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Home My WebLink AboutSDP 09-01; ROBERTSON RANCH PA 22; HYDROLOGIC AND HYDRAULIC ANALYSES; 2006-02-20HYDROLOGIC AND HYDRAULIC ANALYSES FOR ROBERTSON'S RANCH February 20, 2006 Wayne W. Chang, MS, PE ClllllffiOJ[ffi[I]~[trn CMI Engineering • Hydrology • Hydraulics • Sedimentation P.O. Box 9496 Rancho Santa Fe, CA 92067 (858) 692-0760 -TABLE OF CONTENTS - Introduction ................................................................................................................................... 1 Detention Basin BJB Ponding ...................................................................................................... 2 Calavera Creek Floodplain ........................................................................................................... 3 Park Site Ponding ......................................................................................................................... 5 Conclusion .................................................................................................................................... 6 APPENDIX A. 100-Year HEC-1 Analyses and Work Map B. 100-Year HEC-RAS Analyses C. 100-Year WSPGW Analysis and HEC-1 Assuming 8'x8' RCB Open (i \ MAP POCKET I 00-Year Floodplain and Ponding Exhibit 1 /r 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 follows an east-west alignment near the southerly boundary of the East Village. The area north of Cannon Road will be developed with single-and multi-family residential units. The area south of Cannon Road (Planning Area 22) will contain an approximately 4.4 acre pad developed with residential units. Finally, a park site and 5:1 slope grading (for a wildlife corridor) will be constructed within a portion of Robertson's Ranch West Village, which is just west of the East Village, north of Cannon Road, and east of El Camino Real. QTY <F M--~--,\""+~------ NOT TO SCALE PACIFIC OCEAN c SB CITY OF ENCINITAS Figure 1. Vicinity Map 1 The following paragraphs outline the pre-and post-development storm runoff patterns associated with Robertson's Ranch. Under both conditions, the upper Calavera Creek watershed is tributary to Detention Basin BJB, which is immediately east of the East Village. However, the pre-and post-developed conditions have different methods of conveying flow downstream of Detention Basin BJB. Under pre-developed conditions, storm runoff from the East Village footprint and tributary areas flows in a southerly direction towards the lower reach of Calavera Creek, which is located along the boundary of the East Village and RCMHP. A free-standing masonry wall exists along this boundary. A weir has been constructed within the easterly portion of the wall to regulate Calavera Creek flow along the north and south sides of the wall. 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, and then enters Agua Hedionda Lagoon. The base of the wall contains several semi-circular openings along much of its length, which allows some flow migration between either side of the wall. Under developed conditions, the East Village will construct the 84-inch reinforced concrete pipe (RCP) in Cannon Road as well as the Planninw Area 22 grading. The 84-inch RCP is intended to (,-. convey the majority of the East Village runoff as well as flow directed north of the masonry wall ( \. by the weir. The 84-inch RCP will outlet on the north site of Cannon Road just east of El Camino Real. Flow outletting the pipe will not re-enter Calavera Creek. This report contains hydrologic and hydraulic analyses of three areas associated with Robertson's Ranch. First, the 100-year ponding within Detention Basin BJB is analyzed. A recent hydrologic analysis prepared for the city of Carlsbad determined ponding based on upstream improvements that are planned, but not yet constructed. This report revises the analysis to determine the ponding elevation based on current, rather than future, improvements. Next, Calavera Creek below Detention Basin BJB is analyzed to determine the benefits from Robertson's Ranch. Since the proposed 84-inch RCP will reduce flow entering Calavera Creek, the East Village will lower water surface elevations and reduce potential flood inundation in the RCMHP. Finally, the impact of the West Village's park site and 5:1 slope grading in the area north of the El Camino Real and Cannon Road intersection is assessed. The park site and slope grading will place fill in this area, which will reduce its storage capacity. DETENTION BASIN BJB PONDING Rick Engineering Company's (REC) December 13, 2004 report, Rancho Carlsbad Mobile Home Park, Alternative Analysis for Aqua Hedionda Channel Maintenance, contains their latest hydrologic and hydraulic analyses of Agua Hedionda and Calavera Creek. The analyses are ,-based on full implementation of all the improvements associated with the city of Carlsbad's ·" ultimate regional flood control solution for RCMHP, i.e., four detention basins (BJ, BJB, Faraday, Melrose), the aforementioned weir, increased storage capacity in Lake Calavera (with associated modifications to Basin BJB), and channel improvements within Agua Hedionda Creek. As of the date of this Robertson's Ranch report, Detention Basin BJB (without /C 1 modifications), the Faraday Basin, the Melrose Basin and the weir are components of the regional solution that either have been or are nearing completion. The Lake Calavera improvements and Detention Basin BJB modifications have not yet been constructed. In order to more accurately determine the existing ponding in Detention Basin BJB, these two improvements were revised in REC's HEC-1 hydrologic analysis to reflect existing conditions. The existing conditions within Lake Calavera and Detention Basin BJB were obtained from previous studies by REC. The revised HEC-1 analysis is included in Appendix A and shows that the 100-year ponded water surface elevation in Detention Basin BJB will be 77.1 feet. The ponding limits have been delineated on the exhibit in the map pocket. The ponding elevation is slightly higher (less than one foot) than the adjacent ground at the southerly end of the basin. As a result, shallow sheet flow can occur over this area during a 100-year event. In comparison, the ponded water surface elevation based on REC's ultimate condition analysis is 75.2 feet. Once the ultimate improvements are implemented by the city, Detention Basin BJB will provide at least one foot of freeboard over the 100-year ponding limits. CALAVERA CREEK FLOODPLAIN In order to assess the East Village's impact on Calavera Creek, hydrologic analyses were performed to determine pre-and post-development 100-year flow rates. The analyses were based ,,-on the latest HEC-1 analysis in REC's December 2004 report. To model pre-East Village ·l, 1 \. conditions, REC's HEC-1 analysis was modified to reflect completion of only the aforementioned four regional solution components that have been or are nearing completion. This modified HEC-1 analysis and work map is included in Appendix A (see the first HEC-1 analysis in Appendix A), and indicates that the combined 100-year flow rate in Calavera Creek near Agua Hedionda Creek is approximately 1,552 cubic feet per second (571 cfs north of the wall and 981 cfs south of the wall). The 84-inch RCP constructed by the East Village is intended to accomplish essentially the same 100-year flow split as the weir wall under ultimate conditions, i.e., approximately 500 cfs. However, there will be some differences during lower flows. The REC report indicates that 300 cfs will flow south of the wall before the weir begins to split flow north of the wall. On the other hand, the 84-inch RCP will begin to split fl.ow when it reaches approximately 75 cfs. This will provide an overall benefit to RCMHP because more of the lower flows will be directed to the 84- inch RCP rather than Calavera Creek. A HEC-1 analysis based on this post-development condition is included in Appendix A (see the second HEC-1 analysis in Appendix A), and indicates that the I 00-year fl.ow rate in Calavera Creek near Agua Hedionda Creek is approximately 905 cfs. Next, pre-and post-development hydraulic analyses using the revised flow rates were performed to determine the 100-year water surface elevations in Calavera Creek prior to and after construction of the 84-inch RCP and Planning Area 22. The analyses were based on existing Agua H~dionda and Calavera Creek cross-sectional channel geometries from REC's December 1 '(" 2004 report. Future channel improvements proposed by the regional solution were not modeled ' since these will be constructed at a later date. REC's report contains an existing condition HBC- RAS analysis based on field surveys of Calavera Creek in December 200 I, field surveys of Agua Hedionda Creek in May 2002, and updated field surveys downstream of Cannon Road in June 3 2004. The analyses herein use the same basic information (roughness coefficients, cross-section locations/numbering, reach lengths, etc.) as the REC model, and REC's Agua Hedionda Creek cross-sections were not altered. On the other hand, the Calavera Creek cross-sections were modified as discussed next. REC's analysis of Calavera Creek assumed that the mobile home park wall prevents Calavera Creek flow from passing north of the wall. However, pursuant to discussions with the city of Carlsbad, the analyses in this report assume no wall along Calavera Creek because the existing wall has openings and is not FEMA-certified. The area north of the wall was modeled using May 12, 2005 topographic mapping provided by O'Day Consultants. This report's analyses also assume that the Calavera Creek flow is encroached by the homes along the south creek bank since these homes define the southerly effective flow area. A baseline HEC-RAS hydraulic analysis was performed first. This analysis uses the HEC-1 100- year flow rates with the weir and is representative of the pre-Robertson's Ranch condition. Under this baseline, the total Calavera Creek 100-year flow rate below the weir (1,552 cfs) is distributed both north and south of the wall, i.e., there is no differential water surface north and south of the wall in Calavera Creek. The HEC-RAS flow rates assume that flow will be conveyed by the Cannon Road culverts as modeled in the HEC-1 analysis. The HEC-RAS results are included in Appendix B and summarized in Table 1. Appendix B contains tabular output for r both Agua Hedionda and Calavera Creeks and cross-section plots for Calavera Creek. The HEC- ( \\, RAS cross-sections are shown on the exhibit in the map pocket. Next, a HEC-RAS analysis was performed based on O'Day Consultants' Planning Area 22 grading and the reduced Calavera Creek 100-year flow rate (905 cfs) resulting from construction of the 84-inch RCP. This analysis assumes that the flow will not be conveyed to the Cannon Road culverts, and represents the post-development condition. The HEC-RAS results are included in Appendix B and summarized in Table 1. A comparison of the HEC-RAS results indicates that the East Village will reduce water surface elevations throughout much of Calavera Creek. Therefore, flood inundation in RCMHP will also be reduced. It should be noted that the water surface elevations in the lower reach of Calavera Creek are affected by backwater near the confluence with Agua Hedionda Creek. The HEC-RAS analyses indicate that the 100-year flow will overtop El Camino Real under the current channel geometry. As a result, the floodplain extends over a portion of El Camino Real and Cannon Road. The pre-arid post-Robertson's Ranch floodplains are delineated on the exhibit in the map pocket. This exhibit includes a portion of the floodplain exhibit from REC's December 2004 report. REC's exhibit delineates several floodplains based on various scenarios they modeled. The solid green line from REC's exhibit corresponds to the analyses in this report. Consequently, the floodplains developed in this report tie-into the green line on the exhibit. HEC-RAS Elevation without Elevation with East Village, Difference, Cross-section East Villaf!e, ft. 84"RCP & P A-22 Grading, ft. ft. 50.11 49.6 49.5 -0.1 310 49.6 49.5 -0.1 400 49.6 49.5 -0.1 580 49.6 49.5 -0.1 750 49.6 49.6 0 1000 49.6 49.6 0 1230 49.6 49.6 0 1470 49.6 49.6 0 1810 50.8 50.0 -0.8 2100 53.3 51.5 -1.8 2420 56.2 54.5 -1.7 2700 57.2 55.3 -1.9 2980 58.0 56.1 -1.9 3170 59.8 57.3 -2.5 Table 1. Comparison of 100-Year Water Surface Elevations PARK SITE PONDING REC's HEC-1 analysis did not model the storage that can occur in the area north of the intersection of Cannon Road and El Camino Real. This area currently supports a nursery. The West Village development proposes to fill a portion of the area to create a park site and a 5: 1 slope area for a wildlife corridor. In order to assess the park's impact on 100-year ponding in this area, the area's storage-outflow characteristics were added to the HEC-1 analyses. The existing storage volwne was included in the HEC-1 analysis modeling the weir wall (see first HEC-1 analysis in Appendix A), while the reduced storage volume resulting from the park site and 5:1 slope area was included in the HEC-1 analysis modeling the 84-inch RCP (see the second HEC-1 analysis in Appendix A). Furthermore, a third analysis (see the third HEC-1 analysis in Appendix A) was performed assuming completion of the East Village development (84-inch RCP and Planning Area 22), but no grading for the park site or 5: 1 slope area. The existing storage volwne was determined from May 12, 2005, 2-foot contour interval topographic mapping provided by O'Day Consultants, while the proposed storage volwne was determined from the topographic mapping and O'Day Consultants' park and 5: 1 slope grading plans. Flow leaves the area through a culvert under El Camino Real. REC's report indicates that the culvert is an 8-foot by 8-foot reinforced concrete box (RCB). However, a February 16, 2006 field measurement revealed that sediment had deposited in the culvert reducing the height of the opening to 5 feet. The RCB was modeled in the three analyses based on the reduced height. The HEC-1 results indicate that Robertson's Ranch with the East and West Village development will cause a 0.1 foot increase in the ponded 100-year water surface elevation ( 41.3 feet pre- development versus 41.4 feet post-development). Both ponding limits are included on the exhibit in the map pocket. The ponding extends slightly into El Camino Real; however, this area 5 /C- ( experiences greater inundation from the Agua Hedionda Creek floodplain. The results also indicate that Robertson's Ranch with only the East, and not West, Village developed will reduce the ponding to an elevation of 41.1 feet. A WSPGW analysis was performed to determine the 100-year flow velocity in the silted El Camino Real culvert (see Appendix C). The analysis indicates that the velocity can be over 13 feet per second. At this rate, it is possible that accumulated silt will wash out of the RCB. An additional HEC-1 analysis was performed to determine the ponded water surface elevation assuming that Robertson's Ranch was developed and the RCB contained no silt. The analysis is included in Appendix C and indicates that the ponding elevation drops to 35.6 feet, which is several feet lower than the low point in El Camino Real (41.7 feet). CONCLUSION Hydrologic and hydraulic analyses have been performed to determine the ponding in Detention Basin BJB, the 100-year water surface elevations in Calavera Creek, and the ponding in the area north of the El Camino Real and Cannon Road intersection. The analyses assumed completion of only those city of Carlsbad regional solution components that have been, or are currently being, constructed. The remaining components were not modeled since they have yet to be ( r-implemented. The results are summarized in Table 2. ('\ ', 100-Year 100-Year Water 100-Year Ponding NE Ponding in Surface Elevation in of El Camino Real Basin BJB. ft. Calavera Creek. ft. and Cannon Rd .. ft Exist Conditions 77.1 49.6-59.8 41.3 With East Village (84" RCP, PA-22, and 3' of 77.1 49.5-57.3 41.1 Silt in RCB) With East Village, Park Site, and 5:1 Slope Area 77.1 49.5-57.3 41.4 (3' of Silt in RCB) With East Village, Park Site, and 5:1 Slope Area 77.1 49.5-57.3 35.6 (No Silt in RCB) Table 2. Summary of Results The results show that Robertson's Ranch East Village will reduce flood inundation in some areas. The following findings have been made for this development: • The proposed development pads including Planning Area 22 will be above the 100-year floodplain. • Portions of six existing mobile homes in RCMHP will be removed from the 100-year floodplain based on the analyses in this report. The floodplain areas that will be eliminated by the East Village have been shaded on the exhibit in the map pocket • The project will incrementally decrease ponding in the area north of El Camino Real and Cannon Road. • The results also indicate that the park and 5: 1 slope area, to be constructed by the West Village, could slightly increase 100-year ponding onto El Camino Real with the 8'x8' RCB silted to it's current level. On the other hand, it is possible that some silt will be removed as the flow approaches the 100-year magnitude and the velocity increases. In this case, the culvert's capacity would increase, which would prevent ponding onto El Camino Real. Maintenance to remove silt in the culvert would also eliminate 100-year ponding onto the road. ,., APPENDIX A 100-YEAR HEC-1 ANALYSES AND WORK MAP _i. -- ~ / ~ ·-; '"- - ·- - · ... --\ : ~ ._, =~ ... . - ~~ ' 1 .. . •· I · ' ) ' t ,. , ..... 't i ~.- .- · .. --- . ,. _ . .:. · :,, I ...... . . _I_ r-- ·t: _ _. ~ -~ : • '"' ;·· ·:t ' ' ., ---~ ,; 1/) -0 Q) .. r : : . ~ Q) -~ -.; 1/ ) C "i i i C'C I CD Q) 0) CC I C -~ Q 1/ ) C ·u ; C'C I ca