HomeMy WebLinkAboutPD 2023-0034; 7500 CADENCIA ST; DRAINAGE STUDY 7500 CADENCIA ST; 2024-10-11DRAINAGE STUDY – 7500 CADENCIA ST
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DRAINAGE STUDY
7500 CADENCIA ST.
PD2023-0034 GR2023-0041 DWG 545-9A
APN: 223-260-09
7500 CADENCIA ST.
Carlsbad, CA 92009
Prepared By:
John A. Van Ryn, P.E. RCE 80667 EXP: 3-31-25
Van Ryn Engineering, Inc.
16766 Bernardo Center Dr., Ste 115
San Diego, CA 92128
Prepared for:
Rick Guinn
7500 Cadencia St.
Carlsbad, CA 92009
October 11th 2024
"~ VAN RVN
DRAINAGE STUDY – 7500 CADENCIA ST
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TABLE OF CONTENTS
1. INTRODUCTION ................................................................................................................................... 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 ............................................................................................................................ 6
2.4 Tributary Areas .............................................................................................................................. 6
3. CALCULATIONS/RESULTS ................................................................................................................... 6
3.1 Pre & Post Development Peak Flow Comparison ...................................................................... 6
4. CONCLUSION ....................................................................................................................................... 8
Appendix 1 .......................................................... Existing and Proposed Drainage Exhibits
Appendix 2 ................................................................................... Supplemental Documents
DRAINAGE STUDY – 7500 CADENCIA ST.
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Figure 1 Vicinity Map
VICINITY MAP
CITY OF
CITY OF ENCINITAS
NOT TO SCALE
DRAINAGE STUDY – 7500 CADENCIA ST.
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1. INTRODUCTION
1.1 Project Description
The project is located on Cadencia Street approximately 500 feet northeast of the La Costa Ave and
Cadencia St intersection, located in the City of Carlsbad. The site exists today as an existing
developed lot with a single-family residence. The project proposes to construct an additional
dwelling unit, and a covered patio onsite. Also proposed are hardscape and landscape improvement
typical to residential redevelopment.
1.2 Existing Conditions
The existing lot is approximately 34,072 sq-ft or 0.78 acres. Currently a residential home with existing
hardscape that includes an concrete driveway makes up approximately 8,547 square feet of impervious
area. The remaining area is made up of landscaping and natural ground cover on moderate to steep
man-made slopes. Storm water sheet flows across the site in a westerly direction collecting in drainage
ditches until it discharges offsite and directly into a storm drain on Cadencia St. Storm water continues
through the public storm drain system and ultimately conveys westerly to the Pacific Ocean.
For hydrological purposes, the subject property was separated into 2 individual basins: Basin A and B.
Basin A is primarily comprised of the existing residential home, concrete driveway, and landscaping
surrounding the existing PCC brow ditches. Basin B encompasses the remaining, previously graded
pervious area of the existing lot. Due to its composition of existing landscaping and natural ground,
Basin B is deemed self-mitigating.
The project site has a hydrologic soil group type D, according to the USDA web soil survey. The
peak storm water run-off for the site was calculated using the rational method equation (Q=CiA). The
precipitation volume of 2.8 inches was used for design consideration according to the San Diego
County 100yr 6hr Isopluvial Map. A 7.4 in/hr intensity was determined 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= 2.52 CFS. A
weighted runoff coefficient of 0.52 based on the existing 30% of impervious area. See section 3 of
this report for detailed calculations.
1.3 Proposed Conditions
The project proposes to construct a covered patio with planter boxes and a structural retaining wall onsite.
The project proposes a new total of 9,233 square feet of impervious area. Similar to the existing site
condition, storm water will be conveyed in a westerly direction. Drainage will be collected in area drains
and conveyed into the existing onsite drainage channel to the south of the property. The existing PCC
brow ditches, that confluence near the southwest corner of the property, are going to be removed and
replaced with separate brooks box catch basins and splash walls for both the northeast and southeast brow
ditch approaches. Drainage collected in the catch basins will convey water through new PVC storm
drainpipes and into a new onsite brooks box clean out. The cleanout will connect to the existing PVC
storm drainpipe that connects to the adjacent public cleanout on Cadencia St.
As previously stated, the subject property was separated into 2 individual basins for hydrological
purposes. Basin A consists of the existing residential home and driveway, landscaping surrounding
the existing PCC brow ditches, and separate brooks box catch basins and splash walls. Basin B
DRAINAGE STUDY – 7500 CADENCIA ST.
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encompasses the remaining, previously graded pervious area of the existing lot. Due to its
composition of existing landscaping and natural ground, Basin B is deemed self-mitigating.
During the 100-year storm event, runoff will follow the pre-development flow path in a westerly direction
and outlet into a storm drain on Cadencia St. As in the existing condition, the peak storm water runoff
for the site was calculated using the rational method equation (Q=CiA). The precipitation volume of
2.8 inches was used for design consideration according to the San Diego County 100yr 6hr Isopluvial
Map. A 7.4 in/hr intensity was determined 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 post-project runoff of Q= 2.55 CFS. A weighted runoff coefficient of
0.53 based on the proposed 32.4% of impervious area. See section 3 of this report for detailed
calculations.
2. METHODOLOGY
The proposed project has been analyzed to determine the peak runoff flow for the 100 year, 6 hour rainfall
event using the Rational Method per the San Diego County Hydrology Manual. The Weighted Runoff
Coefficient, C, for the existing and proposed conditions were calculated using Table 3-1 of the same
manual. The rainfall intensity for the existing and proposed drainage areas were calculated using a
minimum Tc of 5 min, and the rainfall intensity equation described in section 3.1.3 of the San Diego County
Hydrology Manual which yields an intensity of 7.4 inches per hour, for both the existing and proposed
conditions.
The proposed LID best management practices have been sized and located such that runoff will be directed
to pervious areas where feasible, and into an onsite storm water detention basin before ultimately
discharging to the downstream conveyance 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 = C x I x A
Where:
Q = Flow rate in cubic feet per second (cfs)
C = Runoff coefficient
I = Rainfall Intensity in inches per hour (in/hr)
A = Drainage basin area in acres, (ac)
Rational Method calculations were performed using the San Diego County Hydrology Manual (Section
3.1)
2.2 Runoff Coefficient
The runoff coefficients for the project were calculated using section 3.1.2 of the San Diego County
Hydrology Manual and using a weighted average runoff coefficient based on the actual and proposed
impervious areas which was calculated to be 0.52 and 0.53, respectively.
DRAINAGE STUDY – 7500 CADENCIA ST.
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2.3 Rainfall Intensity
Rainfall intensity was determined using the Rainfall Intensity section of the San Diego County Hydrology
Manual (Section 3.1.3). Based on a 5 min time of concentration, an intensity of 7.4 inches per hour has
been calculated for both the existing and proposed conditions.
2.4 Tributary Areas
Drainage basins are delineated in the Post-Project Hydrology Exhibit in Appendix 1 and graphically portray
the tributary area for each drainage basin.
3. CALCULATIONS/RESULTS
3.1 Pre & Post-Development Peak Flow Comparison
3.1.1 Pre-Development Peak Flow
P100yr,6hr = 2.8
C= 8,547 sf * 0.9 + 19,935 sf *0.35 = 0.52
28,482 sf
Tc=5 min.
i=7.44*P6*D-0.645 → D = 5 min., for minimum time of concentration
i=7.44*2.8*5-0.645 = 7.4 in/hr
Q=C*i*A = 0.52 * 7.4 in/hr * 0.65 ac = 2.52 cfs
3.1.2 Post-Development Peak Flow
Cn = 9,233 sf * 0.9 + 19,249 sf *0.35 = 0.53
28,482 sf
Tc=5 min.
i=7.44*P6*D-0.645 → D = 5 min, for minimum time of concentration
i=7.44*2.8*5-0.645 = 7.4 in/hr
Q=C*i*A = 0.53 * 7.4 in/hr * 0.65 ac = 2.55 cfs
Below are a series of tables which summarize the calculations provided:
SITE IMPERVIOUS AREA COMPOSITION
TOTAL
IMPERVIOUS
AREA
(SQ-FT)
TOTAL
PERVIOUS
AREA
(SQ-FT)
TOTAL
PROJECT
AREA
(SQ-FT)
%
IMPERVIOUS
SURFACES
WEIGHTED
RUNOFF
COEFFICIENT
“C”
Existing 8,547 19,935 28,482 30% 0.52
DRAINAGE STUDY – 7500 CADENCIA ST.
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Proposed 9,233 19,249 28,482 32.4% 0.53
Table 1. Runoff Coefficient “C” Comparison
The table above shows the difference in the weighted runoff coefficient, “C”, between the existing and
proposed condition. Calculations of the weighted runoff coefficients are based on Table 3-1 of the San
Diego County Hydrology Manual and underlying hydrologic soil group D.
EXISTING DRAINAGE FLOWS
DRAINAGE
AREA
DRAINAGE
AREA
(SQ-FT)
WEIGHTED
RUNOFF
COEFFICIENT
(C)
Q100
(CFS)
I100
(IN/HR)
BASIN A 28,482 0.52 2.52 7.4
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.
PROPOSED DRAINAGE FLOWS
DRAINAGE
AREA
DRAINAGE
AREA
(SQ-FT)
RUNOFF
COEFFICIENT
(C)
Q100
(CFS)
I100
(IN/HR)
BASIN A 28,482 0.53 2.55 7.4
Table 3. Proposed Condition Peak Drainage Flow Rates
Table 3 above lists the peak flow rates for the project site for the proposed condition for the respective
rainfall events.
PEAK FLOW COMPARISON
CONDITION
DRAINAGE
AREA
(SQ-FT)
Q100 (CFS)
Existing 28,482 2.52
Proposed 28,482 2.55
Table 4. Proposed Condition Peak Drainage Flow Rates
Table 4 above shows a comparison between the peak flow rates and precipitation volume for the proposed
condition and the existing condition.
DRAINAGE STUDY – 7500 CADENCIA ST.
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4. CONCLUSION
As discussed previously, storm water will be routed toward an existing onsite concrete brow ditch that has
been designed to detain the excess storm water produced during the 100yr storm event. The onsite storm
water routing will assist in the reduction of peak-flows.
The impact of the increase in peak flow from pre-development to post-development was analyzed through
Hydraflow Express simulation. The Brow Ditch Exhibits in Appendix 1 displays how the depth of water
remains the same (0.79 ft) for both development conditions due to the minor increase in peak flow from
2.52 to 2.55 cfs. With a total depth of 1 ft, it was concluded that the brow ditch has the sufficient capacity
to help manage the impact of increased Q100.
Therefore, the project will have no negative impact on downstream facilities or adjacent properties. It is
my professional opinion that the storm drain system proposed in this report and on the precise grading plans
is adequate to intercept, contain and convey Q100 and be beneficial to the private and public drainage
system.
DRAINAGE STUDY – 7500 CADENCIA ST.
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APPENDIX 1
EXISTING & PROPOSED
DRAINAGE EXHIBITS
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LEGEND
PROPERTY LINE
RIGHT-OF-WAY
ROAD CENTERLINE
ADJACENT PROPERTY LINE
DRAINAGE BASIN BOUNDARY
PRE-PROJECT FLO'M.JNE
PRE-PROJECT IMPERVIOUS AREA
AREA CALCULATIONS
TOTAL AREA
IMPERVIOUS AREA
PERVIOUS AREA
% IMP
Cn
28,482 SF (0.65 AC)
8,547 SF
19,935 SF
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0.52
SITE CONDITION
HYDROLOGICAL SOIL GROUP D
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LEGEND
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ADJACENT PROPERTY UN£
DRAINAGE BASIN BOUNDARY
POST-PROJECT Fl.O'MJNE
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AREA CALCULATIONS
TOTAL AREA
IMPERVIOUS AREA
PERVIOUS AREA
% IMP
Cn
28,482 SF (0. 78 AC)
9,233 SF
19,249 SF
32.4%
0.53
SITE CONDITION
HYDROLOGICAL SOIL GROUP D
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DRAINAGE STUDY – 7500 CADENCIA ST.
10 | P a g e
APPENDIX 1
BROW DITCH EXHIBITS
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Friday, Dec 13 2024
Pre-Development Brow Ditch Capacity
Triangular
Side Slopes (z:1) = 0.33, 0.33
Total Depth (ft) = 1.00
Invert Elev (ft) = 245.00
Slope (%)= 16.70
N-Value = 0.012
Calculations
Compute by:Known Q
Known Q (cfs) = 2.52
Highlighted
Depth (ft)= 0.79
Q (cfs)= 2.520
Area (sqft)= 0.21
Velocity (ft/s)= 12.24
Wetted Perim (ft) = 1.66
Crit Depth, Yc (ft) = 1.00
Top Width (ft)= 0.52
EGL (ft)= 3.12
0 .25 .5 .75 1 1.25
Elev (ft) Depth (ft)Section
244.50 -0.50
245.00 0.00
245.50 0.50
246.00 1.00
246.50 1.50
247.00 2.00
Reach (ft)
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Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Friday, Dec 13 2024
Post-Development Brow Ditch Capacity
Triangular
Side Slopes (z:1) = 0.33, 0.33
Total Depth (ft) = 1.00
Invert Elev (ft) = 245.00
Slope (%)= 16.70
N-Value = 0.012
Calculations
Compute by:Known Q
Known Q (cfs) = 2.55
Highlighted
Depth (ft)= 0.79
Q (cfs)= 2.550
Area (sqft)= 0.21
Velocity (ft/s)= 12.38
Wetted Perim (ft) = 1.66
Crit Depth, Yc (ft) = 1.00
Top Width (ft)= 0.52
EGL (ft)= 3.17
0 .25 .5 .75 1 1.25
Elev (ft) Depth (ft)Section
244.50 -0.50
245.00 0.00
245.50 0.50
246.00 1.00
246.50 1.50
247.00 2.00
Reach (ft)
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DRAINAGE STUDY – 7500 CADENCIA ST.
11 | P a g e
APPENDIX 2
SUPPLIMENTAL
DOCUMENTS
Hydrologic Soil Group—San Diego County Area, California
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
5/31/2023
Page 1 of 4
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Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 11N WGS84
0 30 60 120 180Feet
0 5 10 20 30Meters
Map Scale: 1:650 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 18, Sep 14, 2022
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Mar 14, 2022—Mar
17, 2022
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
5/31/2023
Page 2 of 4USDA =
□
D
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Hydrologic Soil Group
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
ExG Exchequer rocky silt
loam, 30 to 70 percent
slopes
D 1.6 87.0%
SnG San Miguel-Exchequer
rocky silt loams, 9 to
70 percent slopes
D 0.2 13.0%
Totals for Area of Interest 1.8 100.0%
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.
Hydrologic Soil Group—San Diego County Area, California
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
5/31/2023
Page 3 of 4USDA =
Rating Options
Aggregation Method: Dominant Condition
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
5/31/2023
Page 4 of 4~
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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* 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. 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 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
3-6
San Diego County Hydrology Manual Section: 3 Date: June 2003 Page: 7 of 26
3.1.3 Rainfall Intensity
The rainfall intensity (I) is the rainfall in inches per hour (in/hr) for a duration equal to the Tc
for a selected storm frequency. Once a particular storm frequency has been selected for
design and a Tc calculated for the drainage area, the rainfall intensity can be determined from
the Intensity-Duration Design Chart (Figure 3-1). The 6-hour storm rainfall amount (P6) and
the 24-hour storm rainfall amount (P24) for the selected storm frequency are also needed for
calculation of I. P6 and P24 can be read from the isopluvial maps provided in Appendix B.
An Intensity-Duration Design Chart applicable to all areas within San Diego County is
provided as Figure 3-1. Figure 3-2 provides an example of use of the Intensity-Duration
Design Chart. Intensity can also be calculated using the following equation:
I = 7.44 P6 D-0.645
Where: P6 = adjusted 6-hour storm rainfall amount (see discussion below)
D = duration in minutes (use Tc)
Note: This equation applies only to the 6-hour storm rainfall amount (i.e., P6 cannot be
changed to P24 to calculate a 24-hour intensity using this equation).
The Intensity-Duration Design Chart and the equation are for the 6-hour storm rainfall
amount. In general, P6 for the selected frequency should be between 45% and 65% of P24 for
the selected frequency. If P6 is not within 45% to 65% of P24, P6 should be increased or
decreased as necessary to meet this criteria. The isopluvial lines are based on precipitation
gauge data. At the time that the isopluvial lines were created, the majority of precipitation
gauges in San Diego County were read daily, and these readings yielded 24-hour
precipitation data. Some 6-hour data were available from the few recording gauges
distributed throughout the County at that time; however, some 6-hour data were extrapolated.
Therefore, the 24-hour precipitation data for San Diego County are considered to be more
reliable.
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Intensity (in/hr)
6-Hour Precipitation (in)
Duration (min)
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Intensity-Duration Design Chart -Example
Directions for Application:
(1) From precipitation maps determine 6 hr and 24 hr amounts
for the selected frequency. These maps are included 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% of 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 to the plotted lines.
(5) This line is the intensity-duration curve for the location
being analyzed.
Application Form:
(a) Selected frequency ~ year
p
(b) Ps = ---in., P24 = ---.~ = %(2)
24
{c) Adjusted p6<2l = ___ in.
{d) Ix = ___ min.
(e) I = ___ in./hr.
Note : This chart replaces the Intensity-Duration-Frequency
curves used since 1965.
PG 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
Duration I I I I I I I I I I I
5 2.63 3.95 5.27 6.59 7.90 9.22 10.54 11.86 13.17 14.49 15.81
7 2.12 3.18 4.24 5.30 6.36 7.42 8.48 9.54 10.60 11.66 12.72
10 1.68 2.53 3.37 4.21 5.05 5.90 6.74 7.58 8.42 9.27 ToTf
15 1.30 1.95 2.59 3.24 3.89 4.54 5.19 5.84 6.49 7.13 7.78
20 1.08 1.62 2.15 2.69 3.23 3.77 4.31 4.85 5.39 5.93 6.46
25 0.93 1.40 1.87 2.33 2.80 3.27 3.73 4.20 4.67 5.13 560
30 0.83 1.24 1.66 2.07 2.49 2.90 3.32 3.73 4.15 4.56 4.98
~o 0.69 1.03 1.38 1.72 207 2.41 2.76 3.10 3.45 3.79 4.13
50 0.60 0.90 1.1 9 1.49 1.79 2.09 2.39 2.69 2.98 3.28 3.58
60 0.53 0.80 1.06 1.33 1.59 1.86 2.12 2.39 2.65 2.92 3.18
90 0.41 0.61 0.82 f 02 1.23 1.43 1.63 1.84 2.04 2.25 2.45
120 0.34 0.51 0.68 0.85 1.02 1.19 1.36 1.53 1.70 1.87 2.04
150 0.29 0.44 0.59 I-0.73 >--0.88 1.03 1.18 1.32 1.47 1.62 1.76
180 0.26 0.39 0.52 0.65 0.78 0.91 1.04 1.18 1.31 1.44 1.57
240 0.22 0.33 0.43 0.54 0.65 0.76 0.87 0.98 1.08 1.19 1.30
300 0.19 0.28 0.38 0.47 0.56 0.66 0.75 0.85 0.94 1.03 1.13
360 0.17 0.25 0.33 0.42 050 0.58 0.67 0.75 0.84 0.92 1.00
FIGU R E ~