7505 SEAWAY BLVD SEPA 016 - 034 - SEAWAY TRANSIT CENTER Land Use Decision Documents 2025-04-22

ti. Technical Memorandum Perteet 2707 Colby Avenue,Suite 900,Everett,WA 98201 f P 425.252.7700 I F 425.339.6018 To: Jane Zimmerman,City of Everett From: Brian Caferro, PE Colin Paulsen Date: August 23,2016 Re: Seaway Transit Center— Drainage Design Approach Introduction Community Transit(CT) proposes to construct a new Transit Center/Transfer Facility on a site located at the intersection of Seaway Boulevard and 75th Street SW in Everett, Washington. See Figure A-1 — Vicinity Map, located in Appendix A of this memorandum. This transit center will serve as the northern terminus for a new SWIFT BRT line and will also accommodate bus routes for Community Transit,Everett Transit,and shuttles from Boeing's adjacent facilities. The new CT facility will be developed with required landscaping,internal circulation, signs, bus shelters, ADA accessibility, security cameras, lighting, stormwater management, and an operator comfort station. There is existing stormwater infrastructure north of the site on 75th Street SW, east and southeast at the PUD Operation Center,and west of the site along Seaway Boulevard,and all existing drainage structures and conveyances eventually drain stormwater to the Narbeck Wetland Sanctuary. The purpose of this technical memorandum is to briefly describe this project's approach to stormwater management and how the project's drainage improvements will satisfy the City's stormwater regulations. Existing Condition The site is approximately 2.72 acres in size. The land surrounding the project site is mainly commercial businesses, including Boeing and PUD. The existing land cover condition of the site is primarily uncut grass and pasture cover, with a gravel maintenance access road in the southwest corner of the property and scattered stands of trees to the south of the site that separate the southern portion of the property from the adjacent PUD site. There is no upstream area that contributes runoff to the project site. According to the geotechnical report completed by HWA GeoSciences, Inc. dated March 9, 2016, the soils on site are primarily classified as Vashon glacial till. This soil type is known as a poorly draining soil. There is also a natural high water table on site. The project site is currently flat along the western portion of the property and is covered with an approximate 10-15 foot mound of fill material along the eastern portion. Stormwater runoff from the site currently sheet flows onto the surrounding roadways and PUD property. Refer to Figure A-2 — Existing Conditions Map, located in Appendix A,for a delineation of the on-site drainage patterns. There is an existing conveyance system running along Seaway Boulevard and 75th Street that collects all of this runoff,conveying it to the southeast and discharging it into the Narbeck Wetland Sanctuary to the east of the site. Refer to Figure A-3—Downstream Map,located in Appendix A of this memorandum,for a delineation of the downstream flow path. The Narbeck Wetland Sanctuary is a 42.67 acre wetland area,having Narbeck Creek run through it to the north. Narbeck Creek is a fish-bearing stream that flows north, eventually flowing underneath Mukilteo Boulevard,and ultimately into the Puget Sound. Proposed Condition In the proposed condition, runoff from the project site will be collected by an on-site conveyance system that will direct water through an Oil Water Separator and Modular Wetland for the purposes of water quality treatment. Once treated, the on-site runoff will be discharged into an underground stormwater concrete Page 1 File location: X:\Community Transit\Projects\20110155-Community Transit On-Call\.016-Seaway Transit Center Preliminary Engineering\Design\Drainage\1_Threshold Analysis\TechMemo 06242016.docx