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HomeMy WebLinkAboutCDP 2018-0038; ALLANSON-SELVIDGE RESIDENCE; HYDROLOGY STUDY; 2021-01-04-------- .. .. -.. --.. ---.. -... -... .. -.. .. 1111 .. .. ... .. ... HYDROLOGY STUDY For ALLANSON-SELVIDGE RESIDENCE 5170 CARLSBAD BL VD CARLSBAD, CA 92008 CDP 2018-0038 DWG520-4A GR 2019-0036 City of Carlsbad, CA PREPARED FOR: Paula Selvidge 2406 Trona Way Carlsbad, CA 92009 Date: January 4, 2021 PREPARED BY: Pasco Laret Suiter & Associates 535 N. Highway 101, Suite A Solana Beach, CA 9207 5 (858) 259-8212 n Vlf1I~ l'.J~ n ltf".tdw 1,.._ __ . f _, sf' \\.. ., ~ . .I JAN O 5 2021 LAN" ... r,~-\':· OPr1El\1T , 1.,1 .I ... 1 t:.:L ..1, .v._ ~ No. 71651 xp. 12/31/ 01/04/2021 BRIAN ARDOLINO, RCE 71651 DATE Executive Summary Introduction Existing Conditions Proposed Project Summary of Results Conclusions References Methodology Introduction TABLE OF CONTENTS County of San Diego Criteria Runoff Coefficient Determination Hydrologic Analyses Existing Hydrologic Analysis Proposed Hydrologic Analysis Detention Calculations Appendix 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 January 2021 .. .. ------.. ----- -.. -.. .. -.. -.. -.. .. .. -.. .. 11111 1111111 11111 -- -.. .. .. .. .. .. ... ... ... .... -... ... .. .. .. .. ... ... -.. .. .. .. .. .. .. .. .. .. .. .. -.. 1.0 EXECUTIVE SUMMARY 1.1 Introduction This Hydrology Study for a proposed single-family 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 event. 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°07'52.61" W 117°20'03.41". The site is bordered by residential development to the north, south and east. Carlsbad Boulevard is located directly to the west of the proposed development. The project site is located in the Carlsbad Hydrologic Unit, Agua Hedionda Hydrologic Area and Los Monos Hydrologic Subarea (904.31) and is underlain with hydrologic type B soils . The existing project site includes 1 residence with an attached garage, shed, concrete driveway, and concrete rear yard patio. The topography of the site generally consists of a gentle slope from east to west. Storm water runoff from the existing site flows overland in a westerly direction onto Carlsbad Boulevard . 1.3 Proposed Project The intent of the proposed project is to construct a new single-family residence with an attached garage, driveway, and associated hardscape improvements . The proposed project generally conveys storm water runoff from impervious areas to landscape areas where it is collected by a series of inlets and conveyed via PVC pipe to a bioretention basin located on the southwest corner of the property. The proposed bioretention basin is designed to provide storm water pollutant control and to mitigate the increase in 100-year storm event peak flows resulting from the proposed development . Runoff conveyed to the bioretention basin will pond and infiltrate into the ground. Any overflow will discharge via an overflow structure to Carlsbad Boulevard, mimicking the existing drainage conditions of the project site . To address the storm water quality goals established for this development, proposed permanent Best Management Practices (BMPs) and treatment methods will be incorporated into the storm water design. The proposed BMPs include multiple landscape areas and a dedicated bioretention basin BMP which is designed to provide storm water pollutant control and to mitigate the 100-year storm event peak flows . January 2021 1.4 Summary of Results Upon performing hydrologic analysis of the project site in both the existing and proposed conditions the following results were produced. s ummary o -r ea ow f 100 Y P k Fl R ates Condition Area (ac) Tc (min) Q100 (cfs) Existing 0.15 5 0.45 Proposed 0.15 5 0.68 Proposed Detained 0.15 5 0.38 Refer to Sections 3.0 and 4.0 for calculations. 1.5 Conclusions The overall peak flow leaving the property is increased by 0.24 cfs due to the increase in impervious area. The existing drainage patterns and discharge point of the site will not be significantly altered as a result of the proposed development. The proposed bioretention basin BMP is adequately designed to treat the runoff from the project site and mitigate the increase in the 100-year storm event peak runoff from the site to below the existing condition. Based on the analyses in this report it is the professional opinion of Pasco Laret Suiter & Associates, Inc. that the proposed drainage system on the corresponding Preliminary Grading Plan will function to adequately intercept, contain and convey flow to the appropriate points of discharge and not cause negative impacts downstream. 1.6 References "San Diego Counry Hydrology Manual': revised June 2003, County of San Diego, Department of Public Works, Flood Control Section. "California Regional Water Qualiry Control Board San Diego Region Order No. R9-2015-0100," California Regional Water Quality Control Board, San Diego Region (SDRWQCB). January 2021 .. .. 1111 .. .. 1111111 .. .. 1111 .. .. 1111 .. .. .. -.. 1111 .. .. .. .. --.. 1111 -.. .. ... -... -.. .. .. .. ... ... -... -... ... ... -... -... -... .. .. ... ... .. -... .. .. ... .... 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 D-0645 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). The RM equation is as follows: Where: Q =CIA Q= flow (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/hr). A = drainage area contributing to the basin (ac) 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 . Per the United States Department of Agriculture Web Soil Survey, the project site is underlain with Hydrologic Soil Group B. Therefore runoff coefficients utilized for the Rational Method calculations are based on B soil . 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) . January 2021 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 (FA .. A) 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 One of the variables of the RM equation is the runoff coefficient, C, which is dependent upon land use and soil type. Table 3-1 Runoff Coefficients for Urban Areas in the County Hydrology Manual 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 for each soil type class. A composite runoff coefficient can also be calculated for an area based on soil type and impervious percentage using the following equation from Section 3.1.2 of the County Hydrology Manual: C = 0.90 X (% Impervious) + Cp X (1 -% Impervious) Where: Cp = Pervious Coefficient Runoff Value for the soil type (shown in Table 3-1 as Undisturbed Natural Terrain/Permanent Open Space, 0% Impervious) The calculations contained herein figure a composite runoff coefficient for the onsite project areas based on the percentage of impervious area and the percentage of pervious or landscape area. Refer to the Section 3.0 for the composite runoff coefficient calculations. January 2021 .. -.. • ---.. -.. .. • 1111 • .. -• .. • .. • 111111 -... -.. -.. -.. -.. -.. -.. ... .. -... -.. ... ... -... -.. - ... -... -... ... ... .. ... .. .. -... ... ... ... ... .. ... 3.0 HYDROLOGIC ANALYSES Rational Method Parameters Runoff Coefficient C= 0.90 x (% Impervious) + Cp (1-% Impervious)* Cp=O.25* for existing condition pervious type "B" soils 10O-Year 6-Hour Storm Precipitation (P6)=2.5 in (see rainfall isopluvial*) I= Intensity in/hr, I=7.44xP6xD-ll 645* Duration (D)= Time of Concentration, Tc (min) Q=Peak Runoff, Q=CIA (cfs) 'From San Diego County Hydrology Manual,Junc 2003 RcYision Section 3 3.1 Existing Condition A=6,66O sf= 0.15 ac Impervious area = 2,119 sf % IMP= 32.1% C= (0.318 x 0.90) + [(1-0.318) x 0.25] C= 0.46 Tc= Assume 5 minutes I 10n=7.44(2.5) (5) -0645 1100=6.59 in/hr Q100= (0.46)(6.59 in/hr) (.15 ac) Qioo=0.45 cfs Proposed Condition A=6,66O sf= 0.15 ac Impervious area = 4,500 sf % IMP= 68.2% C= (0.676 x 0.90) + [(1-0.676) x 0.25] C= 0.69 Tc= Assume 5 minutes 1100=7.44(2.5)(5) 0645 1100=6.59 in/hr Q10n= (0.69)(6.59 in/hr)(O.15 ac) Qioo=0.68 cfs llQ100 = Qpost -Qpre = 0.68 cfs -0.45 cfs 6.Q100 = 0.23 cfs January 2021 4.0 DETENTION CALCULATIONS The 100-year storm event detention analysis was performed using HydroCAD Stormwater Modeling software. The inflow runoff hydrograph to the BMP was modeled using RatHydro which is a Rational Method Design Storm Hydrograph software that creates a hydrograph using the results of the Rational Method calculations. HydroCAD has the ability to route the 100-year storm event inflow hydrograph through the BMP and based on the BMP cross sectional geometry, stage storage and outlet structure data, HydroCAD calculates the detained peak flow rate and detained time to peak. Based on the results of the HydroCAD analysis, the biofiltration basin provides mitigation for the 100-year storm event peak flow rate, detaining the proposed condition to 0.38 cfs which is below the existing condition. January 2021 .. • .. 11111111 .. .. .. • .. • .. -.. • .. • -.. .. .. 11111 .. • .. .. .. ... .. .. .. ... ~ .. .. .. .. ---... -.. .. -... -.... -.... -... -.... -... -... ---.... -.. -- .. -.. .. -... - ~ 1L ~ Inflow to BMP e BMP Routing Diagram for 2912 Prepared by Pasco Laret Suiter & Associates, Printed 4/1/2020 HydroCAD® 10.10-3a s/n 10097 © 2020 HydroCAD Software Solutions LLC 2912 Prepared by Pasco Laret Suiter & Associates HydroCAD® 10.10-3~ s/n 10097 © 2020 HydroCAD Software Solutions LLC Summary for Link 1 L: Inflow to BMP = 0.013 af Printed 4/1/2020 Paqe4 Inflow Primary = 0.66 cfs@ 0.66 cfs@ 4.08 hrs, Volume= 4.08 hrs, Volume= 0.013 af, Atten= 0%, Lag= 0.0 min Primary outflow= Inflow, Time Span= 0.00-96.00 hrs, dt= 0.01 hrs DISCHARGE Imported from RatHydro.csv 0.7·· 0.65 0.6 0.55. 0.5 0.45··· J!! 0.4-· .!:, 3: 0.35 0 ii: 0.3 0.25. 0.2. 0.15. 0.1 0.05 O· 0 ' Link 1 L: Inflow to BMP Hydrograph 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Time (hours) 1111 Inflow II Primary Ill .. Ill .. Ill 11111 .. .. Ill .. .. .. .. 11111 .. .. .. -.. - ... .. .. .. .. • .. """' .. ... .... .. .. .. ... .. ... ... ... .. .... ... ... ... ... ... ... ... ... ... .... ... .... ... .. .. ... .. ... .. .. .. ... -... 2912 Prepared by Pasco Laret Suiter & Associates HydroCAD® 10.10-3a s/n 10097 © 2020 HydroCAD Software Solutions LLC Summary for Pond 2P: BMP = 0.013 af Printed 4/1/2020 Page 5 Inflow Outflow Primary = = 0.66 cfs@ 0.38 cfs@ 0.38 cfs@ 4.08 hrs, Volume= 4.13 hrs, Volume= 4.13 hrs, Volume= 0.013 af, Atten= 42%, Lag= 2.6 min 0.013 af Routing by Dyn-Stor-lnd method, Time Span= 0.00-96.00 hrs, dt= 0.01 hrs Peak Elev= 54.94' @ 4.13 hrs Surf.Area= 93 sf Storage= 85 cf Plug-Flow detention time= 2.0 min calculated for 0.013 af (100% of inflow) Center-of-Mass det. time= 1.8 min ( 238.5 -236.8) Volume Invert Avail.Storage Storage Description #1 52.27' 155 cf Custom Stage Data {Conic) Listed below (Recalc) Elevation {feet) 52.27 54.60 55.80 Device #1 #2 #3 Routing Primary Device 1 Device 1 Surf.Area Voids Inc.Store Cum.Store Wet.Area {sq-ft) {%) { cu bi c-f eet) {cubic-feet) {sq-ft) 93 0.0 0 0 93 93 20.0 43 43 173 93 100.0 112 155 214 Invert Outlet Devices 52.27' 4.0" Round Culvert L= 11.5' RCP, groove end projecting, Ke= 0.200 Inlet/ Outlet Invert= 97.67' / 97.57' S= 0.0087 '/' Cc= 0.900 n= 0.013, Flow Area= 0.09 sf 52.27' 3.0" Vert. Orifice C= 0.600 Limited to weir flow at low heads 55.60' 12.0" x 12.0" Horiz. Grate C= 0.600 in 12.0" x 12.0" Grate (100% open area) Limited to weir flow at low heads Primary Outflow Max=0.38 cfs @ 4.13 hrs HW=54.94' (Free Discharge) 4t._l=Culvert (Passes 0.38 cfs of 0.67 cfs potential flow) c2=Orifice (Orifice Controls 0.38 cfs@ 7.83 fps) 3=Grate ( Controls 0.00 cfs) • • . \'J.. "'lf'\ ~ \'). .,.,\"'\ 'fMC.. .. .. .. 111111 .. 111111 .. -.. .. .. -.. --.. -.. .. .. -.. -.. -.. • .. 1111111 .. -.. -.. -... -.. -... -... -... ... .. ---.... - -... -.... ---.. -... -... -.... -... -... - 2912 Prepared by Pasco Laret Suiter & Associates HydroCAD® 10.10-3a s/n 10097 © 2020 HydroCAD Software Solutions LLC Pond 2P: BMP Hydrograph 0.7 0.65 0.6 0.55 0.5 . 0.45 . .!!! 0.4 ~ ~ 0.35 u:: 0.3 0.25 0.2 0.15 0.1 0.05. o· 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Time (hours) Printed 4/1/2020 Page 6 Ill! Inflow l'J Primary 5.0 APPENDIX April 2020 11111 .. 11111 .. -.. -.. -.. -.. -.. -.. -.. .. --.. ---------------- ' 1 I I ' 1 r , ' 1 San Diego County Hydrology Manual Date: June 2003 f 1 ' 1 r 1 r 1 f 1 r , f 1 f 1 r 1 r 1 Section: Page: Table 3-1 RUNOFF COEFFICIENTS FOR URBAN AREAS Land Use Runoff Coefficient "C' Soil Tv e NRCS Elements Coun , Elements %IMPER. A B C Undisturbed Natural Terrain (Natural) Permanent Open Space O* 0.20 0.25 0.30 Low Density Residential (LDR) Residential, 1.0 DU/ A or less 10 0.27 0.32 0.36 Low Density Residential (LDR) Residential, 2.0 DU/A or less 20 0.34 0.38 0.42 Low Density Residential (LDR) Residential. 2.9 DU/ A or less 25 0.38 0.41 0.45 Medium Density Residential (MDR) Residential, 4.3 DU/A or less 30 0.41 0.45 0.48 Medium Density Residential (MDR) Residential, 7.3 DU/A or less 40 0.48 0.51 0.54 Medium Density Residential (MDR) Residential, 10.9 DU/ A or less 45 0.52 0.54 0.57 Medium Density Residential (MDR) Residential. 14.5 DU/ A or less 50 0.55 0.58 0.60 High Density Residential (HDR) Residential, 24.0 DU/ A or less 65 0.66 0.67 0.69 High Density Residential (HDR) Residential. 43.0 DU/A or less 80 0.76 0.77 0.78 Commercial/Industrial (N. Com) Neighborhood Commercial 80 0.76 0.77 0.78 Commercial/Industrial (G. Com) General Commercial 85 0.80 0.80 0.81 Commercial/Industrial (0.P. Com) Office Professional/Commercial 90 0.83 0.84 0.84 Commercial/Industrial (Limited I.) Limited Industrial 90 0.83 0.84 0.84 Commercial/Industrial (General I. General Industrial 95 0.87 0.87 0.87 ' 1 r 1 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 *'Inc 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 f I f I g 0 ,..._ ..... ..... --0 <;.) . io .... r-.. ..... ..... ,._<!_, a 0 0 ,..._ .... ..... .. Riverside County :· "'"' ..... . . . . ' ., . •,, P:. ~ .,._,'!~ \, .. ·- ..... -'" .. ~ ... :. .. '"'!> .... -~ . . ,() .. ·.·, . '. ~.,," '.. "',,._ •. ·- ", .... io -.:I" 0 co .... .... .. _ ---.... ---~, ...... .. 0 • g 0 co ..... ..... ... ......... ' to .... 0 co ..--.... ,· ! : ~ ,' ,/ I 33°30' ..,.. . . 33•00• -----,-----'ro1'--------;;;.~:-:-~~~~~~::...:__--'-<..~hP.-:~~~c::=_~~~cc........--l-.:.---;:·~;_· -~()--.:.J~~~-{.:._--------L.L' ----... 3~00· :s.D'. COUNfY : 32°45' 32°30' a 'io C") .... • 0 ,..._ ,..._ .... .... .... ..... a 0 0 ,..._ .... .... in -.:I" 0 co .... .... I I •·· .. t,,.-..... I • ... . . . 1· : . \ : ~ ·0. . e X I g 0 co .... .... in ..... ·" co .... .... 32°30' County of San Diego Hydrology Manual Rainfall Isopluvials 100 Year Rainfall Event -6 Hours lsopluvial (inches) ~GIS IS De.oa.rt....-.er:if-:JfP.J;Nci.1.tvka: GG<IIJ,'VP~-•,v; l~m-, ;uX~!i S'),v,i::;:05 THIS MAP IS PROVIDED WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Copyright SanGIS. All Rights Reserved. This products may contain Information from the SANDAG Regional Information System which cannot be reproduced without the written permission of SANDAG. This product may contain Information which has been reproduced with permission granted by Thomas Brothers Maps. Miles 33" 753"N 33" 7 51"N Hydrologic Soil Group-San Diego County Area, California ;: I, ~ Map Scale: 1:277 t printed on A landscape (11" X 8.5'') sheet. 9 N ----===:=..--------========Meiers 24 0 4 8 16 A ---===------=======feet o m ~ ~ oo Map projection: Web Mercator Comerrooronates: WGS84 Edge tics: UTM Zone llN WGS84 USDA Natural Resources Web Soil Survey National Cooperative Soil Survey ":FF Conservation Service 8/6/2018 Page 1 of 4 33" 7 53" N 33" 7 Sl"N Hydrologic Soil Group-San Diego County Area. California MAP LEGEND MAP INFORMATION Area of Interest (AOI) LJ Area of Interest (AOI) Soils Soil Rating Polygons D A D AID DB D BID D C D C/D DD D Not rated or not available Soil Rating Lines A -,,, AID .,,..,,,,_ B -,,, B/D C .,,..,,,,_ D ,. " Not rated or not available Soil Rating Points II A ■ AID ■ B ■ BID N.-t11r"I ResoJirr."" Cc; .. _.~ :ratio ~-ice C C II CID • D □ Not rated or not available Water Features Streams and Canals Transportation H-+ Rails _.., Interstate Highways US Routes Major Roads Local Roads Background • Aerial Photography Web Soj l Survey Natl . oop~oil 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 12, Sep 13, 2017 Soil map units are labeled (as space allows) for map scales 1 :50,000 or larger . Date(s) aerial images were photographed: Nov 3, 2014-Nov 22,2014 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. 8/6/2018 2 of .. .. .. .. .. .. ... ... .... ... ... ... .... ... ... ... .... -.. ... ... ... ... ... ... .. .. ... .. .. ... Hydrologic Soil Group-San Diego County Area, California Hydrologic Soil Group Map unit symbol Map unit name Rating Acres inAOI Percent of AOI MIC Marina loamy coarse B sand, 2 to 9 percent slopes 0.2 I Totals for Area of Interest 0.2 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 (ND, BID, 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 (ND, 8/D, or CID), 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 l!SDA Natural Resources aiii Conservation Service Web Soil Survey National Cooperative Soil Survey 100.0% 100.0% 1 8/6/2018 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 8/6/2018 Page 4 of 4 .. .. .. -.. .. 111111 Ill .. ---.. ------.. -... .. -.. 111111 ---- 11111 -----.. -