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1ST REVIEW CORRECTIONS - PRELIMINARY HYDROLOGY REPORT TTM2019-0005The Wave at Coral Mountain Preliminary Hydrology Report PROJECT DESCRIPTION & LOCATION The project is proposed to consist of residential structures, a multi -story hotel complex, a wave lagoon and various outdoor amenities. The project site comprises approximately 377 acres with an additional 75 acres of offsite tributary area. The subject property is located on the southwest corner of Avenue 58 and Madison Street in the City of La Quinta and is situated in a portion of Sections 27 & 28, Township 6 South, Range 7 East, SBM. Existing dikes 2 & 4 along with the Coral Mountains border the property to the west and south. A vicinity map obtained from the Riverside TLMA website is included as Appendix A. EXISTING SITE CONDITIONS Flood Rate Map The proposed area is covered by FIRM Panel Number 06065C2900G, effective date of August 28, 2008, which indicates the subject property lies within Zone X (Shaded), defined as "Areas of 0.2% annual chance flood; areas of 1% annual chance flood with average depths of less than 1 foot or with drainage areas less than 1 square mile, and areas protected by levees from 1% annual chance flood. Insurance purchase is not required in these areas." Refer to the attached NFIP Flood Insurance Rate Map in Appendix B. Existing Topography The property is generally vacant and undeveloped with CVWD irrigation mains, numerous dirt road and hiking trails. Various desert vegetation is found throughout the site. The site is bounded to the east by Madison Street, to the north by 58th Avenue, existing levees and the Coral Mountains to the west and south, and 60th Avenue to the south. No groundwater was encountered in any of the exploratory borings to a depth of 51 feet. Site drainage appears to be cc,ntrolled via sheet flow and surface infiltration generally from west to east. Sladden Soils Report mentions groundwater Was encountered at about 47ft for BH2 and BH6 PROPOSED FLOOD CONTROL REQUIRni icIN 1 S The purpose of this report is to provide preliminary basin analyses for the subject property. The drainage requirements for this project fall under the jurisdiction of the City of La Quinta. Storm flows are conveyed through the site via street flow to localized low points. Existing improvements along Madison Street have in essence created a situation where there is no emergency out -flow for storm flows exceeding the 100-year storm event. Conversations are on -going with the owners of Andalusia (to the east of the subject property) to allow for an emergency overflow route to be constructed through the development. That being said, the existing retention basins at the low points on Madison Street will need to be preserved. emergency out flow will have to be addressed prior to entitlement HYDROLOGY ANALYSIS DESIGN CRITERIA Storm runoff volumes for the 100-year event were obtained utilizing the Synthetic Unit Hydrograph as described in the RCFC&WCD Hydrology Manual. The hydrologic data used for the calculations are as follows: Hydrologic Soil Group The existing soil is categorized primarily as hydrologic soil groups "A" and "B" for the majority of the subject property with soil group "D" being assigned to the Coral Mountain rock out -cropping as shown per the attached National Cooperative Soil Survey exhibits in Appendix C. For the purposes of this report, Soil Group 'B' was assigned to the analyses with the exception of the rock out -cropping. Soil Group 'B' is defined by RCFCD as — "soils having moderate infiltration rates when thoroughly wetted and consisting chiefly of moderately deep to deep, moderately well to well drained soils with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission". The Wave at Coral Mountain Preliminary Hydrology Report Soil Group `D' is defined by RCFCD as — "soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink -swell potential, soils that have a high- water table, soils that have a claypan or clay layer at or near the surface and soils that are shallow over nearly impervious material". Antecedent Moisture Condition AMC II — Moderate runoff potential, an intermediate condition. Per RCFC & WCD Hydrology Manual (Dated: April 1978): "For the purposes of design hydrology using District methods, AMC II should normally be assumed for both the 10 year and 100 year frequency storm". Land Use Classifications and Runoff Index Numbers Runoff Index Numbers were obtained from RCFCD Plates E6.1, E6.2 and E6.3 are summarized below: Commercial or Residential Landscaping (Soil Group B) 56 Rock Out -cropping (Soil Group D) 93 Existing Open Brush — Poor 76 Percent of Impervious Cover (RCFCD Plate E-6.3): Rock Out -cropping 5% Existing Open Brush — Poor Condition 5% Impervious Areas (Buildings, Pavement, Hardscape) 100% Water Features 100% Commercial Areas 90% SFR — Low Density 30% SFR — Medium Density 40% SFR — High Density 50% Open Space (Landscaping & Retention Basins) 10% Low Loss Rates: 85% Precipitation Frequency Estimates Precipitation depths were obtained from NOAA Atlas 14: 2 Year - 1 Hour Precipitation: 0.358 inches 10 Year — 1 Hour Precipitation: 0.711 inches 10 Year —3 Hour Precipitation: 1.06 inches 10 Year — 6 Hour Precipitation: 1.36 inches 10 Year — 24 Hour Precipitation: 2.23 inches 100 Year —1 Hour Precipitation: 1.44 inches 100Year —3 Hour Precipitation: 2.14 inches 100 Year - 6 Hour Precipitation 2.76 inches 100 Year —24 Hour Precipitation 4.41 inches Slope of Intensity Duration Curve: 0.52 See Appendix D for the NOAA Atlas 14 Point Precipitation Frequency Estimates and respective RCFCD Plates. Site Infiltration No percolation tests have been performed at the current time, therefore, for the purposes of this report a design percolation rate of 1 in/hr was used in the basin sizing calculations. need to submit percolation tests to use percolation rate of 1 in/hr 2 The Wave at Coral Mountain Preliminary Hydrology Report Proposed Land Use Summaries Land uses for each of the drainage areas were analyzed based on the preliminary site plan. In addition, a reconciliation was performed to verify the hydrologic boundary corresponds to the existing property boundary accounting for any differences. Land use worksheets are included in Appendix E. SUMMARY of SYNTHETIC UNIT (SHORTCUT METHOD) ANALYSES The proposed retention basins for the associated drainage areas were sized to retain the 100-year controlling storm event flood volumes and were analyzed utilizing the RCFCD Synthetic Unit (Shortcut Method). Per the manual, for areas of less than 100 — 200 acres, and lag times less than 7 — 8 minutes, the shortcut method is applicable. This method assumes that in a small watershed, response time to effective rainfall is very short. Therefore, runoff rates for a given period of time can be assumed to be directly proportional to effective rain. This method yields only approximate results (on the conservative side) and should be used only for watersheds meeting the criteria noted above. It should be noted that the peak flow for the 1-hour storm is not necessarily representative for peak flow. Per RCFCD, peak discharges from the 3-hour storm should normally compare well with rational peaks. The synthetic unit worksheets are included in Appendix F and are summarized on sheets 1 and 2 of the hydrology exhibits in Appendix H. Drainage Area 'A' This area is located on the westerly portion of the project and consists primarily of Dike Number 2, the Coral Mountains, a portion of Dike Number 4, the south-westerly portion of Planning Area IX and Planning Area X. Storm runoff (approximately 18.4 acre-feet) from this drainage sub -area is directed to the wave lagoon which provides over 73 acre-feet of retention. Shallow basins are proposed to retain 10-year storm flows from drainage areas 'Al' & 'A3'. During the final design process, once a more detailed land plan is provided, first flush storm flows from drainage area 'A2' will be captured prior to being discharged into the lagoon. captured by an above ground or DrainageArea'B' underground basin? Storm runoff of 4.2 acre-feet from this drainage area will be retained within the proposed IaKe. Drainage Area 'C' Storm runoff (roughly 1 acre-foot) for this drainage area will be conveyed to and retained in a temporary retention basin located in Drainage Area V. Drainage Area 'D' Storm runoff of approximately 3.1 acre-feet for this drainage area will be conveyed to and retained in the proposed lake located within the "Farm". Drainage Area 'E' Storm runoff of approximately 5.7 acre-feet for this drainage area will be conveyed to and retained in a a large basin located in along the southerly portion of Planning Area VIII (Area 5). This basin will also provide a discharge location for the lagoon. Drainage Area T' This area is a future low -density residential development. Once developed it is anticipated approximately 4.0 acre-feet of retention will be required. Does the 4.0 acre-feet of retention include the storm runoff from DA-C? The Wave at Coral Mountain Preliminary Hydrology Report Drainage Area 'G' This area is a future low -density residential development. Once developed it is anticipated approximately 3.4 acre-feet of retention will be required. Drainage Area V This area is a future commercial development. Once developed it is anticipated approximately 1.4 acre-feet of retention will be required. Drainage Area T This area is a future low -density residential development. Once developed it is anticipated approximately 4.9 acre-feet of retention will be required. PRELIMINARY WQMP ANALYSES Preliminary design volume and flow for BMP measures were based on Worksheets 1 and 2 from the Riverside County — Whitewater River Region Water Quality Management Plan. Impervious area for the drainage sub- areas are derived from the preliminary land use worksheets. A summary of the BMP results are presented on sheet 1 of the Hydrology Exhibits in Appendix H with the worksheets being included as Appendix G.. RESULTS AND CONCLUSIONS As the above narrative and appendices confirm, the proposed retention basins are sufficiently sized to contain the flood volume from the controlling 100-year storm. During the final design process, a more detailed hydrologic report will be submitted to the City for review and approval. Please include half -street tributary areas on Ave 58 and Madson Street Are all proposed PE a minimum 1 ft above the 100year flood elevation? Please discuss.