3800 RIVERFRONT BLVD 2021-01-28 - Laserfiche WebLink

I IStormwater Pollution Prevention Plan I 2.1 Existing Conditions The existing Simpson Site is cleared and vacant land situated adjacent to the Snohomish River. I The site is divided into three sub basins each draining to a different wetland water body. The site is flat with slopes between 1-3%, with no vegetation. There is a temporary settlement pond in place in the northwest corner of the site. Underlying soil characteristics can be found in the I Geotechnical Report included in the associated Stormwater Site Plan. The three surrounding wetlands all drain to the Snohomish River. The site currently holds open permits to stockpile soil within the existing silt fence limits, with a preload depth of approximately 10' currently in place. 2.2 Proposed Construction Activities I The Simpson Site has been designed to accommodate stockpiled soil to raise the entire site 10' from the proposed final grade. 3:1 maximum slopes have been provided along the perimeter of the site to also accommodate drainage needs. The site has been divided into three sub basin areas I to match the existing condition as closely as possible, with interceptor swales, and multiple outfall locations to promote dispersion and infiltration on site along the perimeter. I Proposed Onsite Stormwater Treatment The runoff from the development will be detained on site and infiltrate the natural soil at a rate of 12"/hour,based on the City of Everett Stormwater Management Manual. Dispersion trench/bio- swale facilities will be utilized for water quality treatment and flow control. The detained and I treated flows will then enter surround wetland water bodies and ultimately discharge to the Snohomish River. IThe following summarizes details regarding site areas: I . Total site area: 10.6 acres • Percent impervious area before construction: 0% I . Percent impervious area after construction: 0% • Disturbed area during construction: 10.6 acres ■ Disturbed area that is characterized as impervious (i.e., I access roads, staging, parking): 0 acres ■ 2-year stormwater runoff peak flow prior to construction 0.031 cfs • 10-year stormwater runoff peak flow prior to construction 0.041 cfs I . 2-year stormwater runoff peak flow after construction(detained) 0.00 cfs ■ 10-year stormwater runoff peak flow after construction(detained) 0.00 cfs IAll stormwater flow calculations were computed using WWHM and are provided in Appendix F. I 1 I4