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SDP 05-826 - Vista DunesHydrology Hydraulics R Vista Dunes Condominiums (Northwest Quadrant of Miles Ave & Adams St) City of La Quinta, State of California MDS 66902 Apri12oo6 Prepared For: City of La Quinta Redevelopment Agency Prepared By: MDS Consulting John W Cavin PE, C 16802 78-900 Avenue 47, Suite 208, La Quinta, CA 92253 (760) 771-4013 Resubmittal: A pfil 2oo6 Hydrology Hydraulics Report Vista Dunes Condominiums M O R S E 78-900 Avenue 47 D Suite 208 Lo Quinto, CA 92253 H S D 0 K I C H Voice: 760-771-4013 FAX: 760-771-4073 S C H U L T Z mdsloquinla®rndsconsulling.net P L A N N E R S E N G I N E E R S S U R V E Y O R S C. ,f Stanley C. Morse t No. 20596 R. C. E. 20596 ' * Exp. 9-30-07 Expires 9/30/07 o v Submittal: AI)ril20o6 4/26/2006 Hydrology & Hydraulics Report Vista Dunes Condominiums MDS 66902 Table of Contents Title Sheet Table of Contents Tab Project Maps & Photographs Tab 1 Project Narrative Tab 2 Summary of Results Tab 3 1. Design Criteria 2. Design Storm Point Precipitation Table (COLA) 3. Tributary Area List 4. Retention Basins Control Elevations 5. Retention Basins Available & Required Storage 6. Storm Storage Summary 7. Street Flow Capacities 8. Curb Inlet / Catch Basin List 9. Nuisance Water Disposal Systems Appendices: Appendix A: Unit Hydrograph Retention Basin Hydrology Tab 4 (100yr —1, 3, 6, & 24hr Design Storms) A-1 Basin 1A: (Onsite Retention Only) Tab 4 Unit Hydrographs Flood Hydrographs A-2 Basin 1: (Existing Offsite Basin Only) Tab 5 Unit Hydrographs Flood Hydrographs A-3 Basin 2: (Existing Offsite Basin With Onsite Drainage) Tab 6 Unit Hydrographs Flood Hydrographs 1 of 2 Table of Contents (continued) Appendix B: Rational Method Storm Drain Hydrology Tab 7 (100yr Design Storm — COLQ Intensities) B-1 Street Capacities Tab 7 B-2 Storm Drain & Curb Inlet Hydrology Tab 8 B-3 Hydraulic Grade Line Calculations Tab 9 Appendix C: Nuisance Water Disposal System Tab 10 Sketches Design Table Appendix D: Design Reference Documents Source: Tab 11 1. Percolation test report Buena Engineers 2. Soil Survey Map & Soil Group Table 12 USDA -SCS 3. Time of Concentration Nomograph, Initial Subarea (Plate D-3 - RCFCD Manua) 4. Rainfall Patterns in % of 3, 6, & 24 hr Storm Volume (Plate E-5.9) 5. Rational Rainfall Intensity Table 10yr & 100 yr Storms (Plate D-4.1) 6. Rational Runoff Index Numbers (Plates D-5.5 &5.6) 7. Runoff Coefficient Curve (Soil Group A, AMC II) (Plate D-5.2, 5.7) 8. Onsite St - - Velocity/Discharge Nomograph (Plate D-7.7) 9. Miles Ave - - Velocity/Discharge Nomograph (Plate D-7.8) 10. Curb Inlet Capacity Street Low Pt (Sag) (BPR #1073.03) 11. Curb Inlet Capacity Continuous Street Slope (LACoRdDept 12. Reinforced Concrete Pipe D -Load Rating Table (LACFCD) 13. Miscellaneous Verification Correspondence Appendix E: Map Exhibit Tab 12 Hydrology Map & Storm Drain Exhibit Existing Topography & Drainage Proposed Street Layout & Elevations Tributary / Drainage Areas Storm Drainage / Retention Facilities 2 Project Maps Photographs Vista Dunes Condominiums .7.M r 0 imp e. 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T � .}.y..!"i �, C�l�.i�.w�i:� `.� .-,•.a�Ac a ♦ i:` � ��M!.L.,�"�'��. } , a.. _ . a _w • L yip m MPn Tw�� 4 -a� ism Project Narrative Vista Dunes Condominiums Vista Dunes MDS 66902 Hydrology and Hydraulic Design Report for Vista Dunes Condominiums (North Side of Miles, West of Adams) In the City Of La Quinta Purpose 4/26/2006 This report identifies the drainage design criteria used, and describes the design of the project drainage system. The analysis identifies the 100 -year storm runoff rates and volumes generated within the boundary of Vista Dunes, to be used in the design of streets, inlets, drainage pipes, and retention basins. Existing Site Description The site measures approximately 1320 by 330 feet, consisting of approximately 10.1 gross acres, located adjacent to the north side of Miles Ave, 330 feet west of Adams St, within the City of La Quinta, County of Riverside, California. The site occupies the highest ground in the vicinity. Existing Drainage Patterns The north 1/3 of Vista Dunes drains north to a concrete flume down the side slope into retention basin 2. Basin 2 is 20 feet lower than the site. The existing secondary overflow for the north 1/3 of the site and existing tract 23269 occurs at the intersection of Fred Waring Ave and Adams St north of the site at approximately elevation 86. The southerly 2/3 of the site drains to Miles Ave. Approximately 100 feet of Adams street drains south to Miles Ave, and thence west to the low point at the center of the Vista Dunes site. The existing secondary overflow for the south 2/3 of the site, existing tract 23268, and surrounding area occurs at the southwest corner of the intersection of Miles Ave and Adams St approximately 330 feet east of the site at approximately elevation 92.5. Overflow then drains south along Adams to the White Water Wash. These overflows are not altered by the Vista Dunes development. The low point of the Miles Ave gutter at the entrance to Vista Dunes is elevation 86.05. The maximum 100 -year, 24 hour design storm water surface elevation in the existing Retention Basin 1 is 89.05. All existing adjacent properties are lower than Vista Dunes. An existing concrete block privacy wall several feet inside the west property line of the site prevents drainage from the site onto neighboring lots. Proposed Project Description Vista Dunes is a project by the City of La Quinta Redevelopment Agency to redevelop a former mobile home park into an 80 unit condominium complex. It is bounded on the west by a large City owned, turfed retention basin / park (designated Basin 1 for this report), and Tract 23268, a single family residential development that drains into Basin 1.; It is bounded on the north by several residential lots and another large, turfed, retention basin / park Vista Dunes 4/26/2006 MDS 66902 (designated Basin 2 for this report). It is bounded on the east by a Fire Station and a church parking lot, which drain to the east to Adams St, and thence north along Adams to inlets near Palma Dr. tthat drain to Basin 2. Site development includes full improvement of the Miles Ave frontage and all onsite areas, and retention of drainage from all these areas in the existing large Basin 2 and in a smaller onsite Basin 1A at the southwest corner of the site. Proposed Drainage Patterns Vista Dunes is designed to collect and convey the 100 -year design storm runoff through the onsite street and drainage piping into the existing retention basin 2 and proposed Basin 1A located in open space areas on the north and southwest sides of the project. This runoff includes drainage from the north half of the Miles Ave frontage. A retention basin is proposed at the southwest corner of the site. Miles Ave will be improved with realigned curb, left turn lane and bus turnout, parkway landscaping, and a stepped retaining / privacy wall. Miles Ave drainage will be collected by an inlet at the entrance into Vista Dunes (CB# 3) and piped to the onsite Retention Basin, (Designated Basin 1A) This will improve the drainage of Miles Ave at this low point. The secondary overflow will continue to be east to the intersection with Adams and thence south along Adams or east along Miles to the Whitewater Wash. All storm drainage from the site and adjacent street is conveyed by surface flow in the rear yard swales and streets to grated area drains and curb inlets. It is then conveyed by storm drain pipes to grated distribution boxes in the bottoms of the retention basins. Large storm flows rise through the top grate of these boxes, spread over the bottom of the retention basins, and percolate into the bottom and sides of each basin. Small Nuisance water flows are disposed of in the Nuisance Water Disposal System (NWDS). (See discussion to follow). The existing flume at the north end of the site is replaced with area drains, a curb inlet, and piping to a new NWSD in the bottom of Basin 2. The existing wall along the west property line will remain. Masonry privacy walls will be constructed along the remaining three sides. These walls will insure that all drainage will be contained within the site. Proposed Drainage System Design Methods All basins are designed for the maximum water surface at overflow to be 1 foot or more below the pad elevation of the lowest buildings within the site. The Riverside County Synthetic Unit Hydrograph Method is used to determine the storm runoff volumes. A Flood Routing program is used to determine required retention basin storage after deduction of percolation from the basin. The on-site soil percolation rate was tested in 1988 for the existing basins 1 and 2 at the time of development of Tracts 23268 & 23269. These tests indicate a percolation rate of 2.64 to 4.65 inches/hr. (See enclosed Appendix D — Tab 10 - KWL Associates hydrology calculations). N Vista Dunes MDS 66902 4/26/2006 A design percolation rate of 2.0 in/hr was used in all design computations per COLA standards. The maximum depth of water storage required in the proposed Retention Basin 1 A is 3.1 feet for the 100 year design storm. (3 hr / 100 yr ). The retention basin drainage areas for the existing offsite basins 1 and 2 and the proposed onsite basin 1A are evaluated with Unit Hydrograph and Flood Routing Hydrographs for the runoff from 100 - yr storms of 1, 3, 6, and 24-hour duration. The storm yielding the largest required storage volume is designated the design storm for each Basin (See Summary 6, 6A, and 6B, Tab 3 and Appendix A, Tabs 4, 5, and 6). The capacity of each existing Basin is calculated and confirmed. A CivilCadd / CivilDesign computer program using the Rational Method hydrology procedure outlined in the Riverside County Flood Control District Hydrology Manual is used in calculating the required capacity of each street, inlet, pipe, and drainage structure (See Rational Method Storm Drain Hydrology in Appendix B, Tab 6). A Water Surface Profile Design program is used to calculate the Hydraulic Grade Line for each pipe system. (See Appendix B-3, Tab 9) Nuisance Water Disposal Nuisance water is defined as silt and contaminant -carrying runoff from landscape irrigation and the debris and contaminants flushed from pavement surfaces during the first minutes of storm runoff. All storm drainage from the site and adjacent streets is collected in the streets and storm drain piping, and flows to junction boxes in the bottoms of the retention basins. Large storm flows will rise through the top grate of these boxes, spread over the bottom of the retention basins, and percolate into the bottom and sides of each basin. Nuisance water flows smaller than the capacity of the NWDS will follow the same path, but will flow from the distribution boxes through small diameter pipes to the NWDS, to be clarified and percolated into the ground. The NWDS is designed to collect nuisance water from hard surface and landscaped areas, remove debris and floating contaminants using Maxwell System pre -clarifiers upstream of each sand filter, remove silt and other non -floating contaminants using sand filters, and dispose of the clarified water in underground percolation chambers beneath the bottom of the basins. (See attached Summary 9, Tab 3)