4620 SEAHURST AVE 2020-05-20 - Laserfiche WebLink

4 +, 2707 Wetmore Avenue e0 M E CSA Everett, WA 98201 ENGINEERING, INC. t 425=387:3820 # 425=259.1958 July 9th, 2018 Chris Thompson Infiltration Trench Sizing for Thompson SP This letter is intended to address infiltration trench sizing for roof area on Lot 1 over the allotted amount described in the drainage report prepared by Omega Engineering May 2011 for the short plat located at. 4618 Seahurst Ave, Everett,WA 98201. Stormwater mitigation for Lots 1 and 2 of the Thompson Short Plat utilizes infiltration. Per the drainage report prepared by Omega Engineering in May 2011, the infiltration trench was sized to_serve roof areas on Lots 1 and 2 up to a total of 4,240 sf. The infiltration trench was designed to be 5.5 feet wide, 60 feet long and 4 feet deep. It is our understanding the roof area for Lot 1 will be constructed over the allotted amount. Therefore, it has been determined that any roof area over the allowed 4,240 sf shall be directed to a separate infiltration trench. The contractor shall install 7.5 linear feet of trench per 500 sf of additional roof area for a 5.5 foot wide infiltration trench. The minimum depth of infiltration trench shall be 4 feet, however per the geotechnical recommendations for this project, the bottom of the trench shall extend a minimum of 1 foot into the gravelly sand layer. Additional trench depth may be necessary based on the soil conditions in the location of the new trench. The new trench shall be located on the southeast side of Lot 1, near TP-8, as shown on Plate 2 of the geotechnical report prepared by Liu &Associates, Inc. The contractor/builder shallfollow all recommendations described in the Geotech report prepared by Liu & Associates, Inc. in July 2011, as well as recommendations described in the Drainage report prepared by Omega Engineering, approved by the City of Everett. Sincerely, Joseph Smeby, P.E. 124e1 191 • Air . . _"`4,•.i toir _ Z gam i Afit ., ..t_f sly, „,,,A2,„ .ftik to,