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

t Technical Memorandum Perteet 2707 Colby Avenue,Suite 900,Everett,WA 98201 I P 425.252.7700 I F 425.339.6018 detention vault. Controlled runoff from the vault will then be discharged into the existing conveyance system along 75th Street,eventually discharging into the Narbeck Wetland Sanctuary. A separate detention tank will be located in the City of Everett right-of-way near the lower entrance into the project site. This detention tank will provide the required flow control for the right-of-way frontage improvements associated with the project. Prior to entering the detention tank,runoff will be treated by a Filterra unit which will be placed in the amenity zone along 75th Street. Runoff from the area to be collected by this detention tank will be a designated equivalent area associated with the project frontage improvements. A Filterra unit will provide the water quality treatment for the runoff collected along 75th Street, before being discharged into the detention tank. The natural discharge point,southeast corner of the project site,will be maintained in the proposed condition. The project area is contained within a single threshold discharge area (TDA), as evidenced in Figure A-3 - Downstream Map. Applicable Standards and Minimum Requirements The project will be designed in accordance with the City of Everett's stormwater standards, including the City's 2010 Stormwater Management Manual. The 2010 Stormwater Manual has been accepted by the Washington State Department of Ecology (Ecology) for use by the City of Everett under their Phase II NPDES municipal discharge permit. Certain elements of the project will also conform to the Ecology's Stormwater Management Manual for Western Washington (2012) as applicable for coverage under Washington State's Construction Stormwater General Permit. The project does not fall within the City of Everett's combined sewer system and will not claim the associated exemptions from stormwater requirements. Threshold Analysis and Minimum Requirements Threshold Analysis Project Level Threshold Analysis As indicated above,the 20I0 Stormwater Management Manual and Ecology's Stormwater Management Manual for Western Washington (20I 2)are being used as the regulatory drainage design manuals for this project. There are a total of nine minimum requirements that apply to this project. The following step-by- step threshold analysis demonstrates which minimum requirements apply to this particular project. The steps for this analysis can also be followed by referring to Chapter 2 of the City of Everett Stormwater Management Manual (COE SWMM). Prior to performing the threshold analysis, existing and new impervious surface areas were delineated and quantified. Tables 1 and 2 show the total existing and total new impervious surface areas (both non-pollution generating(NPGIS)and pollution generating(PGIS) for the entire project site,as well as the portion of the project located within City of Everett right-of-way. The existing impervious surface areas listed below were derived from the area delineations shown in Figures A-4 and A-5,found in Appendix A of this memorandum. The new impervious surfaces listed below were derived from the area delineations shown in Figures B-1 and B-2,found in Appendix B of this memorandum. Table 1 below summarizes the impervious area associated with the existing conditions of the site and City right-of-way,which are shown in Figures A-4 and A-5— Existing Impervious Areas in the Appendix A of this memorandum: Page 2 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