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Home My WebLink AboutARCH-0188-2023 20230420 (Laurel Garden) - Preliminary Hydrology Report Laurel Creek Garden Apartments 1150 Laurel Lane San Luis Obispo, CA 93401 AV Job Number 181058 March 29, 2023 Preliminary Hydrology Report and Post Construction Stormwater Control Plan Prepared for: Patrick Smith 1150 Laurel Creek, LLC San Luis Obispo, CA 93401 Prepared By: Ashley & Vance Engineering, Inc. 1229 Carmel San Luis Obispo, CA 93401 805.545.0010 Laurel Creek Garden Apartments Stormwater Control Plan-181058 Page | 1 March 29, 2023 Attn: Patrick Smith 1150 Laurel Creek San Luis Obispo, CA 93401 Subject: Laurel Creek Garden Apartments Re: Preliminary Stormwater Control Plan-Performance Requirements 1-4 Patrick: Please find enclosed the Stormwater Control Plan for the above-referenced project addressing performance requirements 1-4. Calculations for the volume of water required for treatment and infiltration were prepared using the Post-Construction Stormwater Management Requirements for Development Projects in the Central Coast Region. As detailed in the project conclusion, this project is well suited to meet the City of San Luis Obispo and the Water Board’s goals for stormwater mitigation. Please contact me for any clarifications or supporting information you need concerning this plan. Regards, Juan Alvarez Kathleen Allwine, PE Principal Engineer Laurel Creek Garden Apartments Stormwater Control Plan-181058 Page | 2 Contents Introduction and Background ....................................................................................................................... 3 Site Location Description .............................................................................................................................. 3 Vicinity Map .............................................................................................................................................. 3 Existing Conditions .................................................................................................................................... 4 Proposed Project ....................................................................................................................................... 4 Water Quality Analysis .................................................................................................................................. 4 Performance Requirement 1-Site Design and Runoff Reduction ............................................................. 4 Performance requirement 2-Water Quality Treatment ........................................................................... 4 Performance requirement 3-Runoff Retention ........................................................................................ 4 Performance requirement 4-Peak Management ..................................................................................... 5 Conclusion ..................................................................................................................................................... 5 Drainage Exhibits .......................................................................................................................................... 6 Laurel Creek Garden Apartments Stormwater Control Plan-181058 Page | 3 Introduction and Background This report intends to address the stormwater requirements set forth by the Regional Water Quality Control Board’s Post-Construction Stormwater Management Requirements for Development Projects in the Central Coast Region and meet the City of San Luis Obispo’s requirements outlined in the Drainage Design Manual. Both these goals will be accomplished by incorporating pervious surfaces and providing retention of stormwater via underground storm chambers. The pervious surfaces will comprise of a combination of landscaping and pervious pavers. The combination of the gravel beneath the pavers and the chambers will be sized to contain the 95th events. This project will result in the creation and replacement of over 22,000 square feet of impervious surface area and as a result, is required to meet Performance Requirements 1 through 4. The onsite drainage patterns will shift slightly as a result of this project; however, this report will show that the overall effect to the downstream improvements will be negligible. Site Location Description This site is located at the corner of Orcutt Road and Laurel Lane. The railroad runs parallel to the property along the west side of the property with an existing pedestrian path in the middle. Sydney Creek runs along the northern side of the project site. Vicinity Map Laurel Creek Garden Apartments Stormwater Control Plan-181058 Page | 4 Existing Conditions This Project is located at 1150 Laurel Lane. The existing site consists of a warehouse with parking lots and loading docks around the main warehouse building. The northmost parking lot and driveway slope and drain towards the creek. The main driveway, easterly parking lot and southernmost parking lot drain to an existing storm drain located in the westerly boundary neighboring the railroad. The site does not receive run-on from adjacent lots and does not contribute run-on to adjacent lots. Proposed Project This project will include the construction of four (4) new residential structures and two (2) new mixed- use buildings. Drainage from most of the site will drain to the underground chamber storage system and incorporate pervious pavers in the parking lots. Water Quality Analysis This project creates and replaces over 22,000 square feet of impervious area and thus is required to meet Performance requirements 1 through 4. Performance Requirement 1-Site Design and Runoff Reduction This site minimizes the amount of pervious surface to the maximum extent practicable. Runoff from hardscape and roof drains will outlet to landscaped and pervious paver areas prior to leaving the site. Performance requirement 2-Water Quality Treatment This site is required to treat the 85th percentile storm for the onsite areas. The runoff will be treated via direct infiltration provided by the gravel section located under the pervious pavers or in the underground chambers located around the site. Performance requirement 3-Runoff Retention This project is required to infiltrate the 95th percentile runoff. The onsite ADS Stormtech MC-4500 chambers will retain and infiltrate the 95th percentile storm onsite. The outlet for the chambers will be sized and set to match the pre-improvement flow rates. HydroCAD modeling software was used to calculate the pre- and post-construction runoff from the site and the flow rates for the 2 through 100- year storm events. See the table below. Laurel Creek Garden Apartments Stormwater Control Plan-181058 Page | 5 Performance requirement 4-Peak Management This project will result in an increase in impervious area and a result will increase the flow rates leaving the site for all design storms. The combination use of the pavers and chambers will provide Draining to creek Rainfall Event (year) 2 5 10 25 50 100 Pre Project run off (cfs) 1.77 2.17 3.48 4.52 5.31 6.09 Post Project run off (cfs) 0.19 0.33 1.91 3.54 4.44 5.72 Draining to ECSLO outlet Rainfall Event (year) 2 5 10 25 50 100 Pre Project run off (cfs) 8.46 10.27 16.20 20.85 24.39 27.91 Post Project run off (cfs) 0.88 1.54 9.23 17.48 21.77 24.83 Conclusion This project retains and allows stormwater to infiltrate for the volume of water required by the Regional Water Quality Control Board. Post construction stormwater runoff is reduced for all design storms. All four performance requirements will be met. Final drainage design should not be limited to the drainage facilities proposed in this Preliminary Hydrology Report if equivalent alternatives are provided that meet the criteria of the DDM and the Central Coast RWQCB Post Construction Stormwater Management requirements. Laurel Creek Garden Apartments Stormwater Control Plan-181058 Page | 6 Drainage Exhibits XXXXXXX X X XX X X X 8'' W8'' WXXX8'' W 8'' W 8'' W 8'' W12'' W 6''SS-VCP6''SS-VCPWSTORM DRA IN IN LET : 2 4 6 . 6 5 INV ER T : 2 4 0 . 2 5 DROP INLET : 2 4 5 . 3 6 INVER T : 2 4 2 .91 CO W E250250250260260250250240250 W 260252253 254 255 256 257 258259261 262263 264 4. 7 5 ' 0.6% G GE250242243244245246247248249251252250242242243244245246247248249251252250247248249251252241242243244245246247248249E240240234234235235236236237237237237238238239239240237238239241249248W Ashley&VanceENGINEERING, INC.1413 Monterey StreetSan Luis Obispo, CA 93401 (805) 545-0010 (323) 744-0010www.ashleyvance.comC I V I L S T R U C T U R A LDRAINAGE EXHIBITN18105803/29/20231150 LAUREL LANESAN LUIS OBISPO, CA 93401PRE-CONSTRUCTIONS:\All Jobs\2018 All Jobs\181058 - 1160 Laurel Creek - Braff\02_Working Drawings\Future Buildout\02_Working Drawings\03_EXHIBITS\HYDROLOGY SHEETS-FUTURE.dwg, PRE, Mar 29, 2023 2:17pm, Juan 0100100200HORIZONTAL SCALE: 1" = 100' LEGENDDRAINING TO CREEKPERVIOUS AREA53436 SFIMPERVIOUS AREA23332 SFDRAINING TO STORMDRAINOUTLETPERVIOUS AREA216131 SFIMPERVIOUS AREA129001 SFTOTAL AREA421900 SF(E) HEAD WALL(E) 48" CULVERT(E) STORMDRAIN OUTLET FOR(E) WAREHOUSE ROOF DRAINSAND PARKING LOT DRAINAGE(E) 30" WIDECONCRETE SWALE(E) CATCH BASIN(E) STORMDRAIN COLLECTSROOF DRAINS AND CATCH BASIN(E) STORMDRAIN SDMH(E) WALL PREVENTSSTORMWATER RUN-ONDRAINS TO "ESCORPDRAINAGE CHANNEL"DRAINS TO CREEK(E) WAREHOUSEBUILDING TO REMAIN(E) BIO TREATMENT AREA(E) BIO TREATMENT AREA(E) PERVIOUS PAVERS(E) PERVIOUS PAVERS(E) PERVIOUS PAVERS(E) BIO TREATMENT AREA(E) PERVIOUS PAVERS(E) 4' OPENING IN ASPHALT CURBFOR STORMWATER CONVEYANCE,OUTLETS TO CREEK BANK XXXXXXX X X XX X X X 8'' W8'' WXXX8'' W 8'' W 8'' W 8'' W12'' W 6''SS-VCP6''SS-VCPWSTORM DRA IN INLET : 2 4 6 . 6 5 INV ER T : 2 4 0 . 2 5 DROP IN LET : 245 .36 INVER T : 2 4 2 .91 CO W E250250250260260250250240250 W 260252253 254 255 256 257 258259261 262263 264 4. 7 5 ' 0.6%E250242243244245246247248249251252250242242243244245246247248249251252250 247248249251252 241242243244245246247248249E240240234234235235236236237237237237238238239239240237238239241 249248WDDDDSDSDSD SDSDSDSDSDSDSDSD DD D SD SDDBUILDING 4TYPE B2BUILDING 3TYPE B2BUILDING 1TYPE B1POOL&SPAPROPOSEDBUILDINGTYPE CPROPOSEDBUILDINGTYPE DBUILDING 2TYPE A(P)PARKINGAshley&VanceENGINEERING, INC.1413 Monterey StreetSan Luis Obispo, CA 93401 (805) 545-0010 (323) 744-0010www.ashleyvance.comC I V I L S T R U C T U R A LDRAINAGE EXHIBITN18105803/29/20231150 LAUREL LANESAN LUIS OBISPO, CA 93401POST-CONSTRUCTIONS:\All Jobs\2018 All Jobs\181058 - 1160 Laurel Creek - Braff\02_Working Drawings\Future Buildout\02_Working Drawings\03_EXHIBITS\HYDROLOGY SHEETS-FUTURE.dwg, POST, Mar 29, 2023 2:21pm, Juan 0100100200HORIZONTAL SCALE: 1" = 100' (E) WAREHOUSEBUILDING TO REMAIN(P) STORMTECH CHAMBERSOVERFLOW TO LAUREL CREEK(P) STORMTECH CHAMBERSOVERFLOW TO "ESCORPDRAINAGE CHANNEL"LEGENDDRAINING TO CREEK(P) PERVIOUS AREA49508 SF(P) IMPERVIOUS AREA59648 SFDRAINING TO STORMDRAIN OUTLET(P) PERVIOUS AREA75178 SF(P) IMPERVIOUS AREA237565 SFTOTAL AREA421900 SF(P) STORMTECH CHAMBERSOVERFLOW TO "ESCORPDRAINAGE CHANNEL"(P) STORMTECH CHAMBERSOVERFLOW TO "ESCORPDRAINAGE CHANNEL"(P) PERVIOUS PAVERSOVERFLOW TO LAUREL CREEK(E) STORMDRAIN OUTLETESCORP DRAINAGE CHANNEL(E) CATCH BASIN(E) BIO TREATMENT AREA(E) BIO TREATMENT AREA(E) PERVIOUS PAVERS(E) PERVIOUS PAVERS(E) HEAD WALL(E) 48" CULVERTDRAINS TO "ESCORPDRAINAGE CHANNEL"(E) PERVIOUS PAVERS(P) STORMTECH CHAMBERSOVERFLOW TO "ESCORPDRAINAGE CHANNEL"(P) PERVIOUS PAVERSOVERFLOW TO LAUREL CREEK(E) BIO TREATMENT AREA(E) PERVIOUS PAVERS