Loading...
HomeMy WebLinkAboutReviewed RS_Southport Office Roundabout Drainage Memo_180403_v1.pdfApril 3, 2018 City of Renton-Utility Systems Division 1055 South Grady Way Renton, WA 98057 RE Southport Office Permit Revision to Stormwater Plan Dear Ian Fitz-James: I am writing to inform you of the changes to the stormwater plan resulting from the permit revision to the Southport Office project to add a small roundabout to the onsite private roadway Southport Bristol Avenue. The permit revision proposes to add a roundabout to decrease traffic queuing concerns related to hotel and office events that occur during daily commuting times. The proposed revision increases the overall impervious area of the project by 317 square feet (see figures attached that compare proposed revision conditions to permitted conditions). The increase in impervious area will have a negligible effect on the functionality of the stormwater system. The water quality vault has volume sufficient to treat stormwater runoff from 4.33 acres of impervious surface. The project proposes to route only 3.51 acres of site area to the water quality vault. The project is exempt from flow control requirements through the direct discharge to Lake Washington so the effects on flow control is not a concern. The water quality system for the area being revised consists of a water quality vault that treats the runoff from road, parking, sidewalks, and landscape areas. The vault is designed under the assumption of 100% impervious area for the area it treats. The overall area routed to the water quality vault is not changing under this revision, so the small increase in percentage of impervious area does not affect the design of the water quality system. The water quality plan, water quality areas, and supporting calculations from the TIR are attached for your review. The complete TIR has been submitted for your review as well. I hope you will consider the small magnitude of this design revision and its negligible impact on the stormwater design and permit the revision without further analysis and review. Sincerely, COUGHLIN PORTER LUNDEEN, INC. Bart Balko, PE Project Manager TRUE NORTH 8-1-1 re you -424-5555 GROUND CE (USA) BRISTOL PROPERTY PUGET SOUND ENERGY PROPERTY SHEET x03LEGEND Description Sidewalk Impervious Area Roadway Impervious Area Pervious Area Extents of ASI 023 Revisions Quantity 4,005 SF 22,005 SF 2,744 SF 28,754 SF PERMITTED PROPOSED CONDITIONS TRUE NORTH 8-1-1 re you -424-5555 GROUND CE (USA) BRISTOL PROPERTY PUGET SOUND ENERGY PROPERTY SHEET x03ROUNDABOUT REVISION PROPOSED CONDITIONS LEGEND Description Sidewalk Impervious Area Roadway Impervious Area Pervious Area Extents of ASI 023 Revisions Quantity 3,398 SF 22,929 SF 2,427 SF 28,754 SF ROUNDABOUT ADDITION (TOTAL INCREASE OF 317 SF OF IMPERVIOUS AREA) Scale: 1"=100' FIGURE 5A PROPOSED WATER QUALITY LEGEND Description Area Routed to Ex Wetvault Area Routed to Ex Filterra Quantity 3.51 Acres 2.55 Acres SHEET x01BOEING PROPERTY BRISTOL PROPERTY BRISTOL PROPERTY PUGET SOUND ENERGY PROPERTYSHEET x02THE HOTEL AT SOUTHPORT SHEET x03SHEET x04EXISTING WATER QUALITY VAULT TO TREAT 3.51 ACRES EXISTING STORMFILTER TO TREAT 2.55 ACRES 2001 SOUTHPORT MASTER PLAN2001 MASTER PLAN WATER QUALITY EXISTING WETVAULT CALCULATIONSFROM 2001 SOUTHPORT MASTER PLAN EXISTING WETVAULT CALCULATIONS FROM 2001 SOUTHPORT MASTER PLAN Southport Wetvault Sizing Calculations Following the procedure outlined in 6.4.1.1 of the King County Stormwater Design Manual: Step 1: Identify required wetpool volume factor 𝑒=3 (𝑟𝑖𝑛𝑎𝑒 𝑎𝑎𝑟𝑖𝑎) Step 2: Determine rainfall (R) for the mean annual storm 𝑅𝑟𝑖𝑛𝑒 𝐹𝑖𝑒𝑢𝑟𝑒 6.4.1.𝐴, 𝑅=0.47"= 0.039′ Step 3: Calculate runoff from the mean annual storm 𝑅𝑜=𝑅(0.9𝐴𝑖+0.25𝐴𝑡𝑑+0.10𝐴𝑡𝑑+0.01𝐴𝑜) 𝐴𝑖=190,000 𝑅𝐹 𝐴𝑡𝑑=0.00 𝑅𝐹 𝐴𝑡𝑑=0.00 𝑅𝐹 𝐴𝑜=0.00 𝑅𝐹 𝑅𝑜=( 0.039 𝐹𝑅)(0.9)(190,000 𝑅𝐹) 𝑅𝑜=6,669 𝐶𝐹 Step 4: Calculate required wetpool volume 𝑅𝑏=𝑒𝑅𝑜 𝑅𝑏=(3)(6,669 𝐶𝐹) 𝑅𝑏=20,007 𝐶𝐹 With inside dimensions of 178.5’ x 25’, and a live storage depth of 4.8’, the provided volume of the existing vault is approximately 21,420 CF, so, 21,420 𝐶𝐹>20,007 𝐶𝐹 𝑅𝑜𝑜𝑜𝑡𝑖𝑑𝑑𝑑>𝑅𝑜𝑑𝑜𝑡𝑖𝑜𝑑𝑑 Therefore, the existing vault is adequate for reuse as a wetvault to provide basic water quality treatment. Additionally, the existing wetvault must meet the requirements of City of Renton Amendment to KCSWDM Section 6.4.2. PROJECT Southport Hotel DESIGNED BY BGG DATE 04/01/2015 PROJECT NUMBER C140507-01 CLIENT MG2 CHECKED BY BSB SHEET OF Wetvault sized for 4.33 Acres UPDATED USING EXISTING VAULT SIZE FOR THE 2015 STORMWATER PLAN