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EXHIBIT A <br />SCOPE OF WORK (PHASE 1) <br />CSO STORAGE at 36th Street <br />WO# UP3700 <br />1. Project Background and Understanding <br />Two separate planning and design efforts are in progress that involve combined sewer <br />infrastructure improvements to reduce combined sewer overflow (CSO) discharges at NPDES <br />permitted outfalls SR07 and SR08 to comply with the Department of Ecology (DOE) requirement <br />of no more than one CSO event per year per outfall averaged over a 20-year period. These <br />projects also aim to eliminate chronic sanitary sewer overflow (SSO) events at various locations. <br />Another consultant (BHC) previously completed a modeling study that identified several <br />improvements (grey infrastructure) that would achieve CSO and SSO control. Sewer flow meters <br />were installed in the system to support model calibration and validation. The City plans to <br />implement the following two project elements: <br />CSO Storage <br />Preliminary modeling with CSO storage showed that excess volume can be stored and <br />slowly released over time and is effective in reducing CSOs at SR08 and flooding at various <br />locations within the project area. Exact dimensions of the vault (or other storage <br />structure) orifice, and weir should be evaluated further during design and may be <br />impacted by other site constraints. A pump may be necessary to either allow for deeper <br />storage or to pump out the bottom of the storage vault or to ensure it can drain <br />completely between storm events. <br />Modifications to 36th Street Regulator <br />Modeling indicates that diverting more flow from the 36th Street regulator to the <br />Snohomish River CSO Interceptor (SRI) will decrease flooding at City sewer manholes in <br />the vicinity of this CSO control project by reducing flow in the Summit Interceptor and <br />flattening the hydraulic grade line (HGL). The Sewer 0 projects (being completed by City <br />staff) are to control SSO events at various locations. Modeling for Sewer 0 determined <br />that the proposed improvements would move an SSO problem farther downstream and <br />that additional system improvements are necessary. The City's immediate objective is to <br />ensure that the Sewer 0 project functions without surcharging, while the long-term <br />objective is for CSO control. <br />2. Assumptions <br />This scope of work is based on the following assumptions: <br />a. This project is broken into three phases: <br />• Phase 1 Preliminary Engineering <br />11Page <br />