9530 HOLLY DR 2025-08-05 - Laserfiche WebLink

8. RUN-OFF TREATMENT (MR #6) Runoff treatment for the private road surfaces will take place in bioretention cell #1, as the 2019 DOE SWMMVWV states that 100 percent infiltration within a bioretention cell meets the runoff treatment requirement. For the remainder of the PGHS to be infiltrated via permeable pavement, the underlying 6 inches of soils will be amended if necessary to meet the DOE standard for soil treatment. Therefore, this project will meet the runoff treatment requirement through the utilization of permeable pavement and bioretention. 9. FLOW CONTROL (MR #7) The site design for the improvements is for approximately 29,679 sf of new and replaced hard surfaces, including a new private road, driveways and roofs. The existing and proposed conditions were modeled and compared using VWVHM12 software. Per the Washington State Department of Ecology, the 100-year flow frequency may be analyzed using the 1-hour time step or 15-minute time step. The 15-minute time step was used in this analysis. Therefore, the difference in the 100-year flow frequency from the threshold discharge area must not exceed 0.15 cfs. Due to the LID measures being proposed, the total effective impervious area is below the threshold for MR 7. The following summarizes the areas input into the VWMAH software for the drainage analysis of the predeveloped and developed on -site basins for the project. As previously mentioned, the site is currently developed and consists of an existing house, driveway, unmaintained lawn areas and trees. Existing Site Predeveloped Area: 1.23 ac (disturbed area only) Hydrologic Soils Group C (Till) Impervious Area: 0.09 ac Pervious Area: 1.14 ac (Forest; Type C Soils, Flat Slope) Existing Condition Event Output: WWHM12 Peak Flow cfs Area ac 2- r 0.051 1.23 10- r 0.086 1.23 50- r 0.12 1.23 1 00-r 0.14 1.23 Perez SP March 2022 Revised: June%4ugust2022 Page13