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

! 1 Technical Memorandum Perteet 2707 Colby Avenue,Suite 900,Everett,WA 98201 I P 425.252.7700 F F 425.339.6018 Results of Project Level Threshold Analysis As a result of the project level threshold determination conducted by following Questions 1 and 2 above, this project is required to apply all nine Minimum Requirements as summarized below: • Minimum Requirements#1 thru#9 shall apply to the new and replaced impervious surfaces and the land disturbed. Since project related work to be performed within the City of Everett right-of-way is more of a roadway improvement project,we have analyzed the right-of-way portion of the project as a road related project when considering thresholds. As such,we used Figure 2.2 in the COE SWMM to determine which requirements apply to the improvements located within the right-of-way. All nine minimum requirements still apply to the right- of-way improvements; however, Minimum Requirements #6 and #7 only apply to the new PGIS and new impervious surfaces and not replaced PGIS or replaced impervious surfaces. Water quality treatment and flow control are both required on this project,as the project is adding more than 5,000 square feet of new pollution generating impervious surface (PGIS) and new impervious surface. This project proposes to add 64,977 square feet of new PGIS and 97,301 square feet of new impervious surface. A concrete vault, located on-site, and a separate detention tank, located within City right-of-way, will act as the flow control devices for the project. A Modular Wetland,located on-site,and a Filterra unit,located within City right-of-way, will serve as the water quality treatment features for this project. The site does not fall cleanly within the definitions of high use provided in Ecology or the City of Everett's stormwater manuals and it is not clear whether the site is required to provide oil control. However, conversations with the City of Everett's stormwater reviewer have indicated that the City will likely designate this facility as high use due to frequent trips and "parking" by large vehicles (buses). At this time, it is anticipated that oil control measures will be needed as part of the projects mitigation measures. An Oil Water Separator will be located on-site and will be used to satisfy oil control requirements. Refer to Appendix C of this memorandum for the water quality and flow control sizing calculations. The Minimum Requirements Table below demonstrates how each minimum requirement is being satisfied for this project. _ Table 4—Minimum Requirements Requirement Project Conformance/Method of Compliance 1. Stormwater Site Plan A stormwater site plan will be included as part of this Preparation project. Erosion control measures will be provided as appropriate. See project plans for erosion control measures proposed(plans to be submitted at a later date). 2. Construction A standard Construction Stormwater Pollution Stormwater Pollution Prevention Plan (C-SWPPP)in accordance with Volume II, Prevention Chapter 2 of the COE SWMM will be prepared at a later submittal. The project will seek coverage under the state's Construction Stormwater General Permit. Discharge monitoring and reporting for TSS and pH will occur throughout the duration of the construction phase. 3. Source Control of No activities will occur onsite which necessitate source control measures. However,the contractor shall have Page 4 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