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Home My WebLink AboutCDP 05-46; MOSS RESIDENCE; DRAINAGE STUDY; 2017-12-18.. • • .. .. • • • • • • • .. • .. • .. • .. • • • .. • .. " .. • .. " .. .. .. ◄ .. ◄ COFFEY ENGINEERING, INC . Drainage Study Moss Residence 5015 Tierra Del Oro Street Carlsbad, CA. 92008 CDP05-46 /DWG 508-lA/ GR2017-0061 Prepared For: Steve and Janet Moss and The City of Carlsbad December 18, 2017 ,E,~ORD COPY ~ <2-(-zz.,f I 1 ~ Initial Date DEC 1 9 2017 9666 BUSJNESSPARK AVENUE, SUITE 210 SAN DIEGO, CA 92 1 31 PHONE: (858) 831-01 1 1 FAX: (858) 831-01 79 • .. • • Table of Contents • .. I. Project Description ............................................................................................................................ 3 • 2. Drainage Patterns .............................................................................................................................. 3 3. Purpose and Scope of Report ............................................................................................................ 3 .. 4. Method of Calculations ..................................................................................................................... 3 • 5. Results and Conclusions: .................................................................................................................. 5 • 6. Declaration of Responsible Charge ................................................................................................... 6 • • Appendix A -Reference Drawings & Maps • Moss Residence -Grading & Drainage Plan • • Drainage Map A -Existing On-site Conditions .. • Drainage Map B -Proposed On-Site Conditions Appendix B-Calculations/Evaluations .. • I 00 Year Storm • Weighted Runoff Coefficients .. .. Appendix C -Reference Tables & Figures (County of San Diego Hydrology Manual) • B. I. I -Runoff Coefficients • • Figure 3-1-Intensity-Duration Design Chart ... • I 00-year, 6-hour Isopluvial map • • I 00-year, 24-hour Isopluvial map • • .. .. • • ◄ ... • .. .. .. • 2 .. • • • • • • • • .. • • • • • • • • • • • .. • .. • • • • • .. • ... • • .. • I. Project Description The 13,600 ft2 lot is located at 5015 Tierra Del Oro, Carlsbad CA 92008. The site currently includes a 2, l 00 ft2 single-family residence, swimming pool, and spa. Proposed is the existing house's demolition, and the construction of a 2,575 ft2 single-family residence, associated hardscape, and landscaping areas. A dual sump pump system will be responsible for pumping site storm water to the l 18 ft2 bioretention system located on the southeasterly corner of the site. Existing development west of the deck and patio area will remain undisturbed . 2. Drainage Patterns The site slopes consistently from east to west, where landscape drains and hardscape trench drains capture the runoff and direct it to a sump pump located at the rear yard patio. Storm water is pumped to a 118 ft2 bioretention basin designed for treatment control, where favorable soils conditions support full infiltration. An overflow system will carry any excess ponding to the south end of the bioretention basin planter, where it will release as surface drainage and sheet flow to Tierra Del Oro via a proposed San Diego Regional Standard D-25 curb outlet. Once at the street, runoff travels south along the existing curb & gutter and enter a grated inlet approximately 250' from the site. This inlet is connected to a storm drain that discharges carries runoff along hardened conveyance to the Pacific Ocean. The existing drainage conditions are depicted on Drainage Map A included in Appendix A. The proposed drainage conditions are depicted on Drainage Map B included in Appendix A. 3. Purpose and Scope of Report This report will evaluate the pre-construction hydrologic conditions as well as the post-construction conditions to quantify increases or decreases in runoff from the project and for the design of drainage system components for a 100-yr design storm . 4. Method of Calculations The Rational Method, as defined by the County of San Diego Hydrology Manual (2003), will be used to calculate storm water flow rates. Where noted, the following calculations were used to determine flow properties: Rainfall Characteristics Q = C * I * A, where Q = Flow rate (ft3/sec) C = Runoff coefficient I = Rainfall intensity (in/hr) 3 • • • • • • • • • • .. • .. • .. • .. • .. .. .. ... .. ... -.. A = Area (acres) I= 7.44 * P6 * o-0645, where I = Rainfall intensity (in/hr) P6 = Adjusted 6-hour precipitation (inches) D = Storm duration (min), equal to Tc for time-of-concentration storms Tc= Ti+Tt+Tp (time-of-concentration), where Ti=Over land initial time. Tt=Travel time on natural watersheds . Tp=Travel time on drainage structures (pipes, brow ditch, gutter etc.) Ti= 1.8(1 .1-C) ooso /( so11) (Overland initial time of concentration formula),where D= Watercourse Distance (feet)(see table 3-2 for the max. overland flow length) s =Slope(%) C= Runoff Coefficient Ti=Initial time of concentration (min.) T,=(1 l.9*L3 I 6.H)0385 ( formula for travel time for natural watersheds), where T, = Time of Concentration or Travel time (hours) L = Length of watercourse (miles) 6.H = Change in effective slope height (ft) Pipe and Open Channel Flow Characteristics V = l /n * R213 * S 112 (from Manning), where V = Average cross-sectional velocity (ft/sec) n = Manning roughness coefficient R = Hydraulic radius (ft) S = Slope of water surface (ft height/ft length) ply+ V2/2g + z, + hc =ply+ V2/2g + z2 (from Bernoulli), where p = pressure (lbs/ft2) y = density (lbs/ft3) V = velocity (ft/sec) g = gravity (ft/sec/sec) z = height of fluid (ft) hc = head loss (ft) 4 • • • ... .. • • • • "' .. • .. .. .. • • ◄ .. .. .. .. .. • .. • .. .. .. - ... • .. • 5. Results and Conclusions: The proposed project will result in an increase in runoff from pre-construction levels due to an increase in impervious area. Un-mitigated runoff flowing to the street would increase from 0.25 cfs to 0.55 cfs . Although runoff flow rates increase in post-construction conditions, all drainage from the site will travel along hardened conveyance before discharging to the Pacific Ocean. As a result, flow attenuation is not required in order to reduce post-construction flows to pre-construction values . However, the bioretention basin implemented into the site to satisfy pollutant control requirements will allow for the infiltration of storm water into the native soils underlying the site. When not in overflow conditions, the mitigated runoff reaching the street will be reduced to 0.03 cfs . Full runoff calculations are included in Appendix B . 5 • • • • • • .. • • • • • • • .. • .. • .. • .. • .. .. .. • ... .. .. • .. .. .. " .. .. -.. 6. Declaration of Responsible Charge I hereby declare that I am the Civil 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 design . 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 . -h~ Mic ael Kmnear RCE 76785 Exp. 12-31-18 C--~ . I <--/ l q./ If Date 6 .. • .. • • • .. • .. -.. -- • .. • .. .. • • • • • • .. • .. • .. • .. • .. • .. .. .. • Bibliography • County of San Diego. 2005. Drainage Design Manual http://www.sdcounty.ca.gov/ dpw /floodcontrol/ drainage.html • County of San Diego. 2003. Hydrology Manual http://www.sdcounty.ca.gov/ dpw /floodcontrol/hydrologymanual.html 7 .. .. • • Appendix A -Referenced Plans & Drainage Maps • .. • .. • .. • .. • .. • "' • .. • .. • .. • .. • • .. • .. .. 11 .. ◄ .. 8 I I a I I I I _4 _ _t.17 ff. OROUCHr TOl[RANT VtCETATION PfR lMDSCAl'E PW< <J.67 PONDING 1s· fNG/Nffi/£!) SOIL "£DIA 41-60 ;~ ,,.,__,,.,/fr7J!i IMPfRMEABf.£ ~-~--LINER ALONG S/D£S OF BASIN DETAIL :A ' -BIORETENTION BASIN NO SCALE • SOIi. M£DIA CONS!Sr'S OF 85% WASHED COORS£ SANO, 10% F1N£S (F/JNC£: 8-12%; 8% • 2 IN/HR INF1LTRAOON RAIT, 12% • 1 IN/HR INF1LTRAOON RArE). N/0 5% 0/IGNIIC W,TT£R. FOR ADDITIONAL STANOAIIOS 5££ SAN DIEGO LOW IJJPACT OCVUOPMOO OCSICN MANUAL SECTION 1.2. 4.2 SOIL MEDIA MUST UAJNTNN A MlN/MUM INFILTRA.OON RATE OF S IN/HR > -......, '-~ Lq l) C) I I I I I [ifMJ[j)-~ lie.AN HICH no£ i./N£ AS LOCJ.1!0 0,, FEBRUARY 25. 195.J ( u.s.c d< cs DATUV) '[}@ '[}@ l) ;---. -l, PR BEACH ACCESS EAS£J,J£NT I I :--; u i I ----L I t:?£.IJO'IE A!t/'f 8()IJL0£'1S S£AWA:9D OF' M:l,N 1-'IGH TiDE LINE AS t>C:!RM!.\'W 8Y Tr-IC. II PERMANENT WATER QUALITY TREATMENT FACILITY KEEPING OUR WATERw,ws Q..EN,I MAHT"""""™CAAE·NO..:x:Jff'.A!P§wmo,n:N:B¥:fJff'IAOIJ,J,. lJIJII.: MATERIAL, SIZE ANO PLAC[MENT OF SIGN TO MEU CITY ENGINEER APPROVAL. WATER QUALITY SIGN DETAIL NOSCA/.£ /'!.QI£.: PRIOR TO ANY WORK WfTHIN THE CITY'S RIGHT OF WAY, THE CONTRACfOR WIIU H-11/l: A VAUO RIGHT or WAY PERMIT VACJ.NT EASEMENTS PER F10EUTY NAT/OW.L TITLE COMPANY PR£UMINARY TTTL£ REPORT (ORDER /00167875-994-VNO-RRI, DAITO /MY 11. 2017). THERE ARE EXCEPTIONS Afff.CTING THE SUBJECT PROPERTY. THE FOLLOWING EASEMENT IS USTE:D IN THE J..BOVc REPORT: 0 V, E/,S[MENT FOR LINE OF POLES NID WIRES TOGETHER "1TH THE RICHT OF INGRESS AND £GRESS AND INCIDENTAL PURPOSES RECORDED ON 06/14/1955, IN BOOK 5679. PN;E J67, Of OFFICIAL RECORDS. I ":> ~ 1' "'~ "' ~ '., I ;:.:qj4 h ~ ~ Cl) ~'[} '~ CAf..;.:()RN;J, SI.MI LA-.;{}S C04il.liSSl()N / /~ C(}lJPILATfON Pt.AT M-'7l. ,---,--IN:::·,.,, j Q, I I I (S£PT2016) SD 2016 10-18-2 owe DATED 2/22/17. OiSPOSAL ,r APPROVED SITE OUTS/OE C04STAL Z/PiE I l l_ __________ L_/109.61/ I '[Jt{J MEAN HIGH no£ LINE: EAST£RI.Y UN£: LOCATION OF 11£,W HIGH nDE UN£ AS EST/181.ISHED BY TH£ CALIFORNIA STAT£ LANDS COi/MiSSiON UND£R 1>1£ SJJP[IMSJON OF EVN/ PAGE, PLS 7275. N/0 JIM KOCPKf, PLS 8487, AS SHOWN ON ooct/lJENT CAPTIONED "COMP!tATION PU.T -LOT 15, n[RRA DEL SOL (w,p NO. J052) -CN/t.SBAO, SAN DlfG() COUNTY, OAT£D 02/22/2017. WEST[Ri. Y 1.IN[c LOCATION OF "MEAN HIGH 1/DE UN£ (FEB. 195J)" AS ESTABUSJ<£0 BY THE CALIFORNIA STAIT LANDS COMM/S510N UND£R TH[ SUPUMSION OF EVAN PAGE, Pt.S 7275, AS SHOWN ON IXXUl/£NT CAPTION£D "COMPIU.OON PUT - LOT 15, T1£RRA DEL SOL (w,p NO. J052) -CARI.SBAD. SAN Dl£GO COUNTY. OAT£0 02/22/2017. LOCATION OF ..,,EAN HICH nDE UN£ AS LOCATCll ON FEBRI.AAY 25, 195.J (U.S.C d< G.S DATUM)" PER w,p J052. LOCAOON OF -,,ENI HICH TIDE UN£ AS LOCATED ON FEBRI.AAY 25. 1953" AS PfR RECORD OF SURVfY w,p NO. J150. S£E SAN Dl[G() COUNTY 1/ISCEI.LN/fOUS w,p NO. 287, Fll.£D IN TH£ omCE: OF TH£ COUNTY RECORDER AT TH£ REQUEST OF TH£ STAT£ LANDS COMMISSION ON FEBRUNIY 7, 1955. OEPICTING TH£ LOCATION OF TH£ "ORDINAHY HICH WAT£R MARK OCT. 195.J, • C£RnF1£o ON DECDIBf:R 16. 195.J BY !),\If() K. SPfER, CM/. £NGIN££R, OMSIOH Of STAT£ LANDS. tlfJK.: 1lLlLaL TRW 30.7 8W 29.5 (P)6" TRENCH ORAJH 6' LONG Rll/•41.JO IE•40.80 WATER DISTRICT FORCES TO INSTALL WATER METER & BALL VALV[. CONTRACTOR TO INSTALL REMAINDER. fAP TO MAIN BY CONTRACTOR UNDER DISTRICT SUPERVISION. DRAINAGE NOTES: A/.l UNI/ DRJ/N /J/£S S1l(Jtm TO /£ 6" p;c O 1% MN.IIW SLCY'f (l,(£SS OT1£R'MSC NOTfl) PfR PUN. 2. IWIOSCll'f GPNJfS TO 1£ 2% l/!NMJJl TO OR.NHS NKJ AWAY l1?()r, S1Rt.CTIR£. J. 5. SCFTSCN'E GRAOES TO 8£ 5% AINIWM TO DRNNS (1% "'ll£ FLOW IS CONCENTRATfl)) Am 5% 1i1N1WM A""Y l1?()r, SJJ?I.CJ(I/£ SOit. COlfR -DRJ/H /J/£S SH4U. BE 12" --tn£SS OTIV?MSE NOTfl). NOTIFY CMl ENGHIII FAM' NON-Dl'JJNNG SWP COIOIJ(M5 £/£COi£ lffWIENT OCfl/1,K; CCWSTIIOCOON. HOOS£ /5019 (P)60" /)1,w_ SD MH W/ 251 GPIJ ( 0.56 CFS) OllAL SUIIP PU"P Sl'STEJI WTTH £Jl£RGEHCY ~ UP POWUI SYSIDI "H RI" £!£V-34.48 SD Jlll£T lfaJJ.45 ST11UCTI/Rf 1£•28.48 10 0 ly I 10 ~ AT ALL STOi/11 ""IER asa"'1GC LOWJDl<S. SWAB.£ £IERC'I /JISSl'IJ£RS NIE TO BE JNSTNJ.ED TO Rm.Cf Tl£ DISOWiG£ TD NON-ERW!U 1£1.DOTB -----20 7. NO JlXmONN. F/1>1---0ff IS PR0P0SCD FrYI Tl£ asa"'1GC LOCATIONS. 8. Tl£ Dl1S/1NC ""IER 5UMCC Will 1£ Kl.I.CD AT I.WH NKJ £X1STJIIG ""IER M£T£R RO«MD, A ,£W ""IER 5UMCC CW/£GTK)N TO /MIN ll'lll 8£ I/AOC Am A M:W .. ID? M£T£R INSTA/.l£D. 9. Tl£ Dl1S/1NC Sfl!£R SE1MCC ll'lll - SCALE: 1 "= 1 o· SE COFFEY ENGINEERING, INC. '"' IUSINESsPARI; Av&IJE. S4.IJ( 210 SANDEG0.CAt2131 PHISSS}a)l-0111 FMl8.58~1-Ol19 62716 R.C.£✓CCRT. NO. SPRINKLER PER CITY ~ ff.1TOw-~ I WM 2.5' (/l ~ rricY.}¼6 1:o· I {P) 0-25 ~CUTI.ET 0 2%] llOO (HO MH) o-o.~2 CFS I V-2.6J FPS , 40.97/l "' ~ ~ h CON(; ROU£D CURB. TYP 2 • NC DISCIWiG£ FROM <Ml1fLOWTlfiO(JCH ""11. £-<1.95 (P)4' CURB "'tGUTTUI PER Tso c 2 "' '[j JO I 6/J0/2018 ~ (£)WM TO 8£ REMOVED AHO SEIMCE KII.1.£D AT UAIN " "' 10· ~ r::== \60' 'AS BUILT' POST CONSTRUCTION BMP TABLE n'PE SIZE OY,NERSHIP MOSS __ _ I I I (PRIVATE) I REVIEVE0 BY, BlORETENllON 118 S.F. RC£ ___ EXP. ___ _ JNSPECT□R 1--+--t---------+--t---t---t---1 ~ c11Xr.lf:~fi.Gc:.~.~~AD DATE f 1Nf11AL ~ ltNCINmt or ~ REVISION DESCRIPTION £ATE I t"'lTIAl I DATE j lNJTW. On<ER APPRCNN.. ON APP80YAL GRADING PLANS FOR: MOSS RESIDENCE 5015 TIERRA DEL ORO DATE DATE I SH6ETS I I I I I I I I I I .>, ----. ~ l..1..1 ) Pre -- DMA X (Sheet flow to Street) Y (Sheet flow to Bluff) Z (Ocean) Total 11@ fl Impervious Area 1,712.00 1,830.00 3,542.00 Si COFFEY ENGINEERING, INC. Pervious Area Total (SF} Total (AC} 546.00 2,258.00 0.0521 4,663.00 6,493.00 0.1491 4,849.00 0.111 5,209.00 13,600.00 0.312 11•1~1 C-Value Q (CFS} %Imp. 0.711 0.251 75.821 0.331 0.33[ 28.18j 0.58 I . :"• . ·1 .. .,. SCALE: 1 "=20' LEGEND SYMBOL DESCRIPTION PROPERTY LINE EXISTING CONTOUR PROPOSED CONTOUR DIRECTION OF FLOW PVC PIPE ---90--- OMA LIMIT IMPERVIOUS AREAS PERVIOUS AREAS PACIFIC OCEAN ---90--- SITE CHARACTERISTICS ... ~ -. . --• HYDROLOGIC SOIL GROUP ........... A • APPROXIMATE DEPTH TO GROUND WATER ......... >24' • CRITICAL COARSE SEDIMENT YIELD AREAS ........... NONE PRESENT ON SITE OMA MAP A ': EXISTING CONDITIONS MOSS RESIDENCE 5015 TIERRA DEL ORO ST CARLSBAD CA 92008-4351 I I I I I I I I I I I I >, ----. ~ L1..1 ~ Post DMA A (Biofiltration) B (Sheet flow to Bluff) C (Ocean) D (Sheet Flow to Street) Total if@ Impervious Area 3,607.00 0.00 206.00 3,813.00 ~ COFFEY ENGINEERING, INC. ~ ~ Pervious Area Total (SF) 608.00 4,215.00 4,330.00 4,330.00 4,849.00 0.00 206.00 4,938.00 13,600.00 11•11 11■■11 ~ - Total (AC) C-Value Q (CFS) %Imp. 0.097 0.78 0.52 85.58 0.099 0.10 0.07 0.00 0.111 0.005 0.90 0.03 100.00 0.312 0.62 SCALE: 1 "=20' . # r • LEGEND SYMBOL DESCRIPTION PROPERTY LINE EXISTING CONTOUR PROPOSED CONTOUR DIRECTION OF FLOW PVC PIPE ---90--- ---90--- OMA LIMIT IMPERVIOUS AREAS PERVIOUS AREAS PACIFIC OCEAN SITE CHARACTERISTICS I I ~ • HYDROLOGIC SOIL GROUP ........... A • APPROXIMATE DEPTH TO GROUND WATER ......... >24' • CRITICAL COARSE SEDIMENT YIELD AREAS ........... NONE PRESENT ON SITE DMA MAP 'B:· PROPOSED CONDITIONS MOSS RESIDENCE 5015 TIERRA DEL ORO ST CARLSBAD CA 92008-435 l Appendix B -Calculations/Evaluations 9 100 Year Storm ----------- Table B -Pre Construction Flow Conditions Summary (5 min minimum) Runoff Total time-of-Rainfall Basin Coefficient, concentration, Tc Intensity, I Area, A Flow ID (Basin) C (min) (in/hr) (acres) Q (cfs) Flow ID (Basin) Flow Description X (Sheet flow to Street) 0.71 5.00 6.85 0.05 0.25 X (Sheet flow to Street) Sheet flow to Street Y {Sheet flow to Bluff) 0.33 5.00 6.85 0.15 0.33 Y (Sheet flow to Bluff) Sheet flow to Bluff Sum= 0.58 Table B -Post Construction Flow Conditions Table B -Hydraulics of Proposed Structures Summary (5 min minimum) Runoff Total time-of-Rainfall Basin Coefficient, concentration, Tc Intensity, I Area, A Flow ID (Basin) C (min) (in/hr) (acres) Q (cfs) Flow ID (Basin) Flow Description A {Biofiltration) 0.78 5.00 6.85 0.10 0.52 A (Biofiltration) Pump to Bioretention to Street B {Sheet flow to Bluff) 0.10 5.00 6.85 0.10 0.07 B (Sheet flow to Bluff) Sheet flow to Bluff D {Sheet flow to Street) 0.90 5.00 6.85 0.00 0.03 D (Sheet flow to Street) Sheet flow to Bluff 0.62 Weighted Runoff Coefficients DMA Impervious Area Pervious Area Total (SF) Total (AC) C-Value Q (CFS) %Imp. Pre X (Sheet flow to Street) 1,712.00 546.00 2,258.00 0 .052 0.71 0.25 75.82 Y (Sheet flow to Bluff) 1,830.00 4,663.00 6,493.00 0.149 0.33 0.33 28.18 Z (Ocean) 4,849.00 0.111 Total 3,542.00 5,209.00 13,600.00 0.312 0.58 DMA Impervious Area Pervious Area Total (SF) Total (AC) C-Value Q (CFS) %Imp. Post A (Biofiltration) 3,607.00 608.00 4,215.00 0.097 0.78 0.52 85.58 B (Sheet flow to Bluff) 0 .00 4,330.00 4,330.00 0.099 0.10 0.07 0.00 C (Ocean) 4,849.00 0.111 D (Sheet Flow to Street) 206.00 0.00 206.00 0.005 0.90 0.03 100.00 Total 3,813.00 4,938.00 13,600.00 0.312 0.62 Pre-Construction (CFS) Post-Construction (CFS) I Discharge to Street 0.25 0.55 I Discharge to Bluff 0.33 0.07 Total Discharge from Site 0.58 0.62 Appendix C -Reference Tables & Figures (County of San Diego Hydrology Manual) 10 Appendix B: Storm Water Pollutant Control H ydrologic Calculations and Sizing Methods B.1 DCV DCV is defined as the volume of storm water runoff resulting from the 85'h percentile, 24-hr storm event. The following hydrologic method shall be used to calculate the DCV: Where: DCV = C X d x A X 43,560 s f /ac X 1/12 ft/in DCV = 3,630 X C x d X A DCV = Design Capture Volume in cubic feet C = Runoff factor (unitless); refer to section B.1.1 d = 85th percentile, 24-hr storm event rainfall depth (inches), refer to section B.1.3 A = Tributary area (acres) which includes the total area draining to the BMP, including any offsite or onsite areas that comingles with project runoff and drains to the BMP. Refer to Section 3.3.3 for additional guidance. Street redevelopment projects consult Section 1.4.2. B.1.1 Runoff Factor Estimate the area weighted runoff factor for the tributary area to the BMP using runoff factor (from Table B.1-1) and area of each surface type in the tributary area and the following equation: Where: C = Runoff factor for area X A, = Tributary area X (acres) ICxAx C = I Ax These runoff factors apply to areas receiving direct rainfall only. For conditions in which runoff is routed onto a surface from an adjacent surface, see Section B.2 for determining composite runoff factors for these areas. Table B.1-1: Runoff factors for surfaces draining to BMPs -Pollutant Control BMPs Surface Runoff Factor Roofs1 0.90 Concrete or Asphalt1 0.90 Unit Pavers fo;:outed) 1 0.90 Decomposed Granite 0.30 Cobbles or Crushed A2:2:regate 0.30 Amended, Mulched Soils or Landscape 0.10 Compacted Soil (e.g., unpaved parking) 0.30 1. Surface is considered impervious and could benefit from use of Site Design BMPs and adjustment of the runoff factor per Section B.2.1. B-2 February 2016 Appendix B: Storm Water Pollutant Control Hydrologic Calculations and Sizing Methods I Surface Runoff Factor I Natural (A Soil) 0.10 Natural (B Soil) 0.14 Natural (C Soil) 0.23 Natural (D Soil) 0.30 B.1.2 Offline BMPs Diversion flow rates for offline BMPs shall be sized to convey the maximum flow rate of runoff produced from a rainfall intensity of 0.2 inch of rainfall per hour, for each hour of every storm event. The following hydrologic method shall be used to calculate the diversion flow rate for off-line BMPs: Where: Q=Cx i xA Q = Diversion flow rate in cubic feet per second C = Runoff factor, area weighted estimate using Table B.1-1 i = Rainfall intensity of 0.2 in/hr A = Tributary area (acres) which includes the total area draining to the BMP, including any offsite or onsite areas that corningle with project runoff and drain to the BMP. Refer to Section 3.3.3 for additional guidance. Street redevelopment projects also consult Section 1.4.2. B.1.3 85th Percentile, 24-Hour Storm Event The 85th percentile, 24-hour isopluvial map is provided as Figure B.1 -1. The rainfall depth to estimate the DCV shall be determined using Figure B.1 -1. The methodology used to develop this map is presented below: B.1.3.1 Gage data and calculation of 85th percentile The method of calculating the 85th percentile is to produce a list of values, order them from smallest to largest, and then pick the value that is 85 percent of the way through the list. Only values that are capable of producing run off are of interest for this purpose. Lacking a legislative definition of rainfall values capable of producing runoff, Flood Control staff in San Diego County have observed that the point at which significant runoff begins is rather subjective, and is affected by land use type and soil moisture. In highly-urbanized areas, the soil has a high impermeability and runoff can begin with as little as 0.02" of rainfall. In rural areas, soil impermeability is significantly lower and even 0.30" of rain on dry soil will frequently not produce significant runoff. For this reason, San Diego County has chosen to use the more objective method of including all non-zero 24-hour rainfall totals when calculating the 85th percentile. To produce a statistically significant number, only stations with 30 years or greater of daily rainfall records are used. B-3 February 2016 10.0 9.0 8.0 7.0 6.0 .... ""'" ) .. 5.0 4.0 } " 3.0 ) 2.0 ) -c- .... " r..;; t--. .... .. ~ ..... .. r-.. i... ~--.. ..... ~ .... ·, .... .... ... , ... , "'' i..... .... .... .... .... .... . i..."' .... ....... "' ... i..., . .... .... • ..... ... ,, ... .... "' .... .. ' ""I', " • ...... . ..... ~ .... ... .... ~ ,, ..... "~ . .... ....... .... i... .. r-., ... ..... ....... "' ... ..... ... ~ lo. .. :-., .. ... ~ ... .. .. .. ...... .. .. ...... .. lo. .... .. ..... " .... "' i. .. '~ 5 ~ ... i... """" .. , Ql ..c: -~1.0 -;::o.9 ~0.8 Ql £ 0.7 0.6 ; 0.5 0.4 I 0.3 0.2 0. ' ' ' ' ' ' 5 6 7 8 9 10 ..... .. 15 20 3·0 Minutes .. .... .... .... .. .... .. .... .. .. .. .. .. .. .. 40 50 Duration EQUATION I = 7.44 Ps o-0.645 I = Intensity (in/hr) Ps = 6-Hour Precipitation (in) D = Duration (min) 'r-.. :-,.I""' "' ...... "' ... ... , 1 ... .... ' .... ...... r-.. ' ... r-"' .... l"-,1"-, I• ..... .. .... 1, ...... r" i,. r-/~ .. "'"' :...r,.. "'"" .. ~ .... I• .. I"-, "" ~r-.. ""' r-.. I• .... "'r-. "' .. 1, ,~ r-..... ' .. 1,.., r,.."' .. r,..r-,. "~ ~~ 2 3 4 Hours 5 6 't' I 0 ~ "CJ ~ 0 6.0 -g 5.5 ~ 5.0 g 4.5 '§' 0 4.0 ~ 3.5 ~ 3.0 2.5 2.0 1.5 1.0 Intensity-Duration Design Chart -Template Directions for AppHcatlon: (1) From precipitation maps determine 6 hr and 24 hr amounts for the selected frequency. These maps are included in the County Hydrology Manual (10, 50, and 100 yr maps included in the Design and Procedure Manual). (2) Adjust 6 hr precipitation (if necessary) so that it is 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. Application Form: 100 (a) Selected frequency ___ year (b) Ps = ~ in., P24 = ~ ,;6 = 58 %(2) 24 (c) Adjusted p6<2l = ~ in. (d) Ix= 5 __ min. (e)I= 6.85in./hr. Note: This chart replaces the Intensity-Duration-Frequency curves used since 1965. 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