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HomeMy WebLinkAboutC_Response_Letter_Earth Solutions_1802021-v2.pdfFebruary 21, 2018 ES -4208.01 Dalpay Properties, LLC 4033 Northeast Sunset Boulevard Renton, Washington 98059 Attention: Mr. Jim Dalpay Subject: Response to Review Comments Proposed Dalpay Short Plat 1801 Redmond Avenue Northeast Renton, Washington Reference: Earth Solutions NW, LLC Geotechnical Engineering Study ES -4208, dated December 23, 2015 Earth Solutions NW, LLC Geotechnical Addendum ES -4208.01, dated February 14, 2017 Barghausen Consulting Engineers, Inc. Conceptual Drainage Control Plan Sheet C2 of 3, dated May 17, 2016 City of Renton On -Hold Notice Dated January 31, 2018 Earth I Solutions NW«< Earth Solutions NW LLC 2017 Renton Surface Water Design Manual (RSWD) Appendix C.2.1.1 Dear Mr. Dalpay: • Geotechnical Engineering • Construction Monitoring • Environmental Sciences In accordance with a request made by Wilson Law Group of Washington, Earth Solutions NW, LLC (ESNW) has prepared this response letter for the subject project. Below is the relevant review comment followed by our response. "1805 - 136th Place N.E., Suite 201 0 Bellevue, WA 98005 • (425) 449-4704 • FAX (425) 449-4711 Dalpay Properties, LLC February 21, 2018 Review Comment 2 ES -4208.01 Page 2 The geotech report needs to be revised per the 2017 RSWDM to provide the infiltration rate, recommendations based on actual field criteria, to back up the on-site BMP selection that is mentioned in the RSWDM. ESNW Response The referenced drainage control plan indicates stormwater generated from impervious surfaces will be conveyed to a detention tank, outflow through a filter, tightlined to the base of the adjacent natural drainage ravine to discharge through a flow dissipater system into the stream channel. This proposal represents the only option for gravity method of stormwater conveyance. Based on the soil conditions encountered at the test pit locations described in the referenced report, infiltration is not feasible. Therefore, dispersion is proposed. Review of referenced RSWDM provides criteria for dispersion. The slopes that form the natural drainage ravine are steeper than 20 percent and are about 26 feet in height near the proposed discharge point. However, dispersion is allowed within 200 feet of a landslide, steep slope or erosion hazard provided a qualified professional evaluates the cumulative impacts. The current proposal will include tightline conveyance across the steep slope area and controlled discharge via energy dissipater. The current routing was determined by running the pipe as perpendicular to the slope surface as possible, resulting in the most direct flow path. On this basis, the potential for erosion resulting from discharge will likely be very low. The slope will be protected from erosion on a permanent basis by the proposed drainage system and very little disturbance is required to anchor the pipe to the slope surface during installation. Provided the energy dissipater performs as intended, the likelihood of erosion caused by discharge will be adequately mitigated. This system will also provide an opportunity to recharge the stream alignment with water that would otherwise flow there in a pre -developed condition, thereby preserving the critical areas functions and values. In our opinion, the current proposal represents the most reasonable approach to accommodating runoff for this site. Earth Solutions NW, LLC Dalpay Properties, LLC February 21, 2018 We trust this letter meets your current needs information is required, please call. Sincerely, EARTH SOLUTIONS NW, LLC 1 U , . 5 ott S. Riegel, L.G., L.E.G. Seni r Project Manager ES -4208.01 Page 3 Should you have questions, or if additional Kyle R. Campbell, P.E. Principal Engineer CC' Wilson Law Group of Washington Attention: Ms. Katy Dacanay (Email only) Earth Solutions NW. LLC