimated to be 0.5 feet. Head = 2,400 x 0.00188 + 0.5 = 5 feet The terrain is rather flat along the route of the diversion channel. It is possible to achieve the 5 feet of energy head. However, this will require a lowering and a redesign of the Calavera Creek channel. Inlet Structure - The flow admitted into the diversion channel must be controlled. The inlet structure, as shown schematically in Fig. 7, is used to control the inflow to the diversion channel. This structure may be a guide vane that extends into the main channel to split the total flow into two parts. The flow split must be carefully designed such that the diversion channel carries the flow in excess of the channel capacity for Agua Hedionda Creek. The inlet structure may also be a weir. The shape of the weir must be carefully designed to achieve the object of flow control. Control gates should be avoided if possible. Redesign of Calavera Creek Channel - A preliminary design of the Calavera Creek channel was made some 8 years ago by P&D Consultants. The designer was Mr. Dicken Hall. The previous design assumed a channel alignment outside the fence of the Mobile Home Park. As a result of the flow diversion, the hydraulic geometry for Calavera Lake Creek must be adjusted to handle the augmented flow discharge. This channel must also meet Agua Hedionda Creek at the existing confluence just upstream of the El Camino Real Bridge. The existing bed elevation at the confluence is the downstream control point for Calavera Lake Creek. Any adjustments in channel-bed slope shall still meet this base elevation. (1) Change in flood discharge - With a part of the flow diverted from Agua Hedionda Creek, the flood discharge of Calavera Lake Creek will be augmented. A comprehensive hydrology analysis is required to route the flow from Agua Hedionda Creek into Calavera Creek. The routing study will produce the new flood discharge while considering the timing of these flow events. Flood hydrographs for these two channel as shown in Figs. 1 and 2 indicate that the peak discharges of these two streams occur at nearly the same time. As a rough estimate, the increase in discharge for Calavera Lake Creek may be computed by adding the diverted flow of 2,000 cfs to the 100-yr discharge of 1,750. Thus, the new 100-yr discharge will be 3,750 cfs for Calavera Lake Creek 8 after the confluence of the diversion channel. (2) Change in cross-sectional area - Under the previous design, the improved Calavera Channel would be an environmental channel which maintains a low velocity for the sake of channel stability. With a new discharge more than twice the original value, the cross-sectional area of the channel will be increased by 1.14 times. Other channel dimensions must be adjusted by the designer to accommodate the additional flow. (3) Change in channel slope - The Calavera Lake Creek channel is the downstream control for the diversion channel. In order to provide the required energy head for the diversion channel, the slope of the Calavera Lake Creek channel must be adjusted. Any change in slope will also involve changes in other channel dimensions. (4) Feasibility for channel modifications: The slope of the terrain along the Calavera Lake Creek has a drop in elevation of more than 10 feet from El Camino Real to the northeastern edge of the Park. This slope is considered adequate for the future channel with the required modifications. Cost Estimate - Preliminary cost estimates for this diversion alternative are provided herein. The cost estimate in this study reflects costs for materials and labor, engineering design, and right-of- way acquisition. The material and labor cost was based upon construction costs for local projects. The itemized costs are given in the following: (1) Right of way: The diversion channel will have a surface area of 5.5 acres. The decrease in land value due to channel construction is estimated to be $ 38,000 per acre. Cost for item , $ 209,000 (2) Diversion channel: Construction of the channel will involve excavation of 50,000 cubic yards, which costs $1.2 per cubic yard. Cost for item $ 60,000 (3) Inlet structure: $ 50,000 (4) Additional cost for Calavera Lake Creek channel $ 250,000 (5) Channel maintenance at $ 3000 per year for 100 years . •. $ 300,000 (6) Permit fees: Permits for the project may be required from the City, County, Army Corps of Engineers, U. S. Fish and Wildlife, California Fish and Games, etc $ 150,000 Subtotal $ 1,019,000 Contingency: 10 % of subtotal $ 101,900 Total for this alternative $ 1,120,900 Flow Diversion to the South - This scheme of flow diversion was also suggested by the City of Carlsbad as a possible option. The alignment for this option starts from Don Pablo Drive in the Park, it goes under El Camino Real and then on to join Agua Hedionda Creek south of El Camino Real. Requirements for this diversion option are as follows: (1) Inlet structure at Agua Hedionda Creek - The flow admitted into the diversion channel must be controlled. The inlet structure is used to control the inflow to the diversion channel. This structure may be a guide van that extends into the main channel to split the total flow into two parts. The flow split must be carefully designed such that the diversion channel carries the flow in .excess of the channel capacity for Agua Hedionda Creek. The inlet structure may also be a weir. The shape of the weir must be carefully designed to achieve the object of flow control. Control gates should be avoided if possible. (2) Flood control near Don Juan Drive: The flow diversion will relieve flooding of those areas on the downstream side of Don Pablo Drive, but it does not solve the flooding problem on the upstream side. The Park land in the vicinity of Don Juan Drive still needs flood control works. Possible options include raising of the ground surface elevation to be above the 100-yr flood level, or the construction of a flood control levee along the channel bank adjacent to Don Juan Drive. (3) Diversion channel in the Park: For the diversion channel reach in the Park, it runs along the northwestern side of Don Pablo Drive. In order to save space, a concrete-lined rectangular channel is recommended. The rectangular section, 20 feet wide and 8 feet deep, is an adequate hydraulic geometry for the channel. This channel has a design velocity of 15 feet per second; therefore, fences are needed along both channel banks for safety reasons. To allow space for the 10 channel, a few existing mobile home lots will be sacrificed. (4) Bridges at Don Ricardo and Don Miguel: These bridges are required on the diversion channel in the Park. (5) Underpass at El Camino Real: Double 10' x 71 box culverts with a length of 120 feet are considered adequate. There exist several utility lines under El Camino Real, including those for gas, electricity, sewer, water, telephone, and cable. It will be necessary to raise or to lower these utility lines to make room for the culverts. (6) Concrete-lined channel south of El Camino Real: A concrete-lined trapezoidal channel along the south edge of El Camino Real is a part of the diversion channel. This channel has the bed width of 11 feet, side slope of 1 to 1, channel depth of 8 feet, and surface width of 27 feet. This channel is 1,600 feet long. (7) Energy dissipator at downstream exit of diversion channel: The diversion channel exits into Agua Hedionda Creek south of El Camino Real. The exit is near the proposed Cannon Road Bridge. Because of the high flow velocity in the diversion channel, an energy dissipator is needed at the exit. (8) Feasibility: It is hydraulically feasible to construct the diversion channel in consideration of the total energy head available. The drop in terrain elevation from the entrance of the diversion to its outlet is about 8 feet. This total head is sufficient for the concrete-lined channel described above. However, the concrete-lined channel needs to be kept clean to maintain its function. Cost Estimate for this Option - Cost for required items are listed below: (1) Elimination of existing lots at Park: Lots 53 and 87 will be eliminated to make room for the channel. The lot price is $ 85,000 per lot. Cost for item $ 170,000 (2) Right of way acquisition south of El Camino Real: 1.5 acres at $ 25,000 11 per acre $ 37,500 (3) Inlet structure $ 90,000 (4) Concrete channel in Park: Excavation of 3,900 cubic yards at $ 1.2 per cubic yard yields $4,680; Concrete bed 11,000 ft2 at $ 4 per ft2 gives $ 44,000; Retaining walls 8,800 ft2 at $ 9.5 per square foot yields $ 83,600. Cost for item $ 132,280 (5) Box culverts: 120 feet long at $ 720 per foot $ 86,400 (6) Road work for El Camino Real: 4,800 sqaure feet at $ 3 per square feet $ 14,400 (7) Excavation for culvert placement: 1,250 cubic yards at $1.2 per cubic yard ... $ 1,500 (8) Concrete channel south of El Camino Real: Excavation of 16,600 cubic yards at $1.2 per cubic yard yields $ 19,200; 54,400 square feet of concrete at $ 2.85 per square foot gives $ 155,040. Cost for item $ 174,240 (9) Outlet structure: $ 50,000 (10) Levee for main channel $ 10,000 (11) Bridges at Dom Ricardo Dr. and Don Miguel Dr $ 85,000 Mr (12) Relocation of utility lines under El Camino Real Over $ 1,000,000 (13) Permit fees: Permits for the project may be required from the City, County, Army Corps of Engineers, U. S. Fish and Wildlife, California Fish and Games, etc $ 150,000 Subtotal Over $ 2,001,320 Contingency: 10 % of subtotal $ 200,132 Total for this alternative Over $ 2,201,452 12 IV. LEVEES FOR THE CHANNEL Under this alternative, levees will be constructed along the banks of Agua Hedionda Creek through the Park to contain the 100-yr flood. An analysis has been made for this alternative. Hydraulic requirements for levees have been obtained and provided. The effectiveness and impacts of this alternative on flood control have been analyzed. A cost estimate is also presented. While the levees keep the channel water away from the park land, it must allow the drainage of local storm runoff into the channel. This function can be served by using flap gates installed on the levees that allow one-way flow into the channel. Peak flow in the channel lags behind the local runoff in timing because of the difference in the time of concentration. Therefore, local runoff may be drained through the flap gates before the channel peaks. Levees will prevent flooding of the park land by keeping floodwater in the channel. However, this alternative for flood control has the following considerations: (1) Encroachment of channel flow by levees will raise the flow velocity in the channel. While such raises may be small, they, nevertheless, increase the channel instability. For this reason, the levees may become a legal liability. It is therefore advisable to obtain the consent of residents before proceeding with this alternative. (2) The levees are not used to improve the stability of the flood channel. In fact, the levees are subject to flood damages if its foundation is scoured. Prevention of potential scour will involve extensive construction that is outside the scope of the present study. (3) Presence of levees causes view obstruction which may be considered undesirable environmentally. Now, required features of the levees are described below. (1) Length of levees - Levees are needed along the channel reach where the overbank areas are subject to flooding by the 100-yr flood. Based on this criterion, the levees should start from the 13 wooden bridge at Rancho Carlsbad Drive and ends at El Camino Real for a total channel length of about 2,500 feet. (2) Height of levees - The 100-yr water-surface profile for the channel was computed and presented in the pervious study. Construction of levees at the top of the channel bank will encroach on the width of the floodplain by excluding overbank areas from the channel. Such an encroachment will raise the water-surface elevation. The top of the levee should be the 100-yr flood level plus a freeboard of about 2 feet. Based on this criterion, the levee height varies from 2 feet to 5 feet, with the average being about 3.5 feet. (3) Toe of levees - During the design flood, the levees will be under water pressure from the channel side. The toe entrenchment must be sufficient to resist the turning moment due to hydrostatic pressure. The average toe entrenchment is estimated to be about 3 feet to protect it against turning. (4) Control gates - The levees must be equipped with one-way flow control gates. Such control devices are needed to drain water from the park area into the channel. (5) Water proof - The levees must be impermeable. Cost Estimate - Cost for this alternative include the costs for levees, control gates, etc. (1) Levees, 32,500 square feet, at $ 7.00 per sqaure foot $ 227,500 (2) Flap gates $ 5,000 (3) Decrease in property value, 15 % decrease for 42 lots valued at $ 85,000 per lot $ 535,500 (4) Permit fees: Permits for the project may be required from the City, County, Army Corps of Engineers, U. S. Fish and Wildlife, California Fish and Games, etc $ 300,000 Subtotal $ 1,068,000 Contingency at 10 % of subtotal $ 106,800 Total for this alternative $ 1,174,800 14 c V. CONCLUSIONS AND RECOMMENDATIONS This report provides analyses and cost estimates for three flood control alternatives for the Rancho Carlsbad Mobile Park. The Park has a total area of 154 acres; it is located in the drainage basin of Agua Hedionda Creek that is 17.3 square miles in area at this point. The Park constitutes 1.4 % of the total drainage basin area for the creek. There have been several flood control studies for the Park in recent years. The fundamental problem stems from the fact the channel has the capacity for the 50-yr flood which no longer satisfies the present standard for flood control in the County of San Diego. Since a part of the park land is subject flooding by the 100-yr flood, flood control has thus become a concern. It is also recognized that the duration of flooding for overbank areas of the channel is about 2 hours during the 100-yr flood. Potential damages due to overbank flooding has been found to be limited in value. The existing channel is partially protected with bank protection. While erosion potential for the channel still exists, erosion-related damages by floods may not be accurately estimated. This study does not cover Calavera Lake Creek, for which a flood control measure has already been adopted by the City of Carlsbad. Under this measure, Calavera Lake Creek will be improved to convey the 100-yr flood. Implementation of this measure will benefit the mobile home park. Alternative 1 for flood control is to use floodwater detention basins for flood discharge reduction. Alternative 2 is to reduce the discharge of Agua Hedionda Creek by diverting a part of flow away. It should be noted that the diversion to the north alternative is contingent upon the completion of the proposed Calavera Lake Creek channel improvement. Damages to the park due to flooding has been estimated by the Park manager to be $ 5,000 for each occurrence. In a 100-yr time span, the occurrence of flooding is also the frequency that a 50-yr storm is exceeded. In other words, two flooding occurrences can be expected in the 100- yr time span. With flood control, flooding damages will be eliminated and thus the benefits due to flood control is $ 10,000 in a 100-yr time span. 15 The costs for the alternatives are summarized below. It is clear that the cost for each alternative is much higher than the flood control benefits it provides. The benefit-cost ratios for these alternatives may not justify the undertaking. Alternative 1 . Detention basins 2a. Diversion to the north 2b. Diversion to the south 3. Levees Total Cost $ 3,037,980 $ 1,120,900 Over $2,201 ,452 $ 1,174,800 Benefit-Cost Ratio 0.0033 0.0089 Less than 0.0045 0.0085 Alternative 3 has a lower cost. However, flow encroachment by levees will raise the flow velocity thereby causing channel instability, which may become a legal liability. The levees are subject to damages if its foundation is scoured. All three alternatives considered in this study involve the use of structures for flood control; they are classified as structural flood control measures. These alternatives have been found to be undesirable for different reasons. The alternative to the structural measures is the non-structural flood control measures which involve no structural construction of any kind along Agua Hedionda Creek. Such measures included the following: (1) Advanced flood warning (2) Evacuation (3) Monitoring and maintenance of flood channel. These plans are being developed by others. Recommendations: On the basis of the study, the flood control alternatives are ranked in consideration of the cost, desirability, and other factors. The ranking order is as follows: c First choice: Non-structural flood control but with advanced flood warning and evacuation plans. Second choice: Alternative 2a - Diversion to the north. 16 c Third choice: Alternative 3 - Levees. While this alternative is the less expensive, it is necessary to obtain the consent of the residents who should be informed of the consequences described in this report. Not recommended: Alternative 1 (floodwater detention basins) and Alternative 2b (diversion to the south) are not recommended. REFERENCES 1. Chang, H. H., 1989, "Hydrological Study for Northeastern Carlsbad". LIST OF FIGURES Fig. 1. Hydrographs of 100-yr flood for Agua Hedionda Creek in the Park Fig. 2. Hydrograph of 100-yr flood for Calavera Lake Creek Fig. 3. Location of on-site floodwater detention basins Fig. 4. Picture of flap gates Fig. 5. Alignments of diversion channel Fig. 6. Cross-sectional profile of north diversion channel Fig. 7. Schematic of the inlet structure for diversion channel 17 Agua Hedionda Creek Hydrographs for 100-yr flood Discharge, 1000 cfs Above Calavera Ck Below Calavera Ck 2 - 0- 0 10 12 Time, hours 14 16 10 20 Fig. 1. Hydrographs of 100-yr flood for Agua Hedionda Creek in the Park 2000 Discharge, cfs iaoo - Calavera Lake Creek Hydrograph .for 100-yr flood 8 10 14 1612 Time, hours Fig. 2. Hydrograph of 100-yr flood for Calavera Lake Creek 18 20 V COPY Fig. 4. Picture of flap gates fl^R tai\^ rf* 'VrF*tf&:.\ .."'V* " " 80 feet : 10 feet I 2.5 •30 feet Fig. 6. Cross-sectional profile of diversion channel C Fig. 7. Schematic of the inlet structure for diversion channel Howard H. Chang Consultants Hydraulic, Hydrologic and Sedimentation Engineering P.O. Box 9492 Rancho Santa Fe, CA 92067 (619)756-9050, 594-6380, FAX: (619)756-9460 September 14, 1996 Mr. Lee Anderson Ms. Sue Loftin Mr. Steve Weed Rancho Carlsbad Mobile Home Park Carlsbad, California Dear Friends: Subject: Flood Discharges of Agua Hedionda Creek under Present Conditions Pursuant to your request, I have made a hydrology study for Agua Hedionda Creek (including Calavera Lake Creek as a tributary) in order to determine the 100-yr flood discharges under present conditions of development in the drainage basins. This study may be considered as an extension of the previous study entitled "Hydrological Study for Northeastern Carlsbad" that I made in July, 1989. In the previous study, the 100-yr flood discharges were determined based on the ultimate development of the drainage basin according to the master plan for future development. The study for the present conditions employs the original delineation of subbasins for Agua Hedionda Creek and Calavera Lake Creek. The drainage basins of Agua Hedionda Creek and Calavera Lake Creek were divided into 32 subbasins in the previous study. These subbasins are in different stages of development. Those with more than 50 % of the land area already developed include the following: A3, A6, A7, B, LI, L2, and UF. For other subbasins, the extent of development is less than 50 %. Surface water runoff is directly affected by development. Pavements and drainage facilities associated with a development generally result in increased runoff. In the Soil Conservation Service, or SCS, method for hydrology, surface water runoff is represented by the Curve Number, or CN. Values of CN are specified in the Hydrology Manual published by the County of San Diego. The selection of a CN value depends on the land use, land treatment or practice, soil type, hydrologic condition, and antecedent moisture condition of the area. Summaries of the CN values selected are given in Tables 1 an 2 for the two drainage basins, respectively. ^^^^B Howard H. Chang Consultants Hydraulic, Hydrologic and Sedimentation Engineering P.O. Box 9492 Rancho Santa Fe, CA 92067 (619)756-9050. 594-6380, FAX: (619)756-9460 November 10, 1996 Mr. Lee Anderson Ms. Sue Loftin Mr. Steve Weed Rancho Carlsbad Country Club Estate 5200 El Camino Real Carlsbad, California 92008 Dear Friends: Subject: Technical Review for the Hydraulic Study of Agua Hedionda Creek by Ensign & Buckley in 1992 Pursuant to your request, I have reviewed the hydraulic study for Agua Hedionda Creek in the City of Carlsbad made by Ensign & Buckley in 1992. The hydraulic study was for the purpose of determining the 100-yr flood level and floodplain and floodway boundaries. My review was to assess the accuracy of the study results for Agua Hedionda Creek at Rancho Carlsbad Country Club. In the review, I have found some basic mistakes in the study. Such mistakes resulted in significantly higher 100-yr flood elevations in Agua Hedionda Creek near El Camino Real. The major mistakes include incorrect roughness coefficients, incorrect definitions of the main channel and overbank areas, and improper split flow analysis to account for wen- flows over the roadway of El Camino Real as described below. Incorrect roughness coefficients (or n value) were used for certain cross sections. For example, the n value of 0.08 was used for the cross section just downstream of El Camino Real. The NC record for this section is as follows: NC0.04 0.05 0.08 0.3 0.5 RANCHO CARLSBAD CHANNEL & BASIN PROJECT (Job Number 13182) June 30,1998 BESTCOPY RANCHO CARLSBAD CHANNEL & BASIN PROJECT (Job Number 13182) June 30, 1998 Prepared for: City of Carlsbad 2075 Las Palmas Drive Carlsbad, California 92009-1576 , M.S. R.C.E. #32838 Exp. 6/02 Prepared By: Rick Engineering Company Water Resources Division 5620 Friars Road San Diego, California 92110-2596 (619)291-0707 TABLE OF CONTENTS Introduction 1 Hydrologic Methodology 1 Vicinity Map 2 Hydrologic Results 5 Hydraulics 13 Maintenance Plan 14 Environmental Issues 16 Probable Costs 20 Recommendations 21 References 23 Tables 1. Comparison of 100-year, 24-hour Peak Flow Discharges with Existing Detention Facilities and with Both Existing and Proposed Detention Facilities, Ultimate Development, Rancho Carlsbad Mobile Home Park 7 2. Summary of Proposed Detention Facilities, Rancho Carlsbad Channel and Basin Project, 100-year, 24-hour Storm Event 12 3. Rancho Carlsbad Detention Basins - Direct Impacts (Acres) 17 4. Rancho Carlsbad Detention Basins - Mitigation Requirements (Acres) 18 5. Rancho Carlsbad Detention Basins - Potential Indirect Impacts (Acres) 19 6. Opinion of Probable Costs for Rancho Carlsbad Channel & Basin Project 20 K8j£ Prepared By: DCB:MDL:emn/Report/J-13182.001 Rick Engineering Company - Water Resources Division 07/01/98 Appendices 1. 100-year, 24-hour HEC-1 Analysis for Rancho Carlsbad Mobile Home Park Ultimate Development with Existing Detention Basins (File Name: rcmh24r.hcl) 2. 100-year, 24-hour HEC-1 Analysis for Rancho Carlsbad Mobile Home Park Ultimate Development with Existing and Proposed Detention Basins (File Name: rccbpr.hcl) 3. 100-year Floodplain HEC-2 Analysis for Rancho Carlsbad Mobile Home Park (File Name: FEMA5.hc2) Map Pockets 1. Effective FIRM 2. Watershed Boundary Map 3. Conceptual Plans for the Proposed Melrose Drive Detention Facility 4. Conceptual Plans for the Proposed Faraday Avenue Detention Facility 5. Conceptual Plans for the Proposed BJB Detention Facility 6. Conceptual Plans for the Proposed BJ Detention Facility 7. 100-year Floodplain Map of Rancho Carlsbad Mobile Home Park Prepared By: DCB:MDL:emn/Report/J-13I82.00l Rick Engineering Company - Water Resources Division 07/01/98 Introduction This report has been prepared to summarize the hydrologic and hydraulic studies conducted by Rick Engineering Company for the City of Carlsbad as part of the Rancho Carlsbad Channel and Basin Project. Rancho Carlsbad Mobile Home Park (RCMHP) is located north of El Camino Real midway between College Boulevard and Tamarack Avenue. See the Vicinity Map on the next page. RCMHP contains portions of both Agua Hedionda and Calaveras Creeks. Agua Hedionda Creek flows westerly through the southern portion of RCMHP. Calaveras Creek flows southwesterly along the northern property boundary. Calaveras Creek confluences with Agua Hedionda Creek within RCMHP approximately 300 feet upstream of El Camino Real. The Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) shows that a large portion of RCMHP is inundated by the 100-year storm. See the FIRM in Map Pocket 1. The purpose of this study is to provide recommendations for minimizing the 100-year flooding in RCMHP. These — recommendations include upstream detention basins to decrease the peak flow and on-site creek improvements to increase the creek capacities. ~~ Hydrologic Methodology Hydrologic analyses were prepared to determine the 100-year peak discharge within RCMHP and to analyze proposed detention scenarios. Two hydrologic analyses using the U. S. Army Corps ~ of Engineers' HEC-1 flood hydrograph program are included in this report. The first analysis modeled the existing detention facilities and ultimate development. Ultimate development was assumed in order to account for the maximum anticipated discharge in the watershed. The results _ of the first analysis confirmed that the creeks in RCMHP are inadequate to convey the 100-year e_ Prepared By: " DCB:MDL:emn/Report/J-13182.001 Rick Engineering Company - Water Resources Division 1 07/01/98 I I I I I I I r r r r r r VICINITY MAP RANCHO CARLSBAD MOBILE HOME PARK CITY OF OCEANSIDE HIGHWAY,?*? PROJECT SITE VISTA CITY OF SAN MARCOS NOT TO SCALE PACIFIC OCEAN CITY OF ENCINITAS storm. Therefore, additional analyses were performed in order to study detention scenarios. The HEC-1 analysis containing the most desirable detention scenario is included in this report and is based on the existing and four proposed detention facilities and ultimate development within the entire watershed. The HEC-1 input and methodology are discussed below. The HEC-1 results are discussed in the following section. Prior to preparing the HEC-1 input, previous studies (listed in "References") for RCMHP were reviewed and site visits were performed. The site visit objectives were to verify the watershed boundary and major flow paths of both Agua Hedionda and Calaveras Creeks, determine existing detention locations, and review proposed detention locations. Prior to the site, visits, the watershed boundary and flow paths were delineated on the United States Geological Survey's (USGS) quadrangle maps. The watershed was divided into sub-basins in order to obtain peak flows at existing and proposed detention facility locations and at locations listed in the current Flood Insurance Study. The watershed boundary, flow paths, and sub-basin boundaries were verified during the site visits and adjusted appropriately. See Map Pocket 2 for the RCMHP watershed boundary map. i During the site visits, existing detention facilities such as dams and road embankments were noted. Two dams exist within the RCMHP watershed: Calaveras and Squires. Of these two, only Calaveras dam provides significant detention. It is located within Calaveras Creek and detains the upstream creek flows. On the other hand, Squires Dam is located at the upper end of a drainage basin and provides minimal detention. The plans for Calaveras Dam were obtained from the Division of Safety of Dams (DSOD) and the outlet works and storage capacity were modeled in the hydrologic analyses. Prepared By: : ~ DCB:MDL:cmn/Report/J-J 3 1 82.001 Rick Engineering Company - Water Resources Division 3 07/01/98 e Furthermore, the following road embankments were identified as potential existing detention facilities: Business Park Drive (south of Park Center Drive), Sycamore Avenue (north of Grand Avenue), Shadowridge Drive (north of Antiqua Drive), Melrose Drive (north of Cannon Road), and Melrose Drive (south of Aspen Way). As-built plans for these road crossings were obtained from the appropriate agencies. The culverts and storage capacities of the Sycamore Avenue, Shadowridge Drive, and Melrose Drive (Cannon Road) facilities were modeled in the hydrologic analyses. The Business Park Drive and Melrose Drive (Aspen Way) crossings were not modeled because the culverts at these locations are large enough to convey most of the upstream flows with minimal detention. Two main criteria were considered in selecting potential proposed detention basin sites. First, the facilities listed in the Master Drainage Plan were considered. Second, existing or proposed road crossings were considered. Detention basin construction at road crossings provides several benefits. Road crossings create a natural location for detention. They are cost-effective because the road embankment is used for detention. They do not create a significant increase in environmental impacts. The above-mentioned sub-basins and detention facilities were modeled in the HEC-1 program. The program parameters include sub-basin area, rainfall distribution, lag time, and curve number. These parameters were determined as follows: The sub-basin area was obtained from the USGS watershed boundary map. The rainfall distribution was based on storm duration and frequency, as well as the sub-basin's geographic location. The lag time was based on sub-basin characteristics such as topography, basin shape, vegetative cover, existing development, and storm duration. Both rainfall distribution and lag time were generated by utilizing the criteria outlined in Prepared By: DCB:MDL;emn/Repo«/M3182.001 Rick Engineering Company - Water Resources Division 4 07/01/98 the County of San Diego Hydrology Manual. Curve numbers are a function of land use and soil type. The land use coverages were obtained from the City of Carlsbad's Geographic Information System (GIS). The land use was revised slightly in three locations according to a December 12,1997 exhibit from the City of Carlsbad. In open space areas, land use was based on vegetative cover estimates obtained from the Soil Conservation Service's (SCS) San Diego County Soil Interpretation Study Ground Cover maps, as well as field observations. The soil type coverages are delineated on the SCS's Soil Survey maps. These coverages were obtained from the San Diego Association of Governments (SANDAG) in digital format. Once the land use and soil types were established, the curve numbers were then calculated using the method outlined in the San Diego County Hydrology Manual. The curve number, lag time, rainfall distribution, and area for each sub-basin were generated and input into the HEC-1 program. The HEC-1 program then computed the runoff hydrograph and peak discharge for each sub-basin. The existing detention facilities were modeled in the first HEC-1 analysis, while both existing and proposed detention facilities were modeled in the second HEC-1 analysis. Hydrologic Results The results of the two aforementioned HEC-1 analyses for RCMHP are discussed below. For the first HEC-1 analysis, which modeled the existing detention facilities and ultimate development, both six- and 24-hour, 100-year storms were simulated. The 24-hour storm resulted in higher peak flow discharges at RCMHP for both creeks, thus it was used in all subsequent analyses. Prepared By: DCB:MDL:emn/Report/J-13182.001 Rick Engineering Company - Water Resources Division 5 07/01/98 Appendix 1 contains the 100-year, 24-hour HEC-1 analysis for the RCMHP watershed with the existing detention facilities and ultimate development. The second HEC-I analysis modeled both existing and proposed detention facilities and ultimate development. Several proposed detention scenarios were investigated and it was determined that the most feasible scenario was the combination of four detention basins, all located at proposed road crossings. Two of the proposed detention facilities are listed in the 1994 Master Drainage Plan as Detention Basins BJB and BJ. These facilities are located immediately upstream of RCMHP in Calaveras Creek. Both of the detention basins were designed as flow-by facilities. A flow-by facility detains the higher creek flows, while allowing lower flows to pass through the basin relatively undetained. The other two detention basins are further upstream in Agua Hedionda Creek at the proposed road extensions of Melrose Drive (south of Aspen Way) and Faraday Avenue. Both of the Agua Hedionda detention basins are flow-through types where all of the creek flow is detained. All proposed detention facilities were designed to be outside DSOD's jurisdictional limits, i.e., less than 50 acre-feet of storage volume and less than 25 feet high. Appendix 2 contains the HEC-1 analysis of the 100-year, 24-hour storm for the RCMHP watershed with both existing and proposed detention facilities and ultimate development. Table 1 summarizes the results of both HEC-1 analyses. The table shows that with the proposed detention basins, the peak discharge at RCMHP decreased by approximately 10 to 15 percent. Preliminary design of the proposed detention facilities are discussed below. Prepared By: DCB-.MDL-.emn/Report/JO3182.001 Rick Engineering Company - Water Resources Division O 07/01/98 Table 1 Comparison of 100-year, 24-hour Peak Flow Discharges with Existing Detention Facilities and with Both Existing and Proposed Detention Facilities Ultimate Development Rancho Carlsbad Mobile Home Park Creek CalaverasCreek Agua Hedionda (upstream of confluence with CataverasCreek) Agua Hedionda (downstream of confluence with CalaverasCreek) Peak Discharge with Existing Detention Facilities, *cfs 1,910 8,050 9,950 Peak Discharge with. Existing and Proposed Detention Facilities, *cfs 1,550 7,600 8,970 * cfs = cubic feet per second e Prepared By: Rick Engineering Company - Water Resources Division DCB:MDL:emn/Rcport/M3l82.001 07/01/98 Preliminary designs were performed for each proposed detention facility to determine the outlet works required to achieve maximum detention, while maintaining the height and storage volume below DSOD jurisdictional limits. The preliminary design of each detention facility and the results for each detention facility design are described below. The most upstream proposed detention facility in Agua Hedionda Creek is at Melrose Drive. This facility will be a flow-through detention basin. Melrose Drive runs north-south and currently ends just south of Aspen Way near the Carlsbad Corporate boundary. Future plans call for the extension of Melrose Drive to Palomar Airport Road. An existing reinforced concrete box (RGB) culvert conveys flow under Melrose Drive and is 10 feet wide by 7 feet high. The existing Melrose Drive embankment provides minimal detention because of the RGB's large capacity. Hydrologic calculations show that a 36-inch diameter opening at this location will detain the peak flow discharge from approximately 450 cubic feet per second (cfs) to 180 cfs. There are two alternatives for creating the 36-inch opening. One is to replace the existing culvert with a 36-inch RCP and the other is to construct a concrete barrier at the inlet with a 36-inch diameter opening. The resultant storage volume and ponded water surface elevation (WSEL) with the new outlet works will be approximately 41 acre-feet and 329 feet, respectively. This will create an inundation area of approximately seven acres. The estimated outlet velocities for the first and second alternative will be 25 and 13 feet per second (fps), respectively. The velocity under the first alternative is greater than the maximum desired velocity of 20 fps. The velocity calculation assumed that the proposed 36-inch RCP was constructed at the slope of the existing culvert, which is one percent. If this alternative is selected, the final culvert design should analyze methods for reducing the outlet velocity, such as placing the culvert at a flatter slope or using multiple small diameter culverts. A Prepared By: DCB:MDL:emn/Report/M3182.001 Rick Engineering Company - Water Resources Division o 07/01/98 conceptual plan for the second alternative is included in Map Pocket 3. The other detention facility proposed for Agua Hedionda Creek is the Faraday Avenue flow- through detention basin. Currently, Faraday Avenue runs east-west and ends at Orion Street. The extension of Faraday Avenue to Park Center Drive in the city of Vista is planned as part of Carlsbad Oaks North Business Park. The hydrologic calculations and preliminary design in this report were based on the proposed embankment and topographic information shown on the Tentative Map for Carlsbad Oaks North Business Park by O'Day Consultants, dated April 6, 1998. The calculations show that a single 6-foot wide by 7-foot high RGB culvert will detain approximately 49 acre feet of storage volume and will pond up to an elevation of 240 feet. The inundation area will be approximately seven acres. The 100-year peak discharge of 1,050 cfs entering the detention basin will be detained down to approximately 780 cfs. The approximate calculated outlet velocity will be 19 fps. A conceptual plan for this detention facility is included in Map Pocket 4. The two proposed detention facilities in Calaveras Creek are located just upstream of RCMHP and were designed as flow-by basins. The first facility, Detention Basin BJB, is located north of RCMHP at the proposed College Road extension and west of the proposed Cannon Road extension. College Boulevard currently ends at El Camino Real. North of RCMHP, the proposed College Boulevard extension runs roughly east-west. College Boulevard intersects the proposed Cannon Road extension at the northeast corner of RCMHP. Cannon Road currently ends east of Interstate 5 at Paseo Del Norte. The proposed Cannon Road extension alignment will be parallel to Calaveras Creek and immediately north of RCMHP. The detention basin design consists of an earthen embankment, outlet works, and a small berm. The embankment will have a 10-foot top width and a 76-foot crest elevation with 2:1 (horizontal:vertical) side slopes. The outlet works Prepared By: DCB:MDL:emn/Report/J-l 3182.001 Rick Engineering Company - Water Resources Division " 07/01/98 consist of a single 10-foot wide by 7-foot high RGB and a 48-inch RCP. The 48-inch RCP joins the RGB downstream of the embankment. The RGB then extends to Calaveras Creek. An emergency spillway is also provided. The small berm will run parallel to the creek for approximately 1,200 feet. The berm will have an approximate 74-foot crest elevation, 10-foot top width, 2:1 (horizontal:vertical) side slopes, and a weir section. The weir section, located near the embankment, will allow flow to enter the basin at an approximate WSEL of 73 feet. Hydrologic calculations show that with the outlet works described above, a storage volume of approximately 49 acre feet will be attained. The resultant ponded WSEL will be approximately 75 feet and the inundation area will be approximately 15 acres. The peak discharge of 1,570 cfs entering the basin will be detained down to 1,200 cfs. The approximate outlet velocity will be 19 fps for the RGB. See Map Pocket 5 for a copy of the conceptual design of Detention Basin BJB. The other Calaveras Creek detention facility, Detention Basin BJ, is located northeast of _ RCMHP at the proposed College Boulevard extension and east of the proposed Cannon Road extension. The earthen embankment will have a crest elevation of approximately 81 feet, a top width of 10 feet, and 2:1 side slopes. An emergency spillway will be provided. Approximately 600 feet of — channel improvements upstream of the proposed embankment are necessary. The channel improvements include grading the creek as follows: Trapezoidal-shaped grass-lined channel with a 3-foot bottom width, 4-foot depth, and 2:1 side slopes. The hydrologic calculations showed that a 6-foot wide by 3-foot high RGB would detain the peak flow of 670 cfs down to approximately 350 _ cfs. The inundation area is approximately eight acres and the ponded WSEL is approximately 76 feet. The detention basin stores approximately 48 acre feet of water. The calculated outlet velocity will be approximately 19 fps. See Map Pocket 6 for the conceptual plans for Detention Basin BJ. C Prepared By: ~~ DCB:MDL.emn/Rcport/J-13182.001 Rick Engineering Company - Water Resources Division 1U 07/01/98 As discussed above, with the addition of the proposed detention facilities, the peak discharge at RCMHP is decreased by approximately 10 to 15 percent. All four of the proposed detention facilities were designed to fall below DSOD's jurisdictional limits. Also, all the facilities are located at existing or proposed road crossings and at least one foot of freeboard is maintained at the road embankments. The results are summarized in Table 2, which contains results such as outlet works, velocity, peak flow discharge into and out of the basin (Qin and Q^, storage volume, ponded WSEL, and surface area. Prepared By: DCB:MDL:emn/Report/J-13182.001 Rick Engineering Company - Water Resources Division 11 07/01/98 Table 2 Summary of Proposed Detention Facilities Rancho Carlsbad Channel and Basin Project 100-year, 24-hour Storm Event Facility Name Melrose (south of Aspen Way) Faraday BJB BJ Qw cfs 450 1,050 1,560 670 Q...,rfs 180 780 1,200 350 Outlet Works 36" RCP 6(x7 RCB l-10'x7'RCB & 48" RCP 6'x3' RCB Ponded WSEL, ft 329 240 75 76 Storage, ac-ft 41 49 49 48 Surface Area, ac 7 7 15 8 Outlet Velocity, fps 13 (Alt. 2) 25 (Alt. 1) 19 19 19 Prepared By: Rick Engineering Company - Water Resources Division 12 DCB:MDL:emn/Report/J-13182.001 07/01/98 Hydraulics Hydraulic analyses were performed to determine the amount of silt removal and re-grading required to minimize the 100-year flooding at RCMHP. In order to effectively analyze flood levels in both Agua Hedionda and Calaveras Creeks, the U.S. Army Corps of Engineers HEC-2 Water Surface Profiles program was used. The program is intended for calculating WSELs for steady gradually varied flow in natural or man-made channels. The effects of various obstructions such as bridges and culverts may be considered in the computations. The program also has capabilities available for assessing the effects of channel improvements. The input parameters were based on channel and overbank roughnesses, 100-year discharge, downstream WSEL, and topography. The channel and overbank roughnesses were determined by field observations. The 100-year discharge was obtained from the HEC-1 analysis in Appendix 2 modeling both existing and proposed detention facilities. The downstream WSEL was estimated in the HEC-2 analysis by using the slope-area method. FEMA-approved HEC-2 cross-sections for the area downstream of the site were included in the analysis. The 100-year discharge for the downstream area was obtained using the split-flow analysis from the Flood Insurance Study. The existing topography was based on June, 1995 topographic maps by Manitou Engineering. The topography was used to prepare cross-sections of both creeks, as well as the overbank areas. Since prior studies showed that the creeks were under-capacity, the original grading plans for RCMHP were obtained and modeled in the HEC-2 analysis by using the channel improvement option. The original grading plans were prepared October 15, 1969 and approved by the City on March 24, 1971. The original design consisted of a trapezoidal channel with an overall length of approximately 1.2 miles and included both Agua Hedionda and Calaveras Creeks within Prepared By: DCB-.MDL-.emn/Report/J-13I82.001 Rick Engineering Company - Water Resources Division 13 07/01/98 € RCMHP. The side slopes were 2:1 (horizontahvertical) and the approximate bed slopes were 0.15 and 0.30 percent in Agua Hedionda Creek and Calaveras Creek, respectively. The bottom width of Agua Hedionda Creek varied from 58 feet at the El Camino Real bridge to 44 feet upstream of the confluence. The approximate channel depth was 11.5 feet. The bottom width and channel depth of Calaveras Creek were four feet and nine feet, respectively. A HEC-2 analysis was performed based on the original design. The HEC-2 results showed that a large portion of RCMHP remained inundated by the 100-year flood. In order to increase channel capacity, additional channel improvements were modeled in the HEC-2 analysis for the downstream sections of both creeks. At the El Camino Real bridge, the bottom width was widened to 87 feet. Within the next 1,400 feet upstream of the bridge, the bottom width then tapered down to the original design bottom width of 44 feet in Agua Hedionda Creek and four feet in Calaveras Creek. The results of the hydraulic study are contained in Appendix 3. The results are also depicted on the RCMHP 100-year Floodplain Map in Map Pocket 7. The map shows that with the proposed detention facilities and channel improvements discussed above, a majority of RCMHP will be A outside of the 100-year floodplain. Maintenance Plan This Maintenance Plan contains maintenance requirements for Aqua Hedionda and Calaveras Creek within RCMHP. This plan also contains requirements for the four upstream detention basins. It is vital that the creeks and detention basins be maintained on a regular basis to ensure an acceptable level of flood protection for RCMHP. It is recommended that the maintenance described Prepared By: DCB:MDL:emn/Report/J-13182.001 Rick Engineering Company - Water Resources Division 14 07/01/98 below be performed annually prior to the rainy season and after any storm event exceeding the 10- year peak discharge. Aqua Hedionda and Calaveras Creek must be maintained to prevent adverse siltation in each creek. Siltation will reduce the flow capacity of the creeks and increase the likelihood of inundation within the mobile home park. The first step is to devise a system for monitoring the silt level in each creek. This can be done using metal posts with markings placed six inches apart. The posts should be placed vertically in each creek at intervals not exceeding 500 feet. The posts should extend at least two feet above the creek bed and must be embedded deep enough so that they will not be moved by large creek flows. A geotechnical engineer should be consulted for the required embedment depth. Once the posts are installed, the silt level can be easily monitored by maintenance personnel. As the silt level reaches one foot, the silt should be removed by maintenance crews to the design elevations. The topographic maps have been reviewed to determine the siltation that has occurred in both creeks over the past few years. The design of the creeks within the mobile home park is shown on the grading plan for RCMHP approved March 24,1971. The creek bed elevations on the grading plan served as the base elevations in determining the amount of siltation in each creek. A comparison of the grading plan with a June 1995 topographic map Indicates that the silt in Aqua Hedionda and Calaveras Creek raised the creek beds as much as seven and five feet, respectively. Therefore, siltation has occurred in Agua Hedionda and Calaveras Creek at a rate of up to 0.3 and 0.2 feet per year. Using these rates and an acceptable silt level of one foot indicates that portions of the creeks could require maintenance approximately once every three to five years. It is important to point out that this is a rough approximation because the creek siltation will depend on the Prepared By: DCB'.MDL:enm/Report/J-13182.001 Rick Engineering Company - Water Resources Division 15 07/01/98 frequency and magnitude of future storm events. It is likely that future storm events will not mimic ~ past events. Additionally, it is possible that maintenance has been performed on the creek between 1971 and 1995, which would affect the calculated siltation rates. Maintenance is also required at each of the four detention basins. Maintenance will involve keeping the entrance to each of the detention basin outlet facilities free from silt. Silt should be _ removed from an entrance once the silt level reaches six inches above the entrance's flowline elevation. The amount of deposition should be easy to determine since each outlet facility is a known size. The silt should be removed a distance of 10 feet upstream of the facilities entrance. "~ This will have minimal environmental impacts and will restore the capacity of the outlet facility. The maintenance steps described above are essential for protection of RCMHP. The • maintenance must be performed routinely by qualified personnel and a sufficient budget should be _ established for the maintenance. If any questions arise during the maintenance, a professional _ engineer specializing in water resources should be contacted. Environmental Issues — The environmental issues associated with the Rancho Carlsbad Channel & Basin Project have been addressed by the environmental consultant, RECON, and are summarized below. In regards to the on-site channel silt removal and improvements, it is likely that no environmental ~ mitigation will be necessary. In regards to the four proposed detention facilities, the direct impacts, __ mitigation requirements, and potential indirect environmental impacts are listed by habitat type in Tables 3,4, and 5, respectively. Direct impacts are from embankment construction. As mentioned above, all of the embankments are within footprints of future roadways. Mitigation requirements Prepared By: DCB:MDL:emn/Report/J-13182.001 Rick Engineering Company - Water Resources Division 10 07/01/98 assume 3:1 for wetlands and 2:1 for other. The goal is to mitigate on-site as much as possible. Potential indirect impacts are based on the 100-year inundation area and will be further analyzed during a subsequent design phase. Table 3 Rancho Carlsbad Detention Basins - Direct Impacts (Acres) Habitat Type Sycamore Woodland Willow Woodland/ Freshwater Marsh Willow Woodland Jurisdictional Waters Coastal Sage Scrub Southern Mixed Chaparral Disturbed Totals BJB 0.14 9.4 9.54 BJ 0.07 1.1 1.17 Faraday 0.4 0.12 0.26 0.78 Melrose 0.42 1.50 2.24 4.16 Prepared By: Rick Engineering Company - Water Resources Division 17 DCB:MDL:emn/Report/J-13182.001 07/01/98 Table 4 Rancbo Carlsbad Detention Basins - Mitigation Requirements (Acres)* Habitat Type Sycamore Woodland Willow Woodland/ Freshwater Marsh Willow Woodland Jurisdictional Waters Coastal Sage Scrub Southern Mixed Chaparral Totals Wetland (Upland) BJB 0.42 0.42 BJ 0.14 0.14 Faraday 1.2 0.12 (0.24) 1.2 (0.12) Melrose 1.26 1.50 (3.0) 1.76 (1.64) * Assumes 3:1 wetlands; 2:1 for non-wetland Jurisdictional waters; 1:1 - 2:1 for coastal sage scrub and southern mixed chaparral. Prepared By: Rick Engineering Company - Water Resources Division 18 DCB:MDL:emn/Report/J-13182.001 07/01/98 Table 5 Rancho Carlsbad Detention Basins - Potential Indirect Impacts (Acres) Habitat Type Sycamore Woodland Willow Woodland/ Freshwater Marsh Willow Woodland Jurisdictional Waters Coastal Sage Scrub Southern Mixed Chaparral Disturbed Totals BJB 2.0 14.0 16.0 BJ 0.31 0.6 6.51 7.42 Faraday 3.3 0.35 3.05 6.7 Melrose 2.67 3.92 6.59 € Prepared By: Rick Engineering Company - Water Resources Division 19 DCB:MDL:emn/Report/J-13182.001 07/01/98 Probable Costs Table 6 below summarizes the probable costs associated with the Rancho Carlsbad Channel and Basin Project. These cost estimates are based on the preliminary design discussed in this report, i.e., the four proposed detention facilities and silt removal from Agua Hedionda and Calaveras Creeks, as well as detention facility and channel maintenance. Several mobile homes are within the revised 100-year floodplain. Therefore, costs have also been included for raising the affected mobile homes above the floodplain. These probable costs include construction costs, as well as the following additional cost items: Five percent of the construction costs for mobilization, 15 percent for construction contingencies, and 15 percent for engineering and processing. Note that these costs are based on the preliminary design and that a final cost estimate should be prepared during final design. Table 6 Opinion of Probable Costs for Rancho Carlsbad Channel & Basin Project Detention Facility Faraday Avenue Detention Basin Melrose Drive Detention Basin Detention Basin BJB Detention Basin BJ Silt Removal Raise Affected Mobile Home Sites Total Maintenance of Detention Facilities and Channels per Occurrence Probable Cost $347,000 8,000 834,000 337,000 252,000 341,000 $2,115,000 $25,000 Prepared By: Rick Engineering Company - Water Resources Division 20 DCB:MDL:emn/Report/J-l 3182.001 07/01/98 Recommendations In order to minimize the 100-year flooding in RCMHP, four upstream detention facilities and on-site improvements for Agua Hedionda and Calaveras Creeks are recommended. The detention facilities will decrease the 100-year peak flow in RCMHP and the improvements will increase the on-site channel capacity. The detention facilities are located at the following proposed road extensions: College Boulevard, Melrose Drive, and Faraday Avenue. The preliminary design requirements for the detention facilities and channel improvements are discussed in previous sections. Fourteen mobile homes are within the revised 100-year floodplain limits. These can be removed from the floodplain by temporarily transporting the affected mobile homes and re-grading the sites to one foot above the floodplain. A preliminary cost estimate for this work is presented in the "Probable Costs" section of this report. Also included in the "Probable Costs" section is the estimated maintenance cost for the detention facilities and channels, as outlined in the "Maintenance Plan." Final hydrologic and hydraulic analyses, cost estimates, and environmental assessments should be performed during the final design phase. It is also recommended that a Rural Flood Water Storage Easement be prepared for each detention facility. The purpose of the easement is to define the 100-year inundation limits. The easement should be written so that it can be modified in accordance with future development. An example of the text that can be included in the easement description is as follows: WE HEREBY GRANT TO THE CITY OF CARLSBAD, A MUNICIPAL CORPORATION, EASEMENTS AND RIGHTS-OF-WAY FOR THE STORAGE OF FLOOD WATERS OVER, UPON AND ACROSS THE PORTION OF LOT XX, DESIGNATED AS "RURAL FLOOD WATER STORAGE EASEMENT GRANTED HEREON", ALL AS SHOWN ON THIS MAP WITHIN THIS SUBDIVISION, RESERVING, HOWEVER, TO THE OWNER OF THE FEE UNDERLYING SAID EASEMENT HEREIN GRANTED TO THE CONTINUED USE OF THE SURFACE OF SAID REAL PROPERTY, SUBJECT TO THE FOLLOWING CONDITIONS: THE Prepared By: ~~DCB:MDL:emn/Report/M3182.001 Rick Engineering Company - Water Resources Division /1 07/01/98 ERECTION OF BUILDINGS, WALLS, OR OTHER STRUCTURES SHALL BE PROHIBITED UNLESS IT CAN BE DEMONSTRATED THAT THE STRUCTURE DOES NOT EFFECT THE STORAGE OF FLOOD WATER AS APPROVED BY THE DEPARTMENT OF PUBLIC WORKS. FENCES THAT ALLOW THE PASSAGE OF WATER, OR MINOR GRADING WITH THE BALANCE OF CUT AND FILL SHALL NOT BE PROHIBITED. NOTHING CONTAINED HEREIN SHALL BE CONSTRUED TO ASSIGN ANY MAINTENANCE OR LIABILITY RESPONSIBILITIES TO THE CITY OF CARLSBAD. Prepared By: DCB:MDL:emn/Report/M3182.001 Rick Engineering Company - Water Resources Division 22 07/01/98 c References County of San Diego, Department of Public Works, Flood Control Division Hydrology Manual, revised January, 1985 and minor update April, 1993. Grading Plan Rancho Carlsbad Mobile Home Park JN Y309-4 by South Bay Engineering, 10/15/69, approved for grading within property limits 3/24/71. Flood Insurance Study for San Diego County, California and Incorporated Areas by Federal Emergency Management Agency, June 19,1997. "Flooding Issues Related to Agua Hedionda Creek for Rancho Carlsbad Mobile Home Park" by H. Chang, 5/96. "Floodplain Boundary and Floodway Delineation for Agua Hedionda Creek" by H. Chang, 9/91. "Hydraulic and Scour Studies for Proposed Cannon Road Bridge on Agua Hedionda Creek" by H. Chang, 6/97. "Hydrological Study for Northeastern Carlsbad Basins of CalaverasLake Creek and Agua Hedionda Creek" by H.Chang, 7/89. "Hydrology & Hydraulic Studies for Carlsbad Oaks North Business Park" by H. Chang, April 1997. Master Drainage and Storm Water Quality Management Plan, City of Carlsbad, California, March, 1994. "Rancho Carlsbad Drainage and Floodplain Study" by Rick Engineering Company, JN 8845,6/7/83. "Rancho Carlsbad - Phase n Preliminary Review Study" by Site Design Associates, Inc., April 22, 1997. San Diego County Soil Interpretation Study Ground Cover maps by Soil Conservation Service, 1969. Soil Survey San Diego Area, California maps by Soil Conservation Service and Forest Service, December, 1973. Prepared By: DCB:MDL:emn/Report/J-13182.001 Rick Engineering Company - Water Resources Division 23 07/01/98 RICK I7 T NTf"1 RANCHO CARLSBAD MOBILE HOME PARK UELIMINARY ALTERNATIVES ANALYSIS FOR AGUA HEDIONDA CHANNEL MAINTENANCE J-13182 C JANUARY8,20qi3 Volume 1 of 2 RANCHO CARLSBAD MOBILE HOME PARK PRELIMINARY ALTERNATIVES ANALYSIS FOR AGUA HEDIONDA CHANNEL MAINTENANCE J-13182 C Dennis C. Bowling, M.S. RCE# Prepared by: Water Resources Division Rick Engineering Company 5620 Friars Road, San Diego, California 92110 (619) 291-0707 www.rick engineering, com JANUARY 8, 2000 TABLE OF CONTENTS VOLUME 1: Introduction 1 Vicinity Map 3 Hydrologic Analysis 4 Table 1: 100-year discharges (cfs) in Calavera Creek and Agua Hedionda Creek within the Rancho Carlsbad Mobile Home Park 6 Project Constraints 7 Hydraulic Analysis 9 Table 2. Anticipated Detained 100-year WSELs for the Agua Hedionda Creek Grading Alternatives 13 Table 3. Summary of Inundated Lots Resulting from Each Agua Hedionda Creek Grading Alternative 13 Table 4. Estimated Dredge Volume for Agua Hedionda Creek Grading Alternatives '. 14 Table 5. Approximate Depth of Excavation for Agua Hedionda Creek Grading Alternatives 14 Maintenance Program 15 Conclusion 16 Aerial Photographs —Map^PocketT:Preliminary^Rancho Carlsbad Channel and Basin Project 100-Year Floodplain Alternatives Prepared by: Rjck Engineering Company - Water Resources Division DCB:RC:nd/Report/1305I-C.003.doc 01-08-03 1 i yQJjJME^: Appendix A: Appendix B: Appendix C: Appendix D: Appendix E: Appendix F: Appendix G: TABLE OF CONTENTS «. **-*» . 01-08-03 i *i INTRODUCTION The purpose of this report is to summarize the design alternatives for Agua Hedionda Creek that have been prepared by Rick Engineering Company for the City of Carlsbad as part of the Rancho Carlsbad Channel and Basin Project. The Rancho Carlsbad Mobile Home Park (RCMHP) is an existing residential area located north-east of El Camino Real, south-east of the proposed Cannon Road and south-west of the proposed College Boulevard within the city of Carlsbad, California. RCMHP contains portions of both Agua Hedionda and Calavera Creeks. Agua Hedionda Creek flows westerly through the southern portion of RCMHP. Calavera Creek flows southwesterly along the northern property boundary. Calavera Creek confluences with Agua Hedionda Creek within the RCMHP approximately 300 feet upstream of El Camino Real. Original Channel Design The original constructed Agua Hedionda Creek channel was an earthen trapezoidal channel, as shown on the "Grading Plans for Rancho Carlsbad Mobile Home Park" dated June 27, 1969 prepared by South Bay Engineering. The overall length of the channel was approximately 1.2 miles and included both Agua Hedionda Creek and Calavera Creek. The side slopes were 2:1 (horizontahvertical) and the approximate bed slopes were 0.15 and 0.30 percent in Agua Hedionda Creek and Calavera Creek, respectively. The bottom width of Agua Hedionda Creek varied from 58 feet at the El Camino Real Bridge to 44 feet upstream of the confluence. The approximate channel depth was 11.5 feet. The bottom width and channel depth of Calavera Creek were 4 feet and 9 feet, respectively. Sedimentation Pattern within Channel Historically, Agua Hedionda Creek has been highly subject to sedimentation within the channel along areas of the entire creek. Portions of the channel have experienced up to 6 feet or more of sediment deposition within the Mobile Home Park. This high sediment load may be the result of large agricultural areas upstream of the RCMHP as well as the effects of an increased peak discharge within Agua Hedionda Creek due to development within the upstream watershed. Because of the increased peak discharge and the accumulation of sediment in the channel over time, the capacity of Agua Hedionda Creek to convey storm water through the RCMHP has diminished, and currently a large number of properties within the Mobile Home Park are subject to inundation during a 100-year storm event. Prepared by: DCB:RCmd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01 -08-03 •N m m m Calavera Creek has been subject to erosion within the upstream portion of the channel and sedimentation within the downstream portion of the creek near the confluence with Agua Hedionda Creek. As a result of the sedimentation and the encroachment of homeowners into the channel, Calaver Creek no longer has capacity for the 100-year storm event. Proposed Channel Maintenance In an effort to alleviate flooding within the Mobile Home Park, the City of Carlsbad has investigated several alternatives to increase the capacity of the creeks and improve flood protection in the area. Rick Engineering Company prepared a study entitled, "Rancho Carlsbad Channel and Basin Project," dated June 30,1998 that provided the preliminary design of four detention basins within the watershed tributary to the RCMHP, determined the maintenance and sediment removal required to return Agua Hedionda Channel and Calavera Creeks to their original design configurations per the 1969 Grading Plans, and analyzed the effects of the reduced flow and increased channel capacity on the limits of inundation within the Mobile Home Park. I 4 1 f II Although the original Channel and Basin Project design called for maintenance of Calavera Creek as well as Agua Hedionda Creek, environmental and biological concerns as well as constructability issues determined that the maintenance of Calavera Creek was not practical. However, the maintenance of the Agua Hedionda Creek will provide hydraulic improvement to Calavera Creek and increase the capacity of the channel for flood protection. Prepared by: Rick Engineering Company - Water Resources Division DCB rRCmd/Report/! 3051 -C.003 .doc 01-08-03 i! ft 1 VICINITY MAP RAJNCHO CARLSBAD MOBILE HOME PARK I! I i CITY OF OCEANSIDE HIGHWAY, PROJECT SITE VISTA CITY OF SAN MARCOS CITY OF ENCINITAS i *i Prepared by: Rick Engineering Company - Water Resources Division DCB :KH:no7Report/l 4167.001 01-08-03 HYDROLOGIC ANALYSES * I I I t ri j |f j I If The U.S. Army Corps of Engineers HEC-1 computer program was used to prepare hydrologic models of the watersheds tributary to Agua Hedionda Creek and Calavera Creek upstream of the Rancho Carlsbad Mobile Home Park. The HEC-1 models analyze runoff from the ultimate development of the upstream watershed and take into account the effects of the four proposed detention facilities. The detention facilities are named Melrose, Faraday, BJB and BJ. These analyses were prepared for the "Rancho Carlsbad Channel and Basin Project" report dated June 30, 1998. These HEC-1 analyses and have been revised to model new watershed boundaries related to the drainage across proposed Cannon Road, analysis of the split flow in Calavera Creek, and a new weir structure geometry at the outlet of detention basin BJB. Discussion of these revisions is included below. The revised 100-year ultimate detained condition HEC-1 analysis for the Rancho Carlsbad Mobile Home Park is included in Appendix A. Existing Drainage Pattern The current FEMA model shows the flow in Calavera Creek splitting to the north and south sides of an existing concrete wall that is located along Calavera Creek. The total effective FEMA flow in Calavera Creek is 1350 cfs, which splits into a flow of 805 cfs on the north side of the existing wall and a flow of 545 cfs on the south side of the existing wall. The flow on the north side of the wall is routed to an existing 8'x 8' box culvert under El Camino Real. Proposed Detention Facilities The Calavera Hills and Robertson Ranch development projects are located within the watershed for Calavera Creek, just north of the RCMHP. As these projects are developed, detention Basin BJB will be constructed simultaneously with the construction of Cannon Road and College Boulevard. Due to the restricted capacity of Calavera Creek, a portion of the flows discharging from detention basin BJB will be conveyed within the Robertson Ranch project, north of the existing wall and south of Cannon Road, to honor the existing drainage pattern and reduce the flows entering the Rancho Carlsbad Mobile Home Park on the south side of the wall to approximately 1000 cfs. This will require "plugging" the holes along the wall that currently convey flow north and south of the wall during large storm events. Based on the final design of Detention Basin BJB, the anticipated 100-year outflow from Detention Basin BJB is 1292 cfs. After 625 cfs of this outflow splits to the north side of the wall, the remaining flow is routed to the south side of the wall and combines with the outflow from Detention Basin BJ. Based on the final Prepared by: DCB:RC:nd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01-08-03 I I I I i I design of Detention Basin BJB and the preliminary design of Detention Basin BJ, the total anticipated flow in Calavera Creek on the south side of the wall is 1017 cfs. The amount of flow splitting to the north side of the existing wall is 625 cfs, which combines with tributary watersheds and results in a total flow of 702 cfs at the downstream end of Calavera Creek on the north side of the wall. The flow on the north side of the wall is routed through a triple 10' x 4' box culvert under proposed Canon Road and ultimately to the existing 8' x 8' box culvert under El Camino Real. The 100-year proposed flow of 702 cfs on the north side of the wall is lower than the effective FEMA flow of 805 cfs. However, we prepared hydraulic analyses to determine if the existing 8'x 8' box culvert under El Camino Real has enough capacity to pass the proposed 100-year flow. The results show that the existing 8'x 8' box culvert does not have capacity for the 100-year flow, and inundation over El Camino Real and proposed Canon Road is anticipated. In order to prevent inundation over these road crossings, modification of the box culvert will be required with an 18'xlO'xS' (WxHxL) transition structure. However, this transition structure geometry will not provide any freeboard at El Camino Real. hi order to allow for freeboard along El Camino Real, the 8'x 8' box culvert under El Camino Real would need to be replaced with a larger culvert and further hydraulic analyses will be required. Replacing the existing 8'x 8' box culvert will also require authorization from governing agencies regarding Coastal Permits. Two detention basins are also proposed within the Agua Hedionda Creek watershed upstream of the RCMHP, one at Faraday Avenue and one at Melrose Drive. The preliminary design for these detention facilities is based on-the^evised-HEC-1 analyses that are included in Appendix A of this report. Table 1 shows the results of the HEC-1 analyses tributary to the Mobile Home Park. These results are subject to change based on the final design of the BJ, Faraday, and Melrose detention basins. Prepared by: DCB:RC:nd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01 -08-03 ^ Table 1:100-year discharges (cfs) in Calavera Creek and Agua Hedionda Creek within the Rancho Carlsbad Mobile Home Park Location Calavera Creek upstream of the Mobile Home Park Calavera Creek north of the wall (within Robertson Ranch) Calavera Creek upstream of the confluence with Agua Hedionda Creek Agua Hedionda Creek upstream of the confluence with Calavera Creek Agua Hedionda Creek downstream of the confluence with Calavera Creek Existing Condition (FEMA) 1350 805 545 7810 8080 City of Carlsbad Ultimate Developed Condition (Existing Hydrology) 1908 Data Not Available Data Not Available 8097 9998 Preliminary Ultimate Developed Detained Condition (Proposed Hydrology) 1009 625 1017* 7603* 8596* * The 1998 report stated an ultimate detained flow of 7600 cfs in Agua Hedionda Creek, a flow of 1550 cfs in Calavera Creek, and 8970 cfs downstream of the confluence. I I I Additional Detention Facilities Preliminary investigation of the feasibility for construction of a DSOD Jurisdictional dam upstream of the MPH was prepared to estimate the storage volume required to provide flood protection within the MHP. The construction of additional detention would minimize flooding in the MHP area without jnaintenance within Agua Hedionda Creek. The existing hydraulic capacity of the channel was determined using the HEC-2 hydraulic program by modeling the existing topography in Agua Hedionda Creek with a series of flows until the flow was contained within the channel through most of the MHP. The HEC-2 analyses show that the existing capacity of Agua Hedionda is approximately 4000 cfs. In order to reduce 100-year flows in Agua Hedionda Creek to approximately 4000 cfs, the detention basin atJFaraday would require_2i)0 agiSijggt_pf storage volume, and an additional detention basin would need to be constructed what would provide 895 acre-feet of storage volume. The additional HEC-1 detention analysis is included in Appendix B, along with a hydrology map showing the location of the proposed1 detention basins. i Prepared by: Rick Engineering Company - Water Resources Division DCS :RC md/Report/13051 -C.003 .doc 01-08-03 I I I 1 « i 4 * I PROJECT CONSTRAINTS Additional factors affecting the limits of grading of this project have impacted the proposed Agua Hedionda channel grading. These factors include, but are not limited to: the proximity to the Coastal Zone, the presence of native and non-native trees along the channel banks, and the stability of the channel banks. Coastal Zone Thejimits of the Coastal Zone are locatedjit El Camino Real, and grading within the limits of the Coastal Zone would require Additional permits and coordination. Therefore, grading downstream of El Camino ReaHs not _a_practical alternative. Because the defined maximum limits of grading do not ^extend underneath the El Camino Real Bridge, the proposed flowline of the channel may "ramp up" to match the existing ground' elevations under the El Camino Real Bridge. This "ramp" would diminish the conveyance of low flows through the channel, andjncreasejhe potential for ponded waterjust upstream of the bridge. Therefore, the downstream conveyance of low flows is also a concern for the channel design. Additional discussion of grading and drainage alternatives at the El Camino Real Bridge are included in the following section of this report. Native and Non- Native Trees within the Channel Also, the RCMHP Homeowners Association and the Environmental Resource Agencies have voiced concerns regarding the removal of mature trees and native trees along the Agua Hedionda Creek Bank. In order to preserve as many existing mature trees as possible within Agua Hedionda Creek, a biologist was consulted to identify the native and exotic trees along the channel to determine what trees could be removed during the proposed maintenance. The location of the trees specified to remain determined the maximum limits of grading along Agua Hedionda Creek. Due to the location of these trees along the channel side-slopes, the original channel bottom widths as shown on the "Grading Plans for Rancho Carlsbad Mobile Home Park" dated June 27,1969 prepared by South Bay Engineering can not be completely recovered. After the proposed grading is completed, re-vegetation will be required along the banks of the channel. Prepared by: DCB:RC:nd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01 -08-03 i i 1 i i i i 4 i *i Aerial photographs of the RCMHP area that date back to 1973 are included in the Aerial Photographs section in this report. The RCMHP homeowners association has stated that some maintenance has occurred within the channel in the late seventies and/or early eighties. Slope Stability Soil investigations are provided by GEOCON in a report entitled, "Limited Geotechnical Investigation, Agua Hedionda Creek, Rancho Carlsbad, Carlsbad, California" dated September 5, 2002, as well as an Addendum letter dated October 7,2002. Based on the field investigations, the surface soils suggest that the soils would require significant excavation, recompaction and reinforcement to provide slope stability at 2:1 (per the original 1969 design). This report suggested 3:1 or 4:1 side slopes would provide a more ^^ stable channel configuration. These findings are based on surface investigation, and additional soils •B investigations including borings within the channel would be required to determine if the originally constructed 2:1 banks have been adequately compacted to support a 2:1 side slope during channel maintenance. During final design of the maintenance project, erosion concerns in compliance with the requests and recommendation of the Environmental Permitting Agencies will need to be addressed, such as optional riprap or plantable slope reinforcing fabric at the trees. Due to the above mentioned factors, the original design configuration of Agua Hedionda Creek cannot be completely recovered, several design alternatives have been proposed for the grading in Agua Hedionda Creek that address the above-mentioned issues. The Hydraulic Analysis section of this report includes detailed discussion and hydraulic analyses of five proposed design alternatives for Agua Hedionda Creek. Prepared by: DCB:RC.Tid/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01 -08-03 1 i t HYDRAULIC ANALYSIS Agua Hedionda Creek has historically been subject to sedimentation within the channel along areas of the entire creek. Portions of the channel have experienced up to 6 feet or more of sediment deposition within the Mobile Home Park. This high sediment load may be the result of large agricultural areas upstream of the RCMHP as well as the effects of an increased peak discharge within Agua Hedionda due to development within the upstream watershed. Because of the increased^peajcjischarg^ and the accumulation of sedimentjn the channel over time, the capacity of Agua Hedionda Channel to convey storm water through the RCMHP has diminished, and currently a majority of the Mobile Home Park is,i.i.r i - subject to inundation during a 100-year storm event. The U.S. Army Corps of Engineers HEC-2 computer program was used to determine the hydraulic effects of the maintenance and sediment removal from Agua Hedionda Creek based on the ultimate detained 100-year storm event. Prepared by: PCB:RCmd/Reporf 13051-C.003.doc Rick Engineering Company- Water Resources Division 01-08-03 I i i I l I i i Additional Design Considerations The following are a description of additional concerns associated with the maintenance in Agua Hedionda Creek that have transpired since the June 1998 "Rancho Carlsbad Channel and Basin Project" report. Channel Vegetation The existing condition hydraulic analyses of Agua Hedionda Creek that are discussed later in this report show that velocities in the channel are up to approximately 15 feet-per-second during large storm events. The high velocities as well asi ongomgjnajnt^anceby the residents of the MHP have prevented significant establishment of mature vegetation within the main channel bed. The side slopes of the channel have become vegetated with mostly ornamental and exotic species assumed to be planted by the residents of the Mobile Home Park. Therefore^jnaintenance^of the Agua Hedionda Channel is expected r " to have littlejnipact jo^enyironmentally sensitive habitat. The hydraulic analysis of Agua Hedionda Creek assumes that the channel will continue to be maintained regularly and the establishment of mature vegetation within the channel will be prevented. I Conveyance of Low Flows at the El Camino Real Bridge The limits of grading cannot extend underneath the El Camino Real Bridge due to the proximity of this_-•• "-...I.... iX»^^_BX»^«»^»a»««»»B»E<>«-~i^^ r J H project to the Coastal Zone. The original Agua Hedionda channel design elevation at the El Camino Real Bridge is3L6jfeet_on the 1969 Grading Plans. Due to deposition in the channel, the accumulated H sediment at the El Camino Real Bridge has reached an elevation of 35.4 feet based on a field survey dated June 2002. In order to match the existing ground at El Camino Real Bridge, the flowline of the • channel will "ramp up" a vertical distance of approximately 4 feet. This ramp will prevent low flows from being conveyed through the channel, and will create a potential for ponded water just upstream of I the bridge. It is anticipated that a large storm event will transport the accumulated sediment under the El Camino Real Bridge and provide drainage of the flows for future storms. However, it is difficult to • predict when a storm event adequate to remove the sediment will occur. In order to prevent ponding water_andjflie jrigkjrf vector breeding upstream of the El Camino Real Bridge, a low-flow drainage pipe may be installed upstream of El Camino Real to route low flows from Agua Hedionda Creek to the existing 8' x 8' box culvert under El Camino Real Road downstream of the Prepared by: DCB:RCmd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01 -08-03 10 Robertson Ranch property. Another alternative involves the installation of a pump at the El Camino Real Bridge to pump flows over the ramp and under the El Camino Real Bridge to the Lagoon. Based on conversations with the San Diego Regional Water Quality Control Board (Regional Board) both situations should be presented to the Regional Board in writing, requesting a written response from ^1 the Board. The Regional Board may either require additional permitting for the project based on increased impacts to jurisdictional waters, or determine that no additional permits are required and ^ handle any potential impacts through the 401 Water Quality Certification Process. A written response _i from the Regional Board would alleviate the possibility of differing opinions among regulators and ^' identify a clear path to follow. I™ Alternatively, the existing ground elevation of 35.4 feet at the El Camino Real Bridge could be extended MI as the constant flowline elevation of the proposed channel until it daylights with the existing drop structure in Agua Hedionda Creek located just downstream of the Wood Plank Bridge. This alternative n does not provide as much increase to the capacity in Agua Hedionda Creek, but will maintain conveyance of low flows through the channel without installation of a drainage pipe or pump upstream of the El Camino Real Bridge. M Kelly Ranch Weir Structure • According to the as-built improvement plans entitled, "Agua Hedionda Channel" revised November 13, ! j 1985, there is an existing desilting basin located just downstream of the El Camino Real Bridge that 1 includes a weir structure located approximately 1300 feet downstream of the bridge. A field survey •I performed June 2002 shows that the sediment elevations at the downstream face of the El Camino Real Bridge are at least 6 feet higher than the design flowline elevation of this weir as shown on the I November 1985 improvement plans. The design elevation of the weir structure on the 1985 improvement plans is 23.5 feet. However, the sediment elevation modeled in the HEC-2 analyses at this •( location is approximately 24.5 feet.. Because the drop structure and the weir structure that are associated with the desilting basin have been completely buried under existing sediment, they do not impact the I HEC-2 analyses of Agua Hedionda Creek. I Prepared by: DCB:RCmd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01-08-03 11 Existing Condition Analysis The existing condition HEC-2 hydraulic analysis of Agua Hedionda Creek and Calavera Creek is located in Appendix C of this report. The cross-sectional geometry is based on field surveys of Calavera Creek in December 2001 and Agua Hedionda Creek in May 2002. The hydraulic analysis of the ^- existing topography in the channel includes a water surface profile based on the existing hydrology, as ™: well as a water surface profile based on the proposed detained hydrology that includes the construction ^: of Detention Basins BJB, BJ, Melrose, and Faraday as well as the split flow in Calavera Creek. The ™ map entitled, "Preliminary Rancho Carlsbad Channel and Basin Project 100-Year Floodplain ^ Alternatives" located in Map Pocket 1 shows the limits of flooding in the MHP associated with the ™ existing topography in Agua Hedionda for both the existing condition hydrology and the proposed ta| detained condition hydrology. Iff, Proposed Maintenance Design Alternatives The following is a summary of the design alternatives presented for the proposed maintenance of Agua i i *i Hedionda Creek and the appendices in which the corresponding HEC-2 hydraulic analyses are located. All of the following i and Calavera Creek. All of the following alternatives include the proposed ultimate detained flows in Agua Hedionda Creek • Alternative 1. Trapezoidal channel with 2:1 side-slopes extending from the field surveyed limits of grading to the original design flowline elevation (Appendix D) V Alternative 2. Trapezoidal Channel with 3:1 side-slopes extending from the existing toe of slope to the ) original design flowline elevation (Appendix E) ft Alternative 3. Trapezoidal channel with 2:1 side-slopes extending from the field surveyed limits of grading to the existing ground elevation at the El Camino Real Bridge (Appendix F) Hi Alternative 4. Trapezoidal channel with 3:1 side-slopes extending from the existing toe of slope to the existing ground elevation at the El Camino Real Bridge (Appendix G) • Alternative 5. No Grading in Agua Hedionda Creek (Profile 2 of Existing Condition Analysis located in Appendix C) The map entitled, "Preliminary Rancho Carlsbad Channel and Basin Project 100-Year Floodplain Alternatives" located in Map Pocket 1 shows the HEC-2 cross-section locations, as well as the inundated areas associated with each maintenance alternative. Table 2 shows the anticipated detained 100-year Prepared by: DCB:RC:nd/Report/13051 -C.003.doc Rick Engineering Company - Water Resources Division 01-08-03 12 1 f 1 I I I I water surface elevations. Table 3 shows the anticipated number of inundated lots and the average depth of water within the inundated areas for each of the five Agua Hedionda Creek grading design alternatives. -. Table 2. Anticipated Detained 100-year WSELs for the Agua Hedionda Creek Grading Alternatives fc I I I xsec 2 2.9 3 5.1 6.21 7.58 9.15 12.2 14.65 17.19 19.83 21.51 22.81 25.41 Detained 100-year WSEL, feet Alternative 1 44.41 44.99 45.52 45.61 45.29 45.45 45.57 46.44 46.96 47.35 47.67 47.89 48.23 48.36 Alternative' 2 " 43.68 44.1 45.27 45.86 45.68 45.81 45.92 46.89 47.23 47.61 47.98 48.51 48.85 48.97 Alternatives 43.65 45.66 46.55 46.9 46.48 46.74 46.85 47.84 48.34 48.58 48.84 49.04 49.34 49.37 Alternative 4 (Alternative 5 44.04 45.55 46.91 47.52 47.28 47.44 47.51 48.32 48.64 48.86 49.12 49.47 49.76 49.77 44.63 46.27 47.42 47.94 47.68 47.86 47.86 48.88 49.36 49.6 50.22 50.74 51.32 50.71 Table 3. Summary of Inundated Lots Resulting from Each Agua Hedionda Creek Grading Alternative Number of Inundated Lots Average Depth of Water, feet Alternative 1 102 0.9 Alternative 2 | Alternative 3 111 0.8 148 1.65 Alternative 4 173 2.1 Alternative 5 236 2.6 Prepared by: Rick Engineering Company- Water Resources Division DCB:RCmd/Report/13051-C.003.doc 01-08-03 13 CONCLUSION Agua Hedionda Creek is highly subject to sedimentation for the majority of the creek. Portions of the channel within the Mobile Home Park have experienced up to 6 feet or more of sediment deposition • since the original channel construction. This high sediment load may be the result of agricultural areas upstream of the RCMHP as well as the effects of an increased peak discharge within Agua Hedionda • Creek due to development within the upstream watershed. Because of the increased peak discharge and the accumulation of sediment in the channel over time, the capacity of Agua Hedionda Creek to convey • storm water through the RCMHP has diminished, and currently a significant portion of the lots within the Mobile Home Park is subject to inundation during a 100-year storm event. In an effort to minimize I flooding within the Mobile Home Park, the City of Carlsbad has investigated several alternatives to increase the capacity of the channel and improve flood protection in the area. The design concerns for the maintenance of Agua Hedionda Creek include slope stability of the soil ] material, removal of existing native trees along the channel side slopes, and downstream conveyance of low flows due to the proximity of the project to the coastal zone. The following is a summary of the ! design alternatives presented for the proposed maintenance of Agua Hedionda Creek. All of the Up following alternatives include the proposed ultimate detained hydrologic conditions. I Alternative 1. Trapezoidal channel with 2:1 side-slopes extending from the field surveyed limits of I grading to the original design flowline elevation Alternative 2. Trapezoidal Channel with 3:1 side-slopes extending from the existing toe of slope to the I original design flowline elevation Alternative 3. Trapezoidal channel with 2:1 side-slopes extending from the field surveyed limits of I grading to the existing ground elevation at the El Camino Real Bridge Alternative 4. Trapezoidal channel with 3:1 side-slopes extending from the existing toe of slope to the I existing ground elevation at the El Camino Real Bridge Alternative 5. No Grading in Agua Hedionda Creek Each alternative was analyzed hydraulically to determine the number of lots removed from the floodplain. The resulting water surface elevations were mapped, and the floodplain for each alternative Prepared by: DCB:RC:nd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01 -08-03 16 I • is shown on the map entitled, "Preliminary Rancho Carlsbad Channel and Basin Project 100-year Floodplain Alternatives," dated December 2002 included in Map Pocket 1. This report summarizes the maintenance design alternatives for Agua Hedionda Creek. In order for Rancho Carlsbad Channel and Basin Project to proceed into final design stages, the following issues need to be resolved: ^ 1. The soil stability within the channel and the need for additional geotechnical investigations including borings within Agua Hedionda Creek. 2. The conveyance of low flows at the El Camino Real Bridge. The'environmental permitting through the San Diego Region the U.S. Army Corps of Engineers and The California Department of Fish and Game. I 3. The'environmental permitting through the San Diego Regional Water Quality Control Board, i t Prepared by: DCB:RCmd/Report/l3051-C.003.doc Rick Engineering Company - Water Resources Division 01-08-03 17 RICKENGINEERINGCOMPANY RANCHO CARLSBAD MOBILE HOME PARK ALTERNATIVE ANALYSIS FOR AGUA HEDIONDA CHANNEL MAINTENANCE Job Number 13182-D September 30, 2004 VOL. 2 OF 2 TABLE OF CONTENTS Introduction ................................................................................................................................. 1 Vicinity Map ...... ......................................................................................................................... 3 Flood Insurance Rate Map (FIRM) ............................................................................................. 4 Hydrologic Analysis .................................................................................................................. 5 Table 1: Description of Detention Basins ....................................................................... 7 Table 2: 100-year discharges (cfs) in Calavera Creek and Agua Hedionda Creek within the Rancho Carlsbad Mobile Home Park .................................................. 8 Project Constraints ...................................................................................................................... 9 Hydraulic Analysis ................................................................................................................... 11 Table 3. Anticipated Detained 1 00-year WSELs for the Agua Hedionda Creek Grading Alternatives ............................................................... 15 Maintenance Program ............................................................................................................... 18 Conclusion .......................................................................................................................... . ..... 19 Appendix A: Referenced Plans for Faraday and Melrose Detention Basins, Cannon Road, El Camino Real Appendix B: 100- Year Ultimate Detained HEC-1 Hydrologic Analysis (FN: 5DBJB3.HC1) Appendix C: HEC-RAS Analysis of Existing Condition Channel Geometry with Undetained Flows "No Action" Alternative" (Geometry File: RC_ex) aly^s Flows (Geometry File: RC_ex) Appendix E: HEC-RAS Analysis of Alternative A. Includes "ramp" up to existing ground elevation at El Camino Real (Geometry File: Alt A) Appendix F: HEC-RAS Analysis of Alternative B. Includes grading down to elevation 32 from Cannon Road to El Camino Real (Geometry File: Alt B) Appendix G: HEC-RAS Analysis of Alternative C. Includes 70' bottom width between Cannon Road and El Camino Real (Geometry File: Alt C) Prepared by: DCB:RC:NO:jf/Report/13182-D.001 Rick Engineering Company - Water Resources Division 9-30-04 Map Pocket 1: Preliminary Rancho Carlsbad Channel and Basin Project 100-Year Floodplain Alternatives Map Pocket 2: Preliminary Rancho Carlsbad Channel and Basin Project Grading Exhibits Prepared by: Rick Engineering Company - Water Resources Division DCB:RC:NO:jf/Report/13182-D.001 9-30-04 I I I RICKENGINEERINGCOMPANY RANCHO CARLSBAD MOBILE HOME PARK ALTERNATIVE ANALYSIS FOR AGUA HEDIONDA CHANNEL MAINTENANCE Job Number 13182-D December 13, 2004 Prepared by: Rick Engineering Company - Water Resources Division DCB:KH:jf7Report/13182-D.002 12-13-04 Prepared by: Rick Engineering Company - Water Resources Division DCB :KH :jf7Report/13182-D.002 12-13-04 I I I i i i i I TABLE OF CONTENTS Introduction : 1 Exhibit 1. Rancho Carlsbad Mobile Home Park Vicinity Map 4 Exhibit 2. Agua Hedionda Basin Flood Control Projects 5 Exhibit 3. Agua Hedionda Creek and Calavera Creek Watershed Map 6 Exhibit 4. Flood Insurance Rate Map (FIRM) 7 Hydrologic Analysis 8 Table 1: Description of Detention Basins 10 Table 2: 100-year Discharges in Calavera Creek and Agua Hedionda Creek within the Rancho Carlsbad Mobile Home Park 13 Project Constraints 14 Hydraulic Analysis 16 Table 3. Anticipated Detained 100-year WSELs for the Agua Hedionda Creek Grading Alternatives and Number of Inundated Lots 21 Exhibit 5. Alternative A Approximate Limits of Grading 22 Exhibit 6. Alternative B Approximate Limits of Grading 23 Exhibit 7. Alternative C Approximate Limits of Grading 24 1 Ongoing Maintenance Program , 27 Conclusion 28 I Table 4. Matrix Summary of Proposed Agua Hedionda Creek Maintenance Alternatives 29 Appendix A: Referenced Plans • "Grading Plan Rancho Carlsbad Mobile Home Park," June 27, 1969, South Bay Engineering • "Cannon Road Bridge Over Agua Hedionda General Plan," sheet 59 of 131, February 19, 1998, McDaniel Engineering (Drawing No. 333-2G) • "El Camino Real Bridge Widen Over Agua Hedionda Channel General Plan," sheet 68 of 131, February 19, 1998, McDaniel Engineering (Drawing No. 333-2G) • "Lake Calavera Remedial Improvements General Site Plan," sheet 6 of 35, "Lake Calavera Remedial Improvements Civil Site Plan 1," sheet 7 of 39, "Lake Calavera Remedial Improvements Concrete Spillway Sections," sheet 10 of 39, "Lake Calavera Remedial Improvements New Outlet Pipe Section," sheet 24 of 35, October 2003, CGvL Engineers (Drawing No. 411-6) ^ Prepared by: DCB:KH:jf/Report/13182-D.002 H Rick Engineering Company - Water Resources Division 12-13-04 I I I I I I I • Grading Plans for Melrose Detention Basin Outlet: "Palomar Forum Melrose Drive" sheet 11 of 17, June 3, 2004, O'Day Consultants (Drawing No. 399-4A) • Grading Plans for Faraday Detention Basin Outlet: "Carlsbad Oaks North Faraday Ave," sheet 20 of 37, February 2003, O'Day Consultants (Drawing No. 415-9C) • Grading Plans for Detention Basin BJB: "Calavera Hills Phase II," sheet 5 of 80, October 22, 2002, O'Day Consultants (Drawing No. 390-9A) • Grading Plans for Detention Basin BJ: Base Map "City of Carlsbad Orthophoto Mapping," sheet 55 of 225, Copyright 1991 (Drawing No. 296-5) Appendix B: Back-up Hydraulic Calculations • Detention Basin Outlet Structure at Melrose • Detention Basin Outlet Structure at Faraday • Detention Basin Outlet Structure at BJB • Capacity of 8x8 RGB at El Camino Real Appendix C: Hydrologic Analyses • 100-Year Ultimate Detained HEC-1 (FN: RC100.HC1) • 100-Year Ultimate Undetained HEC-1 (FN: RCUNDET.HC 1) • 100-Year, 24-Hour Precipitation Map • HEC-1 Workmap with USGS Topographic Map Appendix D: HEC-RAS Analysis of No Action Alternative (Executable Files: Project - RanchoCarlsbadAH.prj; Plan - RanchoCarlsbadAH.pO 1; Geometry - RanchoCarlsbadAH.gOl; Steady Flow-RanchoCarlsbadAH.f04) Appendix E: HEC-RAS Analysis of Existing Detained Alternative (Executable Files: Project - RanchoCarlsbadAH.prj; Plan - RanchoCarlsbadAH.p07; Geometry-RanchoCarlsbadAH.gOS; Steady Flow - RanchoCarlsbadAH.fOl) Appendix F: HEC-RAS Analysis of Alternative A (Executable Files: Project - RanchoCarlsbadAH.prj; Plan - RanchoCarlsbadAH.p02; Geometry-RanchoCarlsbadAH.g21; Steady Flow - RanchoCarlsbadAH.fOl) Appendix G: HEC-RAS Analysis of Alternative B (Executable Files: Project - RanchoCarlsbadAH.prj; Plan - RanchoCarlsbadAH.p24; Geometry-RanchoCarlsbadAH.g23; Steady Flow - RanchoCarlsbadAH.fOl) 1 I Prepared by: Rick Engineering Company - Water Resources Division DCS :KH:jf/Report/l 3182-D.002 12-13-04 Appendix H: HEC-RAS Analysis of Alternative C (Executable Files: Project - RanchoCarlsbadAH.prj; Plan - RanchoCarlsbadAH.p27; Geometry - RanchoCarlsbadAH.g26; Steady Flow- RanchoCarlsbadAH.fOl) | MAP POCKETS I Map Pocket 1: Preliminary Rancho Carlsbad Channel and Basin Project 100-Year Floodplain Alternatives I Map Pocket 2: HEC-RAS Workmap (Available on Request) I I i I ( Prepared by: DCB:KH:jf7Report/13182-D.002 Rick Engineering Company - Water Resources Division 12-13-04 Table 3. Anticipated 100-year WSELs for the Agua Hedionda Creek Grading Alternatives and Number of Inundated Lots X-Sec 20.9 50.1 70.58 120.2 170.19 210.51 250.41 No Action Alternative 49.6 50.2 50.4 50.5 50.4 51.3 51.3 Detained 100-year WSEL (feet) Existing Detained 48.6 49.2 . 49.4 49.5 49.5 50.9 51.0 Alternative A 48.5 48.8 48.8 48.8 48.6 49.0 49.1 Alternative B 43.1 43.9 43.9 44.9 46.0 46.6 47.2 Alternative C 42.8 42.0 42.8 44.8 46.0 46.6 47.1 I * Approximate Number of Inundated Lots Maximum Water Depth* Above Channel Bank (feet) 278 •wo 7.4 210 tiJ5 6.5 I4l \<a^> 6.1 45 ?*> 1. 2 12 0.4 * These depths were taken at cross-section 50.1 located just upstream of the Calavera Creek and Agua Hedionda Creek confluence The RCMHP consists of mainly elevated foundations. Therefore, although a lot deemed as inundated may not necessarily mean the home is entirely flooded. The finished floor elevation of the structure may be above the floodplain, in which case an elevation certificate may be prepared to ensure the structure is not within the floodplain. Prepared by: Rick Engineering Company - Water Resources Division 21 DCB:KH:j£'Report/l 3182-D.002 12-13-04 DRAINAGE STUDY FOR ROBERTSON RANCH MP 02-03 Job No. 01-1014 January 26, 2004 Revised: September 7, 2004 Revised: November 12, 2004 Revised: June 28, 2005 Prepared by: O'DAY CONSULTANTS, INC. 2710 Loker Avenue West Suite 100 Carlsbad, California 92008-6603 Tel: (760)931-7700 Fax: (760)931-8680 Keith Hansen RCE 60223 Exp. 06/30/06 Date TABLE OF CONTENTS SECTION 1 SECTION 2 SECTION 3 SECTION 4 INTRODUCTION Purpose of Study Scope Comprehensive Basin Analysis Facilities Proposed in City of Carlsbad Master Plan of Drainage STUDY AREA Soils Groups Land Uses HYDROLOGY Modified Rational Method Description Program Process CONCLUSION Existing Condition Basin Exhibit Proposed Condition Basin Exhibit Vicinity Map Runoff Coefficients Isopluvial Maps 100-Year, 6-Hour 100-Year, 24-Hour Intensity-Duration Design Chart - Figure 3-1 Overland Time of Flow Nomograph - Figure 3-3 Maximum Overland Flow Length & Initial Time of Concentration - Table 3-2 Nomograph for Determination of Tc for Natural Watersheds - Figure 3-4 San Diego County Soils Interpretation Study Hydrology 100 year Analysis Existing Condition (Post Calavera Hills Development) Hydrology 100 year Analysis Proposed Condition SECTION 5 Exhibit A Existing Condition Drainage Map Exhibit B Proposed Condition Drainage Map SECTION 6 Established Flows from Upstream Developments SECTION 7 Hydraulic Analyses for 84" RCP at Robertson's Ranch East Village By: Chang Consultants, April 27, 2005 SECTION 8 Soil Loss Calculations € For the 100-year design storm, the 6-hour rainfall amount is 2.6 inches and the 24-hour rainfall amount is 4.3 inches. San Diego County Rational-Hydrology Program Package Version 7.4, developed by CivilCADD/CIVILDESIGN Engineering Software © (1991-2004), was used to determine the rainfall amount, times of concentration, corresponding intensities and flows for the various hydrologic basins within this model. The program was then used to route flows through drainage conveyance structures and confluence basins per the modified rational method. Established flows from upstream developments are included in Section 6. The 2003 San Diego County Hydrology Manual states "this manual should not be used when there is already an established flow." Although these flows were established by methods that may be less conservative than that of the current method, they will, in the developed condition, continue to drain though the project in existing natural drainage courses where they will combine with onsite flows before draining under El Camino Real. The combination of upstream flows and onsite flows has a tributary area that exceeds one square mile. Rick Engineering's December 13, 2004 HEC-RAS calculations analyze these areas with the assumption that Robertson Ranch will be developed consistent with the general plan land use map. The large area of land that is being preserved as open space rather then being developed as residential makes Rick's analysis a conservative one. Furthermore, Chang's analysis of the 84" Storm Drain modification to Rick's original study is included on Section 7. Program Process The Rational-Hydrology program is a computer-aided design program where the user develops a node link model of the watershed. Developing independent node link models of each interior watershed and linking these submodels together at confluence points create the node link model. The program has the capability of performing calculations for 11 different hydrologic and hydraulic processes. These processes are assigned and printed in the output. They are as follows: 1. Initial sub-area input, top of stream. 2. Street flow through sub-area, includes sub-area runoff. 3. Addition of runoff from sub-area to stream. 4. Street inlet and parallel street and pipeflow and area. 5. Pipeflow travel time (program estimated pipe size). 6. Pipeflow travel time (user-specified pipe size). 7. Improved channel travel - Area add option. 8. Irregular channel travel time - Area add option. 9. User-specified entry of data at a point. 10. Confluence at downstream point in current stream. 11. Confluence of main streams. CONCLUSION The results of the analysis of the Robertson Ranch Storm Drain system are presented on the following page: TABLE1-1 EXISTING CONDITIONS 100-YEAR STORM EVENT (FLOW INTO CALAVERA CREEK NORTH OF FLOOD WALL) BASIN A B C D ACREAGE 10.02 59.71 12.60 22.78 Q 21.18 68.81 15.48 38.20 TABLE1-2 PROPOSED CONDITIONS 100-YEAR STORM EVENT (FLOW INTO CALAVERA CREEK NORTH OF FLOOD WALL) BASIN A-1 B-1 C-1 D-1 ACREAGE 3.59 3.48 1.07 4.05 Q 16.68 13.92 6.01 18.98 TABLE1-3 PROPOSED CONDITIONS 100-YEAR STORM EVENT « < (FLOW INTO 84" STORM DRAIN . „ • NORTH OF CANNON RDi) - BASIN B-2 B-3 B-4 C-2 C-3 D-2 ACREAGE 16.76 10.92 37.39 8.52 1.27 23.02 Q 37.64 24.21 70.62 16.56 3.20 49.46 TABLE1-4 EXISTING CONDITIONS 100-YEAR STORM EVENT (FLOW TOWARDS RCB UNDER EL CAMINO REAL AT KELLY DR.) *' , BASIN E F G ACREAGE 10.12 3.42 58.97 Q 20.98 5.31 60.73 TABLET -5 "" '- PROPOSED CONDITIONS •>.:" 100- YEAR STORM EVENT/ '_ (FLOW TOWARDS RCB UNDER EL CAMINQ 1 ' f REAL AT KELLY DR.1 '-'- ' '' ' ' , BASIN E F G ACREAGE Q SITE TO BE DEVELOPED W/O INCREASE IN FLOW SITE TO BE DEVELOPED W/O INCREASE IN FLOW 57.56 104.01 TABLE1-6 EXISTING CONDITIONS 100-YEAR STORM EVENT (FLOW TOWARDS RCB UNDER EL CAMINO REAL WEST OF CANNON RD.) ~ BASIN H I ACREAGE 268.94 5.61 Q 252.07 11.55 TABLE1-7 PROPOSED CONDITIONS , 100-YEAR STORM EVEN? 1- (FLOW TOWARDS RCB UNDER EL CAMINO REAL WEST OF CANNON RD.) BASIN H 1 ACREAGE 256.47 23.21 Q 271.31 65.34 File: g:\jobs\2002\011014\hydrology\rob-hyd-master.doc HYDROLOGIC AND HYDRAULIC ANALYSES FOR ROBERTSON'S RANCH EAST VILLAGE 84" REINFORCED CONCRETE PIPE ALTERNATIVE April 27, 2005 Wayne W. Chang, MS, PE Chang Civil Engineering ° Hydrology ° Hydraulics ° Sedimentation P.O. Box 9496 Rancho Santa Fe, CA 92067 (858) 692-0760 -TABLE OF CONTENTS - Introduction 1 Proposed Design Criteria 2 Conclusion 3 FIGURES 1. Vicinity Map 2. O'Day Consultants' Preliminary 84-Inch RCP Alignment 3. Conceptual Design of Flow Split APPENDIX A. WSPGW Analyses B. 100-Year HEC-1 Analysis C. 100-Year HEC-RAS Analysis INTRODUCTION Robertson's Ranch is a proposed project by Calavera Hills II, LLC located in the city of Carlsbad (see Vicinity Map). The easterly portion of Robertson's Ranch is named the East Village. The East Village is immediately north of the Rancho Carlsbad Mobile Home Park (RCMHP) and west of College Boulevard. Cannon Road is aligned east-west near the southerly boundary of the East Village. The East Village will be developed with single- and multi-family residential units as well as a portion of a school site. CITY OF" OCEANSIDE HIGHWAY SITE OF VISTA CITY OF SAN MARCOS CITY OF ENCIHIJAS Figure 1. Vicinity Map Detention Basin BJB was recently constructed adjacent to the East Village immediately north of the intersection of Cannon Road and College Boulevard. Detention Basin BJB was designed by Rick Engineering Company (REC) as one part of their regional solution for reducing 100-year flood inundation in the RCMHP. The regional solution also includes the existing weir within the masonry wall immediately downstream of Detention Basin BJB. The weir is intended to control the 100-year flow rate on the north and south sides of the wall. By limiting the flow south of the wall, additional flood protection is provided to RCMHP. According to REC's analyses, ultimate flood protection of RCMHP from Calavera Creek is also dependent on construction of Detention Basin BJ, future modifications to the Calavera Dam outlet structure, additional adjustments to the Detention Basin BJB outlet, and improvements in Agua Hedionda Creek. REC's December 13, 2004 report, Rancho Carlsbad Mobile Home Park Alternative Analysis for Agua Hedionda Channel Maintenance, contains their latest hydrologic and hydraulic analyses for the regional flood control solution. In order to obtain the greatest flood control benefit from REC's regional solution, the masonry wall along the north side of RCMHP must either be adopted as is or replaced with a FEMA- certified floodwall. One criterion for a floodwall to be FEMA-certified is that "all maintenance activities must be under the jurisdiction of a Federal or State agency, an agency created by Federal or State law, or an agency of a community participating in the NFIP [National Flood Insurance Program] that must assume ultimate responsibility for maintenance." It is unlikely that the existing wall can meet this and other FEMA's requirements. Furthermore, a replacement wall could be difficult to design and permit. As a result, an alternative solution has been identified whereby a storm drain pipe will be used to convey the flow that would occur north of the wall. The storm drain will connect to the Detention Basin BJB outlet (an 11-foot by 7-foot reinforced concrete box culvert) and intercept flow that would have been directed north of the wall. The storm drain will be aligned along Cannon Road and outlet adjacent to the box culverts under Cannon Road just east of El Camino Real (see Figure 2 for O'Day Consultants' proposed preliminary storm drain alignment). This report contains proposed criteria to be used for design of the storm drain pipe. PROPOSED DESIGN CRITERIA REC's latest report is the December 13, 2004, Rancho Carlsbad Mobile Home Park Alternative Analysis for Agua Hedionda Channel Maintenance. This report contains REC's most current hydrologic analysis for Detention Basin BJB and Calavera Creek. The report indicates that the 100-year outflow from the Detention Basin BJB 11-foot by 7-foot reinforced concrete box (RGB) culvert will be 901 cubic feet per second (cfs). This assumes ultimate watershed development and future improvements as mentioned above. REC's hydrologic analysis indicates that the existing weir wall will divide the 901 cfs such that approximately 500 cfs flows north of the wall and 401 cfs flows south of the wall. REC's analysis shows that under ultimate conditions with Detention Basin BJ constructed, the 100-year flow rate in Calavera Creek South (Calavera Creek South refers to the channel south of the mobile home park wall) will be approximately 756 cfs, which is below the previously identified target flow rate of 1,000 cfs. The proposed alternative design involves connecting an 84-inch reinforced concrete pipe (RCP) to the 11-foot x 7-foot RCB. The 84-inch RCP invert elevation will be 0.7 feet above the RGB invert so that low flow in the RCB will continue to Calavera Creek South. This will direct approximately 75 cfs to Calavera Creek South prior to splitting into the 84-inch RCP (see Appendix A for the supporting hydraulic analysis). Currently, minor base flow enters Calavera Creek South from urban runoff and other sources in the watershed. This base flow helps maintain existing habitat in the creek. Allowing up to 75 cfs into Calavera Creek South exceeds the current average base flow rate, and will ensure that flow necessary to preserve existing habitat in Calavera Creek South will be maintained. In a March 2, 2005 meeting with city staff, Mr. David Hauser, Deputy City Engineer, indicated that this approach is acceptable subject to environmental review and resource agency approvals, if required. A wall will be constructed within the RGB to regulate the flow split to the 84-inch RCP. A conceptual design is shown in Figure 3. Hydraulic analyses indicate that the 84-inch RCP can convey 450 cfs without pressure (see Appendix A for the 84-inch RCP analysis based on O'Day Consultants latest plan). On the other hand, with 500 cfs the pipe will be under pressure. Therefore, it is proposed that the 901 cfs be split equally between the 84-inch RCP and Calavera Creek South. While this will result in slightly more flow (50 cfs more) entering Calavera Creek South than the latest REC analysis, it will still result in total flow in Calavera Creek South that is below the city's desired threshold of 1,000 cfs. In the March 2, 2005 city meeting, Messrs. David Hauser and Glenn Pruim, City Engineer, indicated that the additional 50 cfs entering Calavera Creek South would be allowed if it does not adversely impact 100-year inundation of the RCMHP mobile homes beyond REC's ultimate regional solution. In order to analyze the impact, REC's 100-year HEC-1 analysis from their December 13, 2004 report was revised to model 450 cfs entering Calavera Creek South rather than 401 cfs (see Appendix B). The updated flow rates were then entered into REC's Alternative C HEC-RAS model, which is the preferred alternative for the Agua Hedionda Creek improvements. The REC and revised results as well as REC's work maps are included in Appendix C, and show that the 100-year water surface elevations increase in the upper portion of Calavera Creek South (at and above Cross-Section 1230, which is approximately 1,200 feet upstream of the confluence with Agua Hedionda Creek) by 0.1 feet. However, this increase is contained in the Calavera Creek South channel and will not affect the mobile homes. The HEC-RAS results also show that the 100-year water surface elevations are unchanged throughout most of Agua Hedionda Creek. Two areas of particular interest are the El Camino Real bridge and the confluence of Agua Hedionda and Calavera Creek. The El Camino Real bridge is between Cross-sections 10.01 and 10.1. The water surface elevation lowers slightly (less than 0.1') below the bridge, then is equal above the bridge. While it seems contrary that the water surface elevation will lower with increased flow, this can occur because of changes in the velocity head. Further upstream at the confluence of the creeks at Cross-section 50.11, the water surface elevations are equal. At Cross-sections 310 and 400 in Calavera Creek immediately above the confluence, the water surface lowers by about 0.1' with the increased flow. Upstream of the confluence in Agua Hedionda Creek, the water surface elevations are essentially equal These comparisons indicate that the water surface elevations will not adversely impact the mobile home park nor other significant locations. Therefore, the flow increase into Calavera Creek South should be acceptable. CONCLUSION An alternative has been identified for conveying 100-year Calavera Creek flows occurring north of the Rancho Carlsbad Mobile Home Park wall. It is unlikely that this wall can meet FEMA's levee criteria; therefore, an alternative was developed, which will avoid the levee issue while still adhering to REC's regional solution for flood protection of RCMHP. The alternative will convey the flow in an 84-inch reinforced concrete pipe along Cannon Road. Preliminary engineering analyses show that two modifications to REC's latest criteria will be necessary for final design of the pipe. First, the low flow entering Calavera Creek South from Detention Basin BJB will be reduced from 300 cfs to 75 cfs. The lower flow rate will still meet the desired goal of providing base flow to "preserve the downstream habitat." Second, the ultimate condition 100-year flow rate into Calavera Creek South will be increased from 401 to 450 cfs. This is necessary to minimize pressure flow in the proposed 84-inch RCP. A larger pipe is not desirable because it will be more difficult to transition to the 11-foot by 7-foot RGB, will require additional cost, and may have potential utility conflicts with a proposed 8-inch sewer located along the westerly-most street in the East Village that will intersect with Cannon Road. REC's hydrologic and hydraulic analyses for the preferred alternative were modified based on this flow revision. The results indicate that the RCMHP mobile homes will not be adversely impacted by the flow increase. Consequently, it is requested that the city approve these revisions for final design of the alternative solution, which will be incorporated into the Robertson's Ranch Master Plan. RICKENGINEERING I COMPANY RANCHO CARLSBAD MOBILE HOME PARK ALTERNATIVE ANALYSIS FOR AGUA HEDIONDA CHANNEL MAINTENANCE Job Number 13182-D December 13, 2004 Revised December 12, 2005 RANCHO CARLSBAD MOBILE HOME PARK ALTERNATIVE ANALYSIS FOR AGUA HEDIONDA CHANNEL MAINTENANCE Job Number 13182-D Dennis C Bowling, M RCE #32838, Exp Prepared For City of Carlsbad David Hauser 1635 Faraday Ave Carlsbad, CA 92008 Prepared by. Water Resources Division Rick Engineering Company 5620 Fnars Road, San Diego, California 92110 (619)291-0707 www rickengmeenng com December 13,2004 Revised December 12,2005 TABLE OF CONTENTS Introduction 1 Exhibit 1 Rancho Carlsbad Mobile Home Park Vicinity Map 4 Exhibit 2 Agua Hedionda Basin Flood Control Projects .... . . 5 Exhibit 3 Agua Hedionda Creek and Calavera Creek Watershed Map 6 Exhibit 4 Flood Insurance Rate Map (FIRM) .... 7 Hydrologic Analysis 8 Table 1 Description of Detention Basins 10 Table 2 100-year Discharges in Calavera Creek and Agua Hedionda Creek within the Rancho Carlsbad Mobile Home Park 13 Project Constraints . 14 Hydraulic Analysis . 16 Table 3 Anticipated Detained 100-year WSELs for the Agua Hedionda Creek Grading Alternatives and Number of Inundated Lots . 21 Exhibits Alternative A Approximate Limits of Grading 22 Exhibit 6 Alternative B Approximate Limits of Grading 23 Exhibit 7 Alternative C Approximate Limits of Grading 24 Ongoing Maintenance Program 27 Conclusion . . 28 Table 4 Matrix Summary of Proposed Agua Hedionda Creek Maintenance Alternatives . 29 APPENDICES (on CD) Appendix A Referenced Plans •"Grading Plan Rancho Carlsbad~MoBrte~flofne~Park," June~27, 1969, South Bay Engineering • "Cannon Road Bridge Over Agua Hedionda General Plan," sheet 59 of 131, February 19, 1998, McDamel Engineering (Drawing No 333-2G) • "El Cammo Real Bridge Widen Over Agua Hedionda Channel General Plan," sheet 68 of 131, February 19, 1998, McDamel Engineering (Drawing No 333-2G) • "Lake Calavera Remedial Improvements General Site Plan," sheet 6 of 35, "Lake Calavera Remedial Improvements Civil Site Plan 1," sheet 7 of 39, "Lake Calavera Remedial Improvements Concrete Spillway Sections," sheet 10 of 39, "Lake Calavera Remedial Improvements New Outlet Pipe Section," sheet 24 of 35, October 2003, CGvL Engineers (Drawing No 411-6) Prepared by DCB KH kw/Rcport/13!82-D002 Rick Engineering Company- Water Resources Division 12-13-04 Revised 12-12-05 • Grading Plans for Melrose Detention Basin Outlet. "Palomar Forum Melrose Drive" sheet 11 of 17, June 3, 2004, O'Day Consultants (Drawing No 399-4A) • Grading Plans for Faraday Detention Basin Outlet "Carlsbad Oaks North Faraday Ave," sheet 20 of 37, February 2003, O'Day Consultants (Drawing No 415-9C) • Grading Plans for Detention Basin BJB "Calavera Hills Phase II," sheet 5 of 80, October 22, 2002, O'Day Consultants (Drawing No 390-9A) • Grading Plans for Detention Basin BJ Base Map "City of Carlsbad Orthophoto Mapping," sheet 55 of 225, Copyright 1991 (Drawing No 296-5) Appendix B Back-up Hydraulic Calculations • Detention Basin Outlet Structure at Melrose • Detention Basin Outlet Structure at Faraday • Detention Basin Outlet Structure at BJB • Capacity of 8x8 RGB at El Cammo Real Appendix C Hydrologic Analyses • 100-Year Ultimate Detained HEC-1 (FN RC100 HC1) • 100-Year Ultimate Undetamed HEC-1 (FN RCUNDET HC1) • 100-Year, 24-Hour Precipitation Map • HEC-1 Workmap with USGS Topographic Map Appendix D HEC-RAS Analysis of No Action Alternative (Executable Files Project - RanchoCarlsbadAH prj, Plan - RanchoCarlsbadAH pOl, Geometry - RanchoCarlsbadAH gOl, Steady Flow - RanchoCarlsbadAH f04) Appendix E HEC-RAS Analysis of Existing Detained Alternative (Executable Files Project - RanchoCarlsbadAH prj, Plan - RanchoCarlsbadAH p07, Geometry - RanchoCarlsbadAH g08, Steady Flow- RanchoCarlsbadAH tt)l) Appendix F HEC-RAS Analysis of Alternative A (Executable Files Project — RanchoCarlsbadAH.prj; Plan — RanchoCarlsbadAH p02, Geometry - RanchoCarlsbadAH g21, Steady Flow - RanchoCarlsbadAH fOl) Appendix G HEC-RAS Analysis of Alternative B (Executable Files Project - RanchoCarlsbadAH prj, Plan - RanchoCarlsbadAH p24, Geometry - RanchoCarlsbadAH g23, Steady Flow - RanchoCarlsbadAH fOl) Appendix H HEC-RAS Analysis of Alternative C (Executable Files Project - RanchoCarlsbadAH prj, Plan - RanchoCarlsbadAH p27, Geometry - RanchoCarlsbadAH g26, Steady Flow - RanchoCarlsbadAH fOl) Prepared by Rick Engineering Company- Water Resources Division DCB KH kw/Report/13182-D 002 12-13-04 Revised 12-12-05 MAP POCKETS Map Pocket 1 Preliminary Rancho Carlsbad Channel and Basin Project 100-Year Floodplam Alternatives Map Pocket 2 HEC-RAS Workmap (Available on Request) Prepared by DCB KH kw/Rcport/l3l82-D002 Rick Engineering Company - Water Resources Division 12-13-04 Revised 12-12-05 INTRODUCTION The purpose of this report is to summarize the design alternatives for Agua Hedionda Creek that have been prepared by Rick Engineering Company for the City of Carlsbad as part of the Rancho Carlsbad Channel and Basin Project The Rancho Carlsbad Mobile Home Park (RCMHP) is an existing residential area located north-east of El Camino Real, south-east of Cannon Road and south-west of College Boulevard Reach 'A,' within the City of Carlsbad, California (see Exhibit 1 on page 4) RCMHP contains portions of both Agua Hedionda and Calavera Creeks Agua Hedionda Creek flows westerly through the southern portion of RCMHP Calavera Creek flows southwesterly along the northern property boundary The confluence of Calavera Creek with Agua Hedionda Creek within the RCMHP is located approximately 300 feet upstream of El Camino Real The Agua Hedionda Creek and Calavera Creek watersheds are shown on Exhibit 3 (page 6) Original Channel Design The original constructed Agua Hedionda Creek channel was an earthen trapezoidal channel, as shown on the "Grading Plans for Rancho Carlsbad Mobile Home Park" dated June 27, 1969 prepared by South Bay Engineering (Appendix A). The overall length of the channel was approximately 1 2 miles and included both Agua Hedionda Creek and Calavera Creek The side slopes were 2 1 (horizontal vertical) and the approximate average bed slopes were 0 15 and 0 30 percent in Agua Hedionda Creek and Calavera Creek, respectively The bottom width of Agua Hedionda Creek varied from 58 feet at the El Camino Real Bridge to 44 feet upstream of the confluence The approximate channel depth was 11 5 feet The bottom width and channel depth of Calavera Creek were 4 feet and 9 feet, respectively Sedimentation Pattern within the Channels Historically, Agua Hedionda Creek has been highly subject to sedimentation within the channel along areas of the entire creek Portions of the channel have experienced up to 6 feet or more of sediment deposition within the Mobile Home Park This high sediment load may be the result of large agricultural areas upstream of the RCMHP as well as the effects of an increased peak discharge within Agua Hedionda Creek due to development within the upstream watershed Prepared by ' DCB KH kw/Report/13182-D 002 Rick Engineering Company - Water Resources Division { 12-13-04 Revised 12-12-05 Because of the increased peak discharge and the accumulation of sediment m the channel over time, the capacity of Agua Hedionda Creek to convey storm water through the RCMHP has diminished, and currently a large number of properties within the Mobile Home Park are subject to inundation during a 100-year storm event Calavera Creek has been subject to erosion within the upstream portion of the channel and sedimentation within the downstream portion of the creek near the confluence with Agua Hedionda Creek As a result of the sedimentation and the encroachment of homeowners into the channel, Calavera Creek no longer has capacity for the 100-year storm event Proposed Channel Maintenance In an effort to alleviate flooding within the Mobile Home Park, the City of Carlsbad has investigated several alternatives to increase the capacity of the creeks and improve flood protection in the area Rick Engineering Company prepared a study entitled, "Rancho Carlsbad Channel and Basin Project," dated June 30, 1998 that provided the preliminary design of four detention basins within the watershed tributary to the RCMHP, determined the maintenance and sediment removal required to return Agua Hedionda and Calavera Creeks to their original design configurations per the 1969 Grading Plans, and analyzed the effects of the reduced flow and restored channel capacity on the limits of inundation within the Mobile Home Park Since the 1998 study, further modifications to the proposed maintenance improvements were required These modifications include updates to the hydrologic model, exclusion of Calavera Creek from maintenance and grading activities, the analysis additional alternatives, as well as final design plans and calculations of three of the four proposed detention basins The "Project Constraints" section of this report addresses in detail the background of the required modifications The existing Calavera Creek channel configuration results in split flow, with some flow being conveyed north of the wall along the RCMHP as shown on Exhibit 4, the Flood Insurance Rate Map (FIRM) panel number 06073C0768 F dated June 19, 1997 (page 7) The original Channel and Basin Project design called for maintenance of Calavera Creek (i e widening the creek), which would force all of the flow into Calavera Creek thereby eliminating the split flow to the Prepared by DCB KM kw/Repon/13182-D 002 Rick Engineering Company - Water Resources Division 2 12-13-04 Revised 12-12-05 north side of the wall Environmental and biological constraints called into question the practicality of the previously proposed maintenance and grading within Calavera Creek Other constraints include the encroachment of homeowners into Calavera Creek, which introduces constructabihty issues of any grading along the channel banks Preserving the current capacity of Calavera Creek though maintaining the split flow situation to convey the 100-year storm can avert maintenance within Calavera Creek. The proposed split flow scenario presented in this report requires less flow be diverted to the north side of the wall than the existing FEMA- mapped condition The maintenance of Agua Hedionda Creek in conjunction with the construction of the proposed detention basins will provide hydraulic benefits to Calavera Creek and increase the capacity of the channel for flood protection This report presents several alternatives for maintenance of Agua Hedionda Creek to restore flood protection within the RCMHP Some alternatives require grading south of El Camino Real, which was not proposed in the 1998 study The locations of the proposed flood control projects discussed in this report are shown on Exhibit 2 on page 5 Prepared by DCB KH kw/Report/13182-D 002 Rick Engineering Company - Water Resources Division 3 12-13-04 Revised 12-12-05 -V.-viA. --'A;, - i •/! vr:>- REK ENGINEERING COMB\NYi4jgjMMhw<»**,. r*ML^.«t,.4.tH»I*i«i*«i47a7 v«M/h E^^CHQ CARLSBAD MOStLB HOME PARK VICINITY MAP FARADAY BASIN EXHIBIT 3 AGUA HEDIONDA CREEK AND CALAVERA CREEK WATERSHED MAP MELROSE BASIN Legend Major Watersheds Detention Basins APPROXIMATE SCALE IN FEET 600 0 600 NATIONAL FLOOD INSURANCE PROGRAM FIRM FLOOD INSURANCE RATE MAP SAN DIEGO COUNTY, CALIFORNIA AND INCORPORATED AREAS PANEL 788 OF 2375 (SEE MAP INDEX FOR PANELS NOT PRINTED) NUM8EB PANEL SIBK CONTAINS COMMUMTY MAP NUMBER 06073C076B F EFFECTIVE DATE: JUNE 19,1997 Federal Emergency Management Agency copyorapMVoncfthMiboMrtiMnMdtoodiniik I :h may M* tern vwto MibMguwK tm lb» AH* on tto HYDROLOGIC ANALYSES The U S Army Corps of Engineers Hydrologic Engineering Center HEC-1 computer program was used to prepare hydrologic models of the watersheds tributary to Agua Hedionda Creek and Calavera Creek upstream of the Rancho Carlsbad Mobile Home Park The hydrology was used to determine preliminary detention basin sizes and outlet structures for four proposed detention basins upstream of the Rancho Carlsbad Mobile Home Park, two within Agua Hedionda Creek, one at Melrose Drive and one at Faraday, and two within Calavera Creek, Basins BJ and BJB, near the intersection of the Cannon Road and College Boulevard extensions The hydrology was also used as a source in the HEC-RAS hydraulic analyses Since the publication of the 1998 study there have been several modifications to the hydrologic analysis of the watershed related to the hydrologic modeling parameters, Calavera Dam, and final design of three of the proposed detention basins Hydrologic Modeling Parameters One aspect of the hydrologic modeling parameters that has been modified since the initial study was the channel Manning's "n" and basin factor values used to calculate the lag time for each watershed Since the hydrology is based on the ultimate development of the watershed, the Manning's "n" and basin factor values were originally based on a typical developed condition for the land-use type dominant m each watershed However, based on current California State Water Resources Control Board requirements for new developments, the tendency is less directly connected impervious surfaces, resulting in a higher Manning's "n" and higher basin factors due to more natural channels Therefore, the hydrologic model was updated to reflect this change Calavera Dam The City of Carlsbad has recently initiated a project involving the restoration and reconstruction of the outlet structure for Calavera Dam The modifications include replacing the existing outlet tower, which currently does not function, with a new one The new outlet tower will allow the lake to be drawn-down in anticipation of large storm events, resulting in a lower peak flow out of Calavera Lake, which has resulted in a change in the hydrology upstream of detention basin BJB The project is currently scheduled to go to bid in the Spring/Summer 2005 The hydrologic Prepared by DCB KH kw/Report/13182-D 002 Rick Engineering Company - Water Resources Division 12-13-04 8 Revised 12-12-05 model was updated based on the following assumptions to reflect the impacts of drawdown of Calavera Dam on the peak discharge in Calavera Creek • Lake elevation is at 209' (the crest of the outlet tower) when the storm begins • The 3 valves are opened once the storm starts • Valve geometry can be found on sheet 24 on the improvement plans prepared by CGvL Engineers titled "Lake Calavera Remedial Improvements" (Drawing # 411-6, dated October 2003, Appendix A) • Spillway elevation is at 214 5' per sheet 6 of the aforementioned plans These analyses result in a 100-year peak flow into Lake Calavera of 1,831 cubic feet per second (cfs) and a peak flow out of Calavera Dam of 967 cfs with a maximum ponded water surface elevation (WSEL) of 219 2' Detention Basin Design The goal of the detention basins proposed by the 1998 study is to detain less than 50 acre-feet of volume, to avoid being within the jurisdiction of the California Department of Water Resources Division of Safety of Dams (DSOD) The modifications to the hydrology, as well as the analysis of the final basin grading, has resulted in some changes to the outlet structure geometries Three of the four proposed detention basin outlet structures have been designed The Melrose detention basin outlet design prepared by O'Day Consultants (Drawing # 399-4A, dated June 3, 2004) has been signed and approved by the City of Carlsbad The Faraday detention basin outlet design prepared by O'Day Consultants (Drawing # 415-9C, dated February 2003) has also been approved Based on the modifications to the hydrologic modeling parameters and the Calavera Dam outlet, as well as the final design information provided by the City of Carlsbad for the Faraday and Melrose basin grading, the detention basin routing has been updated Copies of the plans are included in Appendix A for reference and copies of the hydraulic calculations (prepared by Rick Engineering Company) for the outlet structures are included in Appendix B The construction of detention basin BJB was completed with the construction of the College Boulevard and Cannon Road extensions per the plans prepared by O'Day Consultants dated October 22, 2002 (Drawing # 390-9A sheet 5 of 80, Appendix A) However, the outlet structure Prepared by DCS KH kw/Report/l3!82-D002 Rick Engineering Company - Water Resources Division 12-13-04 9 Revised 12-12-05 for detention basin BJB will ultimately require modifications to further reduce flows once the Calavera Dam improvements are constructed to maximize the flood control benefit of Basin BJB Copies of the hydraulic calculations (prepared by Rick Engineering Company) for the BJB outlet structure are included in Appendix B. Detention basin BJ is still modeled based on the preliminary design outlined in the 1998 report The preliminary plans are based on the map titled "City of Carlsbad Orthophoto Mapping" sheet 55 of 225, Copyright 1991 (Appendix A) Table 1 provides a detailed summary of the detention basins. Table 1: Description of Detention Basins Location of Detention Basin Melrose Faraday BJB* BJ~ Q100 (cfs) In 839 906 1094 629 Out 489 642 901 348 Max ponded WSEL (feet) 3305 241 4 75 16 759 Max Inundation Area (ac) 68 68 24 8 Max Storage Volume (ac-ft) 442 498 499 48 Outlet Structure Details Box culvert with a rectangular orifice 5 6' wide x 4' tall (FL @ 308 0') Box culvert 4 3' wide x 5 7' tall (FL @ 221 8') 72" RCP (FL @ 65 0') and a rectangular box culvert 11' wide x 7' tall with a 10' wide x 7' tall square-edged entrance (FL @ 62 0') * Rectangular box culvert 6' wide x 3' tall (FL@62')~ * The ultimate configuration of detention basin BJB will require construction of a 7 5' x 7' rectangular opening over the 10' x 7' box and a V-notched opening with a 0 1 ' bottom width and a 1 4' top width placed over the 72" RCP to maximize the storage in the basin once the Calavera Lake Dam improvements are constructed ^ Preliminary design per 1998 Channel and Basin Project report Subject to revision pending final design FEMA Drainage Patterns The current effective Federal Emergency Management Agency (FEMA) study for Calavera Creek shown on the FIRM Panel number 06073C0768 F, dated June 19, 1997 (page 7) shows the flow splitting to the north and south sides of the existing concrete wall that is located along the creek Per the FEMA Flood Insurance Study (FIS), the total flow in Calavera Creek is 1350 cfs The FEMA models show approximately 805 cfs conveyed through Calavera Creek on the north Prepared by Rick Engineering Company - Water Resources Division 10 DCB KH kw/Report/13l82-D002 12-13-04 Revised 12-12-05 side of the existing wall, and 545 cfs conveyed on the south side of the wall within the Calavera Creek Channel in the RCMHP Modified Drainage Patterns The analysis in the 1998 report assumed the entire flow from Calavera Creek would be conveyed through the RCMHP because the flow would be concentrated from Basin BJB and the channel would be graded to increase capacity (i e flow would not split to the north side of the wall) However, the project approach to Calavera Creek was modified since the initial study for several reasons The majority of property owners adjacent to Calavera Creek have encroached into the creek with the construction of decks, patios, retaining walls, and landscaping Any grading activities within the channel would result in the removal of the structures which will not be acceptable to many of the RCMHP residents Also, preliminary biological investigations of Calavera Creek identified mature oaks and willows established within the channel The removal of these mature trees would require extensive mitigation If the trees were attempted to be preserved by relocation or structural measures (e g retaining walls, etc ), the engineering and construction costs would be extremely high and the survival rate of the trees is unknown Because the Calavera Creek channel does not have capacity for the entire flow m its current condition, a split flow scenario that more closely mimics the current FEMA model was created by the construction of a weir near the outlet of detention basin BJB at the entrance to Calavera Creek The proposed split flow will result in the conveyance of approximately 500 cfs along the north side of the wall adjacent to the RCMHP and Calavera Creek This flow will be conveyed under Cannon Road through triple 10' wide x 7' tall reinforced concrete boxes (RGB), under El Camino Real through an existing 8' x 8" RGB, and combine with Agua Hedionda Creek upstream of the Cannon Road Bridge prior to discharging into Agua Hedionda Lagoon The 8' x 8' RGB beneath El Camino Real is a restriction of the amount of flow that can split to the north side of the wall at the weir If too much flow splits to the north, flow will overtop El Camino Real at a low point in the road (elevation = 41 3') just upstream of the 8' x 8' RGB The peak discharge to be conveyed north of the wall was determined based on the capacity of the existing 8' x 8' box assuming 500 cfs is diverted to the north side of the wall Calculations included the preparation of a HEC-2 hydraulic model for the north side of the wall extending from the weir to the downstream face of the 8' x 8' box and a WSPGN hydraulic model of the 8' Prepared by DCS KH kWReport/13182-0002 Rick Engineering Company - Water Resources Division 12-13-04 II Revised 12-12-05 x 8' box The following flow rates from the HEC-1 were input into the HEC-2 500 cfs at the weir, 522 cfs at the Cannon Road triple RGBs, and 568 cfs at the 8' x 8' RGB Based on results from the HEC-2 the WSEL at the location of the low point in El Cammo Real was 39 6,' which results in 1 7' of freeboard The HEC-2-calculated WSEL at the upstream face of the 8' x 8' RGB was compared to the WSPGN-calculated WSEL at the same location for verification. Copies of the WSPGN, HEC-2, and HEC-2 workmap are included in Appendix B It should be noted that due to environmental considerations from the resource agencies, frequent storm flows from detention basin BJB that result in a peak discharge of less than approximately 300 cfs will be conveyed in Calavera Creek without flow splitting to the north side of the wall Any modifications to the split flow or the drainage patterns on the north side of the wall should be designed to maintain this scenario, and not intercept any portion of flows less than approximately 300 cfs to preserve the downstream habitat The drainage basin modeling in the HEC-1 has been modified since the preparation of the 1998 study to reflect the split flow scenario Refer to Appendix C for HEC-1 hydrologic calculations Table 2 shows a comparison of the 1998 study hydrologic results to those of the current 2004 study Prepared by DCB KH kw/Repon/13182-D 002 Rick Engineering Company - Water Resources Division 12-13-04 12 Revised 12-12-05 Table 2: 100-year Discharges in Calavera Creek and Agua Hedionda Creek within the Rancho Carlsbad Mobile Home Park Location Calavera Creek upstream of the Mobile Home Park Calavera Creek north of the wall (within Robertson Ranch) Calavera Creek upstream of the confluence with Agua Hedionda Creek Agua Hedionda Creek upstream of the confluence with Calavera Creek Agua Hedionda Creek downstream of the confluence with Calavera Creek Existing Condition (FEMA) 1350 805 545 7810 8080 1998 Hydrology* City of Carlsbad Ultimate Developed Condition (Existing Hydrology) 1910 N/A 8050 9950 Preliminary Ultimate Developed Detained Condition (Proposed Hydrology) 1550 N/A 7600 8970 2004 Updated Hydrology Undetamed Hydrology 1419 1410 7795 9195 Detained Hydrology 749 500 756 7338 8092 * Source "Rancho Carlsbad Channel & Basin Project" dated June 30, 1998 (Rick Engineering Company) Prepared by Rick Engineering Company- Water Resources Division 13 DCB KH kw/Report/13l«2-D002 12-13-04 Revised 12-12-05 e PROJECT CONSTRAINTS Additional factors affecting the limits of grading of this project have impacted the proposed Agua Hedionda channel grading These factors include, but are not limited to the proximity of the project to the Coastal Zone, the presence of native and non-native trees along the channel banks, environmental processing requirements, and channel vegetation Coastal Zone The limits of the Coastal Zone are located along El Camino Real in the vicinity of this project Any maintenance alternative proposing to grade between El Camino Real and Cannon Road would require additional permits and coordination with the Coastal Commission This area is within the Local Coastal Zone administered by the City Council The Coastal Commission could appeal any decision to allow grading or maintenance within the proposed channel downstream of El Camino Real Native and Non Native Trees within the Channel The RCMHP Homeowners Association and the Environmental Resource Agencies have voiced concerns regarding the removal of mature trees and native trees along the Agua Hedionda Creek bank In order to preserve as many existing mature trees as possible within Agua Hedionda Creek, a site visit was conducted with representatives from Rick Engineering Company, RECON, RCMHP, and the City of Carlsbad in the spring of 2002 to identify the native and exotic trees along the channel to determine what trees located on the channel banks could be preserved during the proposed maintenance Survey data was also collected to assist m determining the maximum limits of grading along Agua Hedionda Creek Due to the location of these trees along the channel side-slopes, the original channel bottom widths as shown on the "Grading Plans for Rancho Carlsbad Mobile Home Park" dated June 27, 1969 prepared by South Bay Engineering can not be completely restored Environmental Processing The Agua Hedionda Creek maintenance project involves activities within a junsdictional water Therefore, this project would require processing through the California State Water Resources Control Board, the U S Army Corps of Engineers, as well as the California Department of Fish and Game The detention basins proposed at Melrose, Faraday, BJB, and BJ are subject to Prepared by DCB KH kw/Rcport/13182-D 002 Rick Engineering Company - Wtler Resources Division 12-13-04 14 Revised 12-12-05 separate processing through the resource agencies as part of their associated development projects since their construction is linked to the construction of adjacent developments, not to the maintenance of Agua Hedionda Creek channel Channel Vegetation The existing condition hydraulic analyses of Agua Hedionda Creek that are discussed later in this report show high velocities throughout the creek especially at bridge crossings during large storm events The high velocities as well as ongoing maintenance by the residents of the RCMHP have prevented significant establishment of mature vegetation within the main channel bed of Agua Hedionda Creek upstream of El Cammo Real The side slopes of the channel have become vegetated with mostly ornamental and exotic species presumably planted by the residents of the Mobile Home Park Therefore, maintenance of the Agua Hedionda Channel within RCMHP is expected to have little impact to environmentally sensitive habitat The hydraulic analysis of Agua Hedionda Creek assumes that the channel will continue to be maintained regularly and the establishment of mature vegetation within the channel will be prevented Due to the above-mentioned factors, the original design configuration of Agua Hedionda Creek cannot be completely restored Several design alternatives have been proposed for the grading in Agua Hedionda Creek that address the project constraints The Hydraulic Analysis section of this report includes detailed discussion and hydraulic analyses of three proposed design alternatives for maintenance of Agua Hedionda Creek Prepared by DCB KH kw/Report/13182-D 002 Rick Engineering Company - Water Resources Division 12-13-04 15 Revised 12-12-05 HYDRAULIC ANALYSIS The U S Army Corps of Engineers Hydrologic Engineering Center HEC-RAS computer program was used to determine the hydraulic effects of the maintenance and sediment removal from Agua Hedionda Creek based on the ultimate detained 100-year storm event The existing condition cross-sectional geometry is based on field surveys of Calavera Creek in December 2001 and Agua Hedionda Creek in May 2002 Updated cross-sectional geometry downstream of Cannon Road is based on field surveys m June 2004 The following improvement plans were used to model the Cannon Road Bridge crossing and El Cammo Real Bridge crossing, respectively "Cannon Road Bridge Over Agua Hedionda General Plan," sheet 59 of 131, and "El Cammo Real Bridge Widen Over Agua Hedionda Channel General Plan," sheet 68 of 139, prepared by McDaniel Engineering (Drawing No 333-2G) Copies of these improvement plans are included m Appendix A Description of Alternatives Several alternatives for maintenance of the Agua Hedionda Creek channel within the Rancho Carlsbad Mobile Home Park have been investigated since 1998. There are environmental and/or community impacts within Calavera Creek and Agua Hedionda Creek that may render maintenance within portions of the channels undesirable However, maintenance of the Agua Hedionda Creek channel is imperative to improving conveyance and reducing flooding in the RCMHP The proposed alternatives for Agua Hedionda channel maintenance are described below Exhibits 5, 6, and 7 show the approximate limits of grading for Alternatives A, B, and C, respectively A schematic of the cross-sectional geometry for Alternatives A, B, and C as well as the 100-year floodplam limits are shown on the floodplam alternatives map in Map Pocket 1 The HEC-RAS workmaps are located in Map Pocket 2 No Action Alternative The No Action Alternative models the current channel topography within the RCMHP and models the conveyance of the 100-year undetamed ultimate development hydrology This alternative proposes no changes to the existing drainage patterns and is presented to reflect the current flooding potential within the RCMHP if no channel maintenance Prepared by DCS KH kw/Report/13l82-D002 Rick Engineering Company - Waler Resources Division 12-13-04 16 Revised 12-12-05 occurs and if none of the 4 proposed detention basins were constructed, and no improvements were made to the Lake Calavera outlet structure See Appendix D for the No Action Alternative HEC-RAS model Existing Condition Detained Alternative The Existing Condition Detained Alternative models the current channel topography within the RCMHP and models the conveyance of the 100-year detained ultimate development hydrology This alternative is presented to reflect the potential flooding within the RCMHP if no channel maintenance occurs, but assumes the 4 proposed detention basins, Melrose, Faraday, BJB and BJ, are constructed, Calavera Dam improvements are complete, the subsequent modifications to the BJB outlet structures have been constructed, and 500-cfs is conveyed on the north side of the wall See Appendix E for the Existing Condition Detained Alternative HEC-RAS model Alternative A Alternative A models the current channel topography within Calavera Creek, and models the maintenance of approximately 2,500 feet of Agua Hedionda Creek between El Camino Real and just downstream of Rancho Carlsbad Drive (HEC-RAS cross-sections 10 1 to 250 86) Alternative A models the conveyance of the 100-year detained ultimate development hydrology as discussed in the Existing Condition Detained Alternative Alternative A proposed maintenance includes grading within Agua Hedionda Creek to remove accumulated sediment in the channel bottom The Alternative A channel maintenance will require re-grading Agua Hedionda Creek back to the original trapezoidal channel geometry shown on the "Grading Plans for Rancho Carlsbad Mobile Home Park" dated June 27, 1969 prepared by South Bay Engineering The channel consists of 2 1 (horizontal vertical) side slopes and a 44-foot to 58-foot bottom-width The channel will be graded down to the original channel elevation between the existing drop structure and El Camino Real Bridge At El Camino Real the ground elevation will Prepared by DCB KH k w/Report/13182-D 002 Rick Engineering Company - Water Resources Division 12-13-04 17 Revised 12-12-05 begin to "ramp" up from the original channel flowline of 31 6 feet to the existing ground elevation at cross-section 10 1 (elevation 35 5') This alternative includes possible complications related to the potential for continuous ponded water upstream of El Cammo Real See Appendix E for the Alternative A HEC-RAS model Alternative B Alternative B models the current channel topography within Calavera Creek, and models the maintenance of approximately 3,000 feet of Agua Hedionda Creek between Cannon Road Bridge and just downstream of Rancho Carlsbad Dnve (HEC-RAS cross-sections 8 4 to 250.86) This alternative models the conveyance of the 100-year detained ultimate development hydrology as discussed in the Existing Condition Detained Alternative Alternative B proposed maintenance includes grading within Agua Hedionda Creek to remove accumulated sediment in the channel bottom while protecting the mature trees that have established on the channel banks where possible Upstream of the El Cammo Real Bridge this alternative will require grading a trapezoidal channel with 2 1 side slopes, with the top of the proposed graded slope starting near the toe of the existing channel slope The channel grading will project down to the original channel flowline elevation and longitudinal slope of 0 3%, with approximately a 40-foot bottom width Downstream of the El Cammo Real Bridge to Cannon Road, the channel will be re- graded with a 70-foot bottom width and 2 1 side slopes, to daylight The channel flowline will be graded at a 0% longitudinal slope from the current elevation at the northerly edge of the Cannon Road Bridge of approximately 32 0 feet, upstream until it meett the-ongmaLAgua Hedionda-Creek-flowlme elevation shown^>iv4he 1969 plans, approximately 135 feet upstream of the El Cammo Real Bridge This alternative minimizes impacts to the existing trees within the RCMHP However, grading activities downstream of El Cammo Real are within an area of the Local Coastal Plan (LCP) which is administered by the City and are subject to permitting requirements See Appendix G for the Alternative B HEC-RAS model Prepared by DCB KH kw/Repon/l3l«2-D002 Rick engineering Company - Water Resources Division 12-13-04 IS Revised 12-124)5 Alternative C Alternative C models the current channel topography within Calavera Creek, and models the maintenance of approximately 3,000 feet of Agua Hedionda Creek between Cannon Road Bridge and just downstream of Rancho Carlsbad Drive (HEC-RAS cross-sections 8 4 to 250 86) This alternative also includes widening of a portion of Agua Hedionda Creek between El Cammo Real and the confluence with Calavera Creek to improve the channel capacity This alternative models the conveyance of the 100-year detained ultimate development hydrology as discussed m the Existing Condition Detained Alternative Alternative C proposed maintenance includes grading within Agua Hedionda Creek to remove accumulated sediment in the channel bottom while protecting the mature trees that have established on the channel banks where possible Upstream of the confluence with Calavera Creek this alternative will require grading a trapezoidal channel with 2 1 side slopes, with the top of the proposed graded slope starting near the toe of the existing channel slope The channel grading will project down to the original channel flowhne elevation and longitudinal slope of 0 3%, with approximately a 40-foot bottom width Downstream of the confluence with Calavera Creek to El Cammo Real Bridge, Agua Hedionda Creek will be widened by approximately 27-feet to increase the channel capacity A vertical wall is proposed along a portion of the right channel bank (looking downstream) Downstream of the El Cammo Real Bridge to Cannon Road, the channel will be re-graded with a 70-foot bottom width and 2 1 side slopes, to daylight The -channel jtewline will^be-graded^ta-Q% longitudinal slop&JFom the current elevation at the northerly edge of the Cannon Road Bridge of approximately 32.0 feet, upstream until it meets the original Agua Hedionda Creek flowhne elevation shown on the 1969 plans, approximately 135 feet upstream of the El Cammo Real Bridge Alternative C minimizes impacts to the existing trees within the RCMHP Grading activities downstream of El Cammo Real are within an area of the Local Coastal Plan (LCP) which is administered by the City and are subject to permitting requirements Prepared by DCB KH kw/Report/13182-D 002 Rick Engineering Company - Water Resources Division 12-13-04 19 Revised 12-12-05 The construction of one vertical wall is required just downstream of the confluence of Agua Hedionda Creek and Calavera Creek (between cross-sections 30 and 20) The wall will be up to 10 feet high and 275 feet long along the opposite creek bank from the mobile homes A second vertical wall may be required to protect an existing structure This second vertical wall located along the creek bank closest to the mobile homes will be up to 6 feet tall and 120 feet long to preserve an existing structure In final design, it may be possible to implement an alternative solution to the wall(s), such as adjusting the property line and/or the perimeter wall location, so that the slope can be extended without the need for a retaining wall See Appendix H for the Alternative C HEC-RAS model Bridge Improvements The hydraulic analyses of all alternatives except the No Action Alternative include improvements to the bridges at El Camino Real and Cannon Road These improvements include connecting the piers under the bridge to create one elongated pier to minimize hydraulic losses and debris potential, as well as constructing a debris nose on the upstream face of the bndge piers to minimize the impacts of floating debris on the hydraulics of the bridge crossing Preferred Alternative Alternative C is the preferred alternative by the City of Carlsbad because it removes the largest number-eflots-froffl-the-409-year floodplam, while minimizing impacts to several of-the mature trees along the Agua Hedionda Creek channel banks within the RCMHP Table 3 presents the anticipated 100-year WSELs in Agua Hedionda Creek and the number of inundated lots for each of the proposed alternatives Prepared by DCB KH kw/Rcport/13182-D 002 Rick Engineering Company- Water Resources Division 12-13-04 20 Revised 12-12-05 Table 3. Anticipated 100-year WSELs for the Agua Hcdionda Creek Grading Alternatives and Number of Inundated Lots X-Sec 209 501 (Confluence) 7058 1202 17019 21051 25041 No Action Alternative 496 502 504 505 504 513 51 3 Detained 100-year WSEL (feet) Existing Detained 486 492 494 495 495 509 51 0 Alternative A 485 488 488 488 486 490 49 1 Alternative B 43 1 439 439 449 460 466 472 Alternative C 428 420 428 448 460 466 471 Approximate Number of Inundated Lots Maximum Water Depth Above Channel Bank (feet) X-Sec 501 X-Sec 14065 278 78 62 225 68 5 1 163 64 44 33 1 5 1 1 12 -04* 1 1 * Although the WSEL at X-Sec 50 1 shows that flow is contained within the channel banks, the WSEL at X-Sec 50 11 on Calavera Creek shows the WSEL above the channel banks and inundating lots near the confluence The RCMHP consists of mainly elevated foundations Therefore, although a lot deemed as inundated may not necessarily mean the home is entirely flooded The finished floor elevation of the structure may be above the floodplam, in which case an elevation certificate may be prepared to ensure the structure Ts noTwithin the floodplam Prepared by Rick Engineering Company- Water Resources Division 21 DCB KH kw/Report/l3l«2-D002 12-13-04 Revised 12-12-05 ' V 1 1 'li " . •; X 100 0 \ 200 400 60 GRAPHIC SCALE ft 2JOO' ~7 \-m1m AGUA HEDIONDA CREEK CHANNEL MAINTENANCE EXHIBIT 6 Alternative B Approximate Limits of Grading J-13182D •December 12,2005 AGUA HEDIONDA CREEK -CHANNEL MAINTENANCE EXHIBIT 7 Alternative C Approximate Limits of Grading December 12,2005 Additional Alternatives Investigated but not Analyzed The following alternatives were considered during the development of the proposed improvement alternatives However, preliminary investigations determined several alternatives presented either limited flood control benefit or extensive engineering difficulties Therefore, no detailed analyses were prepared for these alternatives The following provides a discussion of alternatives investigated but not analyzed DSOD Jurisdictional Dam Preliminary analyses of the feasibility for construction of a DSOD Jurisdictional Dam upstream of the RCMHP was investigated to estimate the storage volume required to provide flood protection within the RCMHP The construction of additional detention basins could minimize flooding in the RCMHP area without maintenance within Agua Hedionda Creek The existing hydraulic capacity of the channel was determined using the HEC-RAS hydraulic program by modeling the existing topography in Agua Hedionda Creek with a series of flows until the flow was contained within the channel through most of the RCMHP The HEC-RAS analyses show that the existing capacity of Agua Hedionda is approximately 4000 cfs In order to reduce 100-year flows in Agua Hedionda Creek to approximately 4000 cfs, along with the construction of the Melrose, BJ, and BJB detention basins currently proposed, the detention basin at Faraday would require an increased storage volume from 49 8 acre-feet (currently designed) to 200 acre-feet of storage volume, and an additional detention basin would need to be constructed within the Agua Hedionda watershed that what would provide 895 acre-feet of storage volume The location of this additional detention basin is located in the vicinity of the City of Carlsbad and the City of Vista corporate boundary This preliminary investigation has determined that constructing two DSOD dams as additional detention facilities will result in a significant impact to available open space, would impact not only the City of Carlsbad, but potentially the City of Vista, and is therefore not a practical alternative. Prepared by DCB KH kw/Report/13)82-D002 Rick Engineering Company - Water Resources Division 12-13-04 25 Revised 12-12-05 Levees within RCMHP The impact of constructing levees along the existing channel banks to increase the capacity of Agua Hedionda Creek was investigated for the RCMHP project Water surface elevations for the Existing Detained analysis are 5 feet or more higher than the top of the channel throughout the RCMHP FEMA requires a minimum of 3 feet of freeboard for levees over the 100-year water surface elevation, which would require construction of a berm up to 8 feet higher than the current bank throughout the RCMHP This alternative would not be aesthetically acceptable for many homeowners within the RCMHP, and to construct a berm of this size assuming 2 1 side slopes would require almost a 40' wide footprint impact on each side of the channel The construction of this alternative would also impact approximately 53 property owners by requiring removal of their homes Also, if similar improvements were not made to Calavera Creek several structures and lots would still be subject to inundation Box Culvert along El Cannno Real An alternative to construct a box culvert system to divert flows exceeding the capacity of Agua Hedionda channel around the RCMHP and into Agua Hedionda Lagoon was investigated However, this structure would be required to convey approximately 3,500- cfs and would require approximately 6 ~ 10'wide x 6' high box culverts beneath El Cammo Real for approximately 2,500 linear feet This option is cost prohibitive and has significant constructabihty issues due in part to utility conflicts Prepared by DCB ICH kw/Report/13182-D 002 Rick Engineering Company - Water Resources Division 12-13-04 26 Revised 12-12-05 e ONGOING MAINTENANCE PROGRAM Once the initial channel maintenance and grading outlined by this project has been completed, it is vital that ongoing maintenance including sediment removal and vegetation removal be performed on a regular basis to preserve the capacity of the channel and to ensure an acceptable level of flood protection within the RCMHP The maintenance area will begin at the north edge of the Cannon Road Bridge and extend approximately 2,500 feet upstream to the drop structure underneath the Rancho Carlsbad Drive bridge crossing It is anticipated that ongoing vegetation removal maintenance will need to be performed to ensure no vegetation becomes established between the aforementioned bridge crossings It should be noted, however, that the sediment deposition in the channel is directly related to each rainfall event and therefore, the frequency of maintenance may be more or less frequent than anticipated Monitoring of the sedimentation in the channel over the first few years following the initial channel maintenance will be helpful in determining the future sediment removal maintenance frequency requirements Sediment posts marked in 1-foot increments can be utilized in sections of the channel to assist in monitoring sediment depth Once a depth of approximately 2 to 3 feet of sediment has been accumulated, maintenance including sediment removal between Cannon Road and the drop structure at the Rancho Carlsbad Drive bridge crossing will be needed The maintenance must be performed routinely by qualified personnel and a sufficient budget should be established for the maintenance If any questions arise during the maintenance, a professional engineer specializing in water resources should be consulted The maintenance of Agua Hedionda Creek must be incorporated into the environmental permitting when processed through the California Regional Water Quality Control Board - Region 9, the U S Army Corps of Engineers, the California Department of Fish and Game, and the Coastal Commission if appropriate Prepared by DCS KH kw/Report/13182-D 002 Rick engineering Company - Water Resources Division . 12-13-04 27 Revised 12-12-05 CONCLUSION Agua Hedionda Creek is highly subject to sedimentation for the majority of the creek Portions of the channel within the Rancho Carlsbad Mobile Home Park (RCMHP) have experienced up to 6 feet or more of sediment deposition since the original channel construction This high sediment load may be the result of agricultural areas upstream of the RCMHP as well as the effects of an increased peak discharge within Agua Hedionda Creek due to development within the upstream watershed Because of the increased peak discharge and the accumulation of sediment in the channel over time, the capacity of Agua Hedionda Creek to convey storm water has diminished, and currently a significant portion of the lots within the RCMHP are subject to inundation during a 100-year storm event In an effort to minimize flooding within the Mobile Home Park, the City of Carlsbad has investigated several alternatives to restore the capacity of the channel and improve flood protection in the area Updated hydrology for the watershed tributary to the RCMHP was prepared using HEC-1 to determine the peak 100-year ultimate development runoff to the creeks The detained hydrology for the watershed models the construction of four proposed detention basins upstream of the Rancho Carlsbad Mobile Home Park, two within Agua Hedionda Creek, one at Melrose Drive and one at Faraday, and two within Calavera Creek, BJ and BJB, near the intersection of the Cannon Road and College Boulevard extensions The results of the HEC-1 analyses were modeled in the proposed alternative hydraulic analyses The implementation of the proposed maintenance alternatives for Agua Hedionda Creek includes the following considerations proximity of the project to the coastal zone, native and non-native trees within the channel, environmental processing, and channel vegetation Table 4 shows a matrix summary of the proposed alternatives presented for the maintenance of Agua Hedionda Creek All of the following alternatives include the proposed ultimate detained hydrologic conditions with the exception of the No Action Alternative Prepared by DCB KH kw/Report/13182-D 002 Rick Engineering Company - Water Resources Division 12-13-04 28 Revised 12-12-05 Table 4. Matrix Summary of Proposed Agua Hedionda Creek Maintenance Alternatives Alternative Effective FIRM No Action Existing Detained A B C 100-year Discharge at El Camino Real (cfs) 8,080 9,195 8,092 8,092 8,092 8,092 Grading Downstream of El Camino Real No No No No Yes Yes Bridge Pier Improvements No No Yes Yes Yes Yes Retaining Wall No No No No No Yes Approximate Number of Inundated Lots 316 278 225 163 33 12 Maximum 100- year Water Depth Above Channel Bank (ft) X-Sec 501 N/A 78 68 64 1 5 -04 X-Sec 14065 N/A 62 51 44 1 1 1 1 Each alternative was analyzed hydrauhcally to determine the number of lots removed from the floodplam The resulting water surface elevations were mapped, and the floodplam for each alternative is shown on the map entitled, "Preliminary Rancho Carlsbad Channel and Basin Project 100-year Floodplam Alternatives," dated September 2004 included in Map Pocket 1 Additional alternatives were investigated but not analyzed in detail, including constructing a DSOD Junsdictional Dam, constructing levees within the RCMHP, or constructing a box culvert along El Camino Real This report summarizes the maintenance design alternatives for Agua Hedionda Creek In order for the Rancho Carlsbad Channel and Basin Project to proceed into final design stages, the following issues need to be resolved • Initiate environmental permitting through the California Regional Water Quality Control Board - Region 9, the U S Army Corps of Engineers, the California Department of Fish and Game • Initiate processing a Coastal Development Permit for maintenance of Agua Hedionda Creek channel in-between Cannon Road and El Camino Real ft Prepared by Rick engineering Company - Water Resources Division 29 DCB KH kw/Report/l3l82-D002 12-13-04 Revised 12-12-05 Preliminary Rancho Carlsbad Channel and Basin Project 100-year Floodplain Alternatives AGUA HEDIOMDA CHEEK DOWNSTREAM OF CONFLUENCE AOUA HEDIONDA CREEK UPSTREAM OF CONFLUENCE Alternative^ Cro«.-Sectki Alternative B Croft-Section Matrix Sunmary of Proposed Agi» Htdkwui) Crrck Miintenatm AJtcra»tivM Atanwlive Effective HRM No Action Existing Detained A B C Approximate FtoodpkunDelbwMMD ™— 100-ye«rDiKl-seMEICuninoRM!<C&) 8,080 9,195 8.092 8,092 8,092 8,092 Gnxfog oTEIC-niw Rol No No No No Yes Yes BridfePkf No No Yes Yes Y«Yes (UttininjWill No No No No No Yes ApproxinuleNumber ofImindtted Lob 316 278 225 163 33 12 Appraxrutt 100-ycarW^v DepthAbove Ounnel Bulk (ft)X-Sec50.1 N/A 7.8 6.8 6A1.5 -0.4' X-Sec 140.65 N/A 6.2 5.1 4.4 1.1 1.1 * Although the WSEL al X-Sec 50.1 shows that flow a cfflitained wiltiin the channel bank*. Ibe WSEL at X-Sec 50.11 on Calaven Creek shows the WSEL above the channel banks andinundating lots Dear the confluence. .' "* ":* v' HEC-1 WORKMAP WITH USGS TOPOGRAPHIC MAP 12-8-04 Legend I I Major V\fetersheds I I Drainage Basins Detention Basins ENGINEERING ANALYSES OF DETENTION BASIN BJ October 4, 2005 Chang Civil Engineering ° Hydrology ° Hydraulics - Sedimentation P.O. Box 9496 Rancho Santa Fe, CA 92067 (858) 692-0760 -TABLE OF CONTENTS - Introduction 1 Engineering Analyses 2 Conclusion 3 APPENDIX A. HEC-1 Analyses B. 100-Year HEC-RAS Analyses c INTRODUCTION A primary goal of the Rancho Carlsbad Channel and Basin Project (Project) is to minimize 100- year inundation of the Rancho Carlsbad Mobile Home Park (RCMHP). An initial task recommended by the city of Carlsbad's civil engineering consultant, Rick Engineering Company (Rick), in their 1998 report involved identification of potential detention basin sites upstream of RCMHP in order to reduce creek flows. Four sites were identified: Detention Basin BJ, Detention Basin BJB, at Melrose Drive, and at Faraday Avenue. The first two are in the Calavera Creek watershed and were selected, in part, because they correspond to sedimentation basins identified in the city of Carlsbad's March 1994, Master Drainage and Storm Water Quality Management Plan. The basin locations were also selected because they could be created by the embankments from the Cannon Road and College Boulevard extensions. The last two are in the Agua Hedionda Creek watershed and were determined based on analyses and review of the watershed, and consideration of future road extensions. Conceptual-level analyses indicated that these basins reduced flood inundation in RCMHP. However, additional benefits could be achieved by channel maintenance to remove sediment and restore the creeks within RCMHP to their original design. The initial Project essentially recommended these tasks. To date, Detention Basin BJB has been constructed and the Melrose Drive and Faraday Avenue basins are under construction. During the design and permitting phase for some of the original Project improvements, additional opportunities for reducing flood inundation were realized. The opportunities are as follows. 1. Updated water quality regulations require new development projects to better utilize and preserve natural drainages. Rick revised flow routing routines in their analyses based on the updated regulations, which resulted in lower overall flow rates. 2. Rick performed further hydraulic studies that led to additional recommendations for Agua Hedionda Creek widening and channelization. Their recommendations will be implemented and will further lower water surface elevations in Agua Hedionda as well as Calavera Creek. 3. Proposed restoration and reconstruction of the Calavera Dam outlet structure will result in additional reduction of the Calavera Creek flow. 4. The new Calavera Dam outlet structure will provide the ability to manage the Lake Calavera water level to further control and potentially increase flood attenuation. 5. The reduced flow from Lake Calavera will allow modifications to the Detention Basin BJB outlet facilities, which will reduce downstream flow in Calavera Creek. 6. The 84-inch pipe proposed in Cannon Road will be designed to provide a flow split as similar as possible to the existing weir wall (500 cubic feet per second north of the wall and 400 cfs south of the wall), and will simplify floodplain mapping approval by the Federal Emergency Management Agency. Due to these additional opportunities, the benefit from construction of the final detention basin, BJ, was questioned. The early Rick studies were general in nature and did not include a cost- benefit analysis of Basin BJ. The basin requires relocation of RCMHP's RV parking area, which presents financial as well as logistical burdens. When the costs of relocating the Rancho Carlsbad facilities started to become apparent, further analysis was called for. In order to assess the relative benefit from Detention Basin BJ, this report contains analyses to determine the 100- year floodplain impact on RCMHP from removal of the basin. ENGINEERING ANALYSES A reasonable baseline condition is Alternative C outlined in Rick's December 18, 2004 report titled, Rancho Carlsbad Mobile Home Park Alternative Analysis for Agua Hedionda Channel Maintenance. The report determined Alternative C to be the preferred alternative. As a result, the improvements associated with Alternative C are being pursued by the city of Carlsbad and others. Alternative C consists of the Agua Hedionda Creek channel dredging and improvements, Calavera Dam outlet structure improvements, Faraday and Melrose detention basins, and Detention Basin BJB outlet structure modifications as discussed above. Alternative C also models a conceptual design for Detention Basin BJ. The HEC-RAS hydraulic analysis for Alternative C is included in Appendix B. Rick's floodplain exhibit is included following this report text. The exhibit delineates the Alternative C 100-year floodplain in red. Tables 1 and 2 summarize the modeling parameters and results for Alternative C as well as the additional alternatives, which are described below. Alternative C was revised to include the 84-inch pipe since this is currently being designed and processed. The HEC-1 hydrologic analysis for this revised condition is in Appendix A and the HEC-RAS hydraulic analysis is in Appendix B. The HEC-RAS summary table and cross-section plots in Appendix B include both Rick's (REC RC100) and the revised ultimate condition (ULT) results for easy comparison. Table 2 also compares the 100-year water surface elevations at selected locations. A comparison reveals that the 100-year floodplains are essentially the same. In order to analyze the impacts from eliminating Detention Basin BJ, it was removed from the revised ultimate condition HEC-1 analysis. This additional HEC-1 analysis is included after the revised analysis in Appendix A. A HEC-RAS analysis was created using the new 100-year flow rates. The results from this analysis are included in Appendix B. The HEC-RAS summary table and cross-section plots for this data set are labeled ULTNOBJ. A comparison of the HEC-RAS ULT and ULTNOBJ results reveals that the 100-year water surface elevations in Agua Hedionda Creek are at most 0.1 feet higher. This difference primarily occurs downstream of El Camino Real and has no impact on RCMHP. In fact, a comparison of the ULTNOBJ results with Rick's Alternative C analysis, REC RC100, revealed no water surface impacts along Agua Hedionda Creek within RCMHP. Therefore, the Agua Hedionda floodplain within RCMHP is the same as that shown on Rick's floodplain exhibit. A comparison of the Calavera Creek results reveals that the 100-year water surface elevations have a slight decrease in some locations and increase in others if Detention Basin BJ is removed. A review of the cross-section plots and topographic mapping shows that none of the increases will cause the flow to leave the channel banks and inundate additional homes The ultimate result is that Rick's Alternative C 100-year floodplain delineation essentially represents the floodplain . if Detention Basin BJ is removed. A final analysis (ULTNOBJC) was performed to determine a water level in Lake Calavera that will generally offset the increased flow rates due to removal of Detention Basin BJ. The previous analyses assumed a lake level at elevation 209.0 feet prior to the storm event. If the lake level is lowered, then additional storage and storm attenuation would occur. It was determined that if the lake level is lowered to 206.5 feet, the 100-year flow rate in most locations would be at or below the flow rate based on construction of Detention Basin BJ. The HEC-1 analysis is included in Appendix A and the HEC-RAS analysis is included in Appendix B. The results are also summarized in Table 2. CONCLUSION Engineering analyses have been performed to analyze differences in the 100-year floodplain within the Rancho Carlsbad Mobile Home Park with and without Detention Basin BJ. These analyses are based on assumptions and modeling prepared by Rick Engineering Company. The only revisions made to this modeling were to include the future 84-inch RCP proposed in Cannon Road, to remove Basin BJ from the analyses, and to revise the water level in Lake Calavera. No other verification or validation of Rick's assumptions, modeling, or floodplain delineation was performed. The results show that the 100-year floodplain without Basin BJ is similar to the Alternative C floodplain delineated by Rick. As a result, removal of Basin BJ will not cause additional impacts on insurable structures in RCMHP. C Analysis Rick Alternative C (RC100) Ultimate (ULT) Ultimate without Basin BJ (ULTNOBJ) Ultimate without Basin BJ, with Lake Drawdown (ULTNOBJC) "8 Q,S s Q L. JS 33U 0> C3 Yes Yes Yes Yes CDet. 55 "2 § S- '-C ^ >.o .££-J U 2 81!u s ctf ^ No No No Yes "O • M O S -4—« •4-*SOesPQ V)03PQ Yes Yes Yes Yes 12o> « e h— 1<U.£• JS a 00 No Yes Yes Yes "O-2 s en OU 1-9CO c GA03 Yes Yes No No ene 1PQ 0> o s•a .«e S88 t> "° "S2 e03 0£ 0 Yes Yes Yes Yes one bJD•a -o4> O ® "Q,1 sS3 Q 2 ^u **1 §o >•11?!S Tj <! C8 Yes Yes Yes Yes Table 1. Summary of Modeling Assumptions Analysis Rick Alternative C (RC100) Ultimate (ULT) Ultimate without Basin BJ (ULTNOBJ) Ultimate without Basin BJ, with Lake Drawdown (ULTNOBJC) Top of Left Bank, feet (based on 2005 topo) r-r\s0 E S os_ wo a'C l__) b*W s—- ^ H^^ «. a> s §o .5•§ s 03 U 42.3 42.2 42.3 42.2 44.0 c« 0 -2< ^ r*">0 «<u 2u ^C ^ 3 — ^ Uo -a ^^ H^^ Ctt ® C8 e eo o S3cc » 42.5 42.5 42.5 42.5 43.1 e ^ PH ^ o 'Ca Q ••§ -2U 03 £ S a> s . ^"""^ T"S ^5 p* 2n rt e =2.2 "3 C/D OH 45.1 45.1 45.2 45.1 46.0 If j~l i*Q 2 2" *N"S sS t£ <j c § ^5 pM T~H MM 2S ^e a.2 "o ^9</3 05 48.3 48.1 48.8 48.6 50.0 c^ 03 !? .fa ;> !^^^^"^^ O rneo ua>C/5 57.1 57.1 57.5 57.3 58.0 o *• nJ 2 Q v 2m ^ ,s S3 0 f— \ ^^ 4> >>-4^ f\C8 *"^ E^"5 -M S "^a § 12 12 12 12 — Table 2. Summary of 100-Year Water Surface Elevations Preliminary Rancho Carlsbad Channel and Basin Project 100-year Floodplain Alternatives 7/Y\lRlGK KNClNKi-KING (JOMPANY HEC-1 WORKMAP WITH USGS TOPOGRAPHIC HAP 12-8-04 Legend I i Major Watersheds I I Drainage Basins Detention Basins V Comm. Letter DC Memo To: Bill Arnold From: David Cannon, PE CC: Date: November 22,2005 Re: Robertson Ranch Draft Environmental Impact Report In accordance with the contract between Everest International Consultants and the Rancho Carlsbad Owners Association (Association) dated October 20, 2005 , we have reviewed the Robertson Ranch Draft Environmental Impact Report (DEIR) dated September 2005 and other earlier study reports and related documents (e.g. meeting minutes, e- mail correspondences and letters). Even though the focus of the review was on the DEIR, the other reports provided good background information regarding prior flood studies and infrastructure improvements. Our comments on the DEIR (Main Report and Appendix I) are provided below. COMMENTS ON THE DEIR- MAIN REPORT 1. Page 5.12-6, Section 5.12.1.2 (Flooding)-Thissection summarizes the historical flooding problem for the Rancho Carlsbad Mobile Home Park (RCMHP) and briefly describes the various studies and solutions that have been developed to alleviate the existing flooding problem. It goes on to imply that the proposed project will provide additional flood protection to the RCMHP by limiting flow on the south side of the perimeter wall through construction of the 84" culvert. However, it does not mention that the results of the Chang Consultant study presented in Appendix I (Chang 2005) Indicate that the project will exacerbate the existing flood problem and that the project is relying on full Implementation of the regional flood solution to be done by the City in the future to address this increase in flooding. This section also does not mention that a prior study conducted for the City by Rick Engineering Company (REC) revealed that the existing 8'x8' culvert under El Camino Real is not sufficient to convey the 100-year flood event under existing conditions yet alone proposed conditions. DC1 RESPONSE TO COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 Response to Comment DC1: The Chang Consultants April 27, 2005 study in Appendix I does nol stale that the project will exacerbate the existing flood problem in the Rancho Carlsbad community. The Chang April 27, 2005 study proposed to divert 450 cubic feet per second (cfs) of the Calavera creek flows exiting Basin BJB as opposed to the 500 cfs proposed in the Rick Engineering Company December 13, 2004 study (REC 2004 study). The Chang study also identifies thai the 50 cfs difference would continue to flow into the Calavera Creek drainage through Rancho Carlsbad, which would create a slight insignificant change in surface water elevation over that resulting from the 500 cfs diversion. The issue is immaterial at this point since the Robertson Ranch project has changed the design of Ihe proposed 84-Inch pipe to accept the full 500 cfs diversion as proposed in the REC 2004 study. The December 13, 2004 study by Rick Engineering Company indicates thai the "8' X 8' RGB [reinforced concrete box] beneath El Camino Real is a restriction of the amount of flow thai can split to the north side of the wall at the weir." As a result, the "peak discharge to be conveyed north of Ihe wall was determined based on Ihe capacity of the existing 8' X 8' box . . . ." The 84" storm drain line has been designed to prevent Ihe 100-year flood from overtopping El Camino Real at the RCB. As indicated in response to comment BRE56, as a component of these responses to comments, additional hydrologic and hydraulic analysis was performed by Chang Consultants (February 20, 2006]. This study provides a more comprehensive and detailed study of the 100-year flood as it traverses the project site, and confirms the findings of the previous hydrology studies that have been prepared for the project and are presented in Ihe Draft EIR. Robertson Ranch Master Plan Final EIR RTC-221 April 2006 Page 5.12-15, Section 5.12.2 (Thresholds for Determining Significance) - This section states the criteria that will be used to establish thresholds of significance for impacts related to hydrology (e.g., flooding) and water quality. The following criteria taken from this section deserve some discussion as provided below. a. Substantially alter the existing drainage pattern of the site or area, Including through the alteration of the course of a stream or river, in a manner which would result in substantial erosion or siltation on-site or off-site. ; There was no discussion in Section 5.12.3 (Environmental Impact) of the DEIR regarding how the proposed changes to surface water hydrology would affect erosion and siltation patterns on-slte or off-site. The document (e.g., Page 5.12-24) did indicated that several BMPs (e.g., bioswales and inlet filters) will be incorporated to reduce post-development on-site and off-site erosion and, although not stated, presumably siitation. However, modification of the flow regime (e.g., peak flows) and/or changes in the sediment production coming off the watershed could result in substantial changes in erosion and siltation patterns throughout the downstream and, in some cases, upstream portions of the watershed. It does not appear that this potential mechanism of stream morphology was addressed In the DEIR. As an example, consider the erosion along Calavera Creek adjacent to the RCMHP that was caused by the changes to the hydrology resulting from the Calavera Hills II Project. b. Substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of a stream or river, or substantially increase the rate or amount of surface runoff in a manner which would result In flooding on-site or off-site. The first paragraph of Section 5.12.3.1 (Hydrology) on Page 5.12-16 acknowledges that the proposed project will increase surface water runoff due to increases In impervious surfaces. The results of the hydro-logic analysis (Table 5.12-3) indicate that the proposed project will increase surface water runoff by approximately 78% (257.28 cfs under proposed conditions vs. 143.67 cfs for existing conditions). It does not state whether or not this Increase is considered significant or not. However, mitigation measures are proposed in the form of drainage improvements so, presumably, this Is considered a significant or at least a potentially significant impact. Most Importantly, it seems to imply here and In other places that implementation of DC2 a DC2 b RESPONSE TO COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 (conl'd) Response to Comment DC2a: The development of Robertson Ranch including the construction of the 84-inch storm drain does not substantially alter existing drainage patterns or alter the course of a stream in a manner resulting in substantial erosion or siltation on-site or off-site. Although the post development flows exceed predevelopment flows, the soil loss calculation prepared- for the project by O'Day consultants indicates there will be a substantial reduction in on-site and off-site sedimentation, which will have a beneficial impact lo Agua Hedionda Lagoon which currently is listed as having a sediment impairment. Not withstanding Die erosion problem at the recently constructed weir wall, which is currently being mitigated by a separate project, the Calavera Creek channel through Rancho Carlsbad has historically been subject to minor bank scouring during slorm events. Construction of Ihe 84- inch storm drain will reduce 100-year storm drainage flows in Calavera Creek by approximately 500 cfs, which will help alleviate scouring of these channel banks. The flows discharging from Ihe 84-inch pipe will flow across a flat open space area that will be will be landscaped and planted to prevent any significant erosion. The 84-Inch drainage discharge and other project drainage that flow under El Camino Real through Ihe existing box culvert will travel a short distance across flat terrain having little potential for scour before joining the Agua Hedionda Creek flows. Once the flows join with Ihe larger Agua Hedionda Creek flow, the relatively small differenlial in increased project flows (representing approximately a l% Increase in flow) will not significantly alter the downslreurn morphology. Response to Comment DC2b: The City disagrees with Ihe commentor's assertion thai Ihe project's drainage impacts must be mitigated through implementation of the regional flood control solution. See response BRE 59 and BRE 6). With regard to the comment that the Chang study staled.thai the project exacerbates the flooding within Rancho Carlsbad see response DC I. Robertson Ranch Master Plan Final EIR RTC-222 April 2006 the on-site drainage plan and construction of the 84" culvert will alleviate the flooding problem caused by the project. However, the results of the Chang study (Chang 2005) presented In Appendix I indicate that the project (including the 84" culvert) will exacerbate the existing flooding problem but that the regional solution being implemented in the future by the City will address this increase in flooding as well as other flooding problems. The DEIR should clearly state that the proposed project, including the on-slte drainage plan and 84" culvert, will cause a significant impact to flooding but that this impact will be mitigated through implementation of the regional flooding solution that will be undertaken by the City. This points out that the proposed project is not self-mitigating but, rather, that the project is relying on the future work of others (e.g., City) to address an existing flooding problem that is exacerbated by the proposed project. c. Place within a 100-year flood hazard area structures which would impede or redirect flood flows. In the first paragraph of Section 5.12.3.3 (Flooding) on Page 5.12-32, it states that the proposed project would not place within a 100-year hazard area structures that would impede or significantly redirect flood flows. However, the proposed 84" culvert is, In itself, a structure that would be placed within a 100-year hazard area that would significantly redirect flood flows. Therefore, It seems as though this .potentially significant impact of the 84" culvert itself should be addressed In the DEIR based on strict interpretation of the thresholds of significance criteria presented In Section 5.12.2. 3. Section 5.12.3.1 (Hydrology), Page 5.12.24, Middle Paragraph - At the end of this paragraph there is a bullet list of the storm drain facilities that will be required to mitigate any Impacts to flooding caused by the proposed project as well as to address the existing flooding problem in the area. However, this list represents only a partial list of the required elements of the regional solution to address flooding In the area. This list should be expanded to Include the following elements: (i) Melrose Basin, (ii) Faraday Basin, (iii) BJ Basin, and (iv) maintenance of Agua Hedionda Creek and Calavera Creek. It is important that all of the required elements be completed to provide the required flood protection; therefore, the DEIR should list all of the elements that would be required instead of providing only a partial list. Otherwise, this Information could be taken out of context and interpreted as though only the elements in the bullet list are needed to provide the necessary level of flood protection. DC2 b (conl'd) DC2 c DC2 d RESPONSE TO COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 (conl'd) Response to Comment DC2c: The City does not agree thai the proposed 84-Inch pipe impedes or redirects flood flow. The 84-inch pipe will convey flood flows to a common point of discharge on Ihe west side o( El Camino Real. The Rick Engineering Company hydrologic studies clearly support what the existing FEMA Flood Insurance Rate Map clearly shows, that flood waters north and south of the existing Rancho Carlsbad perimeter wall merge into a common backwater before discharging through the bridge under crossing south of Ihe wall and box culvert north of the wall. The proposed split drainage plan maintains the historic spit of drainage at this location. The drainage then merges back Into one common drainage (low on the west side of El Camino Real. Response to Comment DC2d: The facilities identified by the commentor do not receive drainage from the project that is being constructed and ore therefore not a part of this project or EIR. CEQA does not require analysis of all existing and planned regional flood control projects within the City Inasmuch as the hydrology analysis concludes that the proposed Robertson Ranch project will not contribute to increased flooding at the Rancho Carlsbad community. This conclusion is confirmed by Ihe Chang Consultants, February 20, 2006 study. See responses to comments BRE59 andBRE6l. Robertson Ranch Master Plan Final EIR RTC-223 April 2006 COMMENTS ON THE DEIR - TECHNICAL APPENDIX II General Comment I It appears that Technical Appendix I consists of one document titled, DC3 "Drainage Study for Robertson Ranch MP 02-03° with the latest revision of the document dated September 2, 3005. The document was prepared by O'Day Consultants, Inc. under the direction of Keith Hansen, PE. The document contains eleven sections. Section 1 provides the framework for the report and Sections 2-11 seem Ito provide supporting information to Section 1 but this is not clear since there are no page numbers for Section 1 and the cross-referenceSjbetween documents makes it difficult to follow. The supporting sections of the document (Sections 2-11) provide various calculations (numerical modeling results, hand calculations, maps, etc.) that were apparently used in conducting the drainage study. It is unclear whether the analyses were conducted In conjunction with one or another or whether the analyses were done independently. For example, It Is not clear whether or not the O'Day surface runoff hydrology analysis results for the Robertson Ranch East Village presented in Section 1 were used in the Robertson Ranch East Village analyses conducted by Chang Consultants as presented in Section 7 of the Drainage Study and Attachments 15 and 16 of the Stormwater Management Plan. This should be clarified in the Drainage Study. If the studies were not performed in conjunction with one another (e.g., O'Day study hydrology used by Chang in subsequent analyses) then the rationale needs to be provided to explain the potential discrepancies that might result from differences In hydrology. Section 1 - Drainage StudvfO'Dav Consultants? - no title, author or date) 1. Improper definition of existing conditions Although the document indicates that the drainage analysis was done for DC4 existing and proposed conditions, this does not seem to be supported by the information presented In Technical Appendix I. It appears that "existing conditions" were defined as ultimate build out of the drainage area, completion of the four detention basins, and channel maintenance of Agua Hedionda Creek identified in the 1998 REC report titled, "Rancho Carlsbad Channel & Basin Project" (REC 1998). Only one of the detention basins (BJB) has been completed with two currently under construction and the final basin (BJ) is still pending construction. Moreover, Agua Hedionda Creek has not been dredged to restore the required channel capacity identified in the 1998 REC study. Consequently, It does not appear that the "existing conditions" presented in the document truly represents the existing conditions in the drainage Proper Identification of existing conditions is important because the RESPONSE TO COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 (cont'd) Response to Comment DC3: O'Day Consultants performed hydrologic analyses for design of the storm drain systems (excluding the 84-inch pipe) for the project including Robertson Ranch East Village. Since the overall drainage area was less than a square mile, the City of Carlsbad requires the analyses to be performed using the rational method. Chang Consultants performed regional analyses for the 84-inch pipe because the pipe collects runoff from a large area. Since this overall drainage area was much greater than a square mile, the City of Carlsbad requires the analyses to be performed using a synthetic hydrograph method. The results using these two methods will be different since one is intended for the study of smaller areas and the other is intended for larger areas. The hydraulic analysis used in Ihe Chang and Ihe REC 2004 studies assumed existing General Plan land uses and drainage sub-basin parameters consistent with City standards. The O'Day hydraulic analysis assumed proposed Robertson Ranch land use and sub-basins also consistent with City standards. The City believes the two studies were used appropriately to integrate the regional and local aspects of the project and that the resulting drainage mitigation facilities recommended in the studies and as identified in the Robertson Ranch EIR will adequately mitigate drainage impacts of the projecl. This conclusion is confirmed by the Chang Consultants, February 20, 2006 study. Response to Comment DC4: Existing conditions as used in Ihe Drainage Study refers to the existing conditions of the Robertson Ranch properly. The commentor appears to be concerned that the project has not adequately described the existing conditions within the Rancho Carlsbad community and that by not doing so, Ihe projecl impacts to Rancho Carlsbad cannot be deemed to have less than a significant impact. The City believes that adequate analysis has been conducted through the regional and project specific studies la make an affirmative determination that the project will not significantly impact the Rancho Carlsbad community. The construction of the 84-inch storm drain will reduce a minimum 500 cfs of Ihe 100- year flood flows into Calavera Creek within Ihe Rancho Carlsbad community. This significantly reduces existing flooding potential to Rancho Carlsbad in the assumed fully developed watershed condition. The benefits to the Rancho Carlsbad flooding problem in the existing less than fully developed watershed condition would be proportionately the same or greater and. therefore, there Is no compelling reason to prepare a comprehensive hydrologic analysis of the existing less than fully developed watershed condition. The City disputes the commentor's assertion that the holes within the Rancho Carlsbad perimeter wall create a worse flooding condition than FEMA predicted. The REC 2004 study modeled several flooding scenarios including an existing "no action" condition. This "no action" scenario assumed all Calavera Creek flows to be on the south side of the Rancho Carlsbad perimeter wall, full development of the watershed and no construction of Ihe four other proposed regional basins or Robertson Ranch Master Plan Final EIR RTC-224 April 2006 RESPONSE 10 COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 (cont'd) Response to comment DC4 (conrd): channel improvements. The results of this "no action" analysis, as depicted on the Preliminary Rancho Carlsbad Channel and Basin Project 100-year Floodplain Alternatives map included in Map Pocket I of the REC 2004 study, show that the flooding conditions without the assumption of the flood wall are no worse for the Rancho Carlsbad residents than the flooding conditions assumed in the FEMA 1997 study. Robertson Ranch Master Plan Final EIR RTC-225 April 2006 RESPONSE 10 COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 (conl'd) proposed project should be evaluated biased on a comparison of existing and proposed conditions. If it Is assumed that "existing conditions" includes completion of Infrastructure designed to remediate flood impacts associated with the project then it could "mask" the effects of the project resulting In a potentially significant imp: ct being considered an insignificant impact because It would bf mitigated by completion of the proposed Infrastructure. This means th at the proposed project could cause a significant Impact to flooding but that the project applicant would not be responsible for mitigating these Impacts. It was noted in the drainage study (and elsewhere) that the existing wall running along the northern boundary of the RCMHP was modeled by FEMA as a flood wall, although it is not adequate to function as a lloodwall due to the holes in the wall. The 1997 FEMA study indicated that the RCMHP will be subject to flooding during the 100-year flow based on the assumption that the wall functioned as a floodwall (FEMA 1997). In reality, the holes in the existirjg wall will allow additional flood flows into the RCMHP, which means that flooding under existing conditions will be even worse than indicated by the 1997 FEMA study. This Is another example of how the existing conditions have not been properly defined relative to flooding within the RCMHP. 2. Inadequate capacity of 8'x8' box culvert under El Cam/no Real The drainage plans for the project call for routing of flood flows through an 84" culvert that would run along the northern side of Cannon Road and empty into the low area at the northeast corner of El Camino'Real and Cannon Road. In addition, the 84" culvert is supposed to convey a portion of the flood flows from Calavera iCreek that would currently flow along the wall to the north of the RCMHP. The drainage plans for the project call for the combined flow in the 84" culvert to drain Into the low area at the northeast corner of El Camlno Real and Cannon Road where it will then flow to the north a short distance along El Camino Real before flowing under El Camino Real via an B'x8' box culvert where It will empty into Agua Hedionda Creek on its way to Agua Hedionda Lagoon. The drainage study indicates that approximately 144 cfs and 257 cfs will be conveyed through the system to the existing 8'x8' culvert under existing and proposed conditions, respectively. This indicates thatthe proposed project will increase the runoff to the existing 8'x8' box culvert by approximately 78%. A prior study conducted by REC In 2003 Indicated that the 8'xB' box culvert under El Camino Real does not have capacity for the 100-year flow under the proposed conditions considered for that study, which Included ultimate build out of the drainage area, including the Robertson Ranch Project (REC 2003). The 1997 FEMA flood analysis for existing conditions indicated that the flow (805 cfs) would be greater than the proposed conditions (702 cfs); therefore, by deduction, the 8'x8' culvert lacks capacity to convey the 100-year flow DC4 conl'd) DCS Response to Comment DCS: The December 13. 2004 study by Rick Engineering Company (REC 2004 study] indicates that the "8' X 8' RGB (reinforced concrete box] beneath El Camino Real Is a restriction of the amount of flow that can splil lo the north side of the wall at the weir." As a result, the "peak discharge lo be conveyed north of the wall was determined based on the capacity of the existing 8' X 8' box ..." The project is being designed lo prevent the 100-year flood from overtopping El Camino Real at the RGB. The total maximum 100-year flood How predicted to pass through the 8' x 8' box culvert in the REG 2004 study under the fully developed watershed condition, including the Robertson Ranch project, is 568 cfs. The hydraulic calculallons for the 8' X 8' box culvert contained in Appendix B or the REC 2004 study show that the 100-year flow will pass through the culvert leaving a minimum 1.7' freeboard beneath the lowest surface elevation of El Camino Real. Ihe commenlor appears lo be adding peak flows from the Robertson Ranch sub-basins lo the 500 cfs diversion lo come up with a flow figure lhal he then postulates will lead lo overlapping of El Camino Real. Routing of flood flows are more properly accomplished through Ihe use of flood flow routing programs such as HEC-I where lag limes in flood routing typically produce flow results less than Ihe sum of the peak flows from individual conlribuling sub-basins. Robertson Ranch Master Plan Final EIR RTC-226 April 2006 under existing conditions. This means that there Is an existing flood risk for the area as indicated by the 1997 FEMA analysis, the proposed development will increase the runoff to the area currently susceptible to flooding as Indicated by the drainage study, and that the flooding problem will be exacerbated by the proposed project as indicated by the 2003 REC analysis. 3. Inconsistencies between drainage study and prior studies It appears that much of the work presented in the drainage study is based on prior studies performed by other consultants. This Includes work done by REC for the City of Carlsbad'and developer (Calavera Hills II Development Project) as well as work done by Chang Consultants for the developer. However, in reviewing the prior work that Is referred to In Technical Appendix I there appear to be numerous Inconsistencies in these prior studies that could affect the accuracy and meaningfulness of the analyses conducted for the Robertson Ranch Project. Examples include inconsistencies in methods (e.g., different hydrology manuals and different models), assumptions (e.g., flood wall modeled without holes vs. with holes, variable runoff coefficients for same land use, and different Calavera Lake Dam operations), and drainage area development condition (e.g., ultimate build out, existing development, and variable roughness coefficient). It appears that Information has been selected piecemeal from prior studies; however, this can lead to problems because as the results of prior studies are based on the assumptions underlying those studies. There are also typographical errors or analytical errors in the prior studies that might have affected the analyses performed in the drainage study. For example, the number of inundated lots within the RCMHP shown in Table 4 of the 2004 REC study report is inconsistent with the number of inundated lots within the RCMHP shown in Table 3 of the same report (REC 2004). These types of inconsistencies and potential errors cast doubt on the accuracy of the prior studies that the drainage study was based on. 4. Development is exacerbating an existing flooding problem A fundamental concern is that the developer is relying on the City's regional solution to address flood impacts caused by their project as well as the existing flood problem. However, there is no guarantee that the City will Implement the regional solution in a timely manner nor is there any guarantee that the regional solution will mitigate the existing flood problem at RCMHP yet alone the additional impact caused by the Robertson Ranch Project. 5. Wo citations or references are provided to substantiate prior studies There are no citations or references provided for prior studies that are referred to in the drainage study. Presumably, the drainage study Is DCS (conl'd) DC6 DC7 DCS RESPONSE TO COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 (conl'd) Response to Comment DC6: Many studies have been performed over the area over many years because development of the regional solution has led to many alternative solutions, some of which change or are added over time for various reasons, including public opposition or other factors. Information has not been selected piecemeal, but, in fact, has evolved as the opportunities change or new ones are presented. For instance, at the time the regional solution was initially conceived, the opportunity for additional detention at Lake Calavera was not available. Once the additional detention capacity of Lake Calavera became available, II led to additional opportunities such as improved attenuation at Basin BJB. All of the current studies are based on the most up-to-date information. The hydraulic analysis contained within the REC 2004 study was thoroughly reviewed by the City as well as third parly hydraulic experts and found to be a proper and valid representation of the regional flooding conditions. The Chang and O'Day studies prepared for the project built upon the foundation of the REC 2004 study in a manner consistent with City standards. Finally, the Chang Consultants, February 20, 2006 hydrologic and hydraulic analysis confirms the findings of the studies presented In the EIR. Response to Comment DC7: The project Is not relying on the City's regional solution to address flood impacts caused by the project. The East Village runoff excluding PA23E will not be conveyed to either of the creeks in the Rancho Carlsbad community. Runoff from the East Village will primarily be conveyed by the 84-inch pipe, which will be constructed by the project, and which does not deposit or contribute runoff to the Rancho Carlsbad community. Please refer to responses to comments BRE59 andBRE6l. Response to Comment DCS: The prior study references cited in this comment were identified to give an overall view of the entire drainage basin. These reports were not utilized to determine the project's onsite hydrology. They were provided to provide some context to the O'Day Drainage Study. The O'Day reports will be revised to provide proper referencing. Robertson Ranch Master Plan Final EIR RTC-227 April 2006 based on work documented in these prior studies; therefore, citations and associated references should be provided so that the prior studies can be reviewed In more detail. As aniexample, on (he first page under the section titled, "Comprehensive Basin Analysis" the third paragraph refers to recent studies conducted by Rick (a.k.a. REG) and Chang Consultants; however, there are no citations or references provided to verify the information presented in this paragraph. Moreover, the purpose of this entire section Is unclear as there is no subsequent discussion provided as to how or even if this information was utilized In the drainage study. j 6. Text is unclear and study approach Is confusing The text is not well organized and various sections of the document are I not linked together making It difficult to understand the study approach that was utilized for the drainage study. For example, on the second page under the section titled, "Facilities Proposed in City of Carlsbad Master Plan of Drainage" the flood infrastructure facilities as proposed In the City's 1994 Master Drainage Plan are presented, along with a brief description of the existing facilities. This text describes the differences between what was proposed for these facilities and what was actually built or has yet to be built. However, it is unclear what Information-, If any, from this section was used In the drainage study because it does not explain how this information was used. It Is unclear whether or not the area examined for the drainage study included the entire drainage area or simply the Robertson Ranch property. The text seems to imply that the drainage study included the entire drainage area (i.e., Calavera Creek and Agua Hedlonda Creek Watersheds); however, the analysis results seem to focus only on Robertson Ranch. Section 6: Established flows from upstream developments This section of the document includes information and calculations for the 100-year flow rates at various nodes. The 100-year flow calculations at Node A were taken from an 8-22-2003 document titled, "Drainage Study for Calavera Hills - Village X System 100" that was prepared by O'Day Consultants. The calculations for the 100-year flow at Node B were taken from a 9-20-1982 document titled, The Cape at Calavera Hills, CT 81-47, Unit H. Hydrology & Hydraulic Calculations" that Was prepared by Rick Engineering.. The 100-year flow calculations at Node C were taken from a 4-9-1982 document titled, "Hydrology Study, Area C & D, Calavera Hills" that was prepared by Rick Engineering. The only reference for the 100-year flow calculations at Node D is a map with notations indicating the values to be used and a number (CT 76-12). It appears that the 100-year flow calculations at Nodes E, F, and G are based on prior calculations performed between 1978 and 1980 by Rick Engineering. The 100-year flow calculations at Nodes H and I are based on recent hydrologic analyses conducted with currently accepted DCS cont'd) DC9 RESPONSE TO COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 (conl'd) Response to Comment DC9: The hydrology study in the EIR Includes an analysis and description of the proposed disposition of Master Drainage Facilities thai are within or adjacent to the Robertson Ranch project in accordance with the City Growth Management policies and standards. Response to Comment DC10: Section 1.3 of Ihe 2003 San Diego Hydrology Manual states "This manual should not be used when there is already established flow." Established flows were used for the runoff entering Ihe project. DC10 Robertson Ranch Master Plan Final EIR RTC-228 April 2006 DC10 (cont'd) analytical methods. There Is no justification provided that the older work utilized for some of these flow calculations is still relevant or consistent given changes In site conditions, hydrology manuals, flood protection policy, and analytical methods. Section 7: Hvdrologic and Hvdraulic.Analvses for Robertson's Ranch East Village 84" Reinforced Concrete Pipe Alternative (Chang Consultants. 4/27/2005) Maintenance requirements/responsibilities for the 84" culvert not provided The capacity of the 84" RCP culvert to convey flood flows is dependent upon clean conditions within the culvert. Over time sedimentation, DC11 debris, and vegetation could clog the culvert thereby reducing the capacity of the culvert resulting in flooding of nearby areas, including the RCMHP. The document does not discuss the culvert maintenance requirements, maintenance responsibilities, or the conditions that would trigger maintenance and this information should be Included somewhere In the DEIR. It is our understanding that maintenance of Agua Hedionda Creek and Calavera Creek to maintain flood conveyance capacity is the responsibility of the City; however, the City has not maintained these two creeks since 1998 when the RCMHP was converted from a rental property to ownership property. Moreover, it does not appear these creeks were ever maintained, thereby resulting in a flood risk for the RCMHP. Therefore, if the City will be responsible for maintenance of the 84" culvert then the funding mechanism should be included In the DEIR to provide some assurances that future maintenance will be conducted as required. Stormwater Management Plan (O'Dav Consultants. 9/2/2005): Attachment 15: Hvdroloaic and Hydraulic Analysis for Robertson's Ranch East Village (Chang Consultants. 9/2/2004) Potentially incorrect implied assumption At the top of Page 3 of this document it states that "By using ultimate development flow rates, any regional solutions would appropriately address future conditions In the watershed." This statement implies that the regional solutions when (and if) implemented will adequately address additional flooding caused by this development project and any other development projects In the drainage area because the regional solutions were developed based on analyses that Include ultimate development. However, this statement carries the implicit assumption that the proposed development project and any other development projects are adequately represented by the ultimate development condition utilized in the analysis of regional flood solutions. It does not state whether this Implied assumption was ever verified to determine if DC12 RESPONSE TO COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 (cont'd) Response to Comment DC11: The 84" drainage culvert is inlended to be a public facilily maintained by Ihe City of Carlsbad. The large pipe diameter will reduce the chance thai Ihe facility be clogged with debris. The City will be responsible to maintain Ihe facilily. The City has adequate funding la maintain this drainage facility. The Calavera and Agua Hedionda Creek channels within Rancho Carlsbad were in private ownership and not the maintenance responsibility of the City unlll 1997. The lack of proper maintenance of these channels by the prior properly owner created a situation where extensive dredging (up to 30,000 cubic yards) is required significantly increasing the difficully of obtaining required State and Federal permits. The City has been actively pursuing resolution of Ihe Stale and Federal permitting issues to accomplish the needed maintenance and other channel improvements since taking over maintenance responsibilities. Response to Comment DC12: The development assumptions used in Ihe City's modeling to establish the regional facilities is consistent with City standards. Please refer to responses to comments DC4, BRE59 and BRE61. Robertson Ranch Master Plan Final EIR RTC-229 April 2006 the proposed development project (i.e{, Robertson Ranch) matched the assumptions made in prior studies conducted for the regional solution. Potentially Inconsistent use of hydrology manuals At the bottom of Page 3 of this document it states that REG'S HEC-1 (no mention of which REC study-date or,title) analyses are based on the 1993 San Diego County Hydrology Manual and that the City of Carlsbad required the Robertson Ranch drainage study to be based on the 2003 San Diego County Hydrology Manual. On the next page (Page 4) it indicates that the latest REC HEC-1 analysis was obtained and updated for use on the Robertson Ranch drainage study in order to determine the 100-year flow in Calavera Creek. It does not state whether or not the updated REC HEC-1 analysis was basted on the 1993 or 2003 San Diego County Hydrology Manual. If It was updated to the 2003 San Diego County Hydrology Manual then the results should be identified separately so it is clear how much of the "impact" is due to changing assumptions and modifications in the analysis methods, Including the switch from HEC-2 to HEC-RAS mentioned later, 'f his is necessary so that true project impacts can be discerned fromjthe analysis results. Stormwater Management Plan fO'Dav Consultants, 9/2/2005): . Attachment 16: Hydraulic Analysis for 84" RCP at Robertson's Ranch East Village (Chang Consultants. 4/27/2005) Inconsistent document reference \ The document referenced as Attachment 16 is titled, "Hydraulic Analysis for 84" RCP at Robertson's Ranch Eask Village" and dated April 27, 2005. However, the actual document included in the DEIR as Attachment 16 is titled, "Hydraulic Analysis for 84" RCP at Robertson's Ranch East Village" and dated November 22, 2004. There is another' document prepared by Chang Consultants that Is Included in Technical Appendix I of the DEIR and it is found in Section 7 as mentioned in earlier comments above. The title of that document is "Hydrologic and Hydraulic Analyses for Robertson's Ranch Eat Village 84° Reinforced Concrete Pipe Alternative" and It is dated April 27, 2005 but it is unclear whether that Is intended document referred to as Attachment 16 to the Stormwater Management Plan. These discrepancies (i.e., document titles and dates) should be resolved to allow a proper review of the reference material. DC12 (cont'd) DC13 DC14 RESPONSE TO COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 (conl'dj Response to Comment DC 13: The updated REC 2004 study was based on the 1993 County Hydrology Manual. The City contracted with Rick Engineering Company (REC] to prepare what is now the REC 2004 study before Ihe County revised the hydrology manual. The primary difference in standards between Ihe two manuals for large walershed basin analysis is the application of new rainfall tables. Application of Ihe 2003 hydrology manual standards to the Calavera and Aguo Hedionda Creek watersheds would result in an increase on the projected 100-year drainage flow rates. However, it has been found through comparison of olher drainage study reports, that the difference In project flow rates diminish with increasing size of the watershed and the presence of detention facilities. Given the size and number of existing and proposed detention basins wilhin the Agua Hedionda and Calavera Creek watersheds, the change In projected flow quantities is anticipated to be minimal and not result In a material alteration of Ihe proposed regional flood control mitigation measures identified In Ihe REC 2004 sludy. The City of Carlsbad therefore directed thai Ihe REC 2004 sludy be completed using the 1993 County Hydrology Manual as originally required in Ihe REC contract. Response to Comment DC14: Comment noted. Robertson Ranch Master Plan Final EIR RTC-230 April 2006 RECOMMENDATIONS Based on our review of the information presented in the DEIR, we offer the following recommendations for consideration by the Association. 1. Request an analysis of existing conditions using the same methodology as the analysis of the proposed conditions. This would allow the results to be used to assess the potential impact of the proposed project on existing conditions as well as conditions in the future with full build out of the drainage area and flood infrastructure in place. This analysis should include the wall under existing conditions (i.e., no assumption of a flood wall) and the effects of the semi-circular holes located along the eastern extent of the wall should be evaluated In the analysis. 2. Request a comprehensive analysis of the entire creek system from the headwaters to Agua Hedionda Lagoon. This analysis should be based on the consistent application of one method (e.g., 2003 San Diego County Hydrology Manual, HEC-1, and HEC-RAS) for the entire system by one consultant to avoid problems associated with the piece-meal approach that has taken place thus far. This analysis should also include an assessment of stream morphology along ihe entire reach of all creeks to ' properly address stream bed and stream bank erosion. The work should be done by the City or the City's consultant. If this is not possible then the City should have the work peer reviewed by their consultant through each phase of the study. 3. Request that the City conduct independent analyses to confirm the findings of the developer's consultant (Chang) with regards to the potential elimination of Basin BJ, proposed modifications to the box culvert under Cannon Road near future College Boulevard, and proposed mollifications to the bed and bank protection near the v-notch wisir. 4. Consider contacting an expert (e.g., regulatory specialist or attorney) in the CEQA process to determine if It Is acceptable to defer mitigation for a potentially significant impact to future projects and other parties (e.g., City). It is my understanding that, under CEQA, each project must provide measures to mitigate an) potentially significant impacts to a level that Is considered insignificant unless It Is Infeasible to do so and that the project applicant must commit to implement these measures. I do not believe that it is Infeasible to provide acceptable measures to mitigate for the potentially significant impacts to flooding that the Robertson Ranch Project could cause to the RCMHP, although it might require additional time and money. DC15 DC16 DC17 DC18 RESPONSE TO COMMENT LETTER FROM DAVID CANNON, DATED NOVEMBER 22, 2005 (conl'd) Response to Comment DC1S: Please refer to response lo comment DC4. Response to Comment DCU: Comprehensive analyses of Ihe watershed have been performed and are In the December 13, 2004 report by Rick Engineering Company, and ai confirmed in the Chang Consultants February 20, 2006 study. This report presents analyses In support of their final recommendations for the Rancho Carlsbad community. AI this time, the only proposed change to their recommendation is lo replace Ihe weir wall with Ihe 84-inch pipe. This change will have a negligible impact on the 100-year floodplain within the Rancho Carlsbad communily. Please also refer to responses lo comments DC2a, DC4 and DC 13. Response to Comment DC17: Comment noted. Response to Comment DC 18: The EIR identifies feasible mitigation measures to address project drainage and alleviate regional flooding problems. Please refer lo responses to comments DC2b, BRE59andBRE61. Robertson Ranch Master Plan Final EIR RTC-231 April 2006 REFERENCES Federal Emergency Management Agency (FEMA). June 19, 1997. Flood Insurance Rate Map (FIRM) - San Diego County, California and Incorporated Areas - Panel 768 of 23,75. Map No. 06073C0768F. I Rick Engineering Company. June 30.J1998. Rancho Carlsbad Channel & Basin Project. Prepared for City of Carlsbad. Job Number 13182. Rick Engineering Company. January 8,2003. Rancho Carlsbad Mobile Home Park Preliminary Alternatives Analysis forAgua Hedionda Channel Maintenance. Prepared for City of Carlsbad. Job Number 13182 C. Rick Engineering Company. December 13,2004. Rancho Carlsbad Mobile Home Park Alternative Analysis forAgua Hedionda Channel Maintenance. Prepared for City of Carlsbad. Job Number 13182-D. BRG Consulting, Inc. September 2005. Robertson Ranch Master Plan Draft Environmental Impact Report (EIR 03-03): Main Report. Prepared for City of Carlsbad. State Clearinghouse #2004051039. BRG Consulting, Inc. September 2005. Robertson Ranch Master Plan Draft Environmental Impact Report (EIR 03-03): Technical Appendix I. Prepared for City of Carlsbad. State Clearinghouse #2004051039. Robertson Ranch Master Plan Final EIR lTC-232 APril2006 i NAL CONDITIONAL LETTER OF MAP REVISION DUE TO THE AGUAHEDIONDA & CALAVERA CREEKS DREDGING AND IMPROVEMENTS PROJECT Prepared for City of Carlsbad, California June 2, 2008 € BROWN AND CALD WELL 9665 Chesapeake Drive, Suite 201 San Diego, California 92123 TABLE OF CONTENTS e LIST OF FIGURES. LIST OF TABLES... 1. INTRODUCTION 1-1 1.1 Project Setting 1-2 1.2 Original Site Conditions 1-3 1.3 Existing Conditions 1-4 1.4 Authority and Acknowledgements 1-4 1.5 Coordination with Other Agencies 1-5 2. AREA OF STUDY 2-1 2.1 Scope of Study 2-1 2.2 Community Description 2-1 2.3 Principal Flood Problems 2-2 2.4 Other Features-(Flood Protection Measures) 2-3 3. ENGINEERING METHODS 3-1 3.1' Hydrology Preparation 3-1 3.2 Hydraulic Preparation 3-1 3.3 Model Assumptions 3-2 3.4 Boundary Conditions 3-3 3.5 Progression of Proposed Alternatives with Model Results 3-3 3.6 Modifications 3-4 3.7 Modeling Conclusions and Recommendations 3-4 3.8 Vertical datum 3-5 4. FORMS 4-1 5. OTHER STUDIES 5-1 6. FINDINGS AND CONCLUSIONS 6-1 7. LIMITATIONS 7-1 Report Limitations 7-1 REFERENCES REF-1 BROWN AKDCALDWELL Use cf data contained on this sheet is subject to the limitations specified at (he end of this document. P:\Projects\Carlsbad, City of (CA)V129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc Table Of Contents Conditional Letter of Map Revision APPENDIX A A Application Forms for Conditional Letters of Map Revision A APPENDIX B B HEC-RAS Hydraulic Simulation B APPENDIX C C Chang Consultant Reference Documents C APPENDIX D D Rick Engineering Company Reference Documents D APPENDIX E E Agua Hedionda and Calavera Creeks Floodplain Limits & Work Map E BROWN AND C ALDWELL Use of data contained on this sheet is subject to the limitations specified at the end of this document. F:\Projecis\Carlsbad, City of (CA)\12S309-Agua Hedionda & Calavera Creek Dredging'iWP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc Table Of Contents Conditional Letter of Map Revision LIST OF FIGURES Figure 1 - Vicinity Map 1-1 Figure 2 - Location Map 1-3 Figure 3 - Horizontal Control Map -1 1-7 Figure 4 - Horizontal Control Map - 2 1-8 Figure 5 - Firm Map No, 0607360768 F 2-5 LIST OF TABLES Table 3.1-1. Feature Representation 3-2 Table 3.1-2. Pier Widths from Existing Model 3-2 Table 3.2-1. Steady State Flow Data 3-2 BROWN ANDCALDWELL Hi Use of data contained on ihis sheet is subject to the limitations specified at the end of this document. P:\Projects\Carlsbad, City of (CA)\12930S-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08j.doc Table Of Contents Conditional Letter of Map Revision This Page intentionally left blank BROWN AND CALD WELL iv Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\ProjectsiCarisbad, City of (CAj\12S309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_8asis for FEMA Dredging(06-02-08).doc CONDITIONAL LETTER OF MAP REVISION 1. INTRODUCTION The city of Carlsbad has commissioned Brown and Caldwell to perform a hydrology and hydraulic analysis for Agua Hedionda and Calavera creeks to determine the base 100-year floodplain and its overall impact upon the surrounding residential community, to modify the floodplain via vegetation removal and dredging of the two creeks, and to provide other infrastructure modifications for flood control and protection. This project is built upon a series of past-related projects which similarly were designed to contain a potentially detrimental flood within the watershed for the purpose of restoring flood control and protection to the surrounding community. Figure 1 provides the Vicinity Map for the Agua Hedionda and Calavera Creeks Dredging and Improvements Project. The project information developed for this study will also support the Conditional Letter of Map Revision (CLOMR) and Letter of Map Revision (LOMR) to be submitted to fulfill the requirements of FEMA and the City's Floodplain Management regulations which reference FIRM map requirements. CITY OF OCEANSIDF. CITY OF VISTA CITY OF SAN MARCOS PACIFIC OCEAN CITY OF ENCINrtAS Figure 1 - Vicinity Map BROWN AND CALDWELL 1-1 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projecis\Carisbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\SOQ648.FINAl_Basis for FEMA Dredging(06-02-08).doc € 1: introduction Conditional Letter of Map Revision 1.1 Project Setting Agua Hedionda Creek, as well as Calavera Creek, are within the Carlsbad Hydrologic unit located in the southwest portion of California. Agua Hedionda Creek originates in the hills south of the San Marcos Mountains and generally flows in a westerly direction. The conveyance varies from mild to steep slopes with varying channel widths along its path of travel. The channel bottom and its side slopes have vegetation features that range from sparse to dense ground cover, with an accompanying tree canopy of various types of native species and residentially planted ornamental species. Coupled with its major tributary, Buena Creek, the watershed drains an approximate area of 29 square miles (18,560 acres) where it conveys the collected runoff and discharge to the Agua Hedionda Lagoon, located approximately 1.5 miles downstream of the project site. Agua Hedionda Lagoon then discharges through major arterials, ultimately emptying into the Pacific Ocean. At this discharge point, the lagoon is in the coastal zone, subject to the rules and regulations under Coastal Waters jurisdiction. Calavera Creek originates at Lake Calavera, which receives surface flow from numerous easterly tributaries that originate from the City of Vista. Calavera Creek meanders in a southerly direction through open space. As it approaches the project site, the channel is noticeably smaller than Agua Hedionda Creek. Within the project site, the channel bottom and side slopes are covered with natural vegetative features that range from sparse to dense ground cover, along with an accompanying tree canopy of various types of native species mixed in with ornamental species planted by adjoining residents. Similar vegetative features, as well as, a less dense tree canopy can be found within the Agua Hedionda Creek alignment. Within the Rancho Carlsbad Mobile Home Park (RCMHP) community, Calavera Creek flows southwesterly, along the northwest boundary of the RCMHP community to a point of confluence with Agua Hedionda Creek located approximately 300 feet east of the El Camino Real bridge crossing. During a sufficiently large rainfall event, the channel may overflow into a flood control detention facility designated as Basin BJB. Runoff continues to flow southward through an 11-foot by 7-foot reinforced concrete box culvert under the intersection of College Boulevard and Cannon Road and enters the Rancho Carlsbad community at the point of confluence with a tributary known as Little Encina Creek. The RCMHP community is bounded by both Calavera Creek and Cannon Road to the north, El Camino Real to the south and Rancho Carlsbad Drive to the east. Agua Hedionda Creek conveys runoff in a westerly direction under Rancho Carlsbad Drive and through the center of the community. From this point, the confluence flows in a westerly direction under the Cannon Road Bridge. (See Figure 2 - Location Map) The RCMHP community is comprised of upscale manufactured homes totaling 502 units with typical lot widths of 50 feet by a length of 80 feet. The community has a central recreational center which includes a clubhouse, a pool, tennis courts, with an executive golf course and an open space east of the community. BROWN AKDCALDWELL 1-2 Use of data contained on this sheet is subjeci to the limitations specified at the end of this document. P:\Projects\Carlsbad, City of (CA)\12S30S-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc 1: introduction Conditional Letter of Map Revision CALAVERA CREEK SITE Figure 2 - Location Map 1.2 Original Site Conditions According to construction plans dated in June 1969, the natural flow and channel configuration of Agua Hedionda and Calavera creeks thru the project site were realigned in conjunction with the development of the RCMHP residential community. The channels were reconstructed as man-made, earthen trapezoidal channels as shown in the original "Grading Plans for the Rancho Carlsbad Mobile Home Park" prepared by South Bay Engineering. The plans detail the grading and excavation for the Mobile Home Park development, roadway work, channel reconstruction and extension, including work within both Agua Hedionda and Calavera creeks. The constructed alignment of Agua Hedionda Creek included both a 58-foot and 44-foot channel sections, located immediately upstream of the El Camino Real Bridge and immediately upstream of the confluence between the Calavera and Agua Hedionda creeks, respectively. The original Agua Hedionda channel was typically 11.5 feet deep with 2:1 (horizontal to vertical) side slopes and an average channel bed slope of 0.15 percent (0.0015 ft/ft). Calavera Creek alignment began at the confluence with Agua Hedionda Creek and proceeded upstream to the proposed intersection of Cannon Road and College Boulevard. The original Calavera Creek channel had a typical bottom width of approximately 4 feet, an average channel bed slope of 0.30 percent (0.0030 ft/ft), a channel depth of 9 feet, and 2:1 side slopes. Agua Hedionda Channel improvement plans prepared by VTN Engineer, in August 1985, provided for rock slope protection at the banks of the channel, around the El Camino Real crossing and the Cannon Road Crossing. In addition, a rock slope protection berm 195 feet long by 15 feet wide was installed across the mouth of the channel, 225 feet downstream of the Cannon Road Crossing. In 1998, additional channel enhancement and repair work of Agua Hedionda Creek occurred immediately west of El Camino Real with the construction of the Cannon Road Bridge. In addition, the creek bottom was modified to a width of 80 feet under the Cannon Road Bridge, as well as, providing a width of 100 feet midway between Cannon Road and El Camino Real bridges. The channel width narrows down to 76 feet as the channel approaches the El Camino Real Bridge looking in an upstream direction. BROWN AND CALD WELL 1-3 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carlsbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc 1: introduction Conditional Letter of Map Revision 1.3 Existing Conditions Significant sediment movement and deposition along Agua Hedionda Creek channel bed has been occurring since reconstruction and repair work was accomplished in 1969 and 1998, respectively. To facilitate the discussion, multiple channel stationing is used as reference points (see Figures 3 & 4 — Horizontal Control Maps 1 & 2, respectively) where feasible. The banks along Agua Hedionda Creek are generally sloped at 2:1 and have vegetative ornamental groundcover or trees that provide some slope stability. The channel width reduces from about 70-feet at Station 18+99 to about 40 feet at Station 19+99, continuing at 40-foot width to just upstream of the first pedestrian/roadway crossing ("Line AH" Station 42+99). The upstream portion of Calavera Creek within the RCMHP also has mild to steep slopes along the channel banks. It also shows signs of local scour along its northern and southern bank faces ("Line CC" Station 8+00 to 11+00), as well as, around drainage appurtenances (Station 22+30). Furthermore, the channel capacity has been reduced by sedimentation in the lower reaches of the channel near its confluence ("Line CC" Station 00+00) with Agua Hedionda Creek ("Line AH" Station 18+76.78) and by encroachment of non-native trees and homeowner improvements such as decks, patios, retaining walls, and ornamental landscaping. Towards the El Camino Real Bridge crossing ("Line AH" Station 16+00), a significant amount of sediment has deposited under the structure. Water staining at the underside of the bridge indicates that the water level in Agua Hedionda Creek reaches the bridge bottom under most large rainfall events. The vertical clearance from the ground line to the bridge bottom is approximately 7 to 9-feet, limiting flow volume through the conveyance channel. The channel section between the Cannon Road Bridge crossing ("Line AH" Station 12+50) and the downstream end of the El Camino Real Bridge ("Line AH" Station 15+00) is densely vegetated and has rock slope protection around each bank. Under the Cannon Road Bridge crossing, significant deposition of sediment and deleterious materials exist, restricting flow at this point. Similarly, the vertical clearance from the ground line to the bridge bottom is approximately 6 to 8-feet. The two combined restrictive flow conditions minimize the available open area for efficient discharge creating a backwater effect that may lead to potential flooding upstream. The sediments found within the channel bottom are largely composed of coarse to fine-grained sand with a small percentage of gravel and trace amounts of fine material that can be easily transported downstream at high velocities and deposited when the channel widens and flow velocities decrease. This action elevates the channel bottom, ultimately reducing the conveyance capacity of the channel. 1.4 Authority and Acknowledgements The City of Carlsbad will have signatory authority and will be responsible for the facility in conjunction with the RCMHP residential community. Aerial mapping has been provided by the Photo Geodetic Corporation. Mapping and additional field points have been provided by Right of Way Engineers. Additional mapping has been provided by the City of Carlsbad and Lyle Engineering, Inc. HEC-1 Information for the Agua Hedionda Watershed has been provided by the Rick Engineering Study. Additional modifications to the discharge volumes within the Calavera Creek have been provided by Chang Consultants. BROWN ANDCALDWELL 1-4 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carlsbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc 1: introduction Conditional Letter of Map Revision 1.5 Coordination with Other Agencies As part of on-going work to provide flood protection for the community, the City performed emergency dredging work under the auspices of an Army Corps Regional General Permit No. 63 (File No. 200600151). The emergency dredging work was performed between the limits of the Cannon Road Bridge and 0.16 miles northeast of El Camino Real Bridge. Coordination with other governing agencies, such as the San Diego Regional Water Quality Control Board, Department of Fish and Game, and the Coastal Commission was required to meet individual agency standards of care. BROWN ANDCALDWELL 1-5 Use of data contained on this sheet is subject to the limitations specified at the end of this document, P:\Projects\Car1sbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc 1: introduction Conditional Letter of Map Revision This page intentionally left blank BROWN AnoCALDWELL 1-6 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carisbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP-S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc BASIS OF BEARINGS AND COORDINATES/BENCHMARK NAD 83 (1991.35 EPOCH) POINTS AS SHOWN ONCITY Of CARLSBAD CONTROL RECORD OF SURVtY NO. 17271 USING BEARING: N 56* 44' 52" E POINT 105H 1998400.124 e 8241021.900 ELEV. -26.13' (NOW 1929)3 '•' DISK M NORTHEAST CORNER OF CANNON ROAD BRIDGEOVER AGUA HEDIONDA CREEK. 120' SOUTHVEST OF EL CAMINO REAL POINT 109 N .4BB301.7B7C P239346.102 2.5* DISK IN DRAINAGE BOX INLET OH SOUTH SIDEOF CANNON RD. 0.2 Ml WEST OF EL CAMINO REAL COMBINED SCALE FACTOR AT PONT 109 IS 0.899902440 GRID DISTANCE - (GROUND DISTANCE) X (COMBINED SCALE FACTOR) LINE LI L2 L3 L4 LS LO L7 LB LB L10 L11 L12 L13 L14 L15Lie L17 H LINE TABLE LENGTH 18.70 11.38 45.20 166.16 32.64 1017.71 117.35 233.02 117.89 208.27 121.24 30.04 226.69 450.56 407.54 1130.51as, ei BEARING WrcVWE Se2'57-5B-E S62'5r58-E N23-24-33-ENsrsawE 853p44-28-E N01 •46"We S83-21'53-£ SOOW27-E N87'56T)3*E 324'24'52-W S3B'S3'53-W Se5'4175-W N78*31'03'W 848'1971'W S5V47WW NB9'59WW O CURVE TABLE CURVE C1 C2 C3 C4 CS C6 C7ca C9 CIO C11 C12 C13 LENGTH 399.24 326.61 149.68 660.78 300.04 129.39 203.69 111.90 54.05 225.08 109.58 483.01 133.42 RADIUS 326.00 200.00 300.00 500.00 500.00 500.00 500.00 200.00 200.00 500.00 303.48 500.00 200.00 DELTA ANGLE 70-22W 93'37139' 2fl*35'15" 04*1 6'43' 34'27'Or 14'4ST3r !3'20120> 32'03'29- 15-29-01- aS'47-32" 3S-47-33' syorar 38'13'14' HORIZONTAL COORDINATE TABLE •AH" STATION 7+00 7+18.70 11*17.93 11*74.51 15+1.32 10*09.48 18*19.16 18*52.00 24*12.79 34*30.60 37*31.14 38*48.49 39*77.87 42*11,60 44*15.18 46*33.07 48*44.97 48+53.25 NORTHING 1999303.4911 1999396.3264 1909449.4819 19994J3.76W 1999522.4556 1999076.7708 1908793.9752 1999614.1922 1999660.0073 1999410.3477 1999371.6621 1999300.5730 1999390.3257 1999343.3312 1999299.7907 1999240,6992 19992142707 1999221.8468 EASTING B2404W.8772 82406 13.4742 8240084.2664 8240934.0010 8241209.1234 6241275.9300 0241308.6202 6241392,3997 6241 821.4857 8242634.1517 6243128.6080 8243242.7600 6243371.7770 6243603.8370 6243795.6803 6243897.9970 6244006.1910 6244213,3280 DESCRIPTION BEGIN CONTRACT BEGIN CURVE C1 END CURVE C1 BEGIN CURVE C2 END CURVE C2 BEGIN CURVE C3 END CURVE C3 BEGIN CURVE C4 END CURVE C4 BEQIN CURVE CS END CURVE CS BEGIN CURVE Ct END CURVE CO BEGIN CURVE C7 END CURVE C7 BEGIN CURVE C8 END CURVE C8 END CONTRACT O EXISTING EASEMENT TABLE No. El E2 E3 E4 PURPOSE STORM DRAINAGE STORM DRAINAGE STORM DRAINAGE STORM DRAINAGE OWNER CfTY OF CARLSBAD CfTY Of CARLSBAD CITY OT CARLSBAD CfTY OF CARLSBAD DOCUMENT OOC. 2003-1414301 DOC. 2003-141 4303 FlH 2003-1414301 O.R. Ffl4 2003-1414303 O.R. RECORDED 11-26-2003 11-25-2003 11-25-2003 1 1-25-2003 CALAVERA CREEK - SEE SHEET 5 Off EASEMENT AND RAY FOR TOAD PURPOSES TO GEORGE W. AND EDWINA TARRY RECORDED SEPTEMBER 10. IBS? IN BOOK 0740 PAGE NO. 102. OR AS SHOWN ON PM 17086 <t EASEMENT TO SOG&E RECORDED OCT06ER 29, 1971 AS OOC. NO. 2513B6 OR AS SHOWN ON PM 17985 ' 60' EASEMENT TO JOSEPH tt \ ' , UUJAN CANTARINI | ' AND BANNING CANTARINI j RECORDED APRIL 1, 1967 I AS DOC. NO, 58827 OR AND (' .SEPTEMBER 7. 1995 I AS RLE NO. 1995-0398033 OR M> AS SHOWN ON PM 17985 'AH^JNE STA 7+00.00 N*1099383.49£-6240499.00 DRAINAOE EASEME AND ACCEPTED ON PM 17965 12 EASEMENT TO SOG4E RECORDED NOVEMBER 2B, 1965AS RLE/PAGE NO. 214035 OR AS SHOWN ON PM i?985 _AH* LINE STA 40*53.26 N-1999221 E ^244213,33t 20' EASEMENT TO SDG&E RECORDED MARCH 20, 1956 AS BOOK 6024 PACE NO. 26 OR AS SHOWN ONPM 17985 AQUA HEDIONDA CREEK HOREONTAL CONTROL MAP 100% SUBMITTAL IF THIS BAR DOESNOT MEASURE 1''THEN OftAWHG I!NOT TO SCALE. BROWN wroCALDWELL REVISION DESCRIPTION I CITY OP CAfiLSBAfi II ENGINEERING CCPARUCNT I 22 DIFBOVUBfT PUNa TOIt HORIZONTAL CONTROL MAP - 1 JCTCMtt B.vt X K»» OX* Figure-3 LINE LI L2 L3 L4 L6 LB L7 L6 L9 10 11 12 13 14 15 IB 17 U LINE TABLE LENGTH 18.70 11.36 45.20 i6a.ie 32.04 1017.71 117.35 233.82 117.89 20827 12124 38.04 228.89 460.55 407.84 1130.51 65.61 BEARING N46*39I03-E SaJTSTSTE SM-ST-SB-E N23-24-33-E N5r59-4a-E S63-44-29-E NBtMBTPE Sa3*21'S3*e SeO'OI^T-E N87'55WE 524-24'52-W S39-53-53-W 885*4 1'25-W we-aiww S48M8-21-W S5r4r40-w N89-59WW O CURVE TABLE CURVE C1 C2 C3 C4 C5ca C7 ca CO CIO C11 C12 013 LENGTH 308.24 326.61 148,68 580.79 300.84 129.39 203.69 111.90 54.05 225.08 169.58 483.91 133.42 RADIUS 325.00 200.00 300.00 500.00 500.00 500.00 500.00 200.00 200.00 500.00 303.48 500.00 200.00 DELTA ANGLE 70*221S8" 93'37'29" 26*35'151 64'15'43' 34-27 TO* 14'48W 23'2076- 32-0379- 15*2901- 25-47-3Z- 35'47-33p 53*W3r MM3-14- HORIZONTAL COORDINATE TABLE •CC" STATION 0*00 +2124 +7528 +11.33 +38.40 +7555 +86.64 +01.10 +71.13 +78.82 33+08.33 34+42.75 35+28.36 NORTHING 1998828.9518 1990939.3463 1990984.9645 2000012.6160 2000147.6704 2000248.0213 2000285.8632 2000174.1660 2000289.4521 2000580.5391 2001269.7402 2001302.5605 2001302.5576 EASTING 62414122849 6241482.3856 8241491.0742 6241514.1929 6241691.9610 6241900.8880 8242096.7163 8242521.3655 8242959.3886 62432634876 82441522402 6244275.8897 6244361.6044 DESCRIPTION BEGIN CONTRACT BEGIN CURVE Cs END CURVE CS BEGIN CURVE CIO END CURVE cio BEGIN CURVE Cll END CURVE C11 BEGIN CURVE C12 END CURVE C12 BEGIN LINE L1 8 BEGIN CURVE CIS END CURVE C13 END CONTRACT •CC- LINE STA 38+26 N-2001302.58 £•6244361.8060' EASEMENT AND RIGHT OF WAY FOR ROAD PURPOSES TO GEORGE W. AND EDWINA TARRY RECORDED SEPTEMBER 10. 1957 IN BOOK 8740 PAG£ NO. 102 OR AS SHOWN ON PM 17985 60 EASEMENT AND RIGHT OF WAY FOR ROAD PURPOSES TO GEORGE W. AND EDWINA TARRY RECORDED SEPTEMBER 10. 1957 IN BOOK 67*0PAGE NO. 102 OR AS SHOWN ON PM 17985 PARCEL MAP BOUNDARYAND DRAINAGE EASEMENT LAKE! CALAVERA CREEK DRAINAGE EASEMENT DEDICATED AND ACCEPTED ON PM 17965 EASEMENT TO SDC4£ RECORDED OCTOBER 29. 1971 AS DOC. NO. 251396 OR AS SHOWN ON PM 17965 CALAVERA CREEK HORIZONTAL CONTROL MAP V\aMKJJ^^^/ MMNG 0 1/E 1 IF THIS GAR DOESNOT MEASURE l" NOT TO SCAUL BROWN AND C ALDWELL"S.'srsss'xr(n) nt-tn ra ("I nnm **£ "°™""L """" ri np DATE INSPECTOR OATE [HOMER -f^y*"7K REVISION DESCRIPTION (not >tnovN.mow*. w*1!! CITY OF CARLSBAD If5??5 5 JL ENOKEERINC OePARTMCNT J| 22 HORIZONT AffMnte M^ X HM MPim ontMwanf |S:^=| AL CONTROL MAP - 2 ; 3Ml DMWX Ot/X/W Mlt 3338-1 || 438-2A | Figure-' CONDITIONAL LETTER OF MAP REVISION 2. AREA OF STUDY The ultimate goal of this study is to determine an effective hydraulic capacity of Agua Hedionda and Calavera creeks which will aid in minimizing the floodplain boundary. Based on engineering judgment and assumptions to reduce the 100-year floodplain elevations, the recommendations for design alteration of the existing channel configurations through a proposed dredging methodology, and upon implementation of said recommendations, will remove the threat of flooding to the maximum number of dwelling units within the Rancho Carlsbad Mobile Home Park (RCMHP) community. 2.1 Scope of Study For the purpose of this study, the hydraulic investigation of the Agua Hedionda Creek alignment will originate downstream, at the discharge point 250 feet west of the Cannon Road Bridge crossing ("Line AH" Station 11+50), will incorporate major drainage features and will terminate at the Rancho Carlsbad Road crossing ("Line AH" Station 41+50). The overall alignment length of Agua Hedionda Creek that will be modeled using HEC-RAS will be at least 3,000 feet. The relevant Flood Insurance Rate Map (FIRM - Panel 768 of2375), Map Number 06073C0768 F (with an Effective Date: June 19,1997) is included as Figure 5. FIRM No. 06073C0768 F will be used for comparison of pre-project floodplain boundary for Agua Hedionda Creek. Similarly, the hydraulic investigation of the Calavera Creek alignment will originate at the confluence with Agua Hedionda ("Line CC" Station 0+00), will incorporate proposed channel features where applicable (including gabion structures, rock slope protection, Vmax slope protection), and tie into an existing drop structure feature ("Line CC" Station 34+00). The overall alignment length of Calavera Creek that will be modeled using HEC-RAS will be at least 3,400 feet. Similarly, the FIRM No. 06073C0768 F will be used for comparison of pre-project floodplain boundary for Calavera Creek. The 100-year storm event will be modeled to determine the floodplain boundary of the channels under current conditions. This model will serve as a benchmark for calibration. Modification of the existing channel geometry, coupled with a lower finished grade profile will allow for more volume of runoff within the banks and will provide efficient flow control. The proposed model with suggested improvements will re- establish the floodplain elevation, thus removing the majority of housing units from the floodplain. The extent of localized flooding •will be confirmed after the channel dredging -work is completed. 2.2 Community Description The RCMHP community is mostly made up of single story dwelling units (totaling 502 units) and can be classified as medium to high density residential, based on General Plan Land Uses for the City of Carlsbad. The units have a typical lot width of 50 feet by a length of 80 feet. Approximately 52 units have the rear of the property adjacent to Agua Hedionda Creek, while 41 units have the rear of the property adjacent to Calavera Creek. The dwelling units are typically raised at least 2.0 feet above the ground line of the original grade line. Streets are generally paved with asphalt concrete and have rolled concrete gutters and drainage swales to control onsite runoff. The community has an average gentle slope of 0.50-percent (0.005 foot/foot) that generally drains from the northeast corner at (elevation 60.00+) towards the southwest end of the community (elevation 44.00+). € 2: Area of Study Conditional Letter of Map Revision Existing drainage facilities are typically concrete overside drains and drainage inlets with small diameter culverts with flowlines that provide positive drainage away from the residential areas. The private residential streets which have a 2.00 % crown that force flow lines towards the gutters, were constructed in conjunction with intermittent inlets to provide an approximate 1.0-percent slope in asphalt areas. Other typical drainage features and areas found onsite include rain gutters which would contribute to the overall runoff by directing water from residential area roofs and driveways away from the residences and towards the gutter lines. Based on Parcel Map No. 17985, Parcel 1 contains all the housing units with an area equal to 88.44 acres. Parcel 2, which contains the club house and other structures, has a corresponding area of 6.14 acres. For purposes of this study, the runoff from the community will not have a significant impact to the overall discharge that is being contributed to the Agua Hedionda Creek. Additional runoff generated by this project will be managed within the existing storm water conveyance system. 2.3 Principal Flood Problems Agua Hedionda and Calaveras creeks have historically been subject to varying amounts of unsteady state flow conditions. This has caused the existing channel volume configuration to become inefficient over time. These inefficiencies are a result of deposition, degradation and scour at various locations along each channel alignment. It is proposed to model existing conditions in order to determine areas that are subject to constrained flow, impingement or other conditions that are detrimental to the flow efficiency of the creeks. Evidence of minor local scour caused by the channel runoff was observed around drainage appurtenances (such as culverts and overside drains) and along the bank slopes ("Line AH" Station 19+50, 21+55, 24+85, 29+53). Similar occurrences of localized scour can be seen along the channel alignment where obstructions to the flow path, such as trees or other appurtenances, impinge flow through the channel ("Line AH" Station 16+70,17+50,19+50, 21+50, 24+00, 29+50, 30+50, 41+15). There is evidence of scour around existing concrete aprons, sediment deposition, and movement of the existing rock slope protection. These conditions are typically attributed to high velocities, changes in channel geometry or changes in surface material (hard surface to softer surface). Another example of scour can be found at "Line AH" Station 30+15, where the channel alignment has a slight curve. A railroad tieback retaining wall protects that toe of slope and embankment at "Line AH" Station 30+15. However, due to the flow velocity and height of the water column, the runoff has managed to go behind the appurtenance (tie back wall) and scoured a hole at the bank slope. BROWN AnoCALDWELL 2-2 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carisbad, City of (CAj\129309-Agua Hedionda £ Calavera Creek DredgingWVP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc 2: Area of Study Conditional Letter of Map Revision 2.4 Other Features-(Flood Protection Measures) The Agua Hedionda and Calavera creeks dredging project is a component of on-going floodplain reconstruction work for the purpose of restoring flood control and protection. Several components surrounding the project (depending on the timing of construction) will be incorporated into the design and subsequent construction. These projects features will require construction tie-ins, coordination of design features, or temporary facilities that may require diversion during the construction phase of this project. These project features are briefly discussed below: • Gabion Drop Structures at Agua Hedionda and Calavera Creeks - The project will incorporate five drop down structures within Agua Hedionda (six for Calavera Creek) for velocity control and potential vegetation enhancement. • Desilting Basin — This design feature will provide temporary sediment control measures during construction and will also serve as a permanent sediment control feature that will require long term maintenance. • Maintenance Access Road — The access road will provide an entry way for equipment to initially construct, and ultimately maintain the channels. • Slope and Drainage Repairs. — The proposed work will entail the slope stabili2ation of areas subject to erosion or scour with slope treatment materials such as Vmax or other product, and replacement of damaged headwalls and drainage appurtenances that discharge to the creeks. • Rock Slope Protection at the Maintenance Access Road — The primary purpose of the rock is for protection of the proposed slope that will be subjected to water forces at the bend of the channel. • Ornamental Wall — This feature is to replace "in kind" the removal of the existing wall adjacent to Calavera Creek. Removal of existing wall will allow maintenance access to the channel. This work may require utility relocation of sewer, water and power. • 84" Reinforced Concrete Pipe Culvert - Proposed Diversion upstream of Calavera Creek to reduce flow volume (See Appendix C - Chang Study). BROWN AnoCALDWELL 2-3 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carlsbad, City of (CA)\ 12S30S-Agua Hedionda &. Calavera Creek Dredging\WP\S00648.RNAL_Basis for FEMA Dredging(06-02-08j.doc Uoiunoop siqi jo pus am is pay/pads suogeiiuiii sqi o] pafqns si 13345 siifi uo pauiauoo B)ep so asp fr-Z eajy :3 APPROXIMATE SCALE IN FEET 500 0 500 NATIONAL FLOOD INSURANCE PROGRAM FIRM FLOOD INSURANCE RATE MAP SAN DIEGO COUNTY, CALIFORNIA AND INCORPORATED AREAS PANEL 768 OF 2375 (SEE MAP INDEX FOR PANELS NOT PRINTED] NUMBER PANEL SUFFIX CONTAINS: COMMUNITY CAflLSBAD. CITf OT 060185 0763 MAP NUMBER 06073C0768 F EFFECTIVE DATE: JUNE 19,1997 Federal Emergency Management Agency is an official copy of a portion of tha above referenced flood map. Itwas extracted using F-M1T On-Lina. TWa map doe« not railed change* 01 amendments which may have been made subsequent to the dale on thetills block. For the lat««t product information about National Rood Insurance Figure-5 This page intentionally left blank CONDITIONAL LETTER OF MAP REVISION 3. ENGINEERING METHODS Brown and Caldwell (BC) has been commissioned to prepare HEC-RAS modeling for the existing and proposed conditions of both the Agua Hedionda Creek and Calavera Creek channels (project site). The existing conditions (baseline) model has been prepared with the data provided by the City of Carlsbad, and hydrology information provided for the 100-year event using the estimated flows developed by other consultants (Rick Engineering Company and Chang Consultants). The baseline model, if feasible, will be used to prepare calibration data for the project site. The baseline model will be compared to the existing FEMA mapping to determine floodplain boundaries and other areas of concern. Upon completion of the baseline simulations, proposed and alternative channel alignments will be modeled to determine the best fit channel geometry and mitigation along the banks. The modeling effort is being performed in support of proposed channel modifications. HEC-RAS 3.1.3, developed by the Hydrologic Engineering Center for use by the Army Corps of Engineers, will be utilized for all hydraulic model simulations. The HEC-RAS program is intended for use in calculating water surface profiles for steady state, gradually varied flow. The strength of the program relies upon the basic computational procedures of the one-dimensional energy equation. Through the standard equations, the program can compute subcritical, supercritical and mixed flow regime water surface profiles. Energy losses in a system are evaluated by friction (use of Manning's equation) and contraction/expansion (structure coefficients multiplied by the change in velocity head of the system). The HEC-RAS program is ideal for modeling and studying floodway encroachments, floodplain studies and flood insurance evaluations. Supporting information for the watershed will be provided by the Agua Hedionda Watershed GIS Based Master Plan and the Carlsbad Channel and Basin Project Special Use Permit prepared by Rick Engineering Company. Other supplemental information will be provided by the Conditional Letter of Map Revision Request for Robertson Ranch prepared by Chang Consultants and information provided by the City of Carlsbad. 3.1 Hydrology Preparation The Rick Engineering Company prepared a GIS-based Drainage Plan for the Agua Hedionda watershed portion of the City of Carlsbad. The Drainage Plan calculated the 100-year peak runoff for the watershed tributary to the Agua Hedionda Lagoon and its ultimate discharge to the Pacific Ocean. The computations provided preliminary information that included the Agua Hedionda Creek and Calavera Creek channel flow volumes. The initial data provided from the Rick Engineering documents was compared to the approved FEMA floodplain. Adjustments were made, incorporating the additional hydrology and hydraulic analyses performed by Chang Consultants. These studies are discussed further in Section 5. 3.2 Hydraulic Preparation All model default parameters were used, except where noted. Flowlines provided for the existing conditions were not modified. Features identified in the site walk photos and the survey data were added to the model, as shown in Table 3.1-1. Manning's n values, not shown in Table 3.1-1, were adapted from the existing HEC-RAS model. BROWN AND CALDWELL 3-1 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\ProjecisiCarlsbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP>S00648.FINAL_Basis for FEMA Dredging(06-02-08j.doc 3: Engineering Methods Conditional Letter of Map Revision Table 3.1-1. Feature Representation Water Course Agua Hedionda Creek Calavera Creek Features Represented Explicitly Box Culvert Lower Agua Bridge Cannon Road Bridge El Camino Road Bridge Right bank flood wall(1) Other Feature Representations Houses on the right and left banks are represented with a Manning's roughness of 0.2 Houses on the left bank are represented with a Manning's roughness of 0.2 Riprap in channel is represented with a Manning's roughness of 0.035 Nofes: (1) The Hood wall top elevation was set to the elevation in the survey data: 46 feet at the downstream end and 67 feet at the most upstream cross section. The information for the Agua Hedionda Creek bridges and culvert was obtained from the bridge AutoCAD drawings and supplemented with pier widths from the existing HEC-RAS model. It was assumed that the bridge cross section centerline coincided with the flow line provided. The pier width assumptions are outlined in Table 3.1-2. Table 3.1-2. Pier Widths from Existing Model Structure Box Culvert Lower Agua Bridge Cannon Road Bridge El Camino Road Bridge Pier Width ($* n/a 4.5 1.5 1.3 3.3 Model Assumptions A steady-state flow hydraulic analysis was performed with the flow and boundary conditions shown on Table 3.2-1. Following the previous study by Rick Engineering, Chang Consultants performed a HEC-1 analysis to model an upstream diversion of 500 cfs from Calavera Creek. The flow data used in our initial model runs is a result of the HEC-1 analysis. Table 3.2-1. Steady State Flow Data Water Course Agua Hedionda Creek Calavera Creek 100-yr Event Flow (cfs) • 7,338 upstream of confluence 8,041 downstream of confluence Upstream Flow 629 Diverted Flow 876 Downstream Flow 891 '.,-":,./_-'<• > Boundary Condition*«ft*i,.:f-.|i!'.'i Downstream water surface elevation = 35 feet Downstream water surface elevation = 42.8 feet (from Aqua Hedionda model results) 3-2 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projecls\Carlsbad, City of (CAj\12S309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc 3: Engineering Methods Conditional Letter of Map Revision 100-Year Event Flow Initial hydraulic simulation of Agua Hedionda Creek indicated flooding that extended beyond the cross sections, reaching the Calavera Creek channel from "Line AH" Station 7+00 to 21+99. Therefore, the Agua Hedionda cross sections were extended to the floodwall on the right bank of Calavera Creek up to and including the section at "Line AH" Station 19+99. This point ("Line AH" Station 19+99) was considered the "confluence" during design flow event (see Table 3.2-1) for the steady state flow analysis. Agua Hedionda Creek was modeled with a 100-year storm event discharge volume of 7,338 cfs for the upstream portion of the channel "Line AH" Station 19+99 to 48+50. Agua Hedionda was also modeled with a 100-year storm event discharge volume of 8,041 cfs for the downstream portion of the channel ("Line AH" Station 19+99 to 7+00). Since Agua Hedionda Creek's 100-year flow affects the confluence of Calavera Creek, the water surface elevation of "Line AH" Station 21+99 was used as the downstream control for the Calavera Creek model ("Line CC" Station 4+49) simulation. The Calavera Creek hydraulic simulation was modeled independently (of Agua Hedionda Creek) using the 100-year storm event discharge volume of 629 cfs acquired from an independent study prepared by Chang Consultants (February 2008). 3.4 Boundary Conditions The Agua Hedionda Creek hydraulic model determined that a water surface elevation of 35.0 feet MSL could be achieved at "Line AH" Station 7+00. This water surface elevation was used as the downstream boundary control for determination of back water effects upstream of Agua Hedionda and Calavera creeks. The hydraulic model simulation was run using a subcritical flow regime to determine the water surface elevation and resulting floodplain boundary (footprint of flooding) as shown in the work map Exhibit -1 provided in Appendix E. 3.5 Progression of Proposed Alternatives with Model Results 1. The initial model run illustrated the existing conditions in Agua Hedionda and Calavera creeks. This model simulation generated the floodplain footprint as shown in FEMA mapping. This model simulation showed flooding to impact half of the Rancho Carlsbad community (278 housing units). 2. Proposed modifications to Agua Hedionda Creek began by using a 70' channel width with 2:1 side slopes. This model simulation decreased the amount of flooding within the Rancho Carlsbad community although flooding still would impact 26 housing units. 3. A maintenance access road on the northern bank of Agua Hedionda Creek and a desiltation basin within the channel floor were incorporated into the proposed modification design described in alternative 2 (70* channel width with 2:1 side slopes), just downstream of the confluence. 4. A revision to the Agua Hedionda Creek geometry was proposed after the determination that further protection was necessary for the Rancho Carlsbad community. The modification incorporated an 85' channel width with 1.5:1 side slopes. The desiltation basin was also widened and incorporated into the revised proposal. The proposed maintenance access road described in alternative 3 was withdrawn due to the widening of the channel. 5. The proposed conditions of an 85' channel width with 1.5:1 side slopes incorporated both the widened desiltation basin and the installation of a proposed maintenance access road to a new location, just downstream of the confluence with Calavera Creek. A revised discharge volume was used based on the BROWN AtiDCALDWELL 3-3 Use of data contained on this sheet is subject to the limitations specified at the end of this document. F:\Projecls\Carisbad, City of {CA)\1293Q9-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAl_Basis for FEMA Dredging(Q6-02-08).doc 3: Engineering Methods Conditional Letter of Map Revision modifications of the weir wall upstream of Calavera Creek. The change in condition was based on work by Chang Consultants (November 2005). 6. The final proposed conditions mimic those of alternative #5. In this alternative, drop structures were incorporated into a 200 foot stretch of Calavera Creek for velocity control, just upstream of the confluence with Agua Hedionda Creek and at 200 foot stretch of Calavera Creek almost 2,000 feet upstream of the confluence. Similarly, drop structures were incorporated into a 200 foot stretch for the design at Agua Hedionda Creek, just west of Rancho Carlsbad Drive. 3.6 Modifications The current Agua Hedionda Creek channel geometry was modified in order to contain the 100 year flood flows within the channel banks, lowering the footprint of the floodplain to minimize impacts to the surrounding residential areas. The bottom channel width, between the Cannon Road Bridge ("Line AH" Station 12+75) and the downstream of El Camino Real Bridge ("Line AH" Station 14+88) was widened to 85 feet. This 85 foot width was also incorporated to the upstream sections between the El Camino Real Bridge ("Line AH" Station 15+99) and just west of the confluence between Agua Hedionda and Calavera Creeks ("Line AH" Station 18+99). The channel slope from section "Line AH" Station 18+99 to "Line AH" Station 40+41 was set to a 0.15% grade with the channel elevation at "Line AH" Station 18+99 beginning at 32.01 ft. Manning's roughness values throughout this section were also modified to 0.025 to reflect a clean, uniform channel bottom. 3.7 Modeling Conclusions and Recommendations The modeling performed for the "Rancho Carlsbad Mobile Home Park Alternative Analysis for Agua Hedionda Channel Maintenance", provided several alternatives that depicted undetained flow and detained flow for the channel through the community. The undetained flow revealed flooding beyond the channel banks, impacting at least half the community. The detained flow minimized the flood footprint but still created a significant impact to the community. The additional alternatives, provided in this study were able to reduce the flood footprint, minimizing flood impacts down to approximately 12 housing units. However, the last detained alternative required the construction of retaining flood walls to contain the flow within the channel banks. The hydraulic modeling for the Agua Hedionda Dredging Project incorporated revised topography that reflected different channel flow conditions due to heavy vegetation, changes in flow path and sediment deposition within the channel footprint. Initial field observations revealed localized scour around appurtenances that have encroached in the channel flow path, damage to existing drainage facilities and appurtenances, such as overside drains, concrete aprons and displacement of rock slope protection. Other impacts created within the channel include: emergency dredging within the center of Agua Hedionda Creek to minimize flooding, vegetation removal between the bridges, and adjacent work upstream of Calavera Creek. Future impacts such as new development with storm water facilities will alter drainage flow patterns, create diversions and will make some facilities obsolete. These existing and future channel conditions have been discussed and considered to determine if additional assumptions must be incorporated into the hydraulic modeling for the Agua Hedionda Dredging Project. The most significant of impacts will be the proposed development north of Cannon Road. This development has proposed the design and subsequent construction of an 84-inch Reinforced Concrete Pipe that will drain the development and will divert 500 cfs from the Calavera Creek channel. This will relieve the volume of flow at the upstream basin and will aid in the reduction of the overall volume at the downstream confluence between Agua Hedionda and Calavera Creeks. BROWN ANoCALDWELL 3-4 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carlsbad, City of (CA)\1293Q9-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAl_Basis for FEMA Dredging(06-C2-08).doc 3: Engineering Methods Conditional Letter of Map Revision Based on the above mentioned information, the assumption of reduced flow was introduced into the hydraulic model. Utilizing the initial channel geometry width of 70 feet with 2:1 side slopes, the 100-year discharge flow volume of 7,254 was used upstream of the confluence and 7,983 downstream of confluence. The model results generated a floodplain footprint that exceeded the banks of both channels. Revised hydraulic simulations introduced a desiltation basin and a maintenance access road. The addition of a desiltation basin and the access road altered the geometry of the channel. However, this modification did not have a significant impact to the channel capacity or the floodplain. The next revision changed the geometry of the channel. Utilizing the channel geometry of 85 foot width with 1.5:1 side slopes, and the same discharge flow volumes of 7,254 (upstream) and 7,983 (downstream) respectively. The model resulted in lowering the water surface elevation, as well as, lowering the floodplain footprint. Thus, the flow volume was contained within the banks of both channels. Once this hydraulic simulation became stable, the desiltation basin and maintenance access road were incorporated. Although, the addition of a desiltation basin and the access road altered the geometry of the channel, this modification did not have a significant impact to the channel capacity or the floodplain. Additional features, such as gabion drop structures and rock slope protection, were added to the model for purposes of sediment and velocity control. These control features slightly raised the channel bottom and water surface elevation from the proposed modifications at Calavera Creek. However, this change in water surface did not create a significant impact to the adjoining properties. The current hydraulic model with channel geometry of 85 feet width and 1.5:1 side slope minimizes the flooding potential to about 6 adjacent properties. Therefore, it is recommended to prepare a design that incorporates the design features to minimize potential flooding described above. Details on elements that make up the HEC-RAS hydraulic simulations are found in Appendix B. 3.8 Vertical datum As part of the work order agreement, aerial mapping and field surveys was completed by our sub-consultants, Photo Geodetic Corporation (San Diego, CA.) and Right-of-Way Engineering Services, Inc. (Oceanside, CA.), respectively. Coordinate controlled aerial panels (targets) were set on October 2005 prior to the aerial photography. The aerial photography survey was performed on October 27, 2005, flown at a preset elevation to acquire and generate base mapping at one-foot contour intervals. The base mapping generated for the Agua Hedionda & Calavera creeks project would have one foot contours with a standard accuracy of +_ 0.5 foot on a 1"= 40' scale map. Additional field surveys were conducted to fill in the gaps where the vegetative canopy was too dense to acquire ground elevations through aerial means. Additional supplemental work (spot elevations and tie-in points) were acquired using Total Station electronic equipment. The additional field points were required to tie in survey monuments and aerial panels, and obtain rim and invert elevations for utility vaults, sanitary sewer manholes, cleanouts, headwalls, poles, fences, bridge corners and bottom of bridges, existing flow line elevations for both creeks, and spot elevations in areas that have rock slope protection. The spot elevations generated by the survey personnel have a standard accuracy of +. 0.1 foot on a 1"= 40' scale map. The one-foot contour mapping is justified for an accurate assessment and delineation of the flood plain area for the project. The mapping also meets submittal requirements as outlined by FEMA. Plot files have been created which form the basis of the base map used for the development of the drawings. The basis of bearings and coordinates/benchmarks are based on NAD 83 (1991.35 EPOCH) points as shown on the City of Carlsbad Control Record of Survey No. 17271 using the following two points: • Point No. 105, N1999466.124, E 6241021.960, Elevation=26.13' (NGVD29) located on a 2.5" disk in northeast corner of Cannon Road Bridge over Agua Hedionda Creek, 120' southwest of El Camino Real BROWN ANDCALDWELL 3-5 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carlsbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc 3: Engineering Methods Conditional Letter of Map Revision • Point No. 109, N 1998301.767, E 6239246.162, on 2.5" disk in drainage box inlet on the south side of Cannon Road, 0.2 miles west of El Camino Real A utility contacts list was prepared and a request for information letter sent out to the utilities adjacent to and within the community. The utility search was performed to determine the location of fuel and gas lines, communications such as telephone and cable, and other underground utilities such as electrical, water and sewer. The location and description of these utility features are identified in the design drawings. The City of Carlsbad also provided development drawings from adjacent proposed developments which identified features such as future drainage culverts, utility lines, lot lines and property boundaries. The electronic files were overlaid on the base map as a raster image, and was then fitted to scale and converted into a digitized image. Property boundary lines and easement lines have been incorporated into the design drawings. These survey features are based on the information acquired from Parcel Map No. 17985, recorded on February 4, 1998. Control coordinates for Agua Hedionda and Calavera creeks alignments are as follows: Agua Hedionda Creek, Downstream Station 7+00, elevation 32.00, N=1999383.49, E=6240499.88. Agua Hedionda Creek, Upstream Station 48+53.25, elevation 43.00, N=1999221.85, E=6244213.33. Calavera Creek, Downstream Station 0+00, elevation 31.98, N=1999828.95, E=6241412.29. Calavera Creek, Upstream Station 35+28.36, elevationSO.OO, N=2001302.56, £=6244361.60. 3-6 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projecls\Carlsbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Oredging(06-02-08j.doc CONDITIONAL LETTER OF MAP REVISION 4. FORMS In 1968, the U.S. Congress passed the National Flood Insurance Act, which created the National Flood Insurance Program (NFIP). The NFIP was designed to reduce future flood losses through local floodplain management and to provide protection for property owners against potential losses through flood insurance. As part of the agreement for making flood insurance available in a community, the NFIP requires the participating community to adopt floodplain management ordinances containing certain minimum requirements intended to reduce future flood losses. The NFIP regulations for floodplain management are the minimum criteria a community must adopt for participation in the NFIP. The community is responsible for approving all proposed floodplain development and for ensuring that permits required by Federal or State law have been received. State and community officials, based on knowledge of local conditions and in the interest of safety, may set higher standards for construction or may limit development in floodplain areas. If the State or Community has adopted more restrictive or comprehensive floodplain management criteria, those criteria take precedence over the minimum NFIP requirements. The community is also responsible for submitting data to the U.S. Department of Homeland Security - Federal Emergency Management Agency (DHS-FEMA) reflecting revised flood hazard information so that NFIP maps can be revised as appropriate. This will allow risk premium rates and floodplain management requirements to be based on current data. Submissions to DHS-FEMA for revisions to effective Flood Insurance Studies (FISs), Flood Insurance Rate Maps (FIRMs), or Flood Boundary Floodway Maps (FBFMs) by individual and community requesters will require the signing of application forms. These forms will provide DHS-FEMA with assurance that all pertinent data relating to the revision are included in the submittal. They will also ensure that: (a) the data and methodology are based on current conditions; (b) qualified professionals have assembled data and performed all necessary computations; and (c) all individuals and organizations affected by proposed changes are aware of the changes and will have an opportunity to comment on them. If the submission involves revisions to multiple flooding sources, then separate forms should be completed for each flooding source. For purposes of this study, a Conditional Letter of Map Revision (CLOMR) will be submitted for the community. The appropriate forms have been included in Appendix A and are as follows: Form 1 - Overview & Concurrence Form that provides the basic information regarding the revision request and requires the signatures of the requester, community official, and engineer. Form 2 — Riverine Hydrology & Hydraulics Form provides the basic information on the scope and methodology of hydrologic and/or hydraulic analyses that are prepared in support of the revision request. This form is used for revision requests that involve new or revised hydrologic and/or hydraulic analyses of rivers, streams, ponds, or small lakes. Form 3 - Riverine Structures Form provides the basic information regarding hydraulic structures constructed in the stream channel or floodplain. This form should be used for revision requests that involve new or proposed channelization, bridges/culverts, dams/basins, and/or levees/floodwalls. BROWN AND CALD WELL 4-1 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carlsbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL.Basis for FEMA Dredging(06-02-08).doc 4: Forms Conditional Letter of Map Revision This page intentionally left blank BROWN ANoCALDWELL 4-2 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Cartsbad, City of (CA)M29309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc CONDITIONAL LETTER OF MAP REVISION 5. OTHER STUDIES Brown and Caldwell (BC) was commissioned to prepare HEC-RAS modeling for the existing and proposed conditions of both the Agua Hedionda Creek and Calavera Creek channels (project site). The existing conditions (baseline) model has been prepared with the data provided by the City of Carlsbad, and hydraulic information provided for the 100-year event using the estimated flows developed by others consultants (Rick Engineering Company and Chang Consultants). Initial hydrologic and hydraulic analyses were prepared for the project by Chang Consultants in order to establish the 100-year flow rates and water surface elevations. In particular, the analyses determined the portion of the 100-year flow rate discharging from Detention Basin BJB that will be directed into the 84-inch reinforced concrete pipe versus the portion that will remain in the 11-foot by 7-foot reinforced concrete box (RGB) culvert. A summary of the analyses used for this determination is provided below. A HEC-1 hydrologic analysis was previously performed by Rick Engineering Company (Rick) for the Agua Hedionda Creek and Calavera Creek watersheds, which are tributary to the project site. The analysis is included in Rick's December 13, 2004 report, Rancho Carlsbad Mobile Home Park, Alternative Analysis for Agua Hedionda Channel Maintenance. The watershed was delineated using the United States Geological Survey's quadrangle maps (see the HEC-1 Work map located in Appendix D. Rick performed field investigations to verify the watershed boundaries. The land uses and hydrologic soil groups were based on the adopted land uses and the Soil Survey, San Diego Area, California, respectively. Two HEC-RAS analyses were performed using this 100-year flow rate to determine the flow split in the 11- foot by 7-foot RGB (see HEC-RAS output (Appendix C) and work map). A vertical wall in the RGB will direct a portion of the flow into the 84-inch RCP, while the remainder will continue downstream in the RGB. A HEC-RAS analysis was created starting in the 84-inch RCP, continuing upstream to the split, and ending at the upper end of the RGB. A lid was added to the cross-sections to model the closed conduits and cross- section interpolation was performed to increase the number of cross-sections. A second HEC-RAS analysis extended up the entire length of the RGB, up to and including the flow split wall. The cross-sections upstream of the wall are the same in both HEC-RAS analyses. An iterative procedure was used to determine how the 725 cfs approaching the split would be divided between the 84-inch RCP and the 11-foot by 7-foot RGB. In both HEC-RAS analyses, the wall that splits the flow is located between cross-sections 5 and 6. Cross-section 7 is just upstream of cross-section 6 and the wall. The individual flow rates in the RGB and RCP below the split must add up to 725 cfs since this is the flow rate approaching the split. Therefore, the total flow rate below the split was maintained at 725 cfs, but the individual flow rates were varied in the RGB and RCP until the hydraulic grade lines above the split (above cross-section 7) matched in each analysis. This procedure determined that approximately 364 cfs will be directed to the 84-inch RCP and 361 cfs will continue in the 11-foot by 7-foot RGB. BROWN AND CALD WELL 5-1 Use o? data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projecis\Carlsbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredgingf06-02-08i.doc 5: Other Studies Conditional Letter of Map Revision This page intentionally left blank BROWN AND C ALD WEI L P:\Projects\Carlsbad, 5-2 Use of data contained on this sheet is subject to the limitations specified at She end of this document. City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc CONDITIONAL LETTER OF MAP REVISION 6. FINDINGS AND CONCLUSIONS The proposed hydraulic model of Agua Hedionda and Calavera creeks provides a relatively uniform slope at 0.0015 foot/foot from "Line AH" Station 41+50 to about Station 18+70. Beyond "Line AH" Station 18+70 and through the bridges (El Camino and Cannon Road), the slope matches the original graded slope of 0.0030 foot/foot. The hydraulic modeling shows that the relative velocities through the channel will have a range between 5.50 to 7.50 feet per second. This velocity range can typically generate some sediment movement and scour around bridge abutments and columns during various stages of flow. Part of the design elements incorporated were drop structures to provide steps between grades. This would allow a drop in elevations (one to two feet) between grades so as to maintain mild slopes as part of velocity, scour and sediment controls. This would also aid in reducing the excavation required, particularly in the Calavera Creek, to match existing conditions within the overall channel footprint. It is anticipated that the dredging will remove most of the existing sandy material. However, it will expose the fine material, composed of clay and silt underneath the existing finished grade. The potential sediment movement and or transport within the water column cannot be easily predicted unless a sediment study can be performed. To provide a measure of control with a velocity reduction measure, the introduction of a desilting basin between "Line AH" Station 17+00 to 18+70 has been incorporated into the design. In addition, a Gabion Structure has been added to provide a step down (change in grade elevation) into the basin and to control the velocity of water entering the basin. This feature will provide a measure of sediment/silt protection that can be anticipated based on the relative velocities generated in the modeling of the creeks. An essential component of the desilting basin is the maintenance required to keep it functioning at capacity. To allow for periodic cleaning and sediment removal, a maintenance access road has also been incorporated into the design. Unfortunately, the geometry of the access road has contributed to further excavation along the north side of the slope just west of the confluence between Agua Hedionda and Calavera creeks. The channel widening and access road have also contributed to greater excavation quantities, removal of an ornamental wall that will be replaced, and addition of rock slope protection at the base of the maintenance access road. It is also noted that there is existing rock slope protection around the bridge abutments and in front of the bridges. However, sediment deposition has covered most of the rock within the bottom of the channel. In anticipation of encountering the rock slope protection, a further investigation will have to be performed during construction, since it is difficult to determine the current depth of rock due to its resident time (original rock position at construction in 1969), potential movement due to undermining and settlement due to its own weight. An added feature is the Gabion Structure added upstream of Agua Hedionda Creek. This structure has been added to provide a step down (change in grade elevation) from the existing concrete channel located underneath Rancho Carlsbad Drive ("Line AH" Station 41+50). Hydraulic modeling reveals that velocities are approaching 10 feet per second through this section. There is also a difference in elevation by about three feet due to scour damage downstream of the bridge. The proposal to introduce the gabion structure at ("Line AH" Station 41+50) the tie-in location (Rancho Carlsbad Drive) will mitigate for velocity and elevation difference. This will provide a measure of velocity control and will step down the flow to the milder proposed finished grade. BROWN ANDCALDWELL 6-1 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carlsbad, Ciry of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc 6: Findings and Conclusions Conditional Letter of Map Revision This page intentionally left blank BROWN ANBCALDWELL 6-2 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carisbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging'\WP\S00648.RNAL_Basis for FEMA Dredging(06-02-08).doc CONDITIONAL LETTER OF MAP REVISION 7. LIMITATIONS Report Limitations This document was prepared solely for City of Carlsbad in accordance with professional standards at the time the services were performed and in accordance with the contract between the City of Carlsbad and Brown and Caldwell dated October 3, 2005. This document is governed by the specific scope of work authorized by the City of Carlsbad; it is not intended to be relied upon by any other party except for regulatory authorities contemplated by the scope of work. We have relied on information or instructions provided by the City of Carlsbad and other parties and, unless otherwise expressly indicated, have made no independent investigation as to the validity, completeness, or accuracy of such information. BROWN ANDCALDWELL 7-1 Use of dala contained on this sheet is subject to the limitations specified at the end of this document. P:\Projects\Carlsbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc 7: Findings Conditional Letter of Map Revision This page intentionally left blank BROWN ANDCALDWELL 7-2 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projecis\Carlsbad, City of (CA)\129309-Agua Hedionda & Calavera Creek Dredging\WPiS00648.FINAL_Basis for FEMA Oredging(06-02-08).doc Conditional Letter of Map Revision REFERENCES Bridge Scour and Stream Instability Countermeasures - Experience, Selection and Design Guidance, Hydraulic Engineering Circular 23 (HEC- 23), Publication No. FHWA-NHI-01-003, Second Edition, March 2001. City of San Diego Regional Standard Drawings (Document No. 769332), May 1997. Debris Control Structures - Evaluation and Countermeasures, Hydraulic Engineering Circular 9 (HEC-9), Publication No. FHWA-IF-04-016, Third Edition, October 2005. Geotechnical Evaluation, Cannon Road and El Camino Real Bridges, Carlsbad California (Project No. 105953001) prepared by Ninyo & Moore Geotechnical and Environmental Consultants, San Diego, California, November 30,2006. Guidelines and Specifications for Flood Hazard Mapping Partners, Federal Emergency Management Agency (FEMA), April 2003. HEC-RAS River Analysis System, U. S. Army Corps of Engineers, Hydrologic Engineering Center (HEC), Version 3.1, November 2002 Improvement Plans for Robertson Ranch East Village, (Project No. C.T. 02-16) prepared by O'Day Consultants, Carlsbad, California. Improvement Plans for Cannon Road, (Project No. C.T. 02-16) prepared by O'Day Consultants, Carlsbad, California. Limited Geotechnical Evaluation, Rancho Carlsbad Channel and Basin Project (Project No. 105132001) prepared by Ninyo & Moore Geotechnical and Environmental Consultants, San Diego, California, January 7,2004. Limited Environmental Analysis for Agua Hedionda Creek (Project No. 105760001) prepared by Ninyo & Moore Geotechnical and Environmental Consultants, San Diego, California, February 10,2006. Revisions to National Flood Insurance Program (NFIP) Maps (MT-2), Federal Emergency Management Agency (FEMA), February 2006. Rancho Carlsbad Mobile Home Park Alternative Analysis for Agua Hedionda Channel Maintenance, (Job No. 13182-D) prepared by Rick Engineering Company, December 2004. BROWN AND C ALDWEL L REF-1 Use of data contained on this sheet is subject to the limitations specified at the end of this document. P:\Projecls\Carlsbad, City of (CA)M29309-Agua Hedionda & Ceiavera Creek Dredging\WP\S00648.FINAL_Basis for FEMA Dredging(06-02-08).doc r HYDROLOGIC AND HYDRAULIC ANALYSES I I FOR • ROBERTSON'S RANCH EAST VILLAGE | September 2, 2004 I I fc I I I I I I I I CnangtlosiME: M:QS Civil Engineering ° Hydrology ° Hydraulics ° Sedimentation . P.O. Box 9496 I Rancho Santa Fe, CA 92067 (858) 692-0760 V -TABLE OF CONTENTS -r Introduction 1 Background 2 East Village Analyses 4 Conclusion and Recommendations 6I ! I 1 w. 1 & ™ I APPENDIX I A. 100-Year HEC-1 Analysis B.I 00-Year HEC-RAS Analysis - Calavera Creek Existing Conditions North of Wall C.I 00-Year HEC-RAS Analysis - Calavera Creek Proposed Conditions North of Wall I D.I 00-Year HEC-RAS Analysis - Calavera Creek Existing Conditions South of WallB 8 MAP POCKET HEC-RAS Work Map I IK*" INTRODUCTION Robertson's Ranch is a proposed project by Calavera Hills II, LLC (CHII) located in the city of Carlsbad (see Vicinity Map). The easterly portion of Robertson's Ranch is named the East Village, and is immediately north of the Rancho Carlsbad Mobile Home Park (RCMHP) and west of College Boulevard. Cannon Road is aligned in an east-west direction near the southerly boundary of the East Village. The East Village north of Cannon Road will be developed with single- and multi-family residential units as well as a portion of a school site. The East Village south of Cannon Road will contain an approximately 4.4 acre pad developed with multi-family residential units. In addition, RCMHP intends to add an RV storage area immediately east of this multi-family development. 1C CITY OF OCEANSIDE HIGHWAY SITE VISTA CITY OF SAN MARCOS CITY OF ENCINITAS I Figure 1. Vicinity Map I I (C i I i I 1 Under both existing and proposed conditions, storm runoff from the East Village flows in a southerly direction to Calavera Creek, which is located along the boundary of the East Village and mobile home park. A free-standing masonry wall exists along this boundary. The base of the wall contains several semi-circular openings along much of its length. In general, creek flow south of the wall is conveyed westerly to a confluence with Agua Hedionda Creek within RCMHP, and then continues westerly to Agua Hedionda Lagoon. Creek flow north of the wall is conveyed westerly, passes through culverts under Cannon Road and El Camino Real, then enters Agua Hedionda Lagoon. Several hydro logic and hydraulic analyses have been performed to determine the 100-year flow rates and floodplain limits in Agua Hedionda and Calavera Creek. The analyses have been prepared for various historic as well as proposed conditions. Many of the proposed conditions represent regional solutions for minimizing flood inundation in RCMHP. This report determines the impact of the East Village on Calavera Creek. In particular, the East Village's influence on the established regional solutions is outlined. In order to accomplish this, a background on the relevant analyses and projects in the watershed is presented first. BACKGROUND The following provides background information on historic floodplain conditions in the area. Furthermore, recent studies and completed improvements that are relevant to the East Village are outlined. The Federal Emergency Management Agency (FEMA) has mapped the Calavera and Agua Hedionda Creek 100-year floodplains. The mapping is shown on the June 19, 1997 Flood Insurance Rate Map (see shaded areas in Figure 2). The analyses used for the FIRM mapping were performed prior to recent improvements in the vicinity of Robertson's Ranch. Several of the recent improvements have altered the FEMA floodplains. These improvements include the extension of Cannon Road and College Boulevard and the construction of the Master Drainage Plan facility, Detention Basin BJB. FEMA's analyses show that significant portions of RCMHP are within the 100-year floodplain. It should be noted that the FIRM identifies the masonry wall along the northerly boundary of RCMHP as a floodwall with different 100-year water surface elevations on either side despite the fact that the wall contains many openings. It is unknown whether the differential elevations are based on consideration of the openings. The wall does not meet FEMA's current levee criteria. According to correspondence from Rick Engineering Company (REC), FEMA's 100-year flow rate in Calavara Creek is 1,350 cubic feet per second (cfs) with 545 cfs south of the wall and 805 cfs north of the wall. ' Since the FEMA mapping shows that much of RCMHP is within both floodplains, the city of Carlsbad selected REC to prepare updated hydrologic and hydraulic studies for Calavera and Aqua Hedionda Creek. The result of the studies was the development of regional solutions for minimizing flood inundation in RCMHP. The first step in the city process was to update FEMA's 100-year flow rates. A HEC-1 I hydrologic analysis was performed by REC based on General Plan land uses to determine the 100-year flow rates in both creeks. Since General Plan land uses were used, the resulting flow i rates were, in essence, ultimate development flow rates rather than existing condition flow rates I I By using ultimate development flow rates, any regional solutions would appropriately address future conditions in the watershed. 1C APPROXIMATE SCALE IN FEET 500 0 500 NATIONAL FLOOD INSURANCE PROGRAM FIRM FLOOD INSURANCE RATE MAP SAN DIEGO COUNTY, CALIFORNIA AND INCORPORATED AREAS PANEL 768 OF 2375 (SEE Wf IMOSK FOa SW*EL5 NOT ffrthTEO MAP NUMBER 06073CQ788 F EFFECTIVE 0STE: JUNE 19, 1997 Federal Emergency Management Agency ts an official copy of a portion of the above referenced Scad map extracted using F-MIT Qn-Une. Tbi* map dees not refl at amendments which may ha\e beer; made subsequent to !he dat tilie block. For the latest crcduct Information about National Rood !ris Progcam jlood maps chcc-k Ihc FEMA Flood Map Sto Figure 2. FEMA Fioodplains The hydrologic analysis was then used to identify four potential detention basins that could be constructed to reduce the 100-year flow rates. These basins are at various locations in the watershed. Two of the basins represent Master Drainage Plan Facilities BJ and BJB. To date, only Detention Basin BJB has been constructed. Detention Basin BJ will be constructed when College Boulevard southerly of Cannon Road is constructed. It should be noted that REC's HEC-1 analyses are based on the 1993 County of San Diego Hydrology Manual. This was the current manual at the time the analyses were initially performed. The County updated the Hydrology Manual in 2003. However, REC indicated that they are not planning to nor have they been requested to update their analyses based on the latest criteria. On the other hand, the city of Carlsbad has required the on-site analyses for the East Village to be based on the 2003 criteria. COPY I 1 I I 1 I 1 1 c 1 1 1 I I 1 1f 1 REC also performed hydraulic analyses of Agua Hedionda and Calavera Creek based on the 100- year flow rates from their hydrologic analyses. The hydraulic analyses were used to delineate the floodplain in both creeks and to further identify potential flood control improvements. REC's studies led to their final design and the recent construction of Detention Basin BJB, which is immediately north of the intersection of Cannon Road and College Boulevard. Detention Basin BJB was constructed in order to reduce the 100-year flow rate in Calavera Creek. In addition, REC designed a weir immediately downstream of the Detention Basin BJB outlet. The weir has been constructed within the masonry wall along the north end of RCMHP and is intended to control the flow rates on the north and south sides of the wall. By limiting the flow on the south side of the wall, additional flood protection is provided to RCMHP. It should be noted that, according to REC's analyses, ultimate flood protection of RCMHP from Calavera Creek is dependent on construction of Detention Basin BJ. At this time, the timeframe for construction of this basin is unknown. Furthermore, the regional solution to the RCMHP flooding needs to include the masonry wall openings being plugged or the wall being replaced with a FEMA- certified floodwall. One of the criteria for the floodwall to be FEMA-certified is that "all maintenance activities must be under the jurisdiction of a Federal or State agency, an agency created by Federal or State law, or an agency of a community participating in the NFIP [National Flood Insurance Program] that must assume ultimate responsibility for maintenance." Most recently, REC modified their HEC-1 hydrologic analysis to include impending changes to the outlet facility in Lake Calavera. These changes are intended to provide a greater factor-of- safety against dam failure and have the additional benefit of further reducing the 100-year flow rate in Calavera Creek. The HEC-1 analysis is based on General Plan land uses, REC's final design of Detention Basin BJB, and conceptual design of the remaining three detention basins. EAST VILLAGE ANALYSES In order to assess the impact of the East Village on Calavera Creek, several analyses were performed. First, REC's latest HEC-1 analysis was obtained then updated in order to determine the 100-year flow rate in Calavera Creek. REC's analysis was based on future residential lots within Detention Basin BJB. According to CHII, the lots are no longer planned. As a result, REC's analysis was revised to remove the lots from the basin. The revised basin volume was obtained from previous volume calculations by REC, which assumed no lots in Detention Basin BJB. The updated HEC-1 analysis is included in Appendix A. The analysis shows that the 100- year flow rates into and out of Detention Basin BJB are 1,094 and 878 cfs, respectively. ^ In order to determine how the weir divides the Detention Basin BJB outflow north and south of the wall, two HEC-RAS hydraulic analyses were performed. One analysis models flow through the weir and the other models flow north of the wall. An iterative procedure was used where the total flow rate from both analyses equaled the Detention Basin BJB outflow. The flow rates were then adjusted until the water surface elevations below Detention Basin BJB were equal in each analysis. This procedure determined that approximately 450 cfs is directed north of the wall and 430 cfs is directed south of the wall. The East Village flows confluence with Calavara Creek north of the masonry wall. REC prepared a HEC-2 hydraulic analysis of the creek north of the wall. The analysis assumed that 1C the holes in the masonry wall along the north boundary of RCMHP were plugged and is based on the current condition with Cannon Road constructed. The East Village runoff is directed to this section of the creek under existing and proposed conditions. The revised HEC-1 analysis shows that the flow rate in this area including the East Village contribution is 472 cfs. Since the HEC-1 analysis assumes General Plan land uses, the analysis reflects development of the East Village. REC's HEC-2 analysis was converted to HEC-RAS and revised to reflect the updated HEC-1 results (see Appendix B for the HEC-RAS analysis and the map pocket for the work map). In addition, minor modifications were made to cross-sections at the upstream and downstream limits of the model. This HEC-RAS analysis establishes the baseline for the Calavera Creek 100- year water surface elevations north of the wall. Note that this floodplain is confined between the masonry wall and Cannon Road except where it is conveyed under Cannon Road. The East Village proposes a 4.4 acre pad and berming south of Cannon Road. The pad is planned to be developed with multi-family units. The pad and berm encroach within the aforementioned baseline floodplain. In order to determine the impact from the pad and berm, the HEC-RAS analysis was modified to include these. The analysis (see Appendix C) shows that the 100-year water surface elevation increases up to 2.2 feet between cross-sections 50 to 140, which are along the pad and berm. Beyond this, the water surface elevation reduces back to the baseline level. Table 1 summarizes the HEC-RAS results for the baseline and proposed conditions. HEC-RAS Cross-Section 10 20 30 40 50 60 90 100 110 120 140 150 160 170 180 220 230 240 250 260 270 280 Existing Condition 100- Year Water Surface Elevation, ft. 40.6 41.3 41.4 42.4 44.0 44.3 45.1 46.7 47.4 48.5 49.6 51.6 52.3 53.1 54.6 56.4 57.5 58.0 58.5 58.9 59.9 60.0 Proposed Condition 100-Year Water Surface Elevation, ft. 40.6 41.3 41.4 42.4 45.0 45.5 45.7 47.9 49.6 50.5 50.5 51.1 52.4 53.1 54.6 56.4 57.5 58.0 58.5 58.9 59.9 60.1 Water Surface Elevation Increase From East Village, ft 0 0 0 0 1* 1.2* 0.6* 1.2* 2.2 2.0 0.9 -0.5 0.1 0 0 0 0 0 0 0 0 0.1 *Below water surface south of wall Table 1. HEC-RAS Results - Calavara Creek North of Wall I I i i Finally, an analysis was performed for Calavera Creek south of the wall to establish the 100-year water surface differential on either side of the wall (see Appendix D). REC prepared a HEC-RAS analysis of this area based on surveyed cross-section data. Their analysis was modified for the updated HEC-1 flow rates. Figure 3 provides a comparison of the 100-year water surface elevations north and south of the wall. Figure 3 shows that the water surface south of the wall is higher than the proposed condition north of the wall along the downstream one-third length of the wall. This reverses along the upstream two-thirds of the wall. I I I I 1C I I I s o•JS > CO I i .(•; I' I 65- eo~ 55 50 Weir Q1QQ(with Detention BasinBJB): 472 cfs North of Wall 769 cfs South of Wall Legend ; Prop. WSELN of Wall; Exist. WSELN of Wall Exist. Ground N of Wall &&'$$£"$ li~m COPY HEC-RAS Cross-section o o oS3 8 f5 8 1000 1500 2000 2500 3000 3500 4000 Main Channel Distance North of Wall (ft) Figure 3. 100-Year Water Surface Profiles CONCLUSION AND RECOMMENDATIONS The city of Carlsbad's consultant, Rick Engineering Company, has established General Plan 100- year flow rates throughout the Agua Hedionda and Calavera Creek watersheds. These flow rates were used to develop regional solutions to flooding. The city's studies are based on development of the East Village; therefore, surface runoff impacts generated by the project have been accounted for in REC's analysis of flood control solutions. In particular, the solutions were intended to reduce flood inundation in the Rancho Carlsbad Mobile Home Park. To date, one of the Master Drainage Plan Facilities, Detention Basin BJB, has been constructed. It is our understanding that the city of Carlsbad is considering additional alternatives. These could involve construction of three additional detention basins as well as other channel improvements. I I ^X*****- r Robertson's Ranch East Village will have some impact on 100-year water surface elevations north of the masonry wall along the mobile home park due to the proposed pad and berming south of Cannon Road in part of this area. These increases range from 0.1 to 2.2 feet near the pad and berming. The increases diminish further upstream. In some areas where the increases occur, the water surface elevation south of the wall is higher than north of the wall (between Cross- I sections 40 to 110). Consequently, the East Village will not impact the existing wall in these areas (see below for a discussion on the floodwall). Furthermore, the flow rate north of the wall with the East Village (472 cfs) is significantly less than FEMA's historic flow rate (805 cfs). I I I I I t - This report shows that the regional solutions developed for the city of Carlsbad take into account I the post-development storm runoff from the East Village. Nonetheless, the East Village will have some impacts on the floodplain north of the masonry wall. However, the regional solutions can adequately mitigate these impacts. I I I I I REC's analyses assume that the wall openings do not exist. In reality, the regional solution is to fill the openings or replace the wall with a FEMA-certified floodwall in order to provide adequate flood protection. FEMA certification will be required for FEMA to accept the wall in any future floodplain map revisions. FEMA has several criteria including minimum freeboard, structural rigidity, public agency maintenance, etc. Since the wall has been modeled as a floodwall by REC, the ultimate wall is seen as one component of the regional solution. Once this wall is constructed, flow north of the wall not intermix with flow south of the wall until the flows reach Agua Hedionda Lagoon. As a result, the East Village runoff will not impact the either the Calavara Creek floodplain south of the wall or the Agua Hedionda Creek floodplain. A potential alternative to the floodwall and not intermixing north and south flows could be to install an underground drainage facility within or along Cannon Road. This facility would capture the flow that is intended to be conveyed north of the wall. As a result, the facility would extend from the box culvert outletting Detention Basin BJB to the culverts under Cannon Road just east of El Camino Real. The East Village storm drain systems would connect to this facility. Further engineering analyses would be required to investigate the feasibility of this alternative. I