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HomeMy WebLinkAbout3338; Agua Hedionda & Calavera Creek Dredging; Robertson's Ranch East Village 84" Concrete Pipe; 2005-04-27n HYDROLOGIC AND HYDRAULIC ANALYSES 1. FOR ROBERTSON'S RANCH EAST VILLAGE 1 84" REINFORCED CONCRETE PIPE ALTERNATIVE April 27,2005 Wayne W. Chang, MS, PE Chang@@m8~1k~~B 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 ................................................................................................................................... 4 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 11, 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. 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 Par-k 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 1 1-foot by 7-foot reinforced concrete box culvert under the Cannon Road and College Boulevard intersection, 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 BIB 11-foot by 7-foot reinforced concrete box (RCB) 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 RCB 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 any flow 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 (based on base flow observations during past site visits), 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 RCB 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 40 1 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 by 0.1 feet 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). 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 10.0 1 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 50.1 1, the water surface elevations are equal. At Cross-sections 3 10 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 threshold at which flow from Detention Basin BJB begins to be directed north of the wall will be reduced from 300 cfs to approximately 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. Pressure flow is typically avoided were feasible because it causes increased flow velocities leading to greater pipe wear, results in a lower factor of safety in the event of a pipe obstruction, and is more expensive to construct. A pipe larger than 84-inch is not desirable because it will be more difficult to transition to the 1 1-foot by 7-foot RCB, will require additional cost (a cost estimate has not been performed, but it is intuitive that a pipe that is much larger than the facility it is connecting to will result in higher construction costs), 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. Upon approval of the revisions, final design will be performed for the 84-inch RCP and flow split facility. In addition to addressing the objectives in this report, the final design will need to ensure that Detention Basin BJB is not adversely impacted.