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HomeMy WebLinkAbout3338; Agua Hedionda & Calavera Creek Dredging; Detention Basin BJ-Agua Hedionda Channel; 2005-10-04-TABLE OF CONTENTS . Introduction ................................................................................................................................... 1 Engineering Analyses ................................................................................................................... 2 Conclusion ................................................................................................................................... 3 APPENDIX A . HEC-1 Analyses B . 100-Year HEC-RAS Analyses 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 sediinentatioii basins identified in the city of Carlsbad's March 1994, Master Drainage and Storm Water Qtrali~ Managetnent 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, tlie 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, hrther analysis was called for. In order to assess tlie relative benefit froin Detention Basin BJ, this report contains analyses to determine the 100- year floodplain impact on RCMHP from reinoval 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 Agt~a 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 RCIOO) 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 floodplaiii 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 hture 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.