2320 106TH ST SW 2022-08-29 - Laserfiche WebLink

Stormwater Pollution Prevention Plan 2.0 Site Description 2.1 Existing Conditions The site elevations range from elevation 560 to elevation 548 along the eastern property line of the site. Site grades are such that the site slopes at approximately 7 percent from the west to the east and drains into a wetland area located along the length of the frontage on the eastern property line of the site. Please refer to the site topographic map on the following pages of this report which shows land cover conditions as well as the contour elevations and existing grades on the site. The site is currently densely covered with trees. One area of the site at the southwest corner is covered with several hundred square feet of impervious surface that will be removed for development of the proposed project. Several outbuildings exist very near the property line on the parcel to the east of the site and will remain intact during the development of the proposed project. The parcel to the west of the site drains to a wet pond and detention/water quality vault prior to draining to the northeast corner of the parcel. That site drains to a separate basin and does not drain onto this project site. There are large wetlands with 60-foot and 75-foot buffers located immediately east of the site. These areas will be mitigated for or will remain undisturbed and protected in perpetuity with project development. The on-site soils are classified as Alderwood urban land complex with 2 to 8 percent slopes. This is a 'C' type soil and not conducive for infiltration or stormwater management. 2.2 Proposed Construction Activities Approximately 2.42-acres of the 3.9-acre site will be developed as a warehouse facility of approximately 45,375 square feet of warehouse space and approximately 3,000 square feet of office space along with the associated drive aisles, parking, flow control, water quality and pipe conveyance facilities. The site drains to an existing wetland area located immediately east of the project site with buffers associated with the wetland located east of the site and extending onto the project site. The buffers will either be mitigated for or protected in perpetuity with development of the proposed project. Half-street roadway improvements associated with this development along the project frontage total less than 0.1-acre of new impervious surface and runoff will be collected off-site and routed to the on-site detention and water quality facility. The proposed detention vault is located along the southern property line and will discharge by gravity into and through a 4-foot by 8-foot modular wetland system, which will then discharge to a pump station which will pump detained runoff to a dispersal trench located at the southeast property corner of the site. Water quality is proposed via a modular wetland system, which is classified as General Use Level Designation by the Department of Ecology for Enhanced Water Quality. It also has General Use Level Designation for phosphorus control as well as Basic Water Quality. The conveyance system on this site has been sized to convey the 25-year event based on the Rational Method, which is a highly conservative methodology. 3 17576.003.d oc