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Home My WebLink AboutAMEND 2019-0002; CARLSBAD CORPORATE PLAZA PARKING STRUCTURE; PRELIMINARY HYDROLOGY STUDY; 2019-08-12Prepared By: PRELIMINARY HYDROLOGY STUDY Carlsbad Corporate Plaza Parking Structure 6183 & 6185 Paseo Del Norte Carlsbad, CA 92011 APN: 211-040-18 AUG 2 !! 2019 CITY OF CARLSBAD P!....P.NNING DIVIS!ON RCE 68075 EXP: 06-30-21 Pasco Laret Suiter & Associates, Inc. 535 N. Highway 101, Suite A Solana Beach, CA 92075 PASCO LARET SUITER & ASSOCIATES CIVIL ENGINEERING + LAND PLANNING + LAND SURVEYING Prepared for: NextMed Ill, LLC 6125 Paseo Del Norte, Suite 210 Carlsbad, CA 92011 760-494-9216 Date: August 12, 2019 ----TABLE OF CONTENTS --1.0 INTRODUCTION ....................................................................................................................... 1 -1.1 Project Description ......................................................................................................................... 1 -1.2 Existing Conditions ......................................................................................................................... 1 -1.3 Proposed Conditions ...................................................................................................................... 2 -2.0 METHODOLOGY ....................................................................................................................... 2 --2.1 Rational Method ............................................................................................................................. 3 -2.2 County of San Diego Criteria .......................................................................................................... 3 -3.0 HYDROLOGIC ANALVSIS ............................................................................................................ S -3.1 Existing Condition ........................................................................................................................... 6 -3.2 Proposed Condition ........................................................................................................................ 7 -4.0 CONCLUSION ........................................................................................................................... 8 --APPENDICES -Appendix A: Hydrology Support Material -Appendix B: Existing and Proposed Hydrology Maps ----------------- 1.0 INTRODUCTION This Hydrology Study for the proposed NextMed Parking Garage project has been prepared to analyze the hydrologic characteristics of the proposed project site. This report presents both the methodology and the calculations used for determining the storm water runoff from the project site in the post- developed (proposed) conditions produced by the 100-year, 6-hour storm event. 1.1 Project Description The NextMed Parking Garage project site is approximately 3.8 acres and is located at 6183 & 6185 Paseo Del Norte in the City of Carlsbad. The site currently consists of buildings and a surface parking lot with landscaped islands and an access drive located along the northeastern boundary of the site . The proposed project is a Priority Development Project (PDP) and is subject to storm water pollutant control and hydromodification management (HMP) flow control requirements as set forth by the California Regional Water Quality Control Board San Diego Region municipal storm water permit (Order No. R9-2013-0001, referred to as MS4 Permit) and the City of Carlsbad BMP Design Manual. For detailed pollutant control and HMP calculations refer to the report titled "Priority Development Project Storm Water Quality Management Plan for NextMed Parking Garage" dated March 2019, prepared by Pasco Laret Suiter & Associates. CITY or OCE:41"S10( :,5 VICHTYMAP N.T.S. 1.2 Existing Conditions In the existing condition storm water runoff from the site flows overland across the parking lot in a northwesterly direction and is collected in the existing storm drain located along the northern boundary 1 -------------------------------------- of the parking lot. Runoff from the existing buildings is conveyed in roof drains to the same existing storm drain line. All flow continues northeasterly in the storm drain where it discharges onsite to a grouted rip rap pad, then re-enters a short segment of storm drain prior to discharging via curb outlet to Paseo Del Norte. Runoff flows northerly as gutter flow along Paseo Del Norte, enters a curb inlet and discharges to Encinas Creek which flows westerly and discharges to the Pacific Ocean. No offsite runoff is conveyed through the site. Per the United States Department of Agriculture Web Soil Survey, the project site is underlain with Hydrologic Soil Group D. Refer to Appendix A for soil information. 1.3 Proposed Conditions The project proposes to construct a new parking structure within the existing parking lot. The parking lot will remain at the same grade with some of the parking area being reconfigured including demolition of existing islands and asphalt for parking structure footings and the addition of HMP biofiltration basins for stormwater treatment and flow control. Utility re-routing will be required for the public waterline and private storm drain onsite due to the impact of the proposed parking structure footprint. A new easement dedication will be required for the waterline re-routing coupled with vacation of the abandoned portion of the waterline easement. Storm water runoff from the proposed parking garage and portions of the surrounding surface parking lot including disturbed areas will be collected and conveyed to the HMP biofiltration basins. Flow from the basins will discharge to the onsite existing storm drain and continue as it does in the existing condition. Areas that can not be collected and conveyed to the biofiltration basins will be routed to pervious areas with amended soils for impervious area dispersion. These amended soil areas will be depressed planters within parking islands. Overflow from the amended soils areas will be collected in area drains and routed to the storm drain system. Using the Rational Method Procedure outlined in the San Diego County Hydrology Manual dated June 2003 (SDCHM), the 100-year, 6-hour storm event peak flow rate was calculated for the project site in the existing and proposed condition. In the proposed condition, the drainage pattern mimics the existing condition and the amount of proposed impervious area is slightly less than the existing, therefore the hydrologic results are the same. The table below summarizes the hydrologic analysis. Summary of 100-yr Storm Event Hydrologic Analyses Condition Impervious Area (sf) Pervious Area (sf) QlOO (cfs) Existing 49,566 4,228 7.0 Proposed 47,478 6,316 6.8 Refer to the hydrologic calculations included in Section 3.1 and 3.2. 2.0 METHODOLOGY Pursuant to the San Diego County Hydrology Manual dated June 2003 (SDCHM), the Rational Method is recommended for analyzing the runoff response from drainage areas up to approximately 1 square mile 2 -------------------------------------- in size. The proposed project and associated watershed basins are less than 1 square mile, therefore the Rational Method was used to analyze the project's hydrologic characteristics in the existing and proposed conditions. 2.1 Rational Method The Rational Method (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: 1 = 7 .44 x P6 x o-0-645 I = Intensity (in/hr) PG = 6-hour precipitation (in) D = duration (min -use Tc) Using the Time of Concentration (Tc) 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. 2.2 County of San Diego Criteria The County of San Diego has developed 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). The 100-year 6-hour storm event rainfall isopluvial map is included in Appendix A. 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 SDCHM 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 3 -------------------------------------- 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 SDCHM: 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 project area based on the percentage of impervious area and the percentage of pervious or landscape area. Refer to Section 3.0 for the composite runoff coefficient calculations. 4 ----3.0 HYDRO LOG IC ANALYSIS --The table below summarizes the hydrologic calculations provided in Sections 3.1 and 3.2. -Summary of 100-yr Storm Event Hydrologic Analyses -Condition Impervious Area (sf) Pervious Area (sf) QlOO (cfs) -Existing 49,566 4,228 7.0 -Proposed 47,478 6,316 6.8 ---------------------------5 - 3.1 Existing Condition 3048 NextMed Parking Garage Existing Condition Q =CIA 1 = 7 .44 x PG x o-0·645 P6 (in) 2.5 Area (sq ft) Impervious 49566 Pervious 4228 Total (sf) 53794 Total (ac) 1.23 Tc= (assumed) 5.0 I (in/hr) = 6.6 Q (cfs) = 7.0 C value 0.9 0.35 6 Axe value 44609.4 1479.8 46089.2 Weighted C 0.86 I 3.2 Proposed Condition 3048 NextMed Parking Garage Proposed Condition Q=CIA 1 = 1 .44 x P6 x o·0·645 P6 (in) 2.5 Area (sq ft) Impervious 47478 Pervious 6316 Total (sf) 53794 Total (ac) 1.23 Tc = (assumed) 5.0 I (in/hr)= 6.6 Q (cfs) = 6.8 C value 0.9 0.35 7 A xC value 42730.2 2210.6 44940.8 Weighted C 0.84 I -------------------------------------- 4.0 CONCLUSION Based upon the analyses included in this report, the proposed condition 100-year peak discharge is less than the existing, therefore the project will not impact the existing storm drain system. The proposed storm water HMP biofiltration basins meet the requirements of the MS4 Permit for both pollutant control and hydromodification management resulting in no negative downstream impacts. 8 ------APPENDIX A -------------------------------- I I 33°30'- 33•15•- Orange County I p Lo 32°45' ---- 32°30' ------- b I") 0 ,..._ ..... b (") t--..... .,.... -----------+ io .,.... 0 ,..._ .,.... .,.... io .,.... 0 ,.._ .,.... .,.... b 0 a ,.._ .... .... ~ ,..._ .,.... .,.... io v 0 <D .,.... .,.... Riverside County , ,• , . . . . .. .... .. .......... ,_ .... . ~ . . ~ 0 <D .,.... .,.... . ·· ... ' .. .... M . e X . I C b I") 0 <D ..... .,.... b <") 0 <D .... .... . . .... -........... .. . . ' ,.• ........ .. . . io .,.... 0 <D .,.... .,.... . . . . . ' 33°30' ... ,_,. . .. . . , ........ "· ·····•':-,.--..... .:.1 ~ -33°15' \'2.5· .... •· ····· ........... . io .,.... ~ .,.... .,.... . . ' ·•, 1 3°00' 3 ""O ("[) :::!. 0) 32°45' 32°30' County of San Diego Hydrology Manual Rainfall lsopluvials 100 Year Rainfall Event-6 Hours lsopluvial (inches) DPW ~GIS ~o/A;l;ic\lb1<, {1c•t111,•,:ir,f.11:.l""-'"111t.v1S'XM(;.11,. s:fGIS We Hav~ San D iego CMtrcd! THIS MAP IS PROVIDED WllHOlJT WARIW/TY OF N<V l<INO, EITHER EXPRESS OR IMPI.IEO, INQ.UOINO, BUT NOTLIMITEO TO, lliE IMPLIED WARRA"11ES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ~t SonGIS ,.,_ Rlghla -- Thll pn,dum rney "°""'In inf""""tlon from tt>o SANOAG Regional lnlonnatlon S)ltom......, caMOt bo-wlthout lhO wrltlen -ol SANDAO The prodlct may <Onlaln lnforrnalkwt ~ hal been reproduood .ttti ~gntntedby'lhotnMBrolhO .. M'l)I 3 Miles 33" 70"N 33" 654"N I I i I B j I ~ j !<l j ;:: "' !'l ~ 47Cll8J 47!XBJ ;:: "' !'l ~ N A Hydrologic Soil Group-San Diego County Area, California 470<0) 47043) 47044) 470400 410400 47tHll = 470<0) = 47044) 4708) 410400 = 47ai3J Map Scale: 1:1,100 if printed on A landscape (11" X 8.5") sheet ----=====-------======:::,Meters 0 IB ~ 00 ffi ----====--------=======Feet 0 ~ ~ ~ ~ Map 17()jedion: Web Mercator Comer CDOlt1inates: WGS84 Edge tics: UTM Zroe 11N WGS84 USDA Natural Resources ~iiiF Conservation Service Web Soil Survey National Cooperative Soil Survey 47C6«) 47!ml 470500 471'.BJJ ;:: f:l !'l ~ ;:: f:l !'l ~ I I i I I fil j ~ j !<l j 1/24/2019 Page 1 of 4 33" 7 0"N 33" 654"N Hydrologic Soil Group-San Diego County Area, California MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Rating Polygons D A D N D D B D BID D C D CID D D D Not rated or not available Soil Rating Lines .-.,,, A .-.,,, ND .-.,,, B .-.,,, BID C .-.,,, CID .-.,,, D . ,, Not rated or not available Soil Rating Points ■ A ■ ND ■ B ■ B/D USDA Natural Resources -1ifi Conservation Service C] • C CID ■ D □ Not rated or not available Water Features Streams and Canals Transportation ++t 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 13, Sep 12, 2018 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. 1/24/2019 Page 2 of 4 -------------------------------------- Hydrologic Soil Group-San Diego County Area, California Hydrologic Soil Group Map unit symbol Map unit name Rating Acres inAOI Percent of AOI CfB CfC Chesterton fine sandy D loam, 2 to 5 percent slopes Chesterton fine sandy D loam, 5 to 9 percent slopes 2.6 0.5 Totals for Area of Interest 3.1 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, 8, C, and D) and three dual classes (ND, 8/0, and CID). 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 (ND, 8/0, or C/0), 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. USDA Natural Resources ~ Conservation Service Web Soil Survey National Cooperative Soil Survey 84.0% 16.0% 100.0% 1/24/2019 Page 3 of 4 -... ----APPENDIX B ---------.. ---------------------- PLAN VIEW SCALE: 1• = JO' .30' 15' 0 .30' 60' l~~!!!!!!!!!!!!i!!!!!!!!!!!!l _ ___,I SCALE: 1" = .30' Know what's below. Call before you dig. LEGEND EXISTING STORM DRAIN EXISTING HARDSCAPE/ IMPERVIOUS AREA EXISTING LANDSCAPING/ PERVIOUS AREA DAAtlAGE MANAGEMENT AREA (DMA} BOUNDARY OVERLAND R.OW DIFECllON AAROW STORM DRAtl R.OW DIRECTION AAROW SOIL INFORMA TJON SOIL= TYPED SITE INFORMATION: TOTAL IMPERVIOUS AREA= TOTAL PERVIOU8 AREA: TOTAL AREA= - 49,5668F 4,2288F 53,794SF PASCO LARET SUITER & ASSOCIATES CIVIL ENGINEERING+ LAND PLANNING+ LAND SURVEYING 535 North Highway 101, Suite A Solana Beach, CA 92075 ph 858.259.8212 I fx 858 .259.4812 I plsaengineering.com ==~D::=::=-=== f7/I /Ifill ff/21 ~d@j NEXTMED PARKING STRUCTURE EXISTING HYDROLOGY EXHIBIT CARLSBAD, CALIFORNIA PROJECT NUMBER: 3048 SCALE: 1" = 30' DATE: JUNE 2019 SHEET 1 OF 1 I ' \ D EXISTING BUILDING B c 1 I~ UL4-- \ I I I~ ,----4-~ J '"' /i '-----·-"·.o..·· -~~-~// 30' 15' 0 30' 60' liiiiiij~!!!!!!!!!!!!!!!!!!!l!!!!!!!!!!l _ _____,I SCALE: 1'' = 30' A I I LJ PLAN VIEW SCALE: 1" = 30' \ I I 1 1 ,--J I I ~ I L---------'I I L--_, I ,/ J ~ c:..:.::..::i__ \ = I I __J ~ L Know what's below. Call before you dig. LEGEND PROPOSED STORM DRAIN PROPOSED HAADSCAPE/ IMPERVIOUS AREA PROPOSED L.AfOSCAPINO/ PERVIOUS AFEA DRAINAGE MANAGEMENT AREA (OMA) B01..N)ARY SUB BASIN BOUNDARY TREATMENT CONTROL BMP NU~ OVERLAND R.OW DIRECTION ARROW STORM DRAIN R.OW DIRECTION ARROW TREATMENT CONTROL BMPS 8IORL TRA TlON WITH IMPERMEABLE LINER SOIL INFORMATION SOIL: TYPE D SITE INFORMATION: TOTAL IMPERVIOUS AREA: TOTAL PERVIOUS AREA: TOTAL AREA: 47,478 SF 6,316SF 53,794 SF TOTAL IMPERVIOUS AREA 1A: 21,825 SF TOTAL PERVIOUS AREA 1A: 3,909 SF TOTAL AREA 1A: 25,734 SF TOTAL IMPERVIOUS AFE 2A: TOTAL PERVIOUS AREA 2A: TOTAL AREA 2A: 25,653SF 2,407SF 28,060SF == ~ Cl=:===--~ fl I IIT1//~ /1/J ~~Afa 0 .... ¢:::::i PASCO LARET SUITER & ASSOCIATES NEXTMED PARKING STRUCTURE PROPOSED HYDROLOGY EXHIBIT CIVIL ENG INEER ING+ LANO PLANNING + LAND SURVEYING 535 North Highway 101 , Suite A Solana Beach, CA 92075 ph 858.259.8212 I fx 858.259.4812 I plsaengineering.com CARLSBAD, CALIFORNIA PROJECT NUMBER: 3048 SCALE: 1" = 30' DATE: JUNE 2019 SHEET 1 OF 1