HomeMy WebLinkAboutCDP 05-46; MOSS RESIDENCE; DRAINAGE STUDY; 2017-11-03
COFFEY ENGINEERING, INC.
Drainage Study
Moss Residence
5015 Tierra Del Oro Street
Carlsbad, CA. 92008
Prepared For:
Steve and Janet Moss
and
The City of Carlsbad
TRECE1VE
tO\I 0 6 2.017
LAND DEVELOPMENT
ENGINEERING
c
ujI cr C76785
7
November 3, 2017
9666 BUSINESSPARK AVENUE, SUITE 210 SAN DIEGO, CA 92131 PHONE: (858)831-01 1 1 FAX: (858)831-0179
Table of Contents
Project Description . 3
Drainage Patterns ........................................ 3
Purpose and Scope of Report...................... 3
Method of Calculations............................... 3
Results and Conclusions............................. 5
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
100 Year Storm
Weighted Runoff Coefficients
Appendix C - Reference Tables & Figures (County of San Diego Hydrology Manual)
B. 1.1 - Runoff Coefficients
Figure 3-1— Intensity-Duration Design Chart
100-year, 6-hour Isopluvial map
100-year, 24-hour Isopluvial map
2
Project Description
The 13,600 ft2 lot is located at 5015 Tierra Del Oro, Carlsbad CA 92008. The site currently
includes a 2,100 f12 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 118 ft2 bioretention system located on the southeasterly corner of the site.
Existing development west of the deck and patio area will remain undisturbed.
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.
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.
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* J*A where
Q = Flow rate (ft'/sec)
C = Runoff coefficient
I = Rainfall intensity (in/hr)
3
A = Area (acres)
= 744 * P6 * D °645, where
I = Rainfall intensity (in/hr)
P6= Adjusted 6-hour precipitation (inches)
D = Storm duration (mm), equal to T for time-of-concentration storms
Tc = Ti+T+Tp (time-of-concentration), where
Ti=Over land initial time.
TtTravel time on natural watersheds.
Tp=Travel time on drainage structures (pipes, brow ditch, gutter etc.)
Ti= 1.8(1.1-C) D°5° I( s°33 ) (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
Tt = (11.9*L3 / AH)°385 (formula for travel time for natural watersheds), where
Tc = Time of Concentration or Travel time (hours)
L = Length of watercourse (miles)
AH = Change in effective slope height (ft)
Pipe and Open Channel Flow Characteristics
V = 1/n * R213 * S112 (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 + zi + hL = ply + V2 /2g + Z2 (from Bernoulli), where
p = pressure (lbs/ft2)
y = density (lbs/fl3)
V = velocity (ft/sec)
g = gravity (ft/sec/sec)
z = height of fluid (ft)
hL = head loss (ft)
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.
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.
';'; ~7 C114 C; ~ F
-
Michael Kinnear
RCE 76785
Exp. 12-31-18
on
Bibliography
County of San Diego. 2005. Drainage Design Manual
http: / /www.sdcountv.ca.gov/dpw/floodcontrol/drainage.html
County of San Diego. 2003. Hjidrology Manual
http: / /www.sdcountv.ca.gov/dpw/floodcontrol/hvdrologvmanual.html
Appendix A - Referenced Plans & Drainage Maps
RO4'S
60L1.CE65
,t.., ON (LOPE
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/
/
/ /
\s (P)6' TRENCH
WA' / BRAIN 3' LONG
RIM J9 90
7006"
255
IR 37!1
4.48
LANDSCAPE
AREA
UNDER
STAIRS
34.38 RIM
33.85 IE
Fo PR BASEMENT
FF34.68
PAEVS3.98
(P)INLET
34.38 RIM
33.68 IE
'PVCOI% •',,.
41.2 RIM
39.7 IE
ET /1
RIM!
IT
SUET
"1 #M4 41 44
PAD 42 48 21,
-
1] (P)OS oj I 42.00 IE LNOERDR.AIN
.'.t cI'TA I 1, MAIN '' -
FLOOR
FEX44,68
FORCE MAIN
.a•- j'ft''. I
,K4I
k
.1') 'i"-" "IAD0_0.= =..............._._. TO RIA/I /54.59 '=-=., - ' ORO (P) 5'TRAND
'GUTTESITION TO Z ROLLED
==
STREET h l SERVICE
&FIRE
I SPRINKLER PER W-34
-T \WATER SHOWN
(P) D-25 CU B OUTLET
ON
02%!
-0 0.16 CFS
(M)4' ERRS &
71= 2,3 FPS CONG
40.97 fL
/0/ CU/B,
GUTTE PER TY
STA1L0RDS 1
R 0/S/RICE FRCES TO INSTALL
R MEItR & GALL VALVE.
RACTQI? ED /!YSTALL REMAINDER. I'
TO MA/N BY 4ONTRACTOR UNDER
TIT2T RI(PFRVM)FIN.
778 51/ BI
DETAIL I 'A'
= FREEBOARD;
PONDING; 43.
V BOTTOff: 43.7
PER CI SEA/IL
N
)RETENTION BASIN PER
iNIA
th
I,2I4
----t
CONCRETE
SN,I/E'WAY
)jRDS
TIERRA
JJEL
TY2' FIR GAS
SHOWN ON 741-5
Z
C
z
ABBRE VIA TIONS
BRW (GRADE AT) BOTTOM OF RETAINING WALL PLTR PLANTER/PLANTING AREA
CENTER LINE PP POWER POLE
ELF- (E) EXISTING PR; (P) PROPOSED
FF FINISH FLOOR PVMT PAVEMENT
FL; f FLOWLINE PER PAVERS
FY58 FRONT YARD SETBACK RTSB REAR YARD SETBACK
HP HIGH POINT SORSO S.D. REGIONAL STANDARD DRAWINGS
IE INVERT ELEVATION SF50 SIDE YARD SETBACK
FCC PORTLAND CEMENT CONCRETE 1W TOP OF WALL
EASEMENTS
EASEMENT FOR LINE OF POLES AND WIRES TOGETHER WITH THE RIGHT OF INGRESS AND
EGRESS AND INCIDENTAL PURPOSES RECORDED ON 06/14/1955, IN BOOK 5679, PAGE 367,
OF OFFICIAL RECORDS.
EFFECTIVE I DROUGHT TOLERANT OVERFLOW CONTROL
12' PONDINC
AREA
/Vt LANDSCAPE PLAN
43.67 VEGETATION PER I. E. 42.00
EMGINEf SOIL M
> I =-
TLOCATED
C)
(c) 1
H TIDE LINT AS MEAN HIGH
'III FEBRUARY
I 25. 7953 ( U.S.C. S G.S.
C)
REMOVE ANY BOULDERS SEAWARD OF MEAN
HIGH TIDE LINE AS DETERMINED BY THE
CALIFORNIA SlATE LANDS COMMISSION
COMPILATION FLAT MHTL
(SEPL2016) SD 2016 10-18-2. DWG DATED
2/22117 DISPOSAL AT APPROVED SITE I OUTSIDE COASTAL ZONE.
III
/
/
GRADING PLAN NOTES
PRIOR TO THE ISSUANCE OF ANY CONSTRUCTION PERMIT, THE APPLICANT SMALL INCORPORATE ANY CONSTRUCTION BEST MANAGEMENT PRACTICES NECESSARY TO COMPLY WITH CHAPTER 74,
ARTICLE 2, DIVISION I (GRADING REGULATIONS) OF THE SAN DIEGO MUNICIPAL CODE,INTO THE CONSTRUCTION PLANS OR SPECIFICATIONS.
PRIOR TO THE ISSUANCE OF ANY CONSTRUCTION PERMIT THE APPLICANT SHALT SUBMIT A WATER
POLLUTION CONTROL PLAN (WPCP). THE WPCPW SHALL BE PREPARED IN ACCORDANCE WITH THE
GUIDELINES IN APPENDIX E OF THE CLEFS STORM WATER STANDARDS,
PRIOR ED THE ISSUANCE OF ANY CONSTRUCTION PERMIT, THE OWNER/PERMIrTEE SMALL ENTER INTO A MAINTENANCE AGREEMENT FOR THE ONGOING PERMANENT BMP MAINTENANCE.
SATISFACTORY TO THE CITY ENGINEER.
DRAINAGE NOTES
I. ALL MAIN DRAW LINES SHOWN TO BE 6' PVC 0 IX MINIMUM SLOPE UNLESS OTHERWISE NOTED.
ALL CATCH BASIN LEADS EQ BE 4 PVC 0 2% MINIMUM SLOPE UNLESS OTHERWISE NOTED.
HAROSCAPE GRADES TO BE IX MINIMUM TO DRAINS AND AWAY FROM STRUCTURE.
SOF7SCAPE GRADES TO BE 27 MINIMUM TO DRAINS (TX WHERE FLOW IS CONCENTRATED) AND 2% MINIMUM AWAY FROM STRUCTURE.
SOIL COVER ABOVE DRAIN LINES SMALL BE 72' MINIMUM UNLESS OTHERWISE NOTED.
NOTIFY CIVIL ENGINEER It ANY NON DERRING SWAP CONDITIONS BECOME APPARENT DURING CONSTRUCTION.
AT ALL STORM LETTER DISCHARGE LOCATIONS, SUITABLE ENERGY DISSIPATERS ARE TO BE INSTALLED
TO REDUCE THE DISCHARGE TO NON-ERODIBLE VELOCITIES.
PROPOSED MAINTAIN EXISTING 150W PATTERNS PROPOSED REDUCED EXISTING IMPERVIOUS AREAS. THEREFORE, NO ADDITIONAL RUN-OFF IS PROPOSED FOR THE DISCHARGE LOCATIONS.
LEGEND
DESCRIPTION STD DWG 5MDQL
PROPERTY LINE
ROW CENTERLINE -- -
(E) CONTOUR qo .....................
(P) CONTOUR -90------
(E) SPOT ELEVATION
(P) SPOT ELEVATION
DRAINAGE S WALE OR DIRECTION OF FLOW
(P) RETAINING WALL
(P) CMU STEMWALL .---
(P)WALL
(P) BUILDING FOOTPRINT
(E) BUILDING FOOTPRINT
(P)PVC DRAIN (SIZE NOTED) ------- -
(P) 4 LANDSCAPE DRAIN (NOS OR ED) 0
(P) 4" MARDSCAPE DRAIN (NDS OR EQ)) U
(P) DOWNSPOUT LOCATION .DS
(P)CONCRETE WALKWAY ....
(P) 20 CONC. DRIVEWAY
(P) CURB & GUTTER
W/ 5' TRANSITION TO ROLLED CURB
(P)6 TRENCH DRAIN IIIIIIIIIIIIIIlII
(P) 0-27 SIDEWALK UNDERDRAIN - - -
(M) 24' ORAIN BOX (BROOKS OR ED)
7' WATER SERVICE ,/METER & FIRE PROTECTION W-SA
2.J6' CLASS 11
vo PERMEABLE BASE_____ 38.74
AGGREGATE " 'TA-" " "'' "\ FILTER
STORAGE LATER \ COURSE-
DETAIL A' - BIORETENTION BASIN S
J' CLEAN
AND OVER 3'
NO SCALE NO.8 STONE I
SOIL MEDIA CONSISTS OF 85% WASHED COURSE SAND, 70% FINES (RANGE;
B-129, 8% 2 IN/HR INFILTRATION PATE. 727 = I IN/HR INFILTRATION
RATE), AND 5% ORGANIC MATTER. FOR ADDITIONAL STANDARDS SEE SAN DIEGO ii.. LOW IMPACT DEVELOPMENT DESIGN MANUAL SECTION 7.2.4.2 SOIL MEDIA MUST f,,))
MAINTAIN A MINIMUM INFILTRATION RATE OF 5 IN/HR -
AI DSCA
- -1 -3T1 - N66'54'IO'E '221 69' - ISET_ (P)6D" OHM SD NH W/ LANDSCAPE ------ TRW 415'\ II 156 " BRW 345 's
AREA /,k
IYP
257 GPM ( 056 CFS DUAL SUMP
Alt, UP
PB SYSTEM KITH EMERGENCY 2' PVC :IScnARGE w GALL
(F)6- TRENCH
DRAIN 6'WNG ICRI
ffEI48
PUMP
FROM OVERFLO
THROUGH WALL
RIM 47 IE=40.80 30 gi STRUCTURE IT 2848 It 41 95
I (E) WM TO BE REMOVED AND,
OWNER/APPLICANT SERVICE KILLED AT MAIN
JANET AND STEVE MOSS (E)SURVEY MONUMENT MOSS LIVING TRUST 08-28-00 PROTECT IN PLACE OR I 25408 COLETTE WAY REPLACE BY LICENSED "17 CALABASAS, CA 92067 LAND SURVEYOR
LEGAL DESCRIPTION
LOT IS OF MAP 3052
APR: 210-020-I5
DRAW BE FR
BENCHMARK CIVIL ENGINEER:
/
CHECKED BE JC
CITY OF CARLSBAD CONTROL POINT NO. 57 (CLSB-057). JOHN S. COFFEY ORIGINAL 06/29/17
2. 5 DISK IN DRAINAGE BOX AT THE SOUTHWEST LOANER OF AVE/LISA ENCINAS COFFEY ENGINEERING. INC. R6MSDYI AND CANNON ROAD. 10850 SCRIPPS RANCH BLVD., SUITE 702 77/1/17
SAN DIEGO, CA 92737 - 0851511W 2 TIDY.; 44.07' DATUM. 540 29 (858)831-0111
FAX: (858) 637-0779 088510% 3
20 SOURCE OF TOPOGRAPHY 0 20 40 60 120070N
JOHN S. COFFEY DATE 8151510% 5
COFFEY ENGINEERING, INC. RCE 062716 SCALE: 1 "=20' 9666 BUSINESSPARK AVENUE, SUITE 270
SAN DIEGO, CA 92737 DRAINGE&
(858) 831-0111 _______________________________________________________________________________ JOHN S. COFFEY. LS 8733 GRADING PLAN
DATE MARCH 75, 2077 GRADING TABULATIONS I
DATUM NOTE I TOTAL AMOUNT OF SITE TO BE GRADED: Q,l 0 ACRES X OF TOTAL SITE' 33% I
AMOUNT OF CUT. 300,QY CUBIC YARDS MAXIMUM DEPTH OF CUT I FEET I SCALE !'lO'
THE ELEVATIONS ON THIS PLAN ARE IN REFERENCE TO A CITY OF AMOUNT OF NW 30 CF CUBIC YARDS MAXIMUM 0 OF NW .I FEET I
CARLSBAD=APPROVEO BENCHMARK TO A NAVOBR DATUM. OTHER PLANS IN 11110 I MAXIMUM HEIGHT OF PILL SLOPE(S); _Q_ FEET SLOPE P1170: 1LLL. I
APPLICATION MAY STILL BE REFERENCING THE NC 5029 DATUM. AT THIS SITE I MAXIMUM HEIGHT OF cur SLOPE(S): _._L. FEET SLOPE PATIO: L&_. LOCATION, ELEVATIONS FROM THE NGV029 DATUM WERE FOUND TO BE 2.19 FF07 I AMOUNT OF tR1POftY/ EXPORT SOIL: .,.2.29, CUBIC YARDS (, 1
LOWER THAN ELEVATIONS FROM THE NAV088 DATUM. THEREFORE, SUBTRACT 2. lB I RETAINING! CRIB WALLS: LENGTH__ FEET MAXIMUM HEIGHT' _O,,_, FEET
FEET FROM ANY (EXISTING OR PROPOSED) SPOT ELEVATION OR CONTOUR SHOWN I
ON THIS PLAN TO CONVERT TO THE N0V029 DATUM REFERENCE. FOR EXAMPLE:
40.00 FEET ON THIS PLAN IS AT THE SAME ELEVATION AS 37.82 FEET ON THE
OLD (NGVD29) PLANS. SHT.J.OF_3_.SHTS
7K'
/
- . /
I I
I
1G9.61
N665410E 22769'
L1IN Y
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
LEGEND
DESCRIPTION
PROPERTY LINE
EXISTING CONTOUR
PROPOSED CONTOUR
DIRECTION OF FLOW
PVC PIPE
DMA LIMIT
IMPERVIOUS AREAS
PERVIOUS AREAS
PACIFIC OCEAN
BASIN
/
SCALE: 1"=20'
SYMBOL
go-
SITE CHA RA CTERIS TICS
HYDROLOGIC SOIL GROUP...........A
APPROXIMATE DEPTH TO GROUND
WA TER ......... >24'
CRITICAL COARSE SEDIMENT
YIELD AREAS...........NONE PRESENT ON SITE
DMA MAP A'- EXISTING CONDITIONS
MOSS RESIDENCE
KS: COFFEY ENGINEERING, INC. 5015 TIERRA DEL ORO ST CARLSBAD CA 92008-4351
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) 000 4,330.00 4,330.00 0.099 0.10 0.07 0.00
C (Ocean) 1 4,849.00 0.111
D (Sheet Flow to Street) 1 206.00 O.C)01 206.00 0.0051 0.901 0.03 100.00
Total 3,813.00
_____
4,938.00 13,600.00 0.312 0.62 SCALE: 1 "=20'
LEGEND
DESCRIPTION
PROPERTY LINE
EXISTING CONTOUR
PROPOSED CONTOUR
DIRECTION OF FLOW
PVC PIPE
DMA LIMIT
IMPERVIOUS AREAS
PERVIOUS AREAS
SYMBOL
BASIN
BASINA
J_y
BASIN B
7 -
PACIFIC OCEAN
SITE CHARA CTERIS TICS
HYDROLOGIC SOIL GROUP...........A
APPROXIMATE DEPTH TO GROUND
WA TER ......... >24'
CRITICAL COARSE SEDIMENT
YIELD AREAS ........... NONE PRESENT ON SITE
DMA MAP 'B1 PROPOSED CONDITIONS
MOSS RESIDENCE
Em COFFEY ENGINEERING, INC. 5015 TIERRA DEL ORO ST CARLSBAD CA 92008-4351
Appendix B - Calculations/Evaluations
100 Year Storm
Table B - Pre Construction Flow Conditions
Summary
Flow ID (Basin)
Runoff
Coefficient,
C
(5 min minimum)
Total time-of-
concentration, T
(mm)
Rainfall
Intensity, I
(in/hr)
Basin
Area, A
(acres) Q (cfs) Flow ID (Basin) Flow Description rX (Sheet flow to Street) 0.711 5.001 6.851 0.05 0.25 X (Sheet flow to Street) Sheet flow to Street
V (Sheet flow to Bluff) 0.331 5.001 6.851 0.151 0.33 V (Sheet flow to Bluff) Sheet flow to Bluff
Sum = 0.58
Table B - Post Construction Flow Conditions Table B -Hydraulics of Proposed Structures
Summary
Flow ID (Basin)
Runoff
Coefficient,
C
(5 min minimum)
Total time-of-
concentration, T
(mm)
Rainfall
Intensity, I
(in/hr)
Basin
Area, A
(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
V (Sheet flow to Bluff) 1,830.00 4,663.00 6,493.00 0.149 0.33 0.33 28.18
Z (Ocean) 4,849.001 0.1111
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) 4,849.001 0.111
D (Sheet Flow to Street) 206.00 0.00 206.001 0.0051 0.901 0.03 100.00
Total 3,813.00 4,938.00 13,600.00 0.312 0.62
Pre-Construction (CFS) Post-Construction (CFS)
Discharge to Street 0.25 0.55
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 Hydrologic Calculations and Sizing Methods
B.1 DCV
DCV is defined as the volume of storm water runoff resulting from the 85th percentile, 24-hr storm
event. The following hydrologic method shall be used to calculate the DCV:
DCV = C x d x A x 43,560 sf/ac x 1/12 ft/in
DCV = 3,630 x C d x
Where:
DCV = Design Capture Volume in cubic feet
C = Runoff factor (unitless); refer to section B.l.l
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)
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
Roofs' 0.90
Concrete or Asphalt' 0.90
Unit Payers (grouted)' 0.90
Decomposed Granite 0.30
Cobbles or Crushed Aggregate 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
_
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:
Q = C x i x A
Where:
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 comingle 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:
13.1.3.1 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