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e � <br /> Technical Memorandum Perteet <br /> 2707 Colby Avenue,Suite 900,Everett,WA 98201 P 425.252.7700 I F 425.339.6018 <br /> Requirement Project Conformance/Method of Compliance <br /> Pollution source control measures on site at all times during <br /> construction in case there are any spills. <br /> 4. Preservation of Natural Runoff from the project site,as well as from the <br /> Drainage Systems and equivalent capture area of the roadway improvements, <br /> Outfalls will be discharged to an existing conveyance system along <br /> 75th Street, matching the current discharge route. <br /> 5. On-Site Stormwater On-site stormwater management measures will be <br /> Management incorporated to the maximum extent feasible,but are <br /> generally not appropriate to this project type. BMP T5.13 <br /> — Post Construction Soil Quality and Depth will <br /> incorporated into the landscaped areas on-site and along <br /> the frontage landscaped areas that get disturbed. <br /> 6. Runoff Treatment Runoff treatment is required for this project because <br /> more than 5,000 SF of pollution generating impervious <br /> surface will be added. A Modular Wetland will be used to <br /> treat on-site flows and a Filterra unit will be used to treat <br /> right-of-way runoff. An Oil Water Separator will also be <br /> used on-site for oil control purposes. The treatment <br /> standard to meet will be the enhanced treatment <br /> standard. A minimum of 91%of total runoff from the <br /> pollution generating areas of the site will be treated. Oil <br /> control will be provided for on-site areas subject to bus <br /> traffic. <br /> 7. Flow Control Flow control is required for this project because more <br /> than 5,000 SF of pollution generating impervious surface <br /> will be added. A concrete vault will be utilized on-site to <br /> comply with this requirement. The project will provide <br /> flow control measures which will limit site discharge rates <br /> and duration to match theoretical runoff for a completely <br /> forested site(pre-developed conditions).Duration <br /> matching will be for storms up to the 50 year return <br /> period. A 60-inch diameter detention tank will be located <br /> within the right-of-way and will provide flow control for <br /> the equivalent area associated with the right-of-way <br /> improvements. The City has stated that they will accept a <br /> pasture land cover for the pre-developed condition for <br /> the sizing of the right-of-way flow control facility. <br /> Equivalent areas can be found in Appendix D of this <br /> memorandum. <br /> 8. Wetland Protection There are no wetlands on or immediately adjacent to the <br /> site.Stormwater from the site flows into an existing city <br /> storm sewer system which discharges to the Narbeck <br /> Wetland approximately 400 feet downstream. The <br /> stormwater mitigation measures proposed for the project <br /> will provide sufficient protection for the downstream <br /> Page 5 <br /> File location: X:\Community Transit\Projects\20110155-Community Transit On-Call\.016-Seaway Transit Center Preliminary <br /> Engineering\Design\Drainage\1_Threshold Analysis\TechMemo 06242016.docx <br />