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HomeMy WebLinkAbout3338; Agua Hedionda & Calavera Creek Dredging; Rancho Carlsbad Mobile Home Park Agua Hedionda; 2003-01-08RICK 3 RANCHO CARLSBAD MOBILE HOME PARK PRELIMINARY ALTERNATIVES ANALYSIS FOR AGUA HEDIONDA CHANNEL MAINTENANCE J-13182 C JANUARY 8, 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.rickengineering.com JANUARY 8, 2002 TABLE OF CONTENTS VOLUME 1; Introduction I 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 Pocket 1: Preliminary Rancho Carlsbad Channel and Basin Project 100-Year Floodplain Alternatives Prepared by: DCB:RC:nd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01 -08-03 TABLE OF CONTENTS VOLUME 2: Appendix A: 100-Year Ultimate Detained HEC-1 Hydrologic Analysis (FN: B JBB JDF.HC 1) Appendix B: 100-Year HEC-1 Hydrologic Analysis with Additional Detention (FN: DBTEST2.HC1) Appendix C: HEC-2 Analysis of Existing Channel Geometry with Existing Hydrology and Proposed Hydrology (FN: RCMAINEX.HC2) Appendix D: HEC-2 Analysis of Alternative 1. Side-slopes (2:1) extending to the Original Design Flowline Elevation (FN: RCMAINPF.HC2) Appendix E: HEC-2 Analysis of Alternative 2. Side-slopes (3:1) extending to the Original Design Flowline Elevation (FN: RC3T.HC2) Appendix F: HEC-2 Analysis of Alternative 3. Side-slopes (2:1) extending to the Existing Ground Elevation at the El Camino Real Bridge (FN: RCPFBF.HC2) Appendix G: HEC-2 Analysis of Alternative 4. Side-slopes (3:1) extending to the Existing Ground Elevation at the El Camino Real Bridge (FN: RC3TBF.HC2) Prepared by: DCB:RC:nd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01-08-03 «- INTRODUCTION *"" The purpose of this report is to summarize the design alternatives for Agua Hedionda Creek that havegg 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 to 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 MMM 300 feet upstream of El Camino Real. •HI 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 Mi 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 (horizontal:vertical) 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 M 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 m discharge and the accumulation of sediment in the channel over time, the capacity of Agua Hedionda m Creek to convey storm water through the RCMHP has diminished, and currently a large number of m properties within the Mobile Home Park are subject to inundation during a 100-year storm event. m . • Prepared by: DCB:RC:nd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01-08-03 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 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. 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:RC:nd/Report/13051-C.003.doc 01-08-03 VICINITY MAP RANCHO CARLSBAD MOBILE HOME PARK CITY OF OCEANSIDE PROJECT SITE CITY OF VISTA NOT TOSCALE PACIFIC OCEAN L( CITY OF 1A SAN MARCOS CITY OF ENCINITAS Prepared by: Rick Engineering Company - Water Resources Division DCB:KH:nd/Report/14167.001 01-08-03 HYDROLOGIC ANALYSES 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 *M 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. mm m 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 m 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 ^M 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' boxm 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 Ml over these road crossings, modification of the box culvert will be required with an 18'xlO'x5' (WxHxL) *" transition structure. However, this transition structure geometry will not provide any freeboard at El Camino Real. *"* In 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 revised 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. 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 maintenance 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 at Faraday would require 200 acre-feet of 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 proposed detention basins. Prepared by: Rick Engineering Company - Water Resources Division DCB-.RC :nd/Report/l 3051 -C.003 .doc 01-08-03 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 The limits of the Coastal Zone are located at El Camino Real, and grading within the limits of the Coastal Zone would require additional permits and coordination. Therefore, grading downstream of El Camino Real is 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, and increase the potential for ponded water just 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 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 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:nd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01 -08-03 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 peak discharge and the accumulation of sediment in 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 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: Rick Engineering Company - Water Resources Division DCB:RC:nd/Report/13051-C.003.doc 0 1 -08-03 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 as ongoing maintenance by 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, maintenance of the Agua Hedionda Channel 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. 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 project to the Coastal Zone. The original Agua Hedionda channel design elevation at the El Camino Real Bridge is 31.6 feet on the 1969 Grading Plans. Due to deposition in the channel, the accumulated 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 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. hi order to prevent ponding water and the risk of 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:RC:nd/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 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 from the Regional Board would alleviate the possibility of differing opinions among regulators and identify a clear path to follow. Alternatively, the existing ground elevation of 35.4 feet at the El Camino Real Bridge could be extended 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 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. Kelly Ranch Weir Structure According to the as-built improvement plans entitled, "Agua Hedionda Channel" revised November 13, 1985, there is an existing desilting basin located just downstream of the El Camino Real Bridge that includes a weir structure located approximately 1300 feet downstream of the bridge. A field survey 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 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 HEC-2 analyses of Agua Hedionda Creek. Prepared by: DCB:RC:nd/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 detained condition hydrology. Proposed Maintenance Design Alternatives The following is a summary of the design alternatives presented for the proposed maintenance of Agua Hedionda Creek and the appendices in which the corresponding HEC-2 hydraulic analyses are located. All of the following alternatives include the proposed ultimate detained flows in Agua Hedionda Creek and Calavera 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) Alternative 2. Trapezoidal Channel with 3:1 side-slopes extending from the existing toe of slope to the original design flowline elevation (Appendix E) 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) 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 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 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 i 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 Alternative 3 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 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 Alternative 5 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 [Alternative 2 102 0.9 111 0.8 Alternative 3 148 1.65 Alternative 4 (Alternative 5 173 2.1 236 2.6 Prepared by: Rick Engineering Company - Water Resources Division DCB:RC:nd/Report/13051-C.003.doc 01-08-03 13 m p P, H pt M The proposed maintenance of the channel will result in the removal of sediment for a length of approximately 2400 feet. The dredge volumes for each alternative are shown in Table 4 and the depth of excavation at each cross-section is shown in Table 5. The dredge material in Agua Hedionda is considered good material for the sand mining industry, and will be valuable after separating and drying. Further study and discussion will be required to determine the type of dredging equipment to be used, as well as the location of access paths and delivery locations. Table 4. Estimated Dredge Volume for Agua Hedionda Creek Grading Alternatives Estimated Dredge Volume, cubic yards Alternative 1 30,000 Alternative 2 24,000 Alternative 3 20,000 Alternative 4 18,000 Alternative 5 0 Table 5. Approximate Depth of Excavation for Agua Hedionda Creek Grading Alternatives 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 Average Depth Approximate Depth of Excavation, feet Alternative 1 4.03 3.6 3.38 3.21 3.91 4.18 4.88 5.16 6.14 6.42 6.12 7.1 6.61 7.49 5.16 Alternative 2 4.03 3.6 3.38 3.21 3.91 4.18 4.88 5.16 6.14 6.42 6.12 7.1 6.61 7.49 5.16 Alternative 3 0.27 0.16 0.02 -0.10 0.77 1.25 2.17 2.91 4.25 4.90 5.00 6.23 5.93 7.20 2.93 Alternative 4 0.27 0.16 0.02 -0.10 0.77 1.25 2.17 2.91 4.25 4.90 5.00 6.23 5.93 7.20 2.93 Alternative 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 w m H Prepared by: Rick Engineering Company - Water Resources Division 14 DCB:RC:nd/Report/13051-C.003.doc 01-08-03 MAINTENANCE PROGRAM Once the initial channel maintenance outlined by this project has been completed, it is vital that ongoing maintenance 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. A system for monitoring the silt level in the creek must be implemented to determine the extent of sedimentation within Agua Hedionda Creek. This can be done using metal posts with markings placed approximately six inches apart to determine the sediment depth in the channel at that location. The posts should be placed vertically in the channel at intervals not exceeding 500 feet. The posts should extend at least two feet above the creek bed and must be embedded deep enough to withstand the forces of large creek flows. A geotechnical engineer should be consulted to determine the required embedment depth. Once the posts are installed, maintenance personnel can easily monitor the silt level in the channel. As the silt level reaches two feet the silt should be removed by maintenance crews and the channel returned to the design elevation. 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 Hedinda Creek must be incorporated into the environmental permitting currently being processed through the San Diego Regional Water Quality Control Board, the U.S. Army Corps of Engineers and The California Department of Fish and Game. The maintenance area will extend from the El Camino Real Bridge upstream for approximately 2,400 feet. It is anticipated that maintenance will need to be performed every two to three years. It should be noted, however, that the sediment deposition in the channel is directly related to the storm 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 maintenance frequency requirements. The ongoing maintenance responsibility and funding for this portion of Agua Hedionda Creek is currently being negotiated between the City of Carlsbad and the Rancho Carlsbad Owners Association. Prepared by: DCB:RC:nd/Reporf 13051-C.003.doc Rick Engineering Company - Water Resources Division 01 -08-03 15 •* CONCLUSION m m Agua Hedionda Creek is highly subject to sedimentation for the majority of the creek. Portions of the ^M 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 M 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 m the Mobile Home Park is 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 m 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 Ma 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 them following alternatives include the proposed ultimate detained hydrologic conditions. M» Alternative 1. Trapezoidal channel with 2:1 side-slopes extending from the field surveyed limits of 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 original design flowline elevation 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 Alternative 4. Trapezoidal channel with 3:1 side-slopes extending from the existing toe of slope to the m existing ground elevation at the El Camino Real Bridge Alternative 5. No Grading in Agua Hedionda Creek m 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 m 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. •w j^^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 yu need to be resolved: m 1. The soil stability within the channel and the need for additional geotechnical investigationsmincluding borings within Agua Hedionda Creek. ** 2. The conveyance of low flows at the El Camino Real Bridge. 3. The environmental permitting through the San Diego Regional Water Quality Control Board, *" the U.S. Army Corps of Engineers and The California Department of Fish and Game. Prepared by: DCB:RC:nd/Report/13051-C.003.doc Rick Engineering Company - Water Resources Division 01 -08-03 17 Aerial Photographs SH 11-3-78 ^r* I* ' 2-28-79 8-31-84 r • ' > ' > ^^ ,. \. ^ . 5-15-92 4-21-98