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HomeMy WebLinkAboutCT 16-10; HOME AVENUE; DRAINAGE STUDY; 2016-10-31•• •• -------------- - • • • -.. -------·------------------ DRAINAGE STUDY TENTATIVE SUBDIVISION MAP HOME A VENUE CONDOMINIUMS CITY OF CARLSBAD Home Avenue Condominiums CDP XX-XX Preliminary Drainage Study CT XX-XX PUDXX-XX Prepared for: Edmond T. Shehab 800 Grand MMKLC, LLC 800 Grand Avenue, Suite C16 Carlsbad, CA 92008 Prepared by: bl-fA, Inc land planning, civil engineering, surveying 5115 Avenida Encinas, Suite L Carlsbad, CA 92008-4387 (760) 931-8700 RECEIVED OCT 31 2016 CITY ni= CI' q1 .. SBAD October 31, 2016 w.o. 1026-1383-400 bl-IA, Inc . land planning, civil engineering, surveying .J "'II .. -- -- .. • • .. • TABLE OF CONTENTS Chapter 1-Discussion ............................................................................................................................................. 3 Vicinity Map ................................................................................................................................................. 3 Purpose and Scope ....................................................................................................................................... 4 Project Description ...................................................................................................................................... 4 Study Method ............................................................................................................................................... 6 Conclusions ................................................................................................................................................... 8 Declaration of Responsible Charge ............................................................................................................ 9 Chapter 2 -Existing & Proposed Hydrology Exhibits ......................................................................................... 10 Chapter 3 -Calculations ........................................................................................................................................ 11 Existing Condition Hydrology Calculations ............................................................................................. 11 100-Year Storm ............................................................................................................................. 12 Proposed Condition Hydrology Calculations-Undetained .................................................................... 14 100-Year Storm ............................................................................................................................. 15 Chapter 4 -Detention Routing .............................................................................................................................. 18 Basin Outlet Detail .................................................................................................................................... 19 Chapter 5 -References ........................................................................................................................................... 20 Methodology-Rational Method Peak Flow Determination .................................................................. 20 Home Avenue Condominiums CDP XX-XX . Prelimina_ry Drainage Stugy_ bliA, Inc. land planning, civil engineering, surveying 2 '• --.. • -• .. • ... • .. • - .. • .. • • Home Avenue Condominiums CDP XX-XX f_r:elimj_nary Drainage Study CHAPTERl DISCUSSION bl-IA, Inc. land planning, civil engineering, surveying , _) '11111 .. • ... • ---.. -- -• • -• VICINITY MAP Home Avenue Condominiums CDP XX-XX _Prelim~J)ra~~ll_QY ___ _ 9 VICINITY MAP bl-IA, Inc. land planning, civil engineering, surveying ,, ... • .. .. .. -• • • • .. • • -• PURPOSE AND SCOPE The purpose of this report is to publish the results of hydrology and hydraulic computer analysis for the proposed Home Avenue Condominiums in the City of Carlsbad. The scope of this study is to analyze the results of existing and developed condition hydrology calculations and provide recommendations as to the design and size of various hydraulic systems considered as mitigation of any potential adverse effects of the proposed project. The mitigation measures proposed will include detention calculations and sizing to attenuate the effects of development on storm water discharge. Storms of 100-year frequencies will be analyzed. Information contained in this report will be referred to for the purpose of sizing treatment facilities as proposed in the associated Storm Water Quality Management Plan. PROJECT DESCRIPTION The project is located at Home Avenue in the City of Carlsbad. The proposed project is a 0.415 acre site. The project proposes the development of five condominiums, an amenity site, a driveway, and surrounding landscape. Proposed drainage improvements consist of five (5) flow- through planter BMPs to provide stormwater treatment and maintain the pre-developed runoff characteristics. The project drains to one (1) Point of Compliance (POC) located to the northwest corner of the project site . The disturbed area is 0.415 acres; the existing site is 100% impervious pre-development and 45% impervious post development. The proposed drainage basin matches the existing drainage basin in terms of overall area and basin limits. Storm flows affecting the site are limited to the rainfall that lands directly on this property. Pre-Development Conditions The existing site is approximately 0.415 acres and has been previously graded. The site is currently an impervious parking lot. Storm flows affecting the site are limited to the rainfall that lands directly on the property. Surface runoff sheet flows northwest across the parking lot to Home Avenue. Runoff that drains to Home Avenue will be conveyed northwest via existing curb and gutter to the storm drain system on Grand Avenue. Approximately 100% of the existing site is impervious. The on-site soil classification is Type-B from USGS Web Soil Survey . Post-Development Conditions Storm water runoff from the proposed project site is routed to a single POC located at the northwest of the project site. The proposed drainage pattern will be similar to the existing drainage pattern with some modifications to incorporate the Best Management Practices (BMPs) into the project design to mimic the impacts on storm water runoff and quality. Runoff from the developed project site is drained to five (5) onsite flow-through planter BMPs (WQ- Home Avenue Condominiums bl-IA, Inc . CDP XX-XX land planning, civil engineering, surveying . ·-·-Prelimimiry Draina~ Study_ •• • .. -- BMPs) and a permeable pavement driveway for water quality purposes (the project is not subject to hydromodification requirements). Once flows are routed via the proposed WQ-BMPs, all flows are then conveyed via storm drain to the aforementioned POC. Roof drains will collect runoff from the proposed buildings and discharge into proposed flow- through planters, FTP 1 -FTP 5, located adjacent to the residences. The flow-through planters will provide stormwater treatment and flow detention. Treated water will discharge via storm drain pipes to a proposed storm drain system underneath the permeable pavement driveway . Runoff from the proposed concrete sidewalks and adjacent landscape areas of the amenity site will be directed to yard drains, which will discharge onto the proposed permeable pavement driveway. The permeable pavement will provide stormwater treatment and flow detention. Treated water will discharge via a perforated underdrain pipe underneath the driveway. The underdrain pipe will outlet over rip rap in a landscape area and flow northwest to POC 1. Landscape areas in the back of the proposed residences will gravity flow towards Home Avenue . All runoff will eventually outlet onto Home Avenue at POC 1. The proposed drainage patterns will not alter the existing flow pattern and will discharge from the site at the historic discharge points . STUDY METHOD The method of analysis was based on the Rational Method according to the San Diego County Hydrology Manual (SD HM). The Hydrology and Hydraulic Analysis were done on Hydro Soft by Advanced Engineering Software 2013. The study considers the runoff for a 100-year storm frequency. Methodology used for the computation of design rainfall events, runoff coefficients, and rainfall • intensity values are consistent with criteria set forth in the "2003 County of San Diego Drainage • Design Manual." A more detailed explanation of methodology used for this analysis is listed in Chapter 6 -References of this report. .-Drainage basin areas were determined from the topography and proposed grades shown on the Tentative Map for this site and County of San Diego 200-Scale Topography Maps. - .. • -.. The Rational Method provided the following variable coefficients: Rainfall Intensity-Initial time of concentration (Tc) values based on Table 3-2 of the SD HM. Rainfall Isopluvial Maps from the SD HM were used to determine P6 for 100-year storm, see References. Home Avenue Condominiums CDP XX-XX __ Prelimin~J)rainage St~ ___ -- bl-IA, Inc. land planning, civil engineering, surveying 6 P6 for 100-year storm =2.6" In accordance with the County of San Diego standards, runoff coefficients were based on land use and soil type. The soil conditions used in this study are consistent with Type-B soil qualities. An appropriate runoff coefficient (C) for each type of land use in the subarea was selected from Table 3-1 of SD HM and multiplied by the percentage of total area (A) included in that class. The sum of the products for all land uses is the weighted runoff coefficient (2[CA]). The Proposed Hydrology Exhibit shows the offsite area, proposed on-site drainage system, on- • site subareas, and nodal points. Table 1 summarizes the composite C-values calculated in the ,11 existing and proposed conditions. • TABLE 1-WEIGHTED RUNOFF COEFFICIENT CALCULATIONS • - • -• .. -• -- • - .. .. Proposed Hydrology-Home Avenue Up Node Down Node Total Acreage C1 Al (acres) C2 A2 (acres) Ccomp 1 2 0.10 0.25 0.05 0.87 0.05 3 3 0.03 0.25 0.00 0.87 0.03 Note: C-values taken from Table 3-1 of San Diego County Hydrology Manual, consistent with on-site existing soil types. See References. SUMMARY OF RESULTS TABLE 2 -SUMMARY OF PEAK FLOW RESULTS FOR POC-1 0.56 0.85 The developed site contains five (5) flow-through planters to treat and detain runoff generated by the impervious rooftop surfaces of the proposed residences. The outlet for these planters all discharge to a proposed storm drain system underneath the permeable pavement driveway. The detention values for the planters have not been quantified, due to the fact that enough detention is achieved by decreasing and disconnecting impervious surfaces. Runoff values have been effectively mitigated to below historical values without considering additional flow detention from the flow-through planters. Therefore, we consider the detained flowrate results presented in this Report to be conservative in nature, as the five (5) planters mentioned above will likely further mitigate the site runoff beyond the values shown . Since OlOOex (2.37 cfs) is equal to or greater than OlOOdev (0.86 cfs) then this system achieves the goal of attenuating the storm flows from the proposed development to predevelopment levels. Home Avenue Condominiums CDP XX-XX J:>_relirajn_~_ry Drainage Study . bl-IA, Inc. land planning, civil engineering, surveying • • -• -----.. • - -• --• CONCLUSION The hydraulic calculations in the developed condition shows that the runoff from the proposed residences, which have been treated in flow-through planters sized for treatment control can be routed to discharge through the proposed storm drain system and onto Home Ave. The additional runoff that is generated by the amenity site can be drained directly to the proposed driveway and treated by the permeable surface. The required treatment areas are based on the City of Carlsbad BMP Design Manual. Drainage patterns reflected on the Proposed Hydrology Map will decrease the total developed runoff due to decrease in impervious area. By increasing the overall time of concentration and implementing flow-through planters and permeable pavements, the rate of discharge between the existing condition and the developed condition, can be directly discharged from the site without any further handling. This project is effective at mitigating the potential impacts that development can have on stormwater runoff. By utilizing the proposed LID systems, this project mitigates both the quantity of runoff generated during storm, and the quality of runoff that will ultimately leave the property. It is our professional opinion that the recommendations provided in this report, and the drainage system as proposed effectively intercept, contain, convey, detain and treat the expected storm water runoff generated by this property to mimic pre-development conditions . Home Avenue Condominiums CDP XX-XX _Erelimina_!y_ Drainage Stud~ .. bl-IA, Inc. land planning, civil engineering, surveying 8 • -.. ---• • --... .. -.. -• -• DECLARATION OF RESPONSIBLE CHARGE I hereby declare that I am the Engineer of Work for this project, that I have exercised responsible charge over the design of the project as defined in section 6703 of the business and professions code, and that the design is consistent with current standards. I understand that the check of project drawings and specifications by the City of Carlsbad is confined to a review only and does not relieve me, as Engineer of Work, of my responsibilities for project design. Bruce Rice ~60676 Home Avenue Condominiums CDP XX-XX .. __ Prelimina_ry_ Drainage Stu_Qy _ Date , IQ-3/-1~ bliA, Inc. land planning, civil engineering, surveying I~ -• -• -• • - ---• -• -• -• -• -- • CHAPTER2 EXHIBITS EXISTING CONDITION HYDROLOGY MAP & PROPOSED CONDITION HYDROLOGY MAP bl-tA, Inc. Home Avenue Condominiums CDP XX-XX land planning, civil engineering, surveying ___ p_r:~liminary Drainage Study ___ _ 10 • ... ---.. -• ----.. - • CHAPTER3 CALCULATIONS EXISTING CONDITION HYDROLOGY CALCULATIONS Home Avenue Condominiums CDP XX-XX Preliminary Drainage Study bttA, Inc. land planning, civil engineering, surveying 11 ,. 'HIM • .. - - • • -• -- 100 YEAR STORM **************************************************************************** RATIONAL METHOD HYDROLOGY COMPUTER PROGRAM PACKAGE Reference: SAN DIEGO COUNTY FLOOD CONTROL DISTRICT 2003,1985,1981 HYDROLOGY MANUAL (c) Copyright 1982-2012 Advanced Engineering Software (aes) Ver. 19.0 Release Date: 06/01/2012 License ID 1459 Analysis prepared by: BHA Inc 5115 Avenida Encinas, Suite L Carlsbad CA 92008 ************************** DESCRIPTION OF STUDY************************** * Existing Hydrology Study, 100 Year Storm * Home Avenue * ************************************************************************** FILE NAME: 1383El00.DAT TIME/DATE OF STUDY: 09:57 09/26/2016 USER SPECIFIED HYDROLOGY AND HYDRAULIC MODEL INFORMATION: 2003 SAN DIEGO MANUAL CRITERIA USER SPECIFIED STORM EVENT(YEAR) = 100.00 6-HOUR DURATION PRECIPITATION (INCHES) = 2.600 SPECIFIED MINIMUM PIPE SIZE(INCH) = 3.00 SPECIFIED PERCENT OF GRADIENTS(DECIMAL) TO USE FOR FRICTION SLOPE= 0.95 SAN DIEGO HYDROLOGY MANUAL "C"-VALUES USED FOR RATIONAL METHOD NOTE: USE MODIFIED RATIONAL METHOD PROCEDURES FOR CONFLUENCE ANALYSIS * * * *USER-DEFINED STREET-SECTIONS FOR COUPLED PIPEFLOW AND STREETFLOW HALF-CROWN TO STREET-CROSSFALL: CURB GUTTER-GEOMETRIES: WIDTH CROSSFALL IN-/ OUT-/PARK-HEIGHT WIDTH LIP HIKE NO. (FT) (FT) SIDE / SIDE/ WAY (FT) (FT) (FT) (FT) MODEL* MANNING FACTOR (n) 1 30.0 20.0 0.018/0.018/0.020 0.67 2.00 0.0313 0.167 0.0150 GLOBAL STREET FLOW-DEPTH CONSTRAINTS: 1. Relative Flow-Depth= 0.00 FEET as (Maximum Allowable Street Flow Depth) -(Top-of-Curb) 2. (Depth)*(Velocity) Constraint= 6.0 (FT*FT/S) *SIZE PIPE WITH A FLOW CAPACITY GREATER THAN OR EQUAL TO THE UPSTREAM TRIBUTARY PIPE.* **************************************************************************** FLOW PROCESS FROM NODE 1.00 TO NODE 2.00 IS CODE= 21 >>>>>RATIONAL METHOD INITIAL SUBAREA ANALYSIS<<<<< *USER SPECIFIED(SUBAREA): USER-SPECIFIED RUNOFF COEFFICIENT= .8700 S.C.S. CURVE NUMBER (AMC II) = 0 INITIAL SUBAREA FLOW-LENGTH(FEET) = 66.00 UPSTREAM ELEVATION(FEET) = 59.25 DOWNSTREAM ELEVATION(FEET) 58.00 ELEVATION DIFFERENCE(FEET) 1.25 Home Avenue Condominiums CDP XX-XX __ PreliminaryJ)raina~_ S~----- bl-IA, Inc. land planning, civil engineering, surveying 12 '111 ----------- .. .. .. - • SUBAREA OVERLAND TIME OF FLOW(MIN.) = 2.718 100 YEAR RAINFALL INTENSITY(INCH/HOUR) 6.850 NOTE: RAINFALL INTENSITY IS BASED ON Tc= 5-MINUTE. SUBAREA RUNOFF(CFS) 0.36 TOTAL AREA(ACRES) = 0.06 TOTAL RUNOFF(CFS) = 0.36 **************************************************************************** FLOW PROCESS FROM NODE 2.00 TO NODE 3.00 IS CODE= 51 >>>>>COMPUTE TRAPEZOIDAL CHANNEL FLOW<<<<< >>>>>TRAVELTIME THRU SUBAREA (EXISTING ELEMENT)<<<<< ELEVATION DATA: UPSTREAM(FEET) = 58.00 DOWNSTREAM(FEET) 56.94 CHANNEL LENGTH THRU SUBAREA(FEET) = 128.00 CHANNEL SLOPE 0.0083 CHANNEL BASE (FEET) 50. 00 "Z" FACTOR = 20. 000 MANNING'S FACTOR= 0.015 MAXIMUM DEPTH(FEET) = 1.00 100 YEAR RAINFALL INTENSITY(INCH/HOUR) = 6.572 *USER SPECIFIED(SUBAREA): USER-SPECIFIED RUNOFF COEFFICIENT= .8700 S.C.S. CURVE NUMBER (AMC II) = 0 TRAVEL TIME COMPUTED USING ESTIMATED FLOW(CFS) 1.38 TRAVEL TIME THRU SUBAREA BASED ON VELOCITY(FEET/SEC.) 0.82 AVERAGE FLOW DEPTH(FEET) 0.03 TRAVEL TIME(MIN.) 2.61 Tc(MIN.) = 5.33 SUBAREA AREA(ACRES) 0.35 SUBAREA RUNOFF(CFS) 2.03 AREA-AVERAGE RUNOFF COEFFICIENT 0.870 TOTAL AREA(ACRES) = 0.4 PEAK FLOW RATE(CFS) 2.37 END OF SUBAREA CHANNEL FLOW HYDRAULICS: DEPTH(FEET) = 0.04 FLOW VELOCITY(FEET/SEC.) LONGEST FLOWPATH FROM NODE 1.00 TO NODE END OF STUDY SUMMARY: TOTAL AREA(ACRES) PEAK FLOW RATE(CFS) 0 . 4 TC (MIN. ) = 2.37 END OF RATIONAL METHOD ANALYSIS Home Avenue Condominiums CDP XX-XX -~reliminary Drainage Study ___ _ 1. 09 3.00 = 5.33 194.00 FEET. bl-IA, Inc, land planning, civil engineering, surveying 13 " ' • ---.. -• --- • --• -• • CHAPTER3 CALCULATIONS PROPOSED CONDITION HYDROLOGY CALCULATIONS - UNDETAINED Home Avenue Condominiums CDP XX-XX .. Prelimina!Y_Pra~~ Study bl-IA, Inc. land planning, civil engineering, surveying 14 ''11 - --------• -• -• .. --• -- • 100 YEAR STORM **************************************************************************** RATIONAL METHOD HYDROLOGY COMPUTER PROGRAM PACKAGE Reference: SAN DIEGO COUNTY FLOOD CONTROL DISTRICT 2003,1985,1981 HYDROLOGY MANUAL (c) Copyright 1982-2012 Advanced Engineering Software (aes) Ver. 19.0 Release Date: 06/01/2012 License ID 1459 Analysis prepared by: BHA Inc 5115 Avenida Encinas, Suite L Carlsbad CA 92008 ************************** DESCRIPTION OF STUDY************************** * Proposed Hydrology Study, 100 Year Storm * Home Avenue * Undetained ************************************************************************** FILE NAME: 1383HOMP.DAT TIME/DATE OF STUDY: 11:14 10/31/2016 USER SPECIFIED HYDROLOGY AND HYDRAULIC MODEL INFORMATION: 2003 SAN DIEGO MANUAL CRITERIA USER SPECIFIED STORM EVENT(YEAR) = 100.00 6-HOUR DURATION PRECIPITATION (INCHES) = 2.600 SPECIFIED MINIMUM PIPE SIZE(INCH) = 3.00 SPECIFIED PERCENT OF GRADIENTS(DECIMAL) TO USE FOR FRICTION SLOPE= 0.95 SAN DIEGO HYDROLOGY MANUAL "C"-VALUES USED FOR RATIONAL METHOD NOTE: USE MODIFIED RATIONAL METHOD PROCEDURES FOR CONFLUENCE ANALYSIS *USER-DEFINED STREET-SECTIONS FOR COUPLED PIPEFLOW AND STREETFLOW MODEL* * * * HALF-CROWN TO STREET-CROSSFALL: WIDTH CROSS FALL IN-I OUT-/PARK- NO. (FT) (FT) SIDE I SIDE/ WAY CURB HEIGHT (FT) GUTTER-GEOMETRIES: WIDTH LIP HIKE (FT) (FT) (FT) MANNING FACTOR (n) -------------------------- 1 30.0 20.0 0.018/0.018/0.020 0.67 2.00 0.0313 GLOBAL STREET FLOW-DEPTH CONSTRAINTS: 1. Relative Flow-Depth= 0.00 FEET as (Maximum Allowable Street Flow Depth) -(Top-of-Curb) 2. (Depth)*(Velocity) Constraint= 6.0 (FT*FT/S) *SIZE PIPE WITH A FLOW CAPACITY GREATER THAN OR EQUAL TO THE UPSTREAM TRIBUTARY PIPE.* 0.167 0.0150 **************************************************************************** FLOW PROCESS FROM NODE 1.00 TO NODE 2.00 IS CODE= 21 >>>>>RATIONAL METHOD INITIAL SUBAREA ANALYSIS<<<<< *USER SPECIFIED(SUBAREA): USER-SPECIFIED RUNOFF COEFFICIENT= .5600 S.C.S. CURVE NUMBER (AMC II) = 0 INITIAL SUBAREA FLOW-LENGTH(FEET) = 66.50 UPSTREAM ELEVATION(FEET) 59.25 Home Avenue Condominiums CDP XX-XX Preliminary Drainage Study bHA, Inc. land planning, civil engineering, surveying 15 l ''II • -----• - - -• -• • • -• DOWNSTREAM ELEVATION(FEET) = 58.00 ELEVATION DIFFERENCE(FEET) = 1.25 SUBAREA OVERLAND TIME OF FLOW(MIN.) = 100 YEAR RAINFALL INTENSITY(INCH/HOUR) SUBAREA RUNOFF(CFS) 0.33 6.423 5.829 TOTAL AREA(ACRES) = 0.10 TOTAL RUNOFF(CFS) 0.33 **************************************************************************** FLOW PROCESS FROM NODE 2.00 TO NODE 3.00 IS CODE= 51 >>>>>COMPUTE TRAPEZOIDAL CHANNEL FLOW<<<<< >>>>>TRAVELTIME THRU SUBAREA (EXISTING ELEMENT)<<<<< ELEVATION DATA: UPSTREAM(FEET) = 58.00 DOWNSTREAM(FEET) CHANNEL LENGTH THRU SUBAREA(FEET) = 127.00 CHANNEL SLOPE CHANNEL BASE(FEET) 15.00 "Z" FACTOR= 5.000 MANNING'S FACTOR= 0.030 MAXIMUM DEPTH(FEET) = 1.00 100 YEAR RAINFALL INTENSITY(INCH/HOUR) = 4.217 *USER SPECIFIED(SUBAREA): USER-SPECIFIED RUNOFF COEFFICIENT= .2500 S.C.S. CURVE NUMBER (AMC II) = 0 TRAVEL TIME COMPUTED USING ESTIMATED FLOW(CFS) TRAVEL TIME THRU SUBAREA BASED ON VELOCITY(FEET/SEC.) AVERAGE FLOW DEPTH(FEET) 0.05 TRAVEL TIME(MIN.) Tc(MIN.) = 10.61 0.37 0.51 4.19 57.20 0.0063 0.09 SUBAREA AREA(ACRES) AREA-AVERAGE RUNOFF COEFFICIENT SUBAREA RUNOFF(CFS) 0.09 0. 413 TOTAL AREA(ACRES) = 0.2 PEAK FLOW RATE(CFS) END OF SUBAREA CHANNEL FLOW HYDRAULICS: DEPTH(FEET) = 0.04 FLOW VELOCITY(FEET/SEC.) LONGEST FLOWPATH FROM NODE 1.00 TO NODE 0.51 3.00 = 0.33 193.50 FEET. **************************************************************************** FLOW PROCESS FROM NODE 3.00 TO NODE 3.00 IS CODE= 81 >>>>>ADDITION OF SUBAREA TO MAINLINE PEAK FLOW<<<<< 100 YEAR RAINFALL INTENSITY(INCH/HOUR) = 4.217 *USER SPECIFIED(SUBAREA): USER-SPECIFIED RUNOFF COEFFICIENT= .8500 S.C.S. CURVE NUMBER (AMC II) = 0 AREA-AVERAGE RUNOFF COEFFICIENT= 0.5985 SUBAREA AREA(ACRES) 0.14 SUBAREA RUNOFF(CFS) TOTAL AREA(ACRES) = 0.3 TOTAL RUNOFF(CFS) = TC(MIN.) = 10.61 0.50 0.83 **************************************************************************** FLOW PROCESS FROM NODE 3.00 TO NODE 4.00 IS CODE= 51 >>>>>COMPUTE TRAPEZOIDAL CHANNEL FLOW<<<<< >>>>>TRAVELTIME THRU SUBAREA (EXISTING ELEMENT)<<<<< ELEVATION DATA: UPSTREAM(FEET) = 57.20 DOWNSTREAM(FEET) CHANNEL LENGTH THRU SUBAREA(FEET) = 47.00 CHANNEL SLOPE CHANNEL BASE(FEET) 10.00 "Z" FACTOR= 5.000 MANNING'S FACTOR= 0.040 MAXIMUM DEPTH(FEET) = 1.00 100 YEAR RAINFALL INTENSITY(INCH/HOUR) = 3.932 *USER SPECIFIED(SUBAREA): USER-SPECIFIED RUNOFF COEFFICIENT= .2500 S.C.S. CURVE NUMBER (AMC II) = 0 TRAVEL TIME COMPUTED USING ESTIMATED FLOW(CFS) 0.86 56.94 0.0055 bliA, Inc. Home Avenue Condominiums CDP XX-XX land planning, civil engineering, surveying Preliminary Drainage Study 16 • ,,. -----... • .. • • .. • • TRAVEL TIME THRU SUBAREA BASED ON VELOCITY(FEET/SEC.) 0.65 AVERAGE FLOW DEPTH(FEET) 0.12 TRAVEL TIME(MIN.) 1.21 Tc(MIN.) = 11.82 SUBAREA AREA(ACRES) 0.05 SUBAREA RUNOFF(CFS) 0.05 AREA-AVERAGE RUNOFF COEFFICIENT 0.553 TOTAL AREA(ACRES) = 0.4 PEAK FLOW RATE(CFS) 0.83 END OF SUBAREA CHANNEL FLOW HYDRAULICS: DEPTH(FEET) = 0.12 FLOW VELOCITY(FEET/SEC.) LONGEST FLOWPATH FROM NODE 1.00 TO NODE 0.66 4.00 = 240.50 FEET. **************************************************************************** FLOW PROCESS FROM NODE 5.00 TO NODE 4.00 IS CODE= 81 >>>>>ADDITION OF SUBAREA TO MAINLINE PEAK FLOW<<<<< 100 YEAR RAINFALL INTENSITY(INCH/HOUR) = 3.932 *USER SPECIFIED(SUBAREA): USER-SPECIFIED RUNOFF COEFFICIENT= .2500 S.C.S. CURVE NUMBER (AMC II) = 0 AREA-AVERAGE RUNOFF COEFFICIENT= 0.5305 SUBAREA AREA(ACRES) 0.03 SUBAREA RUNOFF(CFS) TOTAL AREA(ACRES) = 0.4 TOTAL RUNOFF(CFS) = TC(MIN.) = 11.82 END OF STUDY SUMMARY: TOTAL AREA(ACRES) PEAK FLOW RATE(CFS) 0 . 4 TC (MIN. ) = 0.86 END OF RATIONAL METHOD ANALYSIS 11. 82 0.03 0.86 bliA, Inc. Home Avenue Condominiums CDP XX-XX land planning, civil engineering, surveying Preliminary Drainage Study 17 'ti 'If .. Ill ---.. .. • --- - Ill -• -• Home Avenue Condominiums CDP XX-XX Preliminary Drainage Study CHAPTER4 DETENTION ROUTING bl-IA, Inc. land planning, civil engineering, surveying 18 ,II Iii ''Ill - •• - - -- • -- • • - FLOW-THROUGH PLANTER OUTLET DETAIL 111 /JOWNSPOUT---~ STRUCTURAL WALLS W/ WATERPROOF MEMBRANE SPLASH PAD BMP FOOTPRINT L---J8"x78" BROOKS CATCH BASIN W/ GRATED INLET PLANTINGS MULCH LA YER (3" MIN) 78" MIN [NGIN[[R[D SOIL MEDIA ~~~">il:rnL,-j---2" P [ A GR A V[L IMPERMEABLE LINER 6 "-DIA P[RFORA TED PVC PIPE SUBGRAD[ -J" AGGREGA T[ BELOW UNDERDRAIN Home Avenue Condominiums CDP XX-XX . _ereli!!llfl~!Y Drainage Study ___ ----·····-· _ FLOW-THOUGH PLANTER DETAIL, TYPICAL NOT TO SCALE bl-IA, Inc. land planning, civil engineering, surveying 19 11 ••• ,1a - • -- - ---• -• • • CHAPTERS REFERENCES METHODOLOGY -RATIONAL METHOD PEAK FLOW DETERMINATION Home Avenue Condominiums CDP XX-XX ... Prelim~J)rain~eStus!Y __ MtA, Inc. land planning, civil engineering, surveying 20 I I I , t I I I t I I San Diego County Hydrology Manual Date: · June 2003 I I I f • • I I I I • • Table 3-1 I i i j • j Section; Page: RUNOFF COEFFICIENTS FOR URBAN AREAS Land Use I Runoff Coefficient "C" Soil Ti~e NRCS Elements Coun Elements %IMPER. A B lJndisturbed Natural Terrain (Natural) Permanent Open Space O* 0.20 0.25 Low Density Residential (LOR) Residential, 1.0 DU/ A or Jess 10 0.27 0.32 Low Density Residential (LOR) Residential, 2.0 DU/ A or less 20 0.34 0.38 Low Density Residential (LDR) Residential, 2.9 DU/A or less 25 0.38 0.41 Medium Density Residential (MDR) Residential, 4.3 DU/A or Jess 30 0.41 0.45 Medium Density Residential (MOR) Residential, 7.3 DU/A or less 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.S DU/A or less so 0.55 0.58 High Density Residential (HDR) Residential, 24.0 DU/A or less 65 0.66 0.67 High Density Residential (HOR) 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/Tndustrial (0.P. Com) Office Professional/Commercial 90 0.83 0.84 Commercial/Industrial (Limited I.) Limited Industrial 90 0.83 0.84 Commercial/Industrial .{General 1.) General Industrial 95 0.87 0.87 . "' "' C 0.30 0.36 0.42 0.45 0.48 0.54 0.57 0.60 0.69 0.78 0.78 0.81 0.84 0.84 0.87 3 6 of26 D Os 0.35 0.41 0.46 0.49 0.52 0.57 0.60 0.63 0.71 0.79 0.79 0.82 0.85 0.85 0.87 *The values associated with 0% impervious may be used for direct calculation of the nmoff coefficient as described in Section 3.1.2 (representing the pervious nmoff 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 3-6 • • - - - • - • -• -• -• San Diego County Hydrology Manual Date: June 2003 Section: Page: 3 5 of26 Where: C = 0.90 x (%Impervious)+ Cp x (1 -% Impervious) Cp = Pervious Coefficient Runoff Value for the soil type (shown in Table 3-1 as Undisturbed Natural Terrain/Permanent Open Space, 0% Impervious). Soil type can be determined from the soil type map provided in Appendix A. The values in Table 3-1 are typical for most urban areas. However, if the basin contains rural or agricultural land use, parks, golf courses, or other types of nonurban land use that are expected to be permanent, the appropriate value should be selected based upon the soil and cover and approved by the local agency. 3-5 --•• -.. ---• -.... . . • I -'' . • ... , : ..... ...... . .. , .. .. I I ., :, ., . • a, I : \ / ~ -r. : " -/: -~ ,-/.· . ., ., / i#,,,. -\"!) 'I. ' ~- '" , .. ~I\ ~ ,,. .. -.,~ / ,I' . , -.~ / .,, ., ' , ' . . , ._.,, ,, ,. -.,' , .. , .. ., -... /: ,, , .. ' \ ~ .. \ I 4 \. \ v· ".\~ "' .,, ., / \ ~ / \ \ ~ .. / • ' ~ ' ' .. ./ \ : o_, '\ .. .,,.,,...,,, \\ -~ ,, rce, , . ··\ .. : I. \_,.• •O , .... . ... ..... . .... .. I ., I I I .. :, .. ., .• :-11 I .. r. . :c :.1 • ,.:r. •. I I ., ·I ., ~ I ·I :0 ., ........ :. r.u . ;.. .... . ,:;... C'I:":• ·~ ~ . . . I . . . . • . .,· . = Hydrologic Soil Group-San Diego County Area, California MAP LEGEND MAP INFORMATION Area of Interest (AOI) D Area of Interest (AOI) Soils Soll Rating Polygons D A Cl ND l!!!!!!!I B Ill BID D C D CID D D D Not rated or not available Soll Rating Lines -A -ND -B -BID -C -CID -D . ; Not rated or not available Soll Rating Points • A • ND • B • BID Natural Resources Conservation Service • C • CID • D D Not rated or not available Water Features Streams and Canals Transportation +++ Rails ~ Interstate Highways ~ US Routes ---Major Roads Local Roads Background • Aerial Photography Web Soil Survey National Cooperative Soil Survey The soil surveys that comprise your AOI were mapped at 1 :24,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: http://websoilsurvey.nrcs.usda.gov Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area. such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Survey Area Data: San Diego County Area, California Version 9, Sep 17, 2015 Soil map units are labeled (as space allows) for map scales 1 :50,000 or larger. Date(s) aerial images were photographed: Nov 3, 2014-Nov 22, 2014 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. 8/19/2016 Page 2 of 4 --- ,. ,.,. .. - - - - - -.. --- ------ Hydrologic Soil Group-San Diego County Area, California Hydrologic Soil Group Hydrologlc Soll Group-Summery by Map Unit-Sen Diego County ArN.. Callfomla (CAUi) .. Map unit symbol ·· Map unit name Rating I AcreelnAOI Percent of AOI MIC Marina loamy coarse B I 1.2 sand, 2 to 9 percent I slopes I Totals for Area of Interest --~-~j---- 1.2 -~-.-~-~~---' . Description Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms. The soils in the United States are assigned to four groups (A, 8, C, and D) and three dual classes (AID, 8/D, and C/D). The groups are defined as follows: Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Group 8. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission. Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission. Group D. 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. These soils have a very slow rate of water transmission. If a soil is assigned to a dual hydrologic group (AID, 8/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes. Rating Options Aggregation Method: Dominant Condition Component Percent Cutoff: None Specified USDA Natural Resources iiiii Conservation Service Web Soil Survey National Cooperative Soil Survey 100.0% 100.0o/o 8/19/2016 Page 3 of4 .J • -- - - ----- -.. .. - --.. Hydrologic Soil Group-San Diego County Area, California Tie-break Rule: Higher Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/19/2016 Page 4 of 4 Ii 11 I J 11 lJ Ii l I I I I I I I I I I I I i a • 1. J, .. t:i w LL ~ w (.) z ~ en i5 w (/) ~ ::J 0 () a:: w i 100 I , .s I /f/lJ// 1 ~ ; f :/ I .,.,._ 130 ill I-::J z 0 20 ~ z -UJ :JE .:: ~ 0 ...J LL. 10 ~ 5 0:: w 6 ~ 0 EXAMPLE: Given: Watercourse Distance (D) = 70 Feet Slope (s) = 1.3% Runoff Coefficient (C) = 0.41 Overland Flow Time (T) = 9.5 Minutes T = 1.8 (1.1-C) VD 3\fs SOURCE: Airport Drainage, Federal Aviation Administration, 1965 FIGURE Rational Formula -Overland Time of Flow Nomograph 3.3 -------- ----- --- .. - ~E Feet 5000 4000 3000 2000 300 200 100 30 20 10 5 EQUATION Tc '" c:::tt·385 Tc L ~E = Time of concentration (hours) .. Waten::ourse Distance (miles) "' Change In elevation along effective slope line (See Agure 3-5){teet) L MIias Feet 3000 0.5 ' ' 2000 ' ' 1800 ' 1800 ' 1400 300 L SOURCE: California Division of Highways (1941) and Kirpich (1940) Nomograph for Determination of Tc 60 50 40 30 20 18 16 14 12 10 ' 9 a 7 6 5 3 Tc Time of Concentration (Tc) or Travel Time (Tt) for Natural Watersheds FIGURE ~ 4 • • .. • •• •• .... .. ---- - - .. -• - - --- Ql a. 0 en Q) jg U) 0 ~ 0 1-1.s·~1 J -n = .015--. 1 , _ 2% ,_____ n = .0175 ~~~~~--.::..:;:.__~~~.....! 2% Concrete Gutter Paved RESIDENTIAL STREET ONE' SIDE' ONLY 10-t------#---1~----1..--~~--t----1,---t,.r-t--t---t----~ s~-----1-~-+===----=-+---++--+--1---1---1-1--1-----..;:,,.~,-+----+--1'+--""l--r 8-+--- 7-t--- 6 5 2~----..;:;:,,,...d-----+--1----1---+--+--+- 1.8 -f------,,--11--,;:::,,~-1f+---l'---.f.--ll---+ n:Yo-+ __ ..,..,,,__ 1.6 -f:------1---1,----""""p......_ 1.4 -f--__;::"!jt....,.--1----..,fil---l....:::......:: 1.0 -f---,,------i'---+----i~-~ o.e-1..;::::""#.ec-----1--.j~--11---1....:::""""4---'l1--+-1~r-1-~~--------+-1~---' 0.8-f--.j~--,;:::,,~-1-1----11----1--,J--,P,,!.::---+-l~r-1--1--....:::.....,. __ ~,_--~ .1...--i,.-~----1- 0.7 -t-1-------#1----.... 0.5 2 3 4 EXAMPLE: Given: Q a: 10 S = 2.5% Cllart gives: Depth= 0.4, Velocity= 4.4 f.p.s. 5 6 7 8 9 10 Discharge (C.F.S.) 20 30 40 50 SOURCE: San Diego County Department of Special District Services Design Manual FIGURE Gutter and Roadway Discharge -Velocity Chart ~ • ,. ,,.._ ---- -- --,. ----.... -... - '? 0 .E g_ 0.3 0.2 0.15 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.009 0.008 EQUATION: V = 1.49 R213 S112 n U) ::::::, 0.2 0.3 0.4 f 0.5 ~0.6 ~ 0.8 "' 0.9 ~ D 1.0 '6' '" "' 0 2 ~ c2 ~~ 30 20 > ~ 9 /' / -0 8 C / 8 7 Q) Ii) ... 6 Q) a. Y~ Q) ~ .f: Q) ~ .f; lJ.I a.. 0 ..J en ~5 ~ 0.007 / 0.006 c2 _/ ~ ~ ,..4 0.005 O 3/ >- 0.~4()~ ::C ~ 0.002 0.001 0.0009 0.0008 0.0007 0.0006 0.0005 0.0004 0.0003 4 5 6 7 8 9 10 20 SOURCE: USDOT, FHWA, HDS-3 (1961) "'g ..J UJ > GENERAL SOLUTION Manning's Equation Nomograph 3 2 1.0 0.9 0.8 0.7 0.6 0.5 0.01 ()91-/ 0.02 9' C: 0.03 i: .9'1 0 :E Q) 0.04 0 0 Cl) en 0.05 w z I C!) 0.06 ::::::, 0 a::: O.Q7 0.08 0.09 0.10 0.2 0.3 0.4 FIGURE ~ I I I I l I I I I t I I I t • j I t I t I I 10.0 EQUATION A.+--P!-.d-~~N-fl;f't,tftfflit1ttt1fmt11Httttttttt11tt1mm1 .1 = 1.44 P 6 0-0.645 p,..i,.++~-+~H~~~'*H"lffl1'tfl'Hil:!1rttttH"ttttttt1tttl'l11 I = Intensity {in/hr) P 6 = &-Hour Precipitation (in) l-l-!-..+:~!-+-f!~H-~d-+,f'!,,rt:fl~~;fffl+l;!f~~:ltttttlHttt!I D = Duration {min) ~ f I ! .1', ..... "" I ~i" ~ "' l'r,,. "'· I "' "'r,. I" ~ ~ I I' 1111 I ' 1' 1' 1'1""1't I 2.0 I ! ......... I ~ r,,. I' f',, "I' I 11 QI I , .,..._ l'i" ., ,., i• ~ I ::c ,,.,...,. I" ~ • 0 ! ' ""-f',, 1~,. ~ ~ I C I' I' I', ,-~ I ., 'C "r-, i" ~p ~ j ' .. ,.... .... i-.'' .. ~ i,.. ~ ~ 4' 5 i" ... ... r,,.r-. i,..i" ~ ~~ ; ~. '{ii ~ i"r, I' " r 6.0 '2. .! · ..., l"o Ir s.s I g 1.0 ~ ~ , r, r I I 5.0 g -. ~ I' ;:o.9 "" r 4.5 '§' ~ ~ ,. g. c0.8 I' ,.. ~ 4.0 "' ~o.rl I I r.. r a.s ~ -I. !"":-,i" r r ~ 0.6 11 Ii ". i I"' "' I 3.0 II ~ , , I 0 5 , , , 1 1 11 1 ;1 I' ' I I 0,41 I I I I I I I I I I I I I IIIIIIIIHIIIIIJlllllltllllllllllllflflllllHi..lll I 1 !"+-.llllffi!'M.Jllllill'lfl 2.5 I I I I I I' l I 111111nTTTlTTTTlTITT11,HllllllllllllllllllllHllmr,.... I 1111,-.i111111IINJ<11111 'r-. 0.3 I I .._ 2.0 --.. .. 1.5 0.2 . , -· · -1.0 . -·· •• L l=!=f=i=t=J:::J=r~-r:r+r.m!=P+J.'.H-J-mH+1+++1t++ttHitH-rltHittHttttlltt-+++r-t-tt+rt-l+tttlttHtl'ltltl ...... ~ ..... ·tttrtttlt1 trltl11"'' t--t,-1"" ....... J-"· fu++ .. I 1 1-.LLJ_LLI I J l~J nlTlllTTTD: o.JJ-ttttJj#tittBfflll#t · . ' lffllffiJ-#ftfilffffifflll 5 6 7 8 9 10 15 20 30 40 50 2 3 4 5 6 Minutes Hours Duration fntenslty-Ouration Design Chart~ Template I I I I I I I I a • I j Directions for Application: (1) From precipitation maps determine 6 hr and 24 hr amounts for the selected frequency. These maps are included in the Coun1y Hydrology Manual (10, 50, and 100 yr maps tnc:Uded in the Design and F'rocedure Manual). (2) Adjust 6 hr precipitation (if necessary) so thai itis within the range of 45% to 65% of the 24 hr precipitation (not applicaple to Desert). (3) Plot 6 hr precipitation on the right side of the chart. (4) Draw a line through the point parallel to the plotted lines. (5) This line is the intensity-duration curve for the location being analyzed. Appllcatlon Form: {a) Selected frequency ___ year (b) Pe= __ in., P24 = -· _ ;~ = %{2) 24 (c} Adjusted P6<2> = __ in. (d) tx = __ min. (e) f = __ in./hr. Note: This chart replaces the Intensity-Duration-Frequency cut'Ves used since 1965. ! ' l ! . . ··' .. ! :;~miionT + ·:·1t·i ·r-1 )ift: ~:1:3;5· ·r::; \tr f , sl:f.f .: ...... :::~: '1!'.~·rn:1ilt~II {;i / ,:;1 J:~ ~~!+-Viti ~i:~~n:: l;JJ ........ -....................... 1····, ....... : ..... ,. ·T J·· ·· ··· i · , ...... " · ~-:· · ···· ··· ···rn +:-:t~lJ~t Hl1!:;1trl i:f~·1-H!-1··t:~!r{tii1N · .......... ··.! i5 .. [;1~iJ~Ji:.~~1.~:sJt~ll~H.~:~·.1 .. !:~_.;_t~.l··H~•· ·-----ii · ~:·+~~i! t1:1+!}l ~'.~+j:; f{5:+t~· !{:: i ilf{!! .::~:=ii i~~lli? li!.1:!:il!:ii}.iJU::J~ii·l·:J::~:Jtij.:t:t~•!•ll~-. · --1~ 1~ 1g1:iw1ih:I~-;g:~1i>:~I UHHf!··l:!f HJ~ i·+:-I~· :~:: .•.. :~~~ :.~:r.rt~~I~;;JQ,~:;:g:#1•~~r~:.~;••r~:~•l•}~·r-~•:~I::7~ 360 ().17 10,25!0.3310.42·0.50,(),58, 0.67 i 0.75 I 0,84 0,92 i !.:QQ. ~ FJG~UREJ I --·------- I i •• • ... •• ... •• ,,.. •• .... •• ... •• •• .. , . , -·---- San Diego County Hydrology Manual Date: June 2003 3.2 DEVELOPING INPUT DATA FOR THE RATIONAL METHOD Section: Page: 3 20 of26 This section describes the development of the necessary data to perfonn RM calculations . Section 3 .3 describes the RM calculation process. Input data for calculating peak flows and Tc's with the RM should be developed as follows: l. On a topographic base map, outline the overall drainage area boundary, showiTlg adjacent drains, existing and proposed drains, and overland flow paths. 2. Verify the accuracy of the drainage map in the field . 3. Divide the drainage area into subareas by locating significant points of interest. These divisions should be based on topography, soil type, and land use. Ensure that an appropriate first subarea is delineated. For natural areas, the first subarea flow path length should be less than or equal to 4,000 feet plus the overland flow length (Table 3-2). For developed areas, the initial subarea flow path length should be consistent with Table 3-2. The topography and slope within the initial subarea should be generally uniform . 4. Working from upstream to downstream, assign a number representing each subarea in the drainage system to each point of interest. Figure 3-8 provides guidelines for node numbers for geographic information system (GIS)-based studies . 5. Measure each subarea in the drainage area to determine its size in acres (A). 6. Determine the length and effective slope of the flow path in each subarea. 7. Identify the soil type for each subarea . 3-20 I i I • I I I I I J I I I I I 1 I I I f I I I I I I I t I I I i I a I J i j Study Area SC /' ··( I . ·1 ' 't .. ( /' ·. ~/ L.- Study Area LA (D Define Study Areas (Two-Letter ID) 0 Define Maps (or Subregions on Region Basis) 0 Define Model Subareas on Map Basis ' ' L ,• ,• ' ,• .............. --" ..... _ ...... . . •' 0 Define Major Flowpaths in Study Area G) Define Regions on Study Area Basls Subarea ID= (LA010112) Node# 1 Map#--~~ Region#---l study Area (ID)#--- © Define Model Nodes (Intersection of Subarea Boundaries with Flowpath Lines) GIS/Hydrologic Model Data Base Linkage Setup: Nodes, Subareas, Links LA 01 01 03 0 Number Nodes r-·~-~R Ej •• •• • • • -• • ---• - --·---- ---- ·-----• San Diego County Hydrology Manual Date: June 2003 Section: Page: 3 22of26 8. Determine the runoff coefficient (C) for each subarea based on Table 3-l. If the subarea contains more than one type of development classification, use a proportionate average for C. In determining C for the subarea, use future land use taken from the applicable community plan, Multiple Species Conservation Plan, National Forest land use plan, etc . 9. Calculate the CA value for the subarea . I 0. Calculate the 1:(CA) value(s) for the subareas upstream of the point(s) of interest. 11. Detennine P6 and P24 for the study using the isopluvial maps provided in Appendix B. If necessary, adjust the value for P6 to be within 45% to 65% of the value for P24. See Section 3.3 for a description of the RM calculation process. 3.3 PERFORMING RATIONAL METHOD CALCULATIONS This section describes the RM calculation process. Using the input data, calculation of peak flows and Tc's should be performed as follows: 1. Determine Ti for the first subarea. Use Table 3-2 or Figure 3-3 as discussed in Section 3.1.4. If the watershed is natural, the travel time to the downstream end of the first subarea can be added to Ti to obtain the Tc. Refer to paragraph 3.1.4.2 (a). 2. Determine I for the subarea using Figure 3-1. If T; was less than 5 minutes, use the 5 minute time to detennine intensity for calculating the flow. 3. Calculate the peak discharge flow rate for the subarea, where Qp = ~(CA) I. In case that the downstream flow rate is less than the upstream flow rate, due to the long travel time that is not offset by the additional subarea runoff, use the upstream peak flow for design purposes until downstream flows increase again. 3-22 • • • • ... -• -- - --- --- ---------- San Diego County Hydrology Manual Date: June 2003 4. Estimate the T1 to the next point of interest. 5. Add the Tt to the previous Tc to obtain a new Tc . Section: Page: 6. Continue with step 2, above, until the final point of interest is reached . 3 23 of26 Note: The MRM should be used to calculate the peak discharge when there is a junction from independent subareas into the drainage system. 3.4 MODIFIED RATIONAL METHOD (FOR JUNCTION ANALYSIS) The purpose of this section is to describe the steps necessary to develop a hydrology report for a small watershed using the MRM. It is necessary to use the MRM if the watershed contains junctions of independent drainage systems. The process is based on the design manuals of the City/County of San Diego. The general process description for using this method, including an example of the application of this method, is described below. The engineer should only use the MRM for drainage areas up to approximately I square mile in size. If the watershed will significantly exceed I square mile then the NRCS method described in Section 4 should be used. The engineer may choose to use either the RM or the MRM for calculations for up to an approximately I -square-mile area and then transition the study to the NRCS method for additional downstream areas that exceed approximately 1 square mile. The transition process is described in Section 4. 3.4.1 Modified Rational Method General Process Description The general process for the MRM differs from the RM only when a junction of independent drainage systems is reached. The peak Q, Tc, and I for each of the independent drainage systems at the point of the junction are calculated by the RM. The independent drainage systems are then combined using the MRM procedure described below. The peak Q, Tc, and I for each of the independent drainage systems at the point of the junction must be calculated prior to using the MRM procedure to combine the independent drainage systems, as these 3-23 ... • • -------- ----- ----- San Diego County Hydrology Manual Date: June 2003 Section: Page: 3 24 of26 values will be used for the MRM calculations. After the independent drainage systems have been combined, RM calculations are continued to the next point of interest. 3.4.2 Procedure for Combining Independent Drainage Systems at a Junction Calculate the peak Q, Tc, and I for each of the independent drainage systems at the point of the junction. These values will be used for the MR\1 calculations. At the junction of two or more independent drainage systems, the respective peak flows are combined to obtain the maximum flow out of the junction at Tc. Based on the approximation that total runoff increases directly in proportion to time, a general equation may be written to determine the maximum Q and its corresponding Tc using the peak Q, Tc, and I for each of the independent drainage systems at the point immediately before the junction. The general equation requires that contributing Q's be numbered in order of increasing Tc. Let Qi, Ti, and 11 correspond to the tributary area with the shortest Tc. Likewise, let Q2, T2, and Ii correspond to the tributary area with the next longer Tc; Q3, T3, and h correspond to the tributary area with the next longer Tc; and so on. When only two independent drainage systems are combined, leave Q3, T3, and 13 out of the equation. Combine the independent drainage systems using the junction equation below: Junction Equation: Ti < T2 < T3 3-24 - -• -• --- -- -- --- - - --- ----- San Diego County Hydrology Manual Date: June 2003 Section: Page: 3 25 of26 Calculate Qn, Qr2, and Qn Select the largest Q and use the Tc associated with that Q for further calculations (see the three Notes for options). If the largest calculated Q's are equal (e.g., QT1 = QT2 > Q13), use the shorter of the Tc's associated with that Q. This equation may be expanded for a junction of more than three independent drainage systems using the same concept. The concept is that when Q from a selected subarea ( e.g., Q2) is combined with Q from another subarea with a shorter Tc (e.g., Qi), the Q from the subarea with the shorter Tc is reduced by the ratio of the I's (h/11); and when Q from a selected subarea (e.g., Q2) is combined with Q from another subarea with a longer Tc (e.g., Q3), the Q from the subarea with the longer Tc is reduced by the ratio of the Tc's (T2/T3). Note #1: At a junction of two independent drainage systems that have the same Tc, the tributary flows may be added to obtain the Qp- This can be verified by using the junction equation above. Let Q3, T3, and 13 = 0. When T1 and T2 are the same, 11 and hare also the same, and T1/T2 and 12/11 = 1. T1/T2 and h/11 ~re cancelled from the equations. At this point, QT1 = Qn =Qi+ Q2. Note #2: In the upstream part of a watershed, a conservative computation is acceptable. When the times of concentration (Tc's) are relatively close in magnitude (within 10%), use the shorter Tc for the intensity and the equation Q = L(CA)I. Note #3:. An optional method of determining the Tc is to use the equation Tc= [(I: (CA)7.44 P6)/Q] 1.ss This equation is from Q = L(CA)I = I:(CA)(7.44 P6!Tc·645 ) and solving for Tc. The advantage in this option is that the Tc is consistent with the peak flow Q, and avoids inappropriate fluctuation in downstream flows in some cases. 3-25