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Scope of Work <br />2828 Colby Avenue, Suite 401 | Everett, WA 98201 | Phone 425.493.5221 | otak.com <br />S:\1Common\UTILITY PROJECTS\UT 3722 - Evergreen Drainage Study\100 PROJECT MANAGEMENT\Dorn Ave Project\Dorn <br />Ave Drainage Improvements_ Final Design_ Scope_final draft _10-10-24 edit.docx <br />City of Everett <br />Dorn Avenue Drainage Improvements <br />Otak Project No.21001B <br />8/28/2024 <br />Description of Project <br />The goal of this project is to provide final construction documents (plans, specifications, and estimate) to <br />implement flood reduction alternatives 1A and 2 at Evergreen Way near Holly Drive from the Evergreen <br />Way Flood Reduction Alternatives Analysis completed January 19, 2024, by Otak. In addition, water <br />quality treatment for the two locations will be designed to a 30% level and be compatible with the final <br />flood reduction alternatives. <br />Background <br />The primary goal of the City of Everett’s Dorn Avenue Drainage Improvements Project is to reduce <br />flooding that occurs on Evergreen Way without increasing flooding that occurs on downstream private <br />properties and the downstream public conveyance network. Otak developed a hydraulic model of the <br />existing conveyance system and developed solution concepts to reduce the existing flooding conditions. <br />Impacts on downstream flows and velocities at the Dorn Avenue outfall and in North Creek downstream <br />of McGill Avenue were also checked with the model. Most of the study area does not have any existing <br />water quality treatment. Conceptual water quality sizing was completed by extracting water quality flow <br />rates from the hydraulic model. The flows were used to size a typical off-line, regional, underground <br />proprietary biofiltration facility using manufacturer recommendations. Based on hydraulic modeling of the <br />system, the flooding issue at the Evergreen Way low point is caused by the large existing impervious <br />drainage area overwhelming the undersized existing conveyance system. Runoff from a large, <br />predominately commercial, contributing area (55.5 acres) converges near 8824 Evergreen Way with a <br />storm system from southwest of 4th Avenue W. running along both sides of Evergreen Way and <br />collecting area from the west in a system along Holly Drive. This entire system outlets from Dorn Avenue <br />to the mainstem of North Creek through an unknown, poor, and suspect private system within a storage <br />unit. <br />The solution is to upsize the conveyance system from the Evergreen Way Low Point to its existing outlet <br />along Dorn Avenue and to bypass a portion of flows from McGill Avenue to North Creek before they reach <br />the low point. Solution Element 1A consists of 194 LF of new 42-inch diameter pipe, 1805 LF of new 36- <br />inch diameter pipe, and 12 new/replaced structures. A new alignment along Holly Drive is used to <br />minimize work in Evergreen Way. Solution element 2 consists of 1521 LF of new 24-inch diameter pipe <br />and nine new structures along McGill Avenue to North Creek with excavations up to 14 feet deep. <br /> <br /> <br />