1029 52ND ST SW 2022-10-31 - Laserfiche WebLink

6. PRESERVATION OF NATURAL DRAINAGE SYSTEM (MR #4) All of the runoff from the existing site is tributary to the existing storm conveyance system within the right-of-way of Seahurst Ave or 51st PI SW and that will remain the case after this project is completed. 7. ON-SITE STORMWATER MANAGEMENT (MR#5) As previously mentioned the site is currently developed and consists of two existing houses, a garage and parking/driveways. The soils logs provided by the geotechnical engineer found the on-site soils to be silty sand with gravel over glacial till. To address the requirements for mitigation of the impervious surfaces, the applicability and feasibility of infiltration was considered first. The project Geotech performed multiple soil logs and two PIT tests for this site. They have recommended that a site infiltration rate of 0.8"/hr be used for the existing soils above the glacial till. The 2014 DOE manual requires that projects that trigger MR 1-9 show compliance with the.Low Impact Development.Performance Standard and BMP T5.13 or List#2. As such List #2 was used to determine the feasibility for stormwater management BMPs for all impervious surfaces Since infiltration trenches and permeable pavement will be used to fully infiltrate the runoff from the new hard surfaces for this project the only effective impervious surfaces proposed as part of this project is the new sidewalk. Due to the LID measures being proposed, the total effective impervious area is below the threshold for MR 6 and MR 7. Since the new sidewalk is the only new effective impervious surfaces for the project, which totals 800 sf in Basin 'A' and 2,100 sf in Basin,`B' and the fact that this area is too low to be captured and mitigated on-site, this area will be allowed to be collected by the stormwater conveyance system within the right-of-way. As'noted above, roof downspouts for the new/future SFRs will be directed to individual infiltration trenches on each lot.These trenches were sized using the WWHM2012 and the Geotechnical recommended 0.8"/hr long-term infiltration rate. New driveways for lots 3-6 will be constructed with permeable pavement. A typical permeable pavement driveway was analyzed using the 1NWHM2012 software and the recommended 0.8"/hr infiltration rate and found to fully infiltrate the 100-year storm event. Therefore the runoff from the permeable driveways will be considered non-effective surfaces. Finally, any area of the site that will be landscaped must implement post-construction soil quality and depth in accordance with BMP T5.13. Therefore, BMP T5.13 will be applied to all areas of` lawn/landscaping on "this`site to further ;provide filtration and some evapotranspiration of site pervious runoff. Seahurst"SP Page 1 18-0727 September 2019