10521 19TH AVE SE 2022-05-23 - Laserfiche WebLink

Silver Lake Plaza From a Snohomish County Soil Survey done by the USDA NRCS the soils on -site consist of "Alderwood Urban Land Complex" type soils. According to test pits taken by Robinson Noble, Inc. soils on -site resemble silty fine sand type soils that are unsuitable for infiltration. Refer to "Geotechnical Engineering Report" by Robinson Noble, Inc. dated March 23, 2016 within Appendix II for more detailed soil information. The adjacent parcel to the north is developed with existing commercial buildings and parking lots. The adjacent parcels to the south and east are developed with existing residential structures and driveways. The 19th Avenue SE right-of-way lies along the western boundary of the site. Permanent Stormwater Control Plan Existing Site Hydrology — The approximate 0.94 Acre site is currently in a pasture type condition with areas of medium sized trees. Project assumed to be fully forested for flow control modeling purposes. An estimated 0.16 Acres of residential lawn area to the east of the site contributes to the site's upstream runoff. No natural streams or drainage features exist on -site. New detention facility for the site is to be located in the southwestern portion of the site and connect to the existing drainage system along the east side of 19t" Ave SE along the frontage of the site. Refer to Grading and SWPPP plan for a depiction of existing site conditions along with existing flow patterns. Developed Site Hydrology — The developed site is required to satisfy minimum requirements 1 through 9. Refer to the sections later in this report for an explanation of how the proposed development will satisfy each individual minimum requirement. Performance Standards and Goals — Enhanced stormwater treatment is to be provided according to BMPs T8.11 Large Sand Filter Basin and T8.30 Linear Sand filter; refer to MR#6 within this report for more detailed information. Stormwater detention is to be provided by an underground detention pipe; refer to MR#7 within this report for explanation of detention method and sizing calculations. Low Impact Development Features — Due to poor infiltrating soils along with large amounts of hardscape coverage, there are no LID BMP's proposed for this project. The on -site stormwater BMP's mainly consists of T5.13 Post -Construction Soil Quality and Depth. Refer to MR#5 of this report and Appendix IV of this report for for more detailed analysis. WESTERN ENGINEERS, INC. 3 PROJECT: 15-1415-A