HomeMy WebLinkAboutCDP 2021-0025; SWAN RESIDENCE; HYDROLOGY & HYDRAULICS STUDY FOR SINGLE FAMILY RESIDENCE AT 2668 OCEAN ST; 2024-07-01PLSA #3176 July 2024
HYDROLOGY &
HYDRAULICS STUDY
FOR
SINGLE FAMILY RESIDENCE AT
2668 OCEAN STREET
CARLSBAD, CA 92008
PREPARED FOR:
CHRISTOPHER & CRISTA SWAN
2668 OCEAN STREET
CARLSBAD, CA 92008
DATE:
JULY 2024
PREPARED BY:
PASCO LARET SUITER & ASSOCIATES
119 ABERDEEN DRIVE
CARDIFF, CA 92007
_______________________________________7/1/24__
GREGORY W. LANG, RCE 68075 DATE
PLSA #3176 July 2024
TABLE OF CONTENTS
SECTION
Executive Summary 1.0
Introduction 1.1
Existing Conditions 1.2
Proposed Project 1.3
Hydrologic Unit Contribution 1.4
Summary of Results and Conditions 1.5
Conclusions 1.6
References 1.7
Methodology 2.0
Introduction 2.1
County of San Diego Criteria 2.2
Runoff coefficient determination 2.3
Hydrology & Hydraulic Model Output 3.0
Pre-Developed Hydrologic Model Output (100 Year Event) 3.1
Post-Developed Hydrologic Model Output (100 Year Event) 3.2
Attachments 4.0
Isopluvial Maps
Runoff Coefficients
Pipe Sizing
Basin Maps
PLSA #3176 July 2024
1.0 EXECUTIVE SUMMARY
1.1 Introduction
This Hydrology Study for the single-family residence at 2668 Ocean Street 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 project site is located at 2668 Ocean Street, and is bound by residential homes to the
north, south, and east, and Ocean Street to the west. The property is geographically located at
N 33º09’39” W 117º21’19”.
The existing project site includes 1 single-family residence with associated hardscape and
landscape. The site generally slopes from the east to west, toward Ocean Street. The subject
property has an existing total gross area of 3,748 sf or 0.09 acres and has approximately 3,175
sf of impervious surface on-site. Storm water from the east side of the property drains west to
Ocean Street, where it then surface flows north to an existing public storm drain system that
ultimately drains to the Pacific Ocean.
1.3 Proposed Project
The proposed development will include the construction of a new single-family residence,
hardscape, landscape, driveway, and a storm drain system. The proposed development will
result in 2,533 sf of impervious surface, which will translate into a 642 sf decrease in impervious
area.
The proposed storm drain system design includes new PVC stormdrain and area drains, a
channel drain, and swales. Runoff from the site will drain to the west toward Ocean Street 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 project, proposed permanent
Best Management Practices (BMPs) will be incorporated into the storm water runoff design.
The proposed BMPs include multiple landscape planter areas, directing impervious and roof
area runoff to proposed landscape areas, which are intended to mitigate peak flows as well as
serve as settling areas.
1.4 Hydrologic Unit Contribution
The project site is located in the Buena Vista Creek Hydrologic Area, and more specifically, the
El Salto Sub-Area (904.21).
According to the California 1998 and 2002 CWA 303d list published by the San Diego
Regional Water Quality Control Board, there are impaired water bodies that are associated
with this basin, however our project drainage ultimately discharges to the Pacific Ocean, which
is not listed as an impaired water body near the Buena Vista Lagoon.
PLSA #3176 July 2024
1.5 Preliminary Hydrology Calculations
The existing 100 year peak discharge from the site is 0.50 cfs, and is associated with a time of
concentration (Tc) of 5 minutes and a total area of 0.10 acres.
The post-developed 100 year peak discharge from the site is 0.43 cfs, and is associated with a
time of concentration (Tc) of 5 minutes and a total area of 0.10 acres. See sections 3.1 & 3.2.
1.6 Conclusions
The proposed development and proposed storm drain design will be capable of safely
conveying the 100-year storm runoff flow and has included storm drain piping to ensure that
the discharge from the project site is of the best possible quality and will not pose any
significant impact or threats to the water quality of the Pacific Ocean. In addition, the proposed
development and storm drain improvements will not significantly alter the existing drainage
patterns. There is a decrease in storm water runoff will be detained and will not increase the
potential for flooding nor create an increase in erosion.
This project has been deemed a Standard Project and therefore is not subject to
Hydromodification criteria.
It is with these above reasons that it can be concluded that there will be no negative impact to
the downstream storm drain facilities or an increased potential of flooding. Since a major goal
of this project is to ensure that all storm water quality issues are addressed to the maximum
extent practical, the peak discharge for the proposed site will be utilized to adequately size the
storm drain piping system for this project.
1.7 References
“San Diego County Hydrology Manual”, revised June 2003, County of San Diego, Department
of Public Works, Flood Control Section.
”California Regional Water Quality Control Board Order No. 2009-009-DWQ” California
Regional Water Control Board, San Diego Region (SDRWQCB).
PLSA #3176 July 2024
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-0.645
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 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:
Q = CIA
Where:
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 that
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.
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
PLSA #3176 July 2024
limits the flow path length for the initial time of concentration to lengths under 100 feet, and is
dependent on land use and slope.
2.3 Runoff Coefficient Determination
As stated in section 2.2, 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, 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 were calculated
based on the existing and proposed impervious areas for each basin per the County
Hydrology Manual section 3.1.2. See section 3.0 for calculations.
PLSA #3176 July 2024
3.0 HYDROLOGY MODEL OUTPUT
Rational Method Parameters
Runoff Coefficient C= 0.9 x (% Impervious) + Cp (1-% Impervious)*
Cp=0.25* for pervious type “B” soils
100 Year 6 Hour Storm Precipitation (P6)=2.5 in (see rainfall isopluvial*)
I= Intensity in/hr, I=7.44xP6xD-0.645*
Duration (D)= Time of Concentration, Tc
Q=Peak Runoff, Q=C*I*A (cfs)
*From San Diego County Hydrology Manual, June 2003 Revision
3.1 Pre-Developed Hydrologic Model Output (100 Year Event)
Total Basin Area= 4,376 sf A=0.10 acres
Existing Impervious Area= 3,418 sf
CPRE = (0.78* 0.9) + (0.22*0.25)
CPRE =0.76
Tc = 5 min
I = 7.44(2.5)(5) -0.645
I = 6.59 in/hr
Q100= 0.76 * 6.59 in/hr * 0.10 acres
Q100=0.50 cfs
3.2 Post-Developed Hydrologic Model Output (100 Year Event)
Total Basin Area= 4,376 sf A=0.10 acres
Proposed Impervious Area= 2,768 sf
CPOST = (0.63* 0.9) + (0.37*0.25)
CPOST = 0.66
Tc = 5 min
I = 7.44(2.5)(5) -0.645
I = 6.59 in/hr
Q100= 0.66 * 6.59 in/hr * 0.10 acres
Q100=0.43 cfs
TOTAL ∆Q =TOTAL Qpost-TOTAL Qpre
TOTAL ∆Q = 0.43 cfs – 0.50 cfs
TOTAL ∆Q = -0.07 cfs
PLSA #3176 July 2024
4.0 ATTACHMENTS
Hydrologic Soil Group—San Diego County Area, California
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
12/11/2020
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466857 466862 466867 466872 466877 466882 466887 466892 466897 466902 466907
466857 466862 466867 466872 466877 466882 466887 466892 466897 466902 466907
33° 9' 39'' N
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N
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 11N WGS84
0 10 20 40 60Feet
0 3 7 14 21Meters
Map Scale: 1:241 if printed on A landscape (11" x 8.5") sheet.
Soil Map may not be valid at this scale.
USDA =
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Rating Polygons
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Lines
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Points
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: San Diego County Area, California
Survey Area Data: Version 15, May 27, 2020
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Jan 24, 2020—Feb
12, 2020
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Hydrologic Soil Group—San Diego County Area, California
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
12/11/2020
Page 2 of 4USDA =
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Hydrologic Soil Group
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
MlC Marina loamy coarse
sand, 2 to 9 percent
slopes
B 0.1 100.0%
Totals for Area of Interest 0.1 100.0%
Description
Hydrologic soil groups are based on estimates of runoff potential. Soils are
assigned to one of four groups according to the rate of water infiltration when the
soils are not protected by vegetation, are thoroughly wet, and receive
precipitation from long-duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D) and
three dual classes (A/D, B/D, and C/D). The groups are defined as follows:
Group A. Soils having a high infiltration rate (low runoff potential) when
thoroughly wet. These consist mainly of deep, well drained to excessively
drained sands or gravelly sands. These soils have a high rate of water
transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These
consist chiefly of moderately deep or deep, moderately well drained or well
drained soils that have moderately fine texture to moderately coarse texture.
These soils have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist
chiefly of soils having a layer that impedes the downward movement of water or
soils of moderately fine texture or fine texture. These soils have a slow rate of
water transmission.
Group D. Soils having a very slow infiltration rate (high runoff potential) when
thoroughly wet. These consist chiefly of clays that have a high shrink-swell
potential, soils that have a high water table, soils that have a claypan or clay
layer at or near the surface, and soils that are shallow over nearly impervious
material. These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is
for drained areas and the second is for undrained areas. Only the soils that in
their natural condition are in group D are assigned to dual classes.
Rating Options
Aggregation Method: Dominant Condition
Hydrologic Soil Group—San Diego County Area, California
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
12/11/2020
Page 3 of 4USDA =
Component Percent Cutoff: None Specified
Tie-break Rule: Higher
Hydrologic Soil Group—San Diego County Area, California
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
12/11/2020
Page 4 of 4~
San Diego County Hydrology Manual
Date: June 2003
•
Section:
Page:
Table 3-1
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use Runoff Coefficient "C"
Soil Type
NRCS Elements County Elements %IMPER. A B
Undisturbed Natural Terrain (Natural) Permanent Open Space 0* 0.20 0.25
Low Density Residential (LDR) Residential, 1.0 DU/ A or less 10 0.27 0.32
Low Density Residential (LDR) Residential, 2.0 DU/A or less 20 0.34 0.38
Low Density Residential (LDR) Residential, 2.9 DU/A or less 25 0.38 0.41
Medium Density Residential (MOR) Residential, 4.3 DU/A or less 30 0.41 0.45
Medium Density Residential (MOR) Residential, 7.3 DU/A or less 40 0.48 0.51
Medium Density Residential (MDR) Residential, 10.9 DU/A or less 45 0.52 0.54
Medium Density Residential (MOR) Residential, 14.5 DU/ A or less 50 0.55 0.58
High Density Residential (HDR) Residential, 24.0 DU/ A or less 65 0.66 0.67
High Density Residential (HDR) Residential, 43.0 DU/A or less 80 0.76 0.77
Commercial/Industrial (N. Com) Neighborhood Commercial 80 0.76 0.77
Commercial/Industrial (G. Com) General Commercial 85 0.80 0.80
Commercial/Industrial (O.P. Com) Office Professional/Commercial 90 0.83 0.84
Commercial/Industrial (Limited I.) Limited Industrial 90 0.83 0.84
Commercial/Industrial (General I.) General Industrial 95 0.87 0.87
3
6 of26
C D
0.30 0.35
0.36 0.41
0.42 0.46
0.45 0.49
0.48 0.52
0.54 0.57
0.57 0.60
0.60 0.63
0.69 0.71
0.78 0.79
0.78 0.79
0.81 0.82
0.84 0.85
0.84 0.85
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
3-6
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Monday, Jul 1 2024
4 INCH PVC SD
Circular
Diameter (ft)= 0.33
Invert Elev (ft) = 40.70
Slope (%)= 0.50
N-Value = 0.009
Calculations
Compute by:Known Depth
Known Depth (ft) = 0.32
Highlighted
Depth (ft)= 0.32
Q (cfs)= 0.203
Area (sqft)= 0.08
Velocity (ft/s)= 2.40
Wetted Perim (ft) = 0.90
Crit Depth, Yc (ft) = 0.26
Top Width (ft)= 0.13
EGL (ft)= 0.41
0 1 2
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County of San Diego
Hydrology Manual
Rainfall Jsopluvials
100 Year Rainfall Event-6 Hou.rs
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LEGEND
BASIN BOUNDARY
IMPERVIOUS AREA
HYDROLOGIC NODE MAP
2668 OCEAN STREET
PRE-DEVELOPMENT MAP
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POST-DEVELOPMENT MAP
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C ONC RET JF<IYEWAV
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LEGEND
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IMPERVIOUS AREA
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======::±1::=====-=== -----
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V100=2.4 FPS, t
2,768 SF (2,533 SF ON-SITE+ 235 SF OFF-SITE)
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