HomeMy WebLinkAboutCDP 2020-0003; CROWLEY RESIDENCE; DRAINAGE STUDY; 2020-05-18DRAINAGE STUDY
CROWLEY RESIDENCE
CDP2020-0003
APN: 206-180-20, 206-180-46, 206-180-53
4367 HIGHLAND DR.
CARLSBAD, CA 92008
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Prepared By: S No. 73620
C(. Exp. 12/31 /20
5/26/2020
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OF CA\.I~
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William G. Mack, P.E. RCE 73620 EXP: 12-31-20
Pasco Laret Suiter & Associates, Inc.
535 N. Highway IO I, Suite A
Solana Beach, CA 92075
PASCO LARET SUITER
& ASSOCIATES
CIVI FNGlr-EERING + LA\JD PLANNING+ LAND SURVEYl,\JG
Prepared for:
CROWLEY RESIDENCE
4367 HIGHLAND DR.
Carlsbad, CA 92008
May 18, 2020
DRAINAGE STUDY-CROWLEY RESIDENCE MAY 2020
TABLE OF CONTENTS
1. INTRODUCTION ························································································································••oo••oo••· 4
1.1 Project Description ........................................................................................................................ 4
1.2 Existing Conditions ........................................................................................................................ 4
1.3 Proposed Conditions .................................................................................................................... 4
2. METHODOLOGY .................................................................................................................................. 5
2.1 Rational Method ............................................................................................................................ 5
2.2 Runoff Coefficient ......................................................................................................................... 5
2.3 Rainfall Intensity ............................................................................................................................ 5
2.4 Tributary Areas .............................................................................................................................. 5
3. CALCULATIONS/RESULTS ................................................................................................................... 6
3.1 Pre & Post Development Peak Flow Comparison ...................................................................... 6
3.2 Detention ....................................................................................................................................... 6
3.3 Storm Water Quality ..................................................................................................................... 7
4. CONCLUSION ....................................................................................................................................... 7
Appendix 1 ....................................................... Pre & Post Project Hydrology Calculations
Appendix 2 .......................................................... Existing and Proposed Drainage Exhibits
DRAINAGE STUDY-CROWLEY RES![JENCE MAY2020
Figure 1 Vicinity Map
VICINITY MAP
LAGOON
DRAINAGE STUDY-CROWLEY RESIDENCE MAY 2020
1. INTRODUCTION
Project Description
The project is located at 4367 Highland Dr. in Carlsbad California. The site exists today as three lots
with an existing single-family residence on each of the first two lots and vacant land on the third.
The project proposes the demolition of one of the existing single-family residences and constructing
an addition to the remaining existing single-family residence. The project will join the three
properties via a parcel map.
The project is designed in accordance with the San Diego County Hydrology Manual dated June
2003 and complies with the Regional Water Quality Control Board Region 9 MS4 Permit, Order No.
R9-2015-0l 00. The project does not propose work adjacent regulated waters therefore 40 l and 404
permits are not applicable.
Existing Conditions
The 0.96-acre project site is moderately sloped and sheets flows drainage in a westerly direction
towards the adjacent properties and away from Highland Drive. Runoff flows across the adjacent lots
and into Adams Street. Storm water then collects and is conveyed in the PCC ditch on Adams Street
for about 23 feet in a northwesterly direction before it is picked up by a public catch basin located on
the east corner of the intersection of Adams Street and Hoover Street. The storm water continues in
the 21" CMP public stonn drain south westerly parallel to Hoover Street and ultimately discharges to
the Agua Hedionda Lagoon. The existing single-family dwelling roof drains to the ground which
enters the on-site private storm drain system and then outlets at the rear yard on the west end of the
site. The existing houses and hardscape make up the existing impervious area located on the project
site which is approximately 24,117 square feet. The peak storm water run-off was calculated using the
rational method equation (Q=CiA). The precipitation volume of 2.50 inches was used for design
consideration according to the San Diego County 1 00yr 6hr lsopluvial Map. A 6.6 in/hr intensity was
detennined from the San Diego County Hydrology Manual Figure 3-1 assuming the minimum
allowable time of concentration of 5 minutes because the site is relatively small. This resulted in a
peak pre-project run-off of Q= 4.25 CFS using a weighted runoff coefficient of 0.67 based on the
existing impervious area.
Proposed Conditions
The project proposes to construct an addition to the existing single-family residence that is to remain.
The project also proposes a pervious paver parking area and walkways, as well as landscape areas to
minimize the proposed impervious footprint. The total impervious area of the proposed developed
site is 24,267 square feet. The proposed impervious areas will include concrete walkways, parking
and driveway areas, and rooftops. The pervious areas will include landscape areas and permeable
paver areas. The post project condition has been delineated by four (4) drainage areas to model the
post project condition. All roof runoff and on-site storm water runoff will be directed to the
Biofiltration planter which outlets on the west side of the site. The water will then continue down the
slope on the adjacent property and on to Adams Street in the same manner as the existing condition,
ultimately ending up in the Agua Hedionda Lagoon. The post project peak un-detained runoff flow
was calculated to be 4.27 CFS using the Rational Method Q=CiA and a weighted runoff coefficient
of 0.67 based on 58% impervious area.
DRAINAGE STUDY-CROWLEY RESIDENCE MAY 2020
2. METHODOLOGY
The proposed project has been analyzed to determine the peak runoff flow for 100-year, 6-hour rainfall
event using the Rational Method per the San Diego County Hydrology Manual, dated June 2003. The
Runoff Coefficient., C, for the existing and proposed conditions were calculated using a weighted average
of C values from Table 3-1 on page 3-6 in the San Diego Hydrology Manual. The time of concentration
for all existing and proposed drainage areas were calculated using the minimum Tc of 5 min which yields
an intensity of 6.6 inches per hour from using the equation on Figure 3-1.
The proposed best management practices have been sized and located such that all runoff will be directed
to landscape planters or through pervious areas where feasible before ultimately discharging to the
downstream storm drain system.
2.1 Rational Method
As mentioned above, runoff from the project site was calculated for the 100-year storm events. Runoff was
calculated using the Rational Method which is given by the following equation:
Q=CxlxA
Where:
Q = Flow rate in cubic feet per second (cfs)
C = Runoff coefficient (Determined from Table 2, P. 82, City of San Diego Drainage Design Manual)
I = Rainfall Intensity in inches per hour (in/hr)
A= Drainage basin area in acres, (ac)
Rational Method calculations were performed using the City of San Diego Drainage Design Manual
(Section 1-102.3)
2.2 Runoff Coefficient
The Runoff Coefficient, C, for the existing and proposed conditions were calculated using a weighted
average ofC values from Table 3-1 on page 3-6 in the San Diego Hydrology Manual. The weighted average
runoff coefficient is based on the actual impervious area which was calculated to be 0.67 in the post project
condition. See section 3. l .2 in the San Diego County Hydrology Manual.
2.3 Rainfall Intensity
Rainfall intensity was determined using the Rainfall Intensity Duration Frequency Curves from Figure 3-1
in the San Diego County Hydrology Manual, dated June 2003. Based on a 5 min time of concentration, an
intensity of 6.6 inches per hour was determined.
2.4 Tributary Areas
Drainage basins are delineated in the Post-Project Hydrology Exhibit in Appendix I and graphically portray
the tributary area for each drainage basin.
DRAINAGE STUDY-CROWLEY RESIDENCE MAY2020
3. CALCULATIONS/RESULTS
Pre & Post Development Peak Flow Comparison 3.1
Bl ft bl h. h • th I u1 f 'd d . th A • . .. d' fth· • rt
I SITE IMPERVIOUS AREA COMPOSITION
TOTAL TOTAL
IMPERVIOUS PERVIOUS TOTAL % RUNOFF
PROJECT IMPERVIOUS COEFFICIENT AREA AREA
(ACRES) (ACRES) AREA (ACRES) SURFACES "C''
Existing 0.56 0.40 0.96 58% 0.67
Proposed 0.56 0.40 0.96 58% 0.67
Table 1. Runoff Coefficient "C" Comparison
The table above shows the difference in the runoff coefficient, "C", between the existing and proposed
condition.
EXISTING DRAINAGE FLOWS
DRAINAGE RUNOFF DRAINAGE Q,oo I 100 AREA COEFFICIENT AREA (ACRES) (C) (CFS) (IN /HR)
EX-1 0.96 0.67 4.25 6.6
Table 2. Existing Condition Peak Drainage Flow Rates
Table 2 above lists the peak flow rates for the project site in the existing condition for the respective rainfall
events.
T bl 3 P I ed C d'ti P kD • ' Fl Rat -
PROPOSED DRAINAGE FLOWS
DRAINAGE DRAINAGE RUNOFF
COEFFICIENT Q,oo '100 AREA A REA (ACRES) (C) (CFS) (IN/HR)
PR-1 0.65 0.73 3.14 6.6
PR-2 0.18 0.69 0.84 6.6
PR-3 0.10 0.35 0.24 6.6
PR-4 0.02 0.35 0.05 6.6
Total 0.96 0.67 4.27 6.6
The table above lists the peak flow rates for the project site for the proposed condition for the respective
rainfall events.
DRAINAGE STUDY -CROWLEY RESIDENCE MAY2020
Table 4. Pro osed Condition Peak Draina e Flow Rates
DRAINAGE
CONDITION AREA Q,oo V100 C (CFS) (CU-FD
(ACRES)
Existing 0.96 4.25 5800 0.67
Proposed 0.96 4.27 5817 0.67
Table 4 above shows a comparison between the peak flow rates and precipitation volume for the proposed
condition and the existing condition.
As shown in Table 4, the project increases the peak runoff rate and runoff volume for the design storms
analyzed when comparing the pre-project condition to the post-project condition because the proposed
development increases impervious area.
3.2 Detention
The project proposes an increase in total impervious area therefore an increase in peak runoff is anticipated.
The pre-project and post post-project volumes were calculated to show the proposed increase in runoff
volume. The volume detained by the biofiltration basin ponding section is greater than the proposed
increase in runoff volume therefore adequate detention is provided and as a result the post project runoff
peak will be less than the pre-project condition because the biofiltration ponding section will provide
sufficient detention. See the detention calculation in Appendix 1.
3.3 Storm Water Quality
The project's runoff will be treated for storm water quality by site design and source control measures. The
project does propose more than l 0,000 square feet of imp~rvious surface therefore the project is classified
as a "Priority" Development project according to the Storm Water Standards Questionnaire (E-34)
therefore, permanent treatment control BMPs are proposed and a Storm Water Quality Management Plan
has been prepared. Low Impact Design was implemented by locating pervious areas where feasible to
minimize the impervious footprint and to direct storm water runoff towards pervious areas before leaving
the site.
4. CONCLUSION
As discussed previously, the proposed project's peak runoff flow and volume does not increase in
comparison to the existing condition because of the proposed pervious paver areas capture the additional
volume, therefore, the project will not negatively affect downstream facil ities since the overaJI peak flow
rate will remain the same. The project does not negatively impact adjacent properties. It is my professional
opinion that the storm drain systems as proposed in this report and on the grading plans herein is adequate
to intercept, contain and convey QI 00 and will not create negative impacts to the downstream system.
DRAINAGE STUDY-CROWLEY RESIDENCf
PASCO LARET SUITER
& ASSOCIATES
CIVIL ENGINEERING+ LAND PLANNING+ LAND SURVEYING
APPENDIX I
PRE-PROJECT & POST-PROJECT
HYDROLGY CALCULATIONS
MAY 2020
• CROWLEY RESIDENCE
J-3028
5/19/2020
Drainage
Area Arn Description
EX-1 Existing Site
BMP
Location Basin Description
PR-1 Proposed Site
PR-2 Proposed Site
PR-3 Proposed Site
PR-4 Proposed Site
PR·S Proposed Site
Total
An,a
(Ac)
0.96
Total ....
(Ac)
0.65
0.18
0.10
0.02
0.00
ON-SITE PRE-PROJECT HYDOLOGY
lotal 1ota1
..... Impervious " (sq-ft) Area (Sq-Ft) % Impervious Pervious
41645 24117 58% 42%
ON-SITE POST-PROJECT HYDOLOGY .....
Total Impervious ..... Area " (sq-ft) (Sq-Ft) % Impervious Pervlous
28288 19332 68% 32%
8000 4935 62% 38%
4471 0 0% 100%
886 0 0% 100%
215 91 42% 58%
Total 0.96 41860 24358 42%
Note;
100 Yr Storm at 5 Min TC
lntensity:I 6.60
Precip:I 2.50
Detention Calculation:
i
i
n/hr
n
Pre-Project Peak Runoff Volume:
Post-Project Peak Runoff Volume:
Delta Peak Runoff Volume (Post-Pre):
Biofiltration Surface Area:
Biofiltration Ponding Depth:
Detention Volume:
Runoff Coefficient
Impervious 0.90
Landscape 0.35
Permeable Pavers 0.35
5800 cu-ft
5843 cu-ft
43 cu-ft
1132 sq-ft
0.667 ft
755 cu-ft
.. e1g ... ea Peak
Runoff RunoffQ:
Coefficient (CFS)
0.67 4.25
Weighted Peak
Runoff RunoffQ:
Coefficient (CFS)
0.73 3.14
0.69 0.84
0.35 0.24
0.35 0.05
0.58 0.02
0.67 4.29
Therefore, Adequate Detention Provided
J :\Active Jobs\3028 Anchorase lovestment<,\CIVIL \REPORTS\HYDROLOGY\CALCS\3028-DRN.~lsx
Peak Kunu11
Volume:
(cu-ft)
5800
Peak Runoff
Volume:
(cu-ft)
4278
1149
326
65
26
S843
.CROWLEY RESIDENCE
J-3028
5/19/2020
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DRAINAGE STUDY-CROWLEY RESIDENCE
PASCO LARET SUITER
& ASSOCIATES
CIVIL ENGINEERING+ LAND PLANNING+ LAND SURVEYING
APPENDIX2
EXISTING & PROPOSED
DRAINAGE EXHIBITS
MAY 2020
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LEGENQ
PROPERTY LINE
EXISTING CONTOUR
BASIN BOUNDARY
FLOWI..IN£ LINE
....... -----·
TOTAL DRAINAGE AREA: 41,645 SF
EXISTING IMPERVIOUS AREA: 24, 117 SF
% IMPERVIOUS: 58%
RUNOFF COEffiCIENT: 0.67
HYDROLOGY
PRE-PROJECT EXHIBIT
4367 Hl(;HLAND DRIVE
CARLSBAD, CA 92008
DA TE: MAY 2020
PASCO LARET SUITER
SCALE: 1'' = 30' ~ ffi\$${Q)(C ~ffi\if[E$
San Diego I Solana Beach I Orange County
o 30 60 90 Phone 858.259.82121 www.plsaengineering.com
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LEGEND
PROPERTY LINE
PROPOSED CONTOUR
BASIN BOUNDARY
IMPERVIOUS AREA
--
-----14-----
--..-i-----·
~
TOTAL DRAINAGE AREA: 41,860 SF
EXISnNG IMPERVIOUS AREA REMAINING: 10,960 SF
NEW & REPLACED IMPERVIOUS AREA: 13,398 SF
TOTAL POST-PROJECT IMPERVIOUS AREA: 24,.358 SF
% IMPERVIOUS: 58%
RUNOFF COEFFICIENT: 0.67
HYDROLOGY
POST-PROJECT EXHIBIT
4.367 HIGHLAND DRIVE:
CARLSBAD, CA 92008
DA TE: MAY 2020
PASCO LARET SUITER
SCALE: 1" = 30' ~ ffi\~~(Q)(C~t'A\f IE~
San Diego I Solana Beach I Orange County
Phone 858.259.8212 I www.plsaenglneering.com 0 30 60 90