HomeMy WebLinkAboutCDP 16-24; PEELLE RESIDENCE ADDITION; PRELIMINARY HYDROLOGY STUDY; 2017-10-30- RECORD COPY
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PRELIMINARY HYDROLOGY STUDY
For
PEELLE RESIDENCE
2441 BUENA VISTA CIRCLE
CARLSBAD, CA 92008
City of Carlsbad, CA
PREPARED FOR:
Michael & Louise Peelle
2441 Buena Vista Circle
Carlsbad, CA 92008
442.333.9449
Date: October 30, 2017
PREPARED BY:
Pasco Laret Suiter & Associates
535 N. Highway 101, Suite A
Solana Beach, CA 92075
(858) 259-8212
BRIAN ARDOLINO, RCE 71651 DATE
Peelle Residence
TABLE OF CONTENTS
SECTION
Executive Summary 1.0
Introduction 1.1
Existing Conditions 1.2
Proposed Project 1.3
Summary of Results and Conditions 1.4
Conclusions 1.5
References 1.6
Methodology 2.0
Introduction 2.1
County of San Diego Criteria 2.2
Runoff coefficient determination 2.3
Hydrologic Analyses 3.0
Pre-Developed Hydrologic Analysis 3.1
Post-Developed Hydrologic Analysis 3.2
Hydraulic Calculations 4.0
Proposed Outlet Pipe Calculations 4.1
Appendix 5.0
August 2017
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1.0 EXECUTIVE SUMMARY
1.1 Introduction
This Hydrology Study for the Peelle 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 33°09'57.77" W 117°21'06.80". The site is
bordered by residential development to the north and south, Buena Vista Lagoon to the
west and Buena Vista Circle to the east. The project site is located in the Buena Vista Creek
Hydrologic Area and more specifically, the El Salto Sub-Area (904.21).
The existing project site includes an existing single-family residence, and associated paving
and hardscape. The easterly portion of the site consists mostly of a gentle slope from the
east to the west to an existing top of slope. The westerly portion of the site consists of a
steep slope. Drainage from the existing site sheet flows in the westerly direction, down the
existing slope and into the Buena Vista Lagoon.
1.3 Proposed Project
The intent of the proposed project is to construct additions to the existing residence, a new
pool and associated hardscape and driveway improvements.
In general, the proposed drainage design conveys runoff away from the residence and flows
from the east to the west through a PVC stormdrain system to the top of slope, where it is
then spread out along the top of slope by a 1' by 1' gravel spreader with a 4" perf pipe to
maintain the sheet flow condition down the existing slope. Ultimately the storm water ends
up in the Buena Vista Lagoon. The drainage pattern is not altered from the existing
condition.
We believe the proposed storm drain system will not adversely affect the downstream system
negatively.
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.
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In the pre-developed condition one discharge point was analyzed. Outlet point 1.0 indicates
that the 100-year peak flow is 1.32 cfs with a time of concentration of 5.3 min based on an
area of 0.40 AC.
In the post-developed condition one discharge point was analyzed. Outlet point 1.0 indicates
that the 100-year peak flow is 1.12 cfs with a time of concentration of 5.3 nuin based on an
area of 0.40 AC.
1.5 Conclusions
The overall peak flow leaving the property decreases from the existing condition to the
proposed condition due to the reduction in impervious area on site.
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 Tentative Map will
function to adequately intercept, contain and convey flow to the appropriate points of
discharge.
1.6 References
"San Diego County I-.ydrology Manual", revised June 2003, County of San Diego, Department of
Public Works, Flood Control Section.
"Caifornia Regional WaterQualiy Control Board Order No. 2009-0009-D W/Q, "California
Regional Water Control Board, San Diego Region (SDRWQCB).
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Peelle 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 estimates the peak
rate of runoff based on the variables of area, runoff coefficient, and rainfall intensity. The
rainfall intensity (I) is equal to:
I = 7.44 x P6 x D °645
Where:
I = Intensity (in/hr)
P6 = 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 KM 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 KM equation is as follows:
Q=CIA
Where:
Q= flow (in cfs)
C = runoff coefficient, ratio of rainfall that produces storm water
runoff (runoff vs. inflltration/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 KM 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 KM 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 Ration 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
follcws:
Subdivide the watershed into an initial sub-areas and subsequent sub-areas,
which are generally less than 10 acres in size. Assign upstream anc downstream
node numbers to each sub-area.
Estimate an initial Tc by using the appropriate nomograph or overland flow
velocity estimation. The minimum Tc considered is 5.0 minutes.
Using the initial T, determine the corresponding values of I. Then Q = CIA.
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Peelle 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
approximately 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. 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.
August 2017
Peelle Residence
3.0 HYDROLOGIC ANALYSES
Rational Method Parameters
Runoff Coefficient C= 0.9 x (% Impervious) + Cp (1-% Impervious)*
Cp0.25* for existing condition pervious type "B" soils
100 Year 6 Hour Storm Precipitation (P6)=2.5 in (see rainfall isopluvial*)
Tc=(11.9L3/E)0.385 per Figure 3-4 of the County of San Diego Hydrology Manual (L=miles)*
Tt=Ti + Tc
1= Intensity in/hr, I=7.44xP6xD0645*
Duration (D)= Time of Concentration, Tc
Q=Peak Runoff, QC*I*A (cfs)
*From San Diego County Hydrology Manual, June 2003 Revision
3.1 Pre-Developed Hydrologic Model Output
A17,437 sf= 0.18 ac
Impervious area = 7,373 sf
% IMP = 42%
C= (0.42x 0.9) + [(1-0.42)0.25]
C= 0.52
Initial
T1=(11.9(134/5280)3/5.4)0.385
T=0.019 hours = 1.16 mm
T,< 5 mm, therefore use 5 mm
T,=(11.9(76/5280)3/30)0.385
T=0.005 hours = 0.3 min
T= 5+0.3
T=5.3 minutes
I1oo7.44(2.5)(5.3) -0.645
I006.34 in/hr
Qioo 0.52(6.34 in/hr) 0.40 acres
Qioo1.32 cfs
3.2 Post-Developed Hydrologic Model Output
A17,437 sf = 0.40 ac
Impervious area = 5,100 sf
% IMP = 29%
C= (0.29x 0.9) + [(1-0.29)0.25]
C= 0.44
Initial
T= (11.9(163/5280)3/5.9)0.385
T0.024 hours = 1.41 mm
T< 5 min, therefore use 5 mm
T=(11.9(78/5280)3/28.9)0.385
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Peelle Residence
T0.005 hours = 0.3 mm
T= 5+0.3
T=5.3 minutes
I1007.44(2.5)(5.3) -0.645
Ioo6.34 in/hr
Qioo 0.44(6.34 in/hi) 0.40 acres
Qioo1.12 cfs
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Peelle Residence
4.0 HYDRAULIC CALCULATIONS
4.1 Proposed Outlet Pipe Calculations
Southern Outlet
A=3,876 sf = 0.09 ac
Impervious area = 1,577 sf
% IMP = 41%
C (0.41x 0.9) + [(1-0.41)0.25]
C= 0.52
T1< 5 mm, therefore use 5 mm
I1007.44(2.5)(5) -0645
Ioo6.59 in/hr
Qioo 0.52(6.59 in/hr) 0.09 acres
Qi000.31 cfs
Middle Outlet
A2,722 sf = 0.06 ac
Impervious area = 1,807 sf
% IMP = 55%
C (0.55x 0.9) + [(1-0.55)0.25]
C= 0.61
T1< 5 mm, therefore use 5 mm
Iioo7.44(2.5)(5) 0.645
Ioo6.59 in/hr
Qioo 0.61(6.59 in/hr) 0.06 acres
Qioo=0.24 cfs
Northern Outlet
A=3,937 sf = 0.09 ac
Impervious area = 1,713 sf
% IMP = 44%
C (0.44x 0.9) + [(1-0.44)0.25]
C= 0.54
T< 5 mm, therefore use 5 mm
Iioo7.44(2.5)(5) -0.645
Ioo6.59 in/hr
Qioo= 0.54(6.59 in/hr) 0.09 acres
Qioo 0.32 cfs
August 2017
Section
2
I Channel Report
I
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 31D® by Autodesk, Inc.
SOUTHERN 4 INCH PVC OUTLET
I Circular
Diameter (ft)
I
= 0.33
Invert Elev (ft) = 40.90
Slope (%) = 3.15
N-Value = 0.009
I Calculations
Compute by: Known 0
Known Q(cfs) = 0.31
1
Elev (ft)
I
42.00
I
1 41.50
I 41.30
I 40.50
1 40.00
I •
0
Wednesday, Aug 30 2017
Highlighted
Depth (ft) = 0.20
Q (cfs) = 0.310
Area (sqft) = 0.05
Velocity (ft/s) = 5.71
Wetted Perim (ft) = 0.59
Crit Depth, Yc (ft) = 0.31
Top Width (ft) = 0.32
EGL(ft) = 0.71
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Wednesday, Aug 302017
MIDDLE 4 INCH PVC OUTLET
Circular
Diameter (ft) = 0.33
Invert Elev (ft) = 40.90
Slope (%) = 2.00
N-Value = 0.009
Calculations
Compute by: Known Q
Known Q (cfs) = 0.24
Highlighted
Depth (ft)
Q (cfs)
Area (sqft)
Velocity (ftls)
Wetted Perim (ft)
Crit Depth, Yc (ft)
Top Width (ft)
EGL(ft)
= 0.20
= 0.240
= 0.05
= 4.42
= 0.59
= 0.28
= 0.32
= 0.50
Elev (ft) Section
42.30
41.50
41.00
40.50
40.00
0 1 2
Reach (ft)
Highlighted
Depth (ft)
Q (cfs)
Area (sqft)
Velocity (ft/s)
Wetted Perim (ft)
Crit Depth, Yc (ft)
Top Width (ft)
EGL (ft)
= 0.23
= 0.320
= 0.06
= 5.01
= 0.65
= 0.31
= 0.30
= 0.62
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.
NORTHERN 4 INCH PVC OUTLET
Circular
Diameter (ft) = 0.33
Invert Eiev (ft) = 40.90
Slope (%) = 2.22
N-Value = 0.009
Calculations
Compute by: Known Q
Known Q (cfs) = 0.32
Wednesday, Aug 302017
Elev (ft) Section
42.00
41.50
41.00
40.50
40.00
0 1 2
Reach (ft)
Peelle Residence
5.0 APPENDIX
August 2017
San Diego County Hydrology Manual Section: 3
Date: June 2003 Page: 6 of 26
Table 3-1
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use Runoff Coefficient "C"
Soil Type
NRCS Elements County Elements % IMPER. A B C D
Undisturbed Natural Terrain (Natural) Permanent Open Space 0.20 0.25 0.30 0.35
Low Density Residential (LDR) Residential, 1.0 DU/A or less 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 DU/A 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 less 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. Corn) Neighborhood Commercial 80 0.76 0.77 0.78 0.79
Commercial/Industrial (G. Corn) General Commercial 85 0.80 0.80 0.81 0.82
Commercial/Industrial (O.P. Corn) Office Professional/Commercial 90 0.83 0.84 0.84 0.85
Commercial/Industrial (Limited I.) 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
*The 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
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HYDROLOGIC NODE MAP
PEELLE RESIDENCE
PRE-DEVELOPMENT MAP
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PEELLE RESIDENCE
POST-DEVELOPMENT MAP
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"C' CALCULATIONS
PER CO&VTY HYDROLOGY MANUAL 3.12
C= 0.9 x (% IWERVIOUS) + Cp x (1—X IFERVIOUS)
Cp = 025 FOR TYPE B° SOIL
C= (0 29x0 .9) + (0 .71x0 25) C=0.44
SCALE: 1=2O'
BASIN BO(I'VARY
&B-AREA BOL.WARY - - -
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iMCERvIOI.s AREA LT:iIiT] 100 SF (2667 SF TO REMAIN + 2.433 SF NEW