HomeMy WebLinkAboutCDP 2017-0001; PRITCHARD RESIDENCE; HYDROLOGY STUDY; 2017-06-13HYDROLOGY STUDY
for
5098 SHORE DRIVE
CARLSBAD, CA 92008
City of Carlsbad, CA
PREPARED FOR:
Robert & Cathy Pritchard
5098 Shore Drive
Carlsbad, CA 92008
Date: June 13, 2017
Revised:
PREPARED BY:
Pasco Laret Suiter & Associates
535 N. Highway 101, Suite A
Solana Beach, CA 9207 5
(858) 259-8212
F. VE
A..,r-10 2., 7
LAND \,
DATE
RECORDC,?1 ~ i1, f1
Initial ate
Pritchard Residence
TABLE OF CONTENTS
Executive Summary
Introduction
Existing Conditions
Proposed Project
Summary of Results and Conditions
Conclusions
References
Methodology
Introduction
County of San Diego Criteria
Runoff coefficient determination
Hydrologic Analyses
Pre-Developed Hydrologic Analysis
Post-D eveloped Hydrologic Analysis
Hydraulic Calculations
Appendix
2
SECTION
1.0
1.1
1.2
1.3
1.4
1.5
1.6
2.0
2.1
2.2
2.3
3.0
3.1
3.2
4.0
5.0
June 2017
Pritchard Residence
1.0 EXECUTIVE SUMMARY
1.1 Introduction
This Hydrology Study fo r the Pritchard Residence project has been prepared to analyze the
hydrologic and hydraulic characteristics of the existing and proposed project site. This
report intends to present both the methodology and the calculations used for determining
the runoff from the project site in both the pre-developed (existing) conditions and the post-
developed (proposed) conditions produced by the 100 year 6 hour storm. In addition this
report will propose the sizing of all necessary storm drain facilities and storm drain piping
necessary for the storm drain system to safely convey the runoff from the 100-year rainfall
event.
1.2 Existing Conditions
The property is geographically located at N 33007'53.75" W 117°20'07.85". The site is
bordered by residential development to the east and south and Shore Drive to the north and
west. The project site is located in the Agua Hedionda Hydrologic Area and more
specifically, the Los monos Sub-Area (904.31).
The existing project site includes 1 residence associated paving and hardscape. The site
consists mostly of a gentle slope from the east to west. Drainage from the existing site sheet
flows in the westerly direction onto Shore Drive. The runoff is then conveyed through rolled
curb & gutter to an existing cross gutter located just west of the site. Ultimately the cross-
gutter discharges west into the Pacific Ocean.
1.3 Proposed Project
The intent of the proposed project is to construct 1 new single family residence with
associated hardscape improvements and driveways.
The proposed drainage design conveys runoff from the east to the west to landscape BMP
planter areas for LID storm water treatment. O nce treated, the water is discharged through
stormdrain curb underdrains onto Shore Drive as it does in the existing condition.
We believe the proposed storm drain system will not adversely affect the downstream system
negatively.
To address the storm water quality goals established for this development, proposed
permanent Best Management Practice (BMP) and treatment metl1ods will be incorporated
into the storm water runoff design. The proposed BMP's include multiple landscape BMP
planter areas, which are intended to mitigate peak flows as well as serve as settling areas.
June 2017
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Pritchard Residence
1.4 Summary of Results
Upon performing hydrologic analysis of the project site in both the proposed developed and
existing condition the following results were produced. One discharge point was analyzed.
In the pre-developed condition one discharge point was analyzed. Outlet point indicates that
the 100-year peak flow is 0.50 cfs with a time of concentration of 8.0 min based on an area
of 0.14 AC.
In the post-developed condition one discharge point was analyzed. Outlet point indicates
that the 100-year peak flow is 0.40 cfs with a time of concentration of 8.2 min based on an
areaof0.14 AC.
1.5 Conclusions
The overall peak flow leaving the property is decreased by 0.10 cfs.
Based on the discussion in this report it is the professional opinion of Pasco Laret Suiter &
Associates, Inc. that the existing drainage system on the corresponding Preliminary Grading
Plan will function to adeguately intercept, contain and convey flow to the appropriate points
of discharge. See Sections 3.0 and 4.0 for calculations.
1.6 References
''San Diego Counry Hydrology Man11al': revised June 2003, County of San Diego, Department of
Public Works, Flood Control Section.
"California Regional Water Quali(y Control Board Order No. 2009-0009-DWQ., "California
Regional Water Control Board, San Diego Region (SDRWQCB).
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June 2017
Pritchard Residence
2.0 METHODOLOGY
2.1 Introduction
The hydrologic model used to perform the hydrologic analysis presented in this report
utilizes the Ration Method (RM) equation, Q=CIA. The RM formula es timates the peak
rate of runoff based on the variables of area, runoff coefficient, and rainfall intensity. T he
rainfall intensity (I) is equal to:
Where:
I = 7.44 x P 6 x o-0645
I = Intensity (in/hr)
P 6 = 6-hour precipitation (inches)
D = duration (minutes -use Tc)
Using the Time of Concentration (Tc), which is the time required for a given element of
water that originates at the most remote point of the basin being analyzed to reach the point
at which the runoff from the basin is being analyzed. The RM equation determines the
storm water runoff rate (Q) for a given basin in terms of flow (typically in cubic feet per
second (cfs) but sometimes as gallons per minute (gpm)). The RM equation is as follows:
Where:
Q = CIA
Q= flow (in cfs)
C = runoff coefficient, ratio of rainfall that produces storm water
runoff (runoff vs. infiltration/ evaporation/ absorption/ etc)
I = average rainfall intensity for a duration equal to the Tc for the
area, in inches per hour.
A= drainage area contributing to the basin in acres.
The RM equation assumes that the storm event being analyzed delivers precipitation to the
entire basin uniformly, and therefore the peak discharge rate will occur when a raindrop falls
at the most remote portion of the basin arrives at the point of analysis. The RM also
assumes that the fraction of rainfall that becomes runoff or the runoff coefficient C is not
affected by the storm intensity, I, or the precipitation zone number.
In addition to the above Ratio~Method assumptions, the conservative assumption that all
runoff coefficients utilized for this report are based on type "D" soils.
Rational Method calculations were performed using the AES 2010 computer program. To
perform the hydrology routing, the total watershed area is divided into sub-areas which
discharge at designated nodes. The procedure for the sub-area summation model is as
follows:
(1) Subdivide the watershed into an initial sub-areas and subsequent sub-areas,
which are generally less than 10 acres in size. Assign upstream and downstream
node numbers to each sub-area.
(2) Estimate an initial Tc by using the appropriate nomograph or overland fl ow
velocity estimation. The minimum Tc considered is 5.0 minutes.
(3) Using the initial Tc, determine the corresponding values of I. Then Q = CIA.
June 2017
5
Pritchard Residence
(4) Using Q, estimate the travel time between this node and the next by Manning's
equation as applied to particular channel or conduit linking the two nodes.
Then, repeat the calculation for Q based on the revised intensity (which is a
function of the revised time of concentration)
2.2 County of San Diego Criteria
As defined by the County Hydrology Manual dated June 2003, the rational method is the
preferred equation for determining the hydrologic characteristics of basins up to
approxin1ately one square mile in size. The County of San Diego has developed its own
tables, nomographs, and methodologies for analyzing storm water runoff for areas within
the county. The County has also developed precipitation isopluvial contour maps that show
even lines of rainfall anticipated from a given storm event (i.e. 100-year, 6-hour storm).
One of the variables of the RM equation is the runoff coefficient, C. The runoff coefficient
is dependent only upon land use and soil type and the County of San Diego has developed a
table of Runoff Coefficients for Urban Areas to be applied to basin located within the
County of San Diego. The table categorizes the land use, the associated development
density (dwelling units per acre) and the percentage of impervious area. Each of the
categories listed has an associated runoff coefficient, C, for each soil type class.
The County has also illustrated in detail the methodology for determining the time of
concentration, in particular the initial time of concentration. The County has adopted the
Federal Aviation Agency's (FAA) overland time of flow equation. This equation essentially
limits the flow path length for the initial time of concentration to lengths of 100 feet or less,
and is dependent on land use and slope.
2.3 Runoff Coefficient Determination
As stated in section 2.2, the runoff coefficient is dependent upon land use and soil type and
the County of San Diego has developed a table of Runoff Coefficients for Urban Areas to
be applied to basin located within the County of San Diego. The table, included at the end
of this section, categorizes the land use, the associated development density (dwelling units
per acre) and the percentage of impervious area. Low Density Residential coefficient of 0.41
was used for pervious area, and Commercial/Industrial coefficient of 0.87 was used for
impervious area. Weighted runoff coefficients for onsite areas were calculated using the
existing and proposed impervious area for each basin. See Appendix 5.0 for Coefficient
Table and C Value Calculations on the Pre and Post Development Maps.
June 2017
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Pritchard Residence
3.0 HYDROLOGIC ANALYSES
Rational Method Parameters
Runoff Coefficient C= 0.9 x (% Impervious) + Cp (1-% Impervious)*
Cp=0.25* for existing condition pervious type "B" soils
100 Year 6 Hour Storm Precipitation (P6)=2.5 in (see rainfall isopluvial*)
Tc=(11.9L3/ t..E)0.385 per Figure 3-4 of the County of San Diego Hydrology Manual (L=miles)*
Tt=Ti + Tc
I= Intensity in/hr, 1=7.44xP6xD·064S*
Duration (D)= Time of Concentration, Tc
Q=Peak Runoff, Q=C*I*A (cfs)
*From San Diego County I Iydrology Manual,June 2003 Revision
3.1 Pre-Developed Hydrologic Model Output
BASIN
A=6,152 sf= 0.14 ac
Impervious area= 4,574 sf
% IMP= 74%
C= (0.74x 0.9) + [(1-0.74)0.25)
C= 0.73
Initial
Ti for 3% = 7.0 min
T,=(11. 9(58/ 5280)3 /2)o.3ss
T,=0.011 hours = 1 min
Tc= 7.0+1.0
Tc=8.0 minutes
1100=7.44(2.5)(8.0) -0645
1100=4.86 in/hr
Q100= 0.73(4.86 in/hr) 0.14 acres
Qioo=0.50 cfs
3.2 Post-Developed Hydrologic Model Output
BASIN
A=6,152 sf= 0.14 ac
Impervious area = 3,198 sf
% IMP = 52%
C= (0.52x 0.9) + [(1-0.52)0.25)
C= 0.59
Initial
Ti for 3% = 7 .0 min
T,=(11 .9(70/5280)3 /0.8)0.3ss
T,=0.019 hours = 1.2 min
Tc= 7.0+1.2
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June 2017
Pritchard Residence
Tc=8.2 minutes
1100=7.44(2.5)(8.2)-0m
1100=4.79 in/hr
Q100= 0.59(4.79 in/hr) 0.14 acres
Qioo=0.40 cfs
June 2017
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Pritchard Residence
4.0 HYDRAULIC CALCULATIONS
Detention Basin Sizing:
LID BMP Sizing Requirement
LID BMP area required is equal to 4% of the impervious are being removed and replaced or
added
Required BMP = 0.04 (3,198 sf)
= 128 SF
Proposed BMP Area = 1,549 SF
Proposed BMP Area > Required BMP Area
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Pritchard Residence
5.0 APPENDIX
June 2017
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..
San Diego County Hydrology Manual Section: 3
Date: June 2003 Page: 6of26
Table 3-1
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use I Runoff Coefficient "C"
Soil Type
NRCS Elements Coun Elements %IMPER. A B C D
Undisturbed Natural Terrain (Natural) Pennanent Open Space O* 0.20 0.25 0.30 0.35
Low Density Residential (LDR) Residential, 1.0 DU/A or Jess 10 0.27 0.32 0.36 0.41
Low Density Residential (LDR) Residential, 2.0 DU/A or less 20 0.34 0.38 0.42 0.46
Low Density Residential (LDR) Residential, 2.9 DUiA or less 25 0.38 0.41 0.45 0.49
Medium Density Residential (MDR) Residential, 4.3 DU/A or less 30 0.41 0.45 0.48 0.52
Medium Density Residential (MDR) Residential, 7.3 DU/A or less 40 0.48 0.51 0.54 0.57
Medium Density Residential (MDR) Residential, 10.9 DU/A or Jess 45 0.52 0.54 0.57 0.60
Medium Density Residential (MDR) Residential, 14.5 DU/A or less 50 0.55 0.58 0.60 0.63
High Density Residential (HDR) Residential, 24.0 DU/ A or less 65 0.66 0.67 0.69 0.71
High Density Residential (HDR) Residential, 43.0 DU/A or less 80 0.76 0.77 0.78 0.79
Commercial/Industrial (N. Com) Neighborhood Commercial 80 0.76 0.77 0.78 0.79
Commercial/Industrial (G. Com) General Commercial 85 0.80 0.80 0.81 0.82
Commercial/Industrial (O.P. Com) Office Professional/Commercial 90 0.83 0.84 0.84 0.85
Commercial/Industrial (Limited l.) Limited Industrial 90 0.83 0.84 0.84 0.85
Commercial/Industrial (General I.) General Industrial 95 0.87 0.87 0.87 0.87
*'Ille values associated with 0% impervious may be used for direct calculation of the runoff coefficient as described in Section 3.1.2 (representing the pervious runoff
coefficient, Cp, for the soil type), or for areas that will remain undisturbed in perpetuity. Justification must be given that the area will remain natural forever (e.g., the area
is located in Cleveland National Forest).
DU/A= dwelling units per acre
NRCS = National Resources Conservation Service
3-6
San Diego County Hydrology Manual
Date: June 2003
Section:
Page:
3
12 of26
Note that the Jnitial Time of Concentration should be reflective of the general land-use at the
upstream end of a drainage basin. A single lot with an area of two or less acres does not have
a significant effect where the drainage basin area is 20 to 600 acres.
Table 3-2 provides limits of the length (Maximum Length (LM)) of sheet flow to be used in
hydrology studies. Initial Ti values based on average C values for the Land Use Element are
also included. These values can be used in planning and design applications as described
below. Exceptions may be approved by the "Regulating Agency" when submitted with a
detailed study.
Table 3-2
MAXIMUM OVERLAND FLOW LENGTH (LM)
& INITIAL TIME OF CONCENTRATION (T1)
Element* DU/ .5% 1% 2% 3% 5% 10%
Acre LM Ti LM Ti LM Ti LM Ti LM Ti LM Ti
Natural 50 13.2 70 12.5 85 10.9 100 10.3 100 8.7 100 6.9
LOR 1 50 12.2 70 11.5 85 10.0 100 9.5 100 8.0 100 6.4
LOR 2 50 11.3 70 10.5 85 9.2 100 8.8 100 7.4 100 5.8
LOR 2.9 50 10.7 70 10.0 85 8.8 95 8.1 100 7.0 100 5.6
MOR 4.3 50 10.2 70 9.6 80 8.1 95 7.8 100 6.7 100 5.3
MOR 7.3 50 9.2 65 8.4 80 7.4 95 7.0 100 6.0 100 4.8
MOR 10.9 50 8.7 65 7.9 80 6.9 90 6.4 100 5.7 100 4.5
MOR 14.5 50 8.2 65 7.4 80 6.5 90 6.0 100 5.4 100 4.3
HOR 24 50 6.7 65 6.1 75 5.1 90 4.9 95 4.3 100 3.5
HOR 43 50 5.3 65 4.7 75 4.0 85 3.8 95 3.4 100 2.1
N.Com 50 5.3 60 4.5 75 4.0 85 3.8 95 3.4 100 2.7
G.Com 50 4.7 60 4.1 75 3.6 85 3.4 90 2.9 100 2.4
O.P./Com 50 4.2 60 3.7 70 3.1 80 2.9 90 2.6 100 2.2
Limited I. 50 4.2 60 3.7 70 3.1 80 2.9 90 2.6 100 2.2
General I. 50 3.7 60 3.2 70 2.7 80 2.6 90 2.3 100 1.9
*See Table 3-1 for more detailed description
3-12
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Minutes Hours
Duration
Intensity-Duration Design Chart -Template
Directions for Application:
(1) From precipitation maps detennine 6 hr and 24 hr amounts
for the selected frequency. These maps are induded 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% or 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 If) the plotted lines.
(5} This line is the intensity-duration curve for the location
being analyzed.
Application Form:
(a) Selected frequency ___ year
p
(b) P6 = in. P24 = _§_ = •;.<21 --' --'P24 --
(c) Adjusted p6(2) = __ in.
(d) Ix = __ min.
(e) I = __ in./hr.
Note: This chart replaces the Intensity-Duration-Frequency
curves used since 1965.
PS 1 1.5 2 2.5 3 3.5 4 4.S 5 5.5 6
'DIJration I I I I 1 · I I I I I I
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County of San Diego
Hydrology Manual
Rainfall Isopluvials
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HYDROLOGIC NODE MAP
5098 SHORE DRIVE
PRE-DEVELOPMENT MAP
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LEGEND
BASIN BOUNDARY
FLOW LINE
IMPERVIOUS AREA
•c• CALCULATIONS
4,574 SF
PER COUNTY HYDROLOGY MANUAL 3.1.2
C= 0 .9x (:XIMPERVIOUS)+Cpx ( j-:XIMPERVIOUS)
Gp = 0 .25 FOR TYPE 'B ' SOIL
C= (0 .7 4x0 .9) + (0 .26x0 .25)
C=0 .73
SCALE: 1•=20'
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HYDROLOGIC NODE MAP
5098 SHORE DRIVE
POS~DEVELOPMENTMAP
SHORE DRIVE
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LEGEND
BASIN BOUNDARY
FLOW LINE
IMPERVIOUS AREA ,, --~ ~" -I 3,198 SF
•c11 CALCULATIONS
PER COUNTY HYDROLOGY MANUAL 3. 1 .2
C= 0 .9x (%IMPERVIOUS)+Cpx (1-%IMPERVIOUS)
Cp = 0 .25 FOR TYPE 'D ' SOIL
C= (0 .52x0 .9) + (0 .4Bx0 .25)
C=0.59
SCALE: 1•=20'