HomeMy WebLinkAboutPUD 2020-0001; HEMLOCK COAST HOMES; PRELIMINARY HYDROLOGY STUDY; 2020-09-01PRELIMINARY HYDROLOGY STUDY
FOR
TENTATIVE PARCEL MAP I MINOR SITE DEVELOPMENT PLAN/
COASTAL DEVELOPMENT PERMIT
HEMLOCK COAST HOMES -320 HEMLOCK A VENUE
DEV 2020-0023
CITY OF CARLSBAD, CA
PREPARED FOR:
KMJ REAL ESTATE
2888 LOKER A VENUE EAST, SUITE 200
CARLSBAD, CA 92010
PH: (949) 678-4927
PREPARED BY:
PASCO LARET SUITER & AS SOCIA TES, INC.
535 N. HIGHWAY 101, SUITE A
SOLANA BEACH, CA 92075
PH: (858) 259-8212
Prepared: September 2020
BRYAN ANDREW KNAPP, PE #86542 DATE
No. 86542
p. 03/31/
Executive Summary
Introduction
Existing Conditions
Proposed Project
Conclusions
References
Methodology
Introduction
TABLE OF CONTENTS
County of San Diego Criteria
Runoff Coefficient Determination
Hydrology & Hydraulic Model Output
Pre-Developed Hydrologic Calculations (100-Year Event)
Post-Developed Hydrologic Calculations (100-Year Event)
Pre-Developed vs. Post-Developed Peak Flows (100-Year Event)
Detention Analysis
Appendix
Isopluvial Maps
Intensity Duration Design Charts
Runoff Coefficients
Hydrologic Soil Group -USDA Web Soil Survey
Pre-Development Hydrology Node Map
Post-Development Hydrology Node Map
SECTION
1.0
I.I
1.2
1.3
1.4
1.5
2.0
2.1
2.2
2.3
3.0
3.1
3.2
3.3
3.4
4.0
PLSA 3392-01
1.0 EXECUTIVE SUMMARY
1.1 Introduction
This Preliminary Hydrology Study for the proposed development at 320 Hemlock
A venue, known as Hemlock Coast Homes, has been prepared to analyze the hydro logic
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.
1.2 Existing Conditions
The subject property is located along Hemlock A venue, northeast of the intersection of
Hemlock A venue and Garfield Street, in the City of Carlsbad. The site is bound by a
combination of existing single-family and multi-family developments to the west, north,
and east, as well as Hemlock A venue, a public road, to the south. The existing site
consists of a single-family residence, driveway, walkways, landscaping, and
miscellaneous improvements typical of this type of development. The project site is
located in the Agua Hedionda Hydrologic Area, and, more specifically, the Los Monos
Hydro logic Sub-Area (904.31) of the Carlsbad watershed.
The subject property has an approximate area of 0.189 acres and is approximately 34%
impervious in the existing condition. Per the Web Soil Survey application available
through the United States Department of Agriculture, the basin is generally categorized to
have type B hydrologic soils. Based on the existing impervious area and land use type, a
pre-development weighted runoff coefficient of 0.47 was calculated for the subject
property using the methodology described in section 3.1.2 of the San Diego County
Hydrology Manual and the formula provided therein.
After review of the site edge conditions, existing topography, and adjacent developments,
it was determined that there is no additional offsite runon entering the site from the
neighboring properties. The existing site can be categorized into one (1) major drainage
basin, with one (1) primary discharge location leaving the property. Drainage basin EX-1
consists of the entirety of the project site and primarily drains from the northeast comer
of the property to the southwest, discharging to the Hemlock A venue right-of-way by
surface and sheet flow methods. Once in the Hemlock Avenue right-of-way, runoff
leaving the site travels west in the existing curb and gutter toward an existing storm drain
sump curb inlet approximately 100 feet west of the property. Once surface flow enters
the curb inlet, it is conveyed east in the underground public storm drain system toward
the Coastal Rail Trail and outlets on the surface of the SDNR right-of-way. From there,
runoff is ultimately conveyed south to outlet in the Agua Hedionda Lagoon prior to
entering the Pacific Ocean.
Using the Rational Method Procedure outlined in the San Diego County Hydrology
Manual, a peak flow rate and time of concentration was calculated for the 100-year, 6-
Page 1 of 11
PLSA 3392-01
hour storm event for the onsite drainage basin. Table 1 below summarizes the results of
the Rational Method calculations.
EXISTING DRAINAGE FLOWS
DRAINAGE DRAINAGE AREA Q100 lioo
AREA (ACRES) (CFS) (IN/HR)
EX-1 0.189 0.58 6.59
Table 1. Existing Condition Peak Drainage Flow Rates
Refer to pre-development hydrology calculations included in Section 3 .1 of this report for
a detailed analysis of the existing drainage basin, as well as a pre-development hydrology
node map included in the appendix of this report for pre-development drainage basin
delineation and discharge locations.
1.3 Proposed Project
The proposed project includes the demolition of all existing onsite improvements and the
construction of two (2) new multi-family/ duplex structures consisting of four (4) total
condominium units. The project proposes a driveway to provide vehicular access to the
proposed garages consisting of two types of pervious pavers, delineated by color, style
and finish to demarcate proposed walkway locations. Two private onsite parking stalls
interior to the site are also shown for site residents. The proposed pad elevations for the
two structures are 56.6, which appears to be roughly the same elevation as the existing
residence. Site grading, drainage and utility improvements typical of this type of multi-
family residential development will also be constructed as can be seen on the Preliminary
Grading Plan prepared by Pasco, Laret, Suiter & Associates under separate cover.
The subject property is approximately 53% impervious in the proposed condition. Based
on the proposed impervious area and land use type, a post-development weighted runoff
coefficient of 0.59 was calculated for the subject property using the methodology
described in section 3 .1.2 of the San Diego County Hydrology Manual and the formula
provided therein. The proposed site consists of one (1) major drainage basin, with one
(1) primary discharge location from the southwest comer of the property similar to the
existing condition. Drainage basin PR-1 consists of the entirety of the site and will
continue to convey runoff generally from a high point in the northeast comer of the site to
the Hemlock Avenue right-of-way by means of surface flow. A concrete ribbon gutter
located at the center of the pervious paver driveway will also serve to capture and convey
drainage to the existing curb and gutter along Hemlock A venue.
Using the Rational Method Procedure outlined in the San Diego County Hydrology
Manual, a peak flow rate and time of concentration were calculated for the 100-year, 6-
hour storm event for the onsite drainage basin. Table 2 below summarizes the results of
the Rational Method calculations in the post-developed condition.
Page 2 ofll
PLSA 3392-01
PROPOSED DRAINAGE FLOWS
DRAINAGE DRAINAGE
Q100 AREA hoo
AREA (ACRES) (CFS) (IN/HR)
PR-1 0.189 0.74 6.59
Table 2. Proposed Condition Peak Drainage Flow Rates
Refer to post-development hydrology calculations included in Section 3 .2 of this report
for a detailed analysis of the proposed drainage basin, as well as a post-development
hydrology node map included in the appendix of this report for post-development
drainage basin delineation and discharge locations.
In an effort to comply with the City of Carlsbad storm water standards, the proposed site
has incorporated LID design techniques to optimize the site layout. Runoff from
proposed roofs and new or removed and replaced hardscape areas will be directed to
landscaped areas or pervious pavers in an effort to disperse drainage to pervious surfaces.
The landscaped areas will assist to remove sediment and particulate-bound pollutants
from storm water. In addition, the storage layers of the pervious pavers will help to
mitigate peak runoff by providing detention volume as well as slightly increasing the
site's overall time of concentration. Refer to Section 3.4 of the report for a static
detention analysis showing that the increase in overall volume generated by the slight
increase in hardscape is accounted for in the gravel storage layer of the pervious pavers.
1.4 Conclusions
Based upon the analysis included in this report, there is an increase of ~0.16 cfs in peak
runoff as a result of the proposed development due to the 1,605 square feet of additional
hardscape in the proposed condition as compared the existing site. Similar to the existing
condition, the project will discharge from one (1) location leaving the property in the
southwest comer to the Hemlock Avenue right-of-way. Thus, water will not be diverted
away from existing drainage patterns, and the increase in proposed impervious area and
resulting peak runoff -before taking into account the detention qualities of the proposed
pervious pavers -will not have an adverse effect on the downstream watershed. The
inherent detention characteristics and storage volume provided in the gravel layers of the
pervious pavers will assist to detain and mitigate peak runoff to pre-project conditions.
The proposed project will be exempt from hydromodification compliance requirements as
it does not qualify as a Priority Development Project as defined in the Regional
Municipal Separate Storm Sewer System (MS4) Permit and the San Diego County Model
BMP Design Manual.
Page 3 ofll
PLSA 3392-01
1.5 References
"San Diego County Hydrology Manual", revised June 2003, County of San Diego,
Department of Public Works, Flood Control Section.
"San Diego County Hydraulic Design Manual", revised September 2014, County of San
Diego, Department of Public Works, Flood Control Section
"City of Carlsbad Engineering Standards, Volume 5: Carlsbad BMP Design Manual
(Post Construction Treatment BMPs) ", revised February 2016
"Low Impact Development Handbook-Stormwater Management Strategies", revised
July 2014, County of San Diego, Department of Public Works
Soil Survey Staff, Natural Resources Conservation Service, United States Department of
Agriculture. Web Soil Survey. Available online at http://websoilsurvey.nrcs.usda.gov.
Accessed September 2, 2020
Page 4 ofll
PLSA 3392-01
2.0 METHODOLOGY
2.1 Introduction
The hydrologic model used to perform the hydrologic analysis presented in this report
utilizes the Rational 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:
Where:
I= 7.44 x P6 x o-0·645
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:
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 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.
Page 5 of 11
PLSA 3392-01
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 under
100 feet, and is dependent on land use and slope. The time of concentration minimum is
5 minutes for purposes of calculating rainfall intensity per the County of San Diego
Hydrology Manual requirements.
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
in the Appendix of this report, categorizes the land use, the associated development
density ( dwelling units per acre) and the percentage of impervious area.
For this study, a weighted runoff coefficient was used for both the pre-project and post-
developed site in accordance with the equation provided in Section 3.1.2 of the County of
San Diego Hydrology Manual. The weighted runoff coefficient is a function of the total
impervious and pervious surface areas in the existing and proposed conditions.
Page 6 ofll
PLSA 3392-01
3.0 HYDROLOGY MODEL OUTPUT
3.1 Pre-Developed Hydrologic Model Output (100-Year Event)
Pre-Development:
*Rational Method Equation Q=CIA
P100= 2.5 * 100-Year, 6-Hour Rainfall Precipitation
Basin EX-1
Total Area= 8,241 sf-+ 0.189 Acres
Impervious Area= 2,760 sf-+ 0.063 Acres
Pervious Area= 5,481 sf-+ 0.126 Acres
Cn, Weighted Runoff Coefficient
-0.25, Cn value for natural ground, Type B soils
* Per San Diego Hydrology Design Manual (SDHDM) Section 3.1.2
-0.9, C value for developed/ impervious surface
* Per SDHDM Section 3.1.2
Cn = 0.9 x 2,760 sf+ 0.25 x 5,481 sf= 0.47
8,241 sf
Tc= 5.0 Min
r = 7.44 x P6 x o-0·645
*Minimum Tc for Rainfall Intensity per SDHDM
*Rainfall Intensity Equation
r = 7.44 x 2.5 x 5.o-0·645 ::::: 6.59 in/hr
Q100 = 0.47 x 6.59 in/hr x 0.189 Ac= 0.58 cfs
Page 7 ofll
PLSA 3392-01
3.2 Post-Developed Hydrologic Model Output (100-Year Event)
Post-Development (without considering HMP/BMP treatment):
*Rational Method Equation Q=CIA
P100= 2.5 * 100-Y ear, 6-Hour Rainfall Precipitation
Basin PR-1
Total Area= 8,241 sf-+ 0.189 Acres
Impervious Area= 4,365 sf-+ 0.100 Acres
Pervious Area= 3,876 sf-+ 0.089 Acres
Cn, Weighted Runoff Coefficient
-0.25, Cn value for natural ground, Type B soils
* Per San Diego Hydrology Design Manual (SDHDM) Section 3.1.2
-0.9, C value for developed/ impervious surface
* Per SDHDM Section 3.1.2
Cn = 0.9 x 4,365 sf+ 0.25 x 3,876 sf= 0.59
8,241 sf
Tc= 5.0 Min
1 = 7.44 x P6 x o-0·645
*Minimum Tc for Rainfall Intensity per SDHDM
*Rainfall Intensity Equation
1 = 7.44 x 2.5 x 5.o-0·645 :::::: 6.59 in/hr
Q100 = 0.59 x 6.59 in/hr x 0.189 Ac= 0.74 cfs
Page 8 ofll
3.3 Pre-Developed vs. Post-Developed Peak Flows (100-Year Event)
Total Pre-Development (Discharge Leaving Site)
Basin EX-1 Q100 = 0.58 cfs
*Discharging from the site at the southwest comer to Hemlock A venue
Total Q100 = 0.58 cfs
Total Post-Development (Discharge Leaving Site)
Basin PR-1 Q100 = 0.74 cfs
*Discharging from the site at the southwest comer to Hemlock A venue
Total Q100 = 0.74 cfs
Pre-Development vs. Post-Development (Discharge Leaving Site):
Delta
PLSA 3392-01
Pre-Development
Q100 = 0.58 cfs
Post-Development
Q100 = 0.74 cfs 0.16 cfs increase
3.4 Detention Analysis
The 100-year, 6-hour storm event detention analysis was performed in accordance with
Chapter 6 of the San Diego County Hydrology Manual (SDCHM). The results of the
analysis provide a static detention requirement by comparing the total volume produced
by the 100-year, 6-hour storm in the existing and post-project conditions. The total
volume can be calculated after producing a hydrograph for each event (pre-and post-
developed) with the results of the Rational Method analysis. Calculations and results of
this analysis can be shown below.
The proposed pervious pavement driveway provides mitigation of the 100-year, 6-hour
storm event peak flow rate. Underneath the paver units and bedding layer is a 6-inch
gravel storage layer that provides sufficient detention volume to offset the increase in
total volume generated in the proposed condition due to the increase in overall hardscape.
See sections 3.4.1 and 3.4.2 below for the results of these calculations.
Page 9 ofll
3.4.1 Static (No Routing) Detention Calculation
Pre-Development: 100-Year Event (Basin EX-I)
PLSA 3392-01
Volume = C*P6* A *Equation 6-1 of SDCHM
Volume= 0.47 * (2.5 in* 1 ft/ 12 in)* (8,241 SF)= 807 CF
Post-Development: 100-Year Event (Basin PR-I)
Volume= C*P6*A *Equation 6-1 of SDCHM
Volume= 0.59 * (2.5 in* 1 ft/ 12 in)* (8,241 SF)= 1,013 CF
Required Detention Volume = Total Post-Development -Total Pre-Development
= Basin PR-I -Basin EX-I
= 1,013 CF-807 CF= 206 CF
-> 206 CF static storage required for proposed basin PR-I
3.4.2 Pervious Pavement Drive Aisle -Storage
Detention Volume Provided = Driveway Area x 6" Gravel Layer x 40% Voids
= 2,930 SF* (6 in* 1 ft/ 12 in)* 0.40
= 586 CF
PERVfOUS PAVERS • TYPICAL DETAIL
586 CF storage in gravel layers under pervious paver driveway > 206 CF static storage
required for drainage basin PR-1 per SDCHM.
** As stated in the conclusion of the report, the slight increase in total peak flow leaving
the project site will be mitigated by the storage volume provided by the pervious pavers
proposed throughout the site, as shown above. Refer to Sheet 3 of the Civil plans
prepared by Pasco, Laret, Suiter & Associates for the Tentative Parcel Map (MS) and
minor Site Development Plan application under separate cover for a cross-section of the
pervious pavers showing a gravel storage layer underneath the paver and bedding layers.
Page 10 of 11
PLSA 3392-01
4.0 APPENDIX
Page 11 of 11
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Minutes Hours Duration
Intensity-Duration Design Chart -Template
Directions for Appllcatlon:
( 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) p = 2.5 in. p = 4.5 _§_ = 55.6 %(2) 6 -· --• 24 --•p24 --
(c) Adjusted p6<2> = ~ in.
(d) tx = ~min,
(e) I = 6.59 in.lhr.
Note: This chart replaces the Intensity-Duration-Frequency
curves used since 1965.
3-1
San Diego County Hydrology Manual
Date: June 2003
Table 3-1
Section:
Page:
RUNOFF COEFFICIENTS FOR URBAN AREAS
Land Use Runoff Coefficient "C"
Soil T~ee
NRCS Elements Coun Elements % IMPER. A 8
Undisturbed Natural Terrain (Natural) Pennanent Open Space 0* 0.20 1 0.25 I
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 (MDR) Residential, 4.3 DU/A or less 30 0.41 0.45
Medium Density Residential (MDR) Residential, 7.3 DU/A or less 40 0.48 0.51
Medium Density Residential (MDR) Residential, I 0.9 DU/ A or less 45 0.52 0.54
Medium Density Residential (MDR) 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
C
0.30
0.36
0.42
0.45
0.48
0.54
0.57
0.60
0.69
0.78
0.78
0.81
0.84
0.84
0.87
3
6 of26
D
0.35
0.41
0.46
0.49
0.52
0.57
0.60
0.63
0.71
0.79
0.79
0.82
0.85
0.85
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
Hydrologic Soil Group-San Diego County Area, California
Map Scale: 1:396 if pmted on A portrait (8.5" X 11") sheet
N ----=====--------========Meter.; :I) 0 5 10 20
A ---===------======feet 0 ~ :II 00 00
Map projection: ~ MercalDr C.orner CDOrdinates: WGS84 Edge tic;: UTM Zone llN WGS84
!J_SDA Natural Resources
;..-Conservation Service
Web Soil Survey
National Cooperative Soil Survey
9/2/2020
Page 1 of 4
Hydrologic Soil Group-San Diego County Area, California
MAP LEGEND MAP INFORMATION
Area of Interest (AOI) D Area of Interest (AOI)
Soils
Soil Rating Polygons
• A
D AID .B
BID
□ C
D CID
D D
._J Not rated or not available
Soil Rating Lines
A
AID
~ B
~ B/D
C
CID
D
,. ~ Not rated or not available
Soil Rating Points
■ A
a AID
■ B
■ B/D
USDA Natural Resources
:iiiiii Conservation Service
C
C
C
CID
C D
C Not rated or not available
Water Features
Streams and Canals
Transportation
+-+-+
~
Rails
Interstate Highways
_., US Routes
Major Roads
local Roads
Background
• Aerial Photography
Web Soil Survey
National Cooperative Soil Survey
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.
9/2/2020
Page 2 of 4
Hydrologic Soil Group-San Diego County Area, California
~
Hydrologic Soil Group
Marina loamy coarse
sand, 2 to 9 percent
slopes
Totals for Area of Interest
Description
0.3
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 (AID, 8/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 8. 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 (AID, 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
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
100.0%
9/2/2020
Page 3 of 4
Hydrologic Soil Group-San Diego County Area, California
Component Percent Cutoff: None Specified
Tie-break Rule: Higher
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
9/2/2020
Page 4 of 4
..
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