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Home My WebLink AboutPUD 2021-0001; GARFIELD HOMES; PRELIMINARY HYDROLOGY STUDY; 2020-12-01- • - - - •· - - - -------.. -- 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 ( l 00-Year Event) Pre-Developed vs. Post-Developed Peak Flows ( 100-Year Event) Appendix lsopluvial Maps Intensity Duration Design Charts Runoff Coefficients Hydrologic Soil Group -USDA Web Soil Sutv'ey 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 4.0 ----- - --- • - - - - • - • - • - - --- -- PLSA 3391-01 1.0 EXECUTIVE SUMMARY 1.1 Introduction This Preliminary Hydrology Study for the proposed development at 4008 Garfield Street, known as the Garfield Homes, 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 l 00-ycar, 6-hour stonn. 1.2 Existing Conditions The subject property is located along Garfield Street, southeast of the Garfield Street and Chinquapin A venue intersection, in the City of Carlsbad. The site is bound by Garfield Street to the west, Chinquapin A venue to the north, and residential development to the south and east. The existing site consists of a single-family residential structure, detached garage, and other hardscape and landscape improvements typical to the surrounding residential sites. The project site is located in the Agua Hedionda Hydrologic Area, and, more specifically, the Los Monos Hydrologic Sub-Area (904.3 l). The subject property has an approximate area of 0.138 acres and is approximately 51 % 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 0.58 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 existing site can be categorized into a single drainage basin designated as basin EX-I, and comprises the entire property. Drainage basin EX-I primarily drains from west to east, discharging onto Chinquapin Avenue by surface and sheet flow methods. Once on Chinquapin Avenue, runoff leaving the site travels east along Chinquapin Avenue and enters an existing storm drain inlet at the end of the public street. Storm water collected in the drain at the end of Chinquapin Avenue continues to be conveyed through public storm water facilities until ultimately reaching the Pacific Ocean . 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 stonn event for the onsite drainage basins. Table 1 below summarizes the results of the Rational Method calculations. Page I of9 PLSA 3391-01 EXISTING DRAINAGE FLOWS DRAINAGE Q,oo DRAINAGE 1100 AREA (CFS) (IN/HR) AREA (ACRES) EX-1 0.138 0.51 6.42 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 three (3) new single-family residential condominiums. Construction activities such as grading and onsite hardscape improvements are also proposed. Hardscape improvements include the construction of concrete driveways and walkways. The development project also includes offsite improvement in the right-of-way such as public concrete walkways, new curb and gutter, and driveway aprons. Miscellaneous landscape improvements typical to residential development are also being proposed. As in the existing condition, stonn water will flow from west to cast in the post-development condition. Storm water collection from structures and other onsite hardscape features will be dispensed into landscaped areas around proposed building and conveyed via landscaped swales to brooks box catch basins. Catch basin will be connected to 3" PVC underdrain pipes directing water onto Chinquapin A venue. As in the pre-development condition, water will then continue east through public facilities until ultimately reaching the Pacific Ocean. The subject property is approximately 70% impervious in the proposed condition. Based on the proposed impervious area and land use type, a post-development weighted runoff coefficient of 0.71 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 a single drainage basin designated as PR-I. Drainage basin PR-I consists of the entire project property, and as previously discussed will all drain into the Chinquapin right-of-way. 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 I 00-year, 6- hour storm event for the onsite drainage basins. Table 2 below summarizes the results of the Rational Method calculations in the post-developed condition. Page 2 of9 .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. ~ .. ~ .. ... • • • • • - - • ►, --------- ► PLSA 3391-01 PROPOSED DRAINAGE FLOWS DRAINAGE DRAINAGE Qwo 1,oo AREA AREA (ACRES) (CFS) (IN/HR) PR-l 0.138 0.49 5.03 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 hardscapc areas will be directed to landscaped areas in an effort to disperse drainage to pervious surfaces. The landscaped areas will assist to remove sediment and particulate-bound pollutants from storm water . 1.4 Conclusions Based upon the analysis included in this report, there is a difference of ~0.02 cfs in peak runoff between the proposed development and the existing condition. Similar to the existing condition, the project will discharge onto Chinquapin Avenue. Water will not be diverted away from existing drainage patterns, and the difference in runoff will not have any adverse effect on the downstream watershed and can be considered negligible. 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. An additional detention analysis was not performed at this time as it was determined that the project decreases runoff in the proposed condition from the pre- project condition . Page 3 of9 PLSA 3391-01 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 -Stonnwater 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://websoi!survey.nrcs.usda.gov. Accessed May 20, 2020 Page 4 of9 - ------.. .. .. .. .. .. -.. -.. ... .. ... .. -.. .. .. - --- ... .. - - • -- • -- ------- • - • - - • - - - -.. -- PLSA 3391-0l 2.0 METHODOLOGY 2.1 Introduction The hydrologic mode] 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 (J) is equal to: I= 7.44 X P, X o-0 "" 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: 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 stonn 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 of9 PLSA 3391-01 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 stonn 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 of9 .. -.. -.. -.. -.. -.. --.. -.. --.. -.. .. .. - - - • - .. - • - ..... - - - - • - --- - - ----- -- PLSA 3391-0 I 3.0 HYDROLOGY MODEL OUTPUT 3.1 Pre-Developed Hydrologic Model Output (JOO-Year Event) Pre-Development: *Rational Method Equation Q=CIA P100= 2.5 * 100-Year, 6-Hour Rainfall Precipitation Total Site Tota1Area=6,00I sf~ 0.138Acres Impervious Area= 3,032 sf-+ 0.070 Acres Pervious Area= 2,969 sf-+ 0.068 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 3,032 sf+ 0.25 x 2,969 sf= 0.58 6,001 sf Tc=ti+tt ti@ 100'=8.3% '=> 4.0 min from Table 3-2 tt => L=l56'-IOO'= 56' 6E=58.5-57.6=0.9' tt= [ { 1 l.9(L/5,280)'3 }/6E]'0.385 11= [ { 11.9(56/5,280)'3 //0.9]'0.385 = 0.02 0.02 x 60 = 1.2 Min. Tc = 4.0 + 1.2 = 5.2 Min Tc= 5.2 Min I= 7 .44 x P6 x o-M45 *Rainfall Intensity Equation I= 7.44 x 2.5 x 5.2-"645 " 6.42 in/hr Q,oo-0.58 x 6.42 in/hr x 0.138 Ac= 0.51 cfs Page 7 of9 PLSA 3391-0l . ' 4.0 APPENDIX • • • • • ------.. ------Page 9 of9 -- - - - - • • • ---- -------- San Diego County Hydrology Manual Date: June 2003 Section: Page: ) 12 of26 Note that the Initial Time of Concentration should be reflective of the general land-use at the upstream end of a drainage basin. A single lot with an area of two or less acres does not have a significant effect where the dramage basin area is 20 to 600 acres_ Table 3-2 provides limits of the length (Maximum Length (LM)) of sheet flow to be used in hydrology studies. Initial T1 values based on average C values for the Land Use Element are also included. These values can be used in planning and design applications as described below. Exceptions may be approved by the "Regulating Agency" when submitted with a detailed study. Element* Natural LDR LOR LDR MOR MOR MOR MOR HDR HOR N.Com G.Com O.P./Com Limited l. General I. Table 3-2 MAXIMUM OVERLAND FLOW LENGTH (LM) & INITIAL TIME OF CONCENTRATION (T1\ DU/ .5% 1% 2% 3% 5% Acre LM T, LM T, LM T, LM T, LM T, 50 I 3.2 70 12.5 85 10.9 100 10.3 100 8.7 I 50 12.2 70 11.5 [ 85 10.0 100 9.5 100 8.0 2 50 11.3 70 10.5 ; 85 9.2 100 8.8 100' 7.4 2.9 50 10.7 70 10.0 85 8.8 95 8.1 100 7.0 4.3 50 10.2 70 9.6 80 8.1 95 7.8 l00 6.7 7.3 50 9.2 65 8.4 80 7.4 95 7.0 100 6.0 l0.9 50 8.7 65 7.9 80 6.9 90 6.4 l00 5.7 14.5 50 8.2 65 7.4 80 6.5 90 6.0 100 5.4 24 50 6.7 65 6.1 75 5.1 90 4.9 95 4.3 43 50 5.3 65 4.7 75 4.0 85 3.8 95 3.4 50 5.3 60 4.5 75 4.0 85 3.8 95 3.4 50 4.7 60 4.1 75 3.6 85 3.4 90 2.9 50 4.2 60 3.7 70 3.1 80 2.9 90 2.6 50 4.2 60 3.7 70 3.1 80 2.9 90 2.6 50 3.7 60 3.2 70 2.7 80 2.6 90 2.3 10% LM T, 100 6.9 100 6.4 100 5.8 100 5.6 100 5.3 100 4.8 100 4.5 100 4.3 l00 3.5 JOO 2.7 100 2.7 100 2.4 100 2.2 100 2.2 100 1.9 *See Table 3-1 for more detailed description 3-12 D CJ Data used for Pre-Developement hydrology Data used for Post-Developement hydrology 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 e NRCS Elements Count Elements %IMPER. A B Undisturbed NaturaJ Terrain (Natural) Permanent Open Space o• 020 0.25 Low Density Residential (LOR) Residential, 1.0 DU/A or less JO 027 0.32 Low Density Residential (LOR) Residential, 2.0 DUI A or less 20 0.34 0.38 Low Density Residential (LOR) 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 (MOR) Residential, 7.3 DU/A or less 40 0.48 0.51 Medium Density RcsidentiaJ (MDR) Residential, 10.9 DU/A or Jess 45 0.52 0.54 Medium Density Residential (MDR) Residential, 14.5 DU/A or less 50 0.55 0,58 High Density Residential (HOR) Residential, 24.0 DUiA or less 65 0.66 0.67 High Density Residential (HOR) Residential. 43.0 DU/ A or less 80 0,76 0,77 Commcrcial/lndustrial (N. Com) Neighborhood Commercial 80 0.76 0.77 C'ommercial/IndustriaJ (G. Com) General C.ommercial 85 0.80 0,80 Commercial/Industrial (O.P. Com) Otlice Professional/Commercial 90 0,83 0,84 Commercial/Industrial (Limited I.) Limited Industrial 90 0,83 0,84 Commcrcialilndustrial General T. General Industrial 95 0.87 0.87 3 6 of26 C D 0.30 0.35 0.36 0.41 0.42 0.46 OAS 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 inay he used for direct calculation of the runoff coefficient as described in Section 3. J .2 (representing the pervious runoff coefficient, Cp, for the soil type), or for areas that will remain undisturbed in perpetuity. Justification mu»t 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 l I l I l I I I I I I I I I I I I I I I I I l I l I I I l I l I I I I I I I ~ Hydrologic Soil Group-San Diego County Area, California MAP LEGEND MAP INFORMATION Area of Interest (AOI) D Area of Interest (AOI) Soils Soll Rating Polygons lli:} A e] ND ti:)B (E! BID {!:I C e!J CID ~ D D Not rated or not available Soil Rating Lines ,_.,,,;. A .,,,..., AID .,...,,,. B ,..,,. BID ~ C """"' CID ,,,.; D ---··-Not rated or not available Soll Rating Points El A ■ AID • 8 ■ BID Natural Resources Conservation Service El c II CID • 0 C Not rated or not available Water Features Streams and Canals Transportation H-4 Rails _,,, Interstate Highways """'11' US Routes ,,_.~ Major Roads i"tJ# 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 Soll map units are labeled (as space allows) for map scales 1 :50,000 or larger. Date(s) aerial images were photographed: Jan 23, 2020--Feb 13,2020 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. 11/16/2020 Page 2 of4 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 11/16/2020 Page4 of4