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HomeMy WebLinkAboutGPA 2017-0003; OAKMONT OF CARLSBAD; PRELIMINARY HYDROLOGY; 2018-01-01Table of Contents INTRODUCTION ....................................................................................................................... 2 Purpose of Study Scope -Existing and Proposed Drainage STUDY AREA .................................................................................................••..•.••••.•...••••.•.•...• 2 Soils Groups Land Uses HYDROLOGY ...............................................................................................••••••.•...••.••••...•.•••.•• 3 Modified Rational Method Description Program Process RESULTS ...............................................................................................•...••••.•.•...••••.•..•....••.••.•• 4 CONCLUSION ......................................................................................................................•..•. 4 List of Appendices APPENDIX A -Vicinity Map APPENDIX B -Site Development Plan APPENDIX C -Rational Method Reference Charts APPENDIX D -lsopluvial Map (100-Year, 6-Hour & 100-Year, 24-Hour) APPENDIX E -Hydrologic Soils Group Map APPENDIX F -Hydrologic Modeling Output (CivilDesign)-Developed Condition 100-Year APPENDIX G -Existing Condition Reference Plans APPENDIX H -Developed Condition Hydrology Map 1 INTRODUCTION Purpose of Study The Oakmont assisted living facility project is located on the north side of Faraday Avenue, west of the intersection at El Fuerte Street in the City of Carlsbad, San Diego County {see Appendix A for Vicinity Map). In the current state, the project is a mass graded lot (Lot 1) per Grading plans 415-9A, Appendix G. Lot 1 is approximately 6.38 acres. This study will consider the proposed disturbed site area consisting of 5.64 acres. The purpose of this hydrology study is to assess peak flowrates for the proposed Oakmont facility (see Appendix B). The developed site will consist of a three-story luxury assisted living facility, a two-story memory care building, and a one-story models building. A pool, parking areas, pickle ball court, bocci ball court, outdoor picnic area, various landscaped areas, and associated infrastructure are also proposed. Scope -Existing and Proposed Drainage The existing mass graded pad for Oakmont of Carlsbad drains to an existing desilting basin located inside the southeastern corner of the property near the street intersection of Faraday Avenue and El Fuerte. A spillway outlets the onsite desilting basin and runs south to an existing diverter box that splits drainage from higher magnitude stormwater flows. An existing water quality basin is located just off site to the east of the property for low flows (see Sheet 20 of Dwg No 415-9C Appendix G). High flows run to an existing 24" stormdrain that runs beneath Faraday Ave to the southeast corner of the El Fuerte and Faraday intersection where they are discharged to the natural channel and canyon below. Prior to discharge into the offsite canyon, stormwater flows from the Oakmont project site are comingled within the 24" drain with street flows from Faraday Ave and El Fuerte. All existing onsite and offsite stormdrain infrastructure, including the 24" RCP, was originally designed for an industrial land use site previously proposed on this property. The industrial site was only rough graded with drainage items installed and was never constructed. The developed condition site will not utilize the offsite water quality basin for low flows. Proposed structural BMPs will be used to treat onsite flows, and include two biofi!tration basins and one media filter. All treated flows will be routed to a new manhole installed just downstream of the existing diverter box at southeast corner of the site. The diverter box will remain, however it will be plugged with brick and mortar at the existing 12" low flow outlet, and the outlet line will be abandoned. The treated flows will drain south across Faraday Avenue through the existing 24" RCP public storm drain system (see Appendix G, Sheet 20 of As-Built Storm drain Plans for Dwg No 415-9(). It should be noted that the 100-year discharge for the proposed Oakmont project is shown not to exceed the design capacity of the existing 24" RCP shown on Sheet 20 of Dwg No 415-9C. 2 STUDY AREA Soils Groups The site is characterized by soil group D. Refer to Hydrologic Soils Group Map, Appendix E. Hydrologic group D soils have very slow percolation rates when thoroughly wet. The rate of water transmission is very slow. Land Use The proposed land use is a mix of High Density Residential use (24DU/AC or less) and open space for the landscaped portion. The composite land use and soil group have been used to determine a runoff coefficient of 0. 71 for the proposed hydrology calculations. HYDROLOGY The hydrologic analyses are performed according to the 2003 San Diego County Hydrology Manual. The overall drainage area is less than one square mile and includes junctions of independent drainage systems; therefore, the Modified Rational Method is being used for the analysis. The Modified Rational Method is applicable to a 6-hour storm duration because the procedure uses Intensity-Duration Design Charts that are based on a 6-hour storm duration. In some cases, the 6-hour precipitation must be adjusted based on the ratio of the 6-to 24-hour precipitation. This will be performed where necessary. Modified Rational Method Description The modified rational method, as described in the 2003 San Diego Flood Control/Hydrology Manual, is used to estimate surface runoff flows. The basic equation: Q = CIA C = runoff coefficient {varies with surface) I= intensity (varies with time of concentration) A= area in acres For the 100-year design storm, the 6-hour rainfall amount is 2.8 inches and the 24-hour rainfall amount is 5.1 inches (see Appendix D}. San Diego County Rational-Hydrology Program Package Version 7.9, developed by CIVILCADD/CJVILDESIGN Engineering Software © (1991-2012}, was used to determine the rainfall amount, times of concentration, corresponding intensities and flows for the various hydro logic basins within this model. The program was then used to route flows through drainage conveyance structures and confluence basins per the modified rational method. 3 Program Process The Rational-Hydrology program is a computer-aided design program where the user develops a node link model of the watershed. Developing independent node link models of each interior watershed and Ii n king these subareas together at co nfl ue nee points create the node Ii n k mode I. The program has the capability of performing calculations for 11 different hydrologic and hydraulic processes. These processes are assigned and printed in the output. They are as follows: 1. Initial sub-area input, top of stream. 2. Street flow through sub-area, includes sub-area runoff. 3. Addition of runoff from sub-area to stream. 4. Street inlet and parallel street and pipeflow and area. 5. Pipeflow travel time (program estimated pipe size}. 6. Pipeflow travel time (user-specified pipe size). 7. Improved channel travel -Area add option. 8. Irregular channel travel time -Area add option. 9. User-specified entry of data at a point. 10. Confluence at downstream point in current stream. 11. Confluence of main streams. RESULTS The results of the analysis of the runoff from Oakmont of Carlsbad are presented below: 100-YEAR STORM EVENT Acreage Q (cfs) Existing Condition (Allowable) 5.52 28.1 Developed Condition 5.64 23.5 ,Q, 0.12 -4.6 CONCLUSION Site drainage patterns in the developed condition will drain southeast to an existing underground 24" storm drain line along Faraday Ave, and there will be no increase in peak flowrate on the existing storm drain infrastructure. Drainage will be routed to water quality basins and a biofi!tration unit not shown in this report. Therefore, developed Qs are conservative. Detailed 0100 routing analysis through on site water quality basins will be included in final design hydrology report. The developed condition 100-year peak flowrates do not exceed the design ca pa city of the existing 24" RCP shown on Sheet 20 of Dwg No 415-9C. The existing underground storm drain the project outlet drains to will not directly discharge into the ultimate receiving water body, Agua Hedionda Lagoon. Therefore, this project is deemed not exempt from hydromodification, and is discussed in detail in the SWQMP Report associated with this project. This project is considered to have no significant hydrologic impacts to the surrounding environment and is acceptable for development. 4 San Diego County Hydrology Mwrnal Date: June 2003 Table 3-1 Section: Page: RUNOFF COEFFICIENTS FOR URBAN AREAS Land Use Runoff Coefficient "C" Soil Type NRCS Elements Coun Elements % IMPER. A B Undisturbed Natural Terrain (Natural) Permanent Open Space o• 0.20 0.25 Low Density Residential (LOR) Residential, 1.0 DU/A or less IO 0.27 0J2 Low Density Residenlial (LDR) Residential, 2.0 DU/A or less 20 0.34 OJS Low Density Residential (LOR) Residential, 2.9 DU/A or less 25 0.38 0.41 Medium Density Residential (MOR) Residential, 4.3 DU/A or less 30 0.41 0.45 Medium Density Residential (MDR) Residential, 7 3 DU/A or Jess 40 0.48 0.51 Medium Density Residential (MDR) Residential, 10.9 DU/A or less 45 0.52 0.54 Medium Density Residential (MOR) Residential, 14.5 DU/A or Jess 50 0.55 0.58 High Density Residential (HOR) Residential, 24.0 DU/A or less 65 0.66 0.67 High Density Residential (HDR) Residential, 43.0 DU/A or less 80 0.76 0.77 Commercial/Industrial (N. Com} Neighborhood Commercial 80 0.76 0.77 Commercial/Industrial (G. Com) General Commercial 85 0.80 0.80 Commercial/Industrial (0.P. Com) Office Professional/Commercial 90 0.83 0.84 Commercial/Industrial (Limited I.) Limited Industrial 90 0.83 0.84 Commercial/lndusbial General I. General Industrial 95 0.87 0.87 3 6 of26 C D 0.30 0.35 0.36 0.41 0.42 0.46 0.45 0.49 0.48 0.52 0.54 0.57 0.57 0.60 0.60 0.63 0.69 0.71 0.78 0.79 0.78 0.79 0,81 0.82 0.84 0.85 0.84 0.85 0.87 0.87 *The values associated with 0% impervious may be used for direct calculation of the runoff coefficient as described in Section 3.1.2 (representing the pervious runoff coefficient, Cp, for the soil type), or for areas that will remain undisturbed in perpetuity. Justification must be given that the area will remain natural forever (e.g., the area is located in Cleveland National Forest). DU/A "'dwelling units per acre NRCS "'National Resources Conservation Service (24.0 Du/A or Less ) Impervious value, Ai = 0.650 sub-Area c value= 0.710 Time of concentration= 5.96 min. Rainfall intensity= 6.585(In/Hr) for a 100.0 year storm Effective runoff coefficient used for total area (Q=KCIA) is C = 0.710 CA= 2.130 Subarea runoff 7.378(CFS) for 1.710(Ac.) Total runoff= 14.026(CFS) Total area= 3.000(Ac.) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Process from Point/Station 3.000 to Point/Station 5.000 **** CONFLUENCE OF MINOR STREAMS**** Along Main stream number: 1 in normal stream number 1 Stream flow area= 3.000(Ac.) Runoff from this stream 14.026(CFS) Time of concentration 5.96 min. Rainfall intensity= 6.585(In/Hr) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Process from Point/Station 3.000 to Point/station 4.000 **''* INITIAL AREA EVALUATION **** Decimal fraction soil group A Decimal fraction soil group B Decimal fraction soil group C Decimal fraction soil group o [HIGH DENSITY RESIDENTIAL (24.0 D~/A or Less ) Impervious value, Ai = 0.650 sub-Area C Value= 0.710 0.000 0.000 0.000 1.000 Initial subarea total flow distance Highest elevation= 60.000(Ft.) Lowest elevation= 58.000(Ft.) 490.000(Ft.) Elevation difference= 2.000(Ft.) slope 0.408 % INITIAL AREA TIME OF CONCENTRATION CALCULATIONS: The maximum overland flow distance is 50.00 (Ft) for the top area slope value of 0.41 %, in a development type of 24.0 DU/A or Less In Accordance With Figure 3-3 Initial Area Time of Concentration= 6.69 minutes TC= [1.8*(1.l-C)*distance(Ft.)A.5)/(% slopeA(l/3)] TC= [1.8*(1.1-0.7100)*( 50.000A.5)/( 0.408A(l/3)]= 6.69 Rainfall intensity (I)= 6.112(rn/Hr) for a 100.0 year storm Effective runoff coefficient used for area (Q"'KCIA) is c = 0. 710 subarea runoff= ll.457(CFS) Total initial stream area= 2.640(Ac.) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Process from Point/Station 4.000 to Point/station 5.000 *''** PIPEFLOW TRAVEL TIME (Program estimated size) **** upstream point/station elevation"' 55.000(Ft.) Downstream point/station elevation 50.lOO(Ft.) Pipe len9th 405.00(Ft.) slope 0.0121 Manning's N No. of pipes= 1 Required pipe flow 11.457(CFS) Nearest computed pipe diameter = 18.00(In.) Calculated individual pipe flow 11.457(CFS) Normal flow depth in pipe= 14.63(In.) Flow top width inside pipe= 14.0S(In.) 0.013 critical Depth= 15.48(rn.) Pipe flow velocity= 7.45(Ft/s) Travel time through pipe= 0.91 min. Time of concentration (TC) 7.60 min. ++++++++++++++++++++++++++++~+++++++++++++++++++++++++++++++++++++++++ Process from Point/station 4.000 to Point/Station 5.000 **** CONFLUENCE OF MINOR STREAMS**** Along Main stream number; 1 in normal stream number 2 stream flow area= 2.640(Ac.) Runoff from this stream ll.457(CFS) Time of concentration~ 7.60 min. Rainfall intensity= 5.632(In/Hr) Summary of stream data: Stream Flow rate TC Rainfall Intensity No. (CFS) (min) (In/Hr) 1 14.026 5. 96 6. 585 2 11. 457 7.60 5.632 Qmax(l) 1.000 * 1.000 ,. 14.026) + 1.000 * 0.785 ., 11.457) + = 23.018 Qmax(2) 0.855 * 1. 000 * 1.000 * 1.000 * Total of 2 streams to confluence: 14.026) + 11.457) + Flow rates before confluence point: 14.026 11.457 23.454 Maximum flow rates at confluence using above data: 23.018 23.454 Area of streams before confluence: 3.000 2.640 Results of confluence: Total flow rate = 23.454(CFS) Time of concentration= 7.598 min. Effective stream area after confluence End of computations, total study area= 5.640(Ac.) 5.640 (Ac.) OAKMONT OF CARLSBAD APPENDIX H DEVELOPED CONDITION HYDROLOGY MAP