Otak, Inc. 11/13/2024 - Laserfiche WebLink

Scope of Work continued City of Everett 6 Dorn Ave. Drainage Improvements Otak  No changes to the existing condition model will be needed.  Downstream impacts modeling will be limited to comparisons in peak flow and velocity. Wetland hydroperiod analysis will not be performed. 5. Options Analysis 5.1 Options Analysis for Solution Element 2 Outfall This task will determine the ultimate outfall location for the McGill Avenue bypass. The prior alternatives analysis identified an option 2a that directs the outfall to McGill Pond, or option 2b which outlets directly to North Creek. The city has indicated that McGill Pond is suspected to overflow and cause flooding of McGill Avenue just south of the pond. The options analysis will include a solution to this flooding – likely construction of a conveyance from the pond overflow spillway to North Creek that precludes road flooding. 5.2 Options Analysis for Water Quality Treatment The size and cost, at the alternatives analysis, were developed using the Contech Modular Wetland System (MWS) which is Department of Ecology GULD certified for Enhanced Treatment. Conceptually, this off-line treatment would involve a flow splitter at the depth of the proposed conveyance improvement. The MWS has an upper size limit at approximately 0.9 cfs. To treat larger flows, multiple vaults would need to be joined in parallel requiring additional connecting structures and space. Therefore, regional treatment is more challenging at the Solution Element 1 (Dorn Avenue) location and multiple treatment locations in the watershed or other water quality treatment techniques will be explored further in this task. For Solution Element 2 (McGill Avenue) the configuration would change if the outfall was to North Creek (less space would be available) instead of McGill Pond. The sizing performed for the alternatives analysis was meant to approximate cost of water quality retrofit treatment for planning purposes for the entire contributing area (public and private). This task will revisit the water quality configuration at Dorn Avenue and provide configuration options that provides maximum treatment area within or adjacent to the Solution Element 1 R/W limits. The water quality treatment configuration options will also be completed at McGill (Solution Element 2). These will be completed in tandem with the outfall options for Solution Element 2 – Task 5.1. The configuration options will focus on: · available area in roadway or R/W · other area adjacent to the R/W · vertical drop