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6. DEVELOPMENT AND EVALUATION OF ALTERNATIVES 6.1 Conveyance System Alternatives A rigorous analysis was made of potential wastewater collection system flow management alternatives as part of the September 2000 MWMC Wet Weather Flow Management Plan (WWFMP) [CH2M HILL, 2000]. The results of that analysis are summarized below. 6.1.1 Basic Alternatives The WWFMP considered several technologies for managing wet weather flow in the wastewater collection system: system rehabilitation to reduce RDII; storage; conveyance; and wet weather treatment. This section summarizes the analysis, evaluates the four technologies and combinations of the four, and develops solutions for most cost-effectively managing wet weather flow. The analysis included many iterations of hydraulic model simulations, as well as economic optimization coupling hydraulic modeling results (flow management effectiveness) and cost estimates. The WWFMP analysis primarily addressed two objectives: · Objective 1: Manage flows and water surface elevations in manholes in the collection system to eliminate overflows and basement flooding. · Objective 2: Provide the most cost-effective means to manage excess flow rates at the WPCF. The alternatives analysis/system optimization proceeded in two phases directed at these two objectives. In the first phase, sub-area alternatives and solutions were developed and evaluated to eliminate basement flooding and overflows in the collection system. With implementation of successful sub-area solutions, the flow arriving at the WPCF was estimated to still exceed the maximum primary treatment capacity. Therefore, the second phase dealt with additional effort beyond the sub-area solutions to manage the excess peak flow arriving at the WPCF. To meet Objective 1 (described above) city staff used the hydraulic model and local knowledge to evaluate sub-area alternatives and develop sub-area solutions that would reduce maximum hydraulic grade (water surface elevations in manholes) to acceptable levels. Because potential problems were location-specific, alternatives and solutions to address them were referred to as "sub-area" alternatives and solutions. Sub-area alternatives were comprised of one or more of three technologies: system rehabilitation to reduce RDII, in-line and off-line storage facilities, and additional conveyance capacity. System rehabilitation was considered separately as either public only or public and private. Public only consisted of rehabilitation that would occur only in the public right-of-way. Public and private included system rehabilitation on portions of the private system within sub-basins identified for public rehabilitation. Once sub-area alternatives were refined such that they adequately accomplished Objective 1, they became sub-area solutions. To facilitate analysis of sub-area alternatives, the collection system was divided into East and West portions. The East portion comprised the entire system upstream of the Willakenzie pump station. This included all of Springfield and Eugene's Willakenzie sub- basins. The West portion included the rest of the Eugene system. Sub-area alternatives were evaluated separately for both the East and West portions. Each sub-area alternative was MWMC_6.0_REVI 1 DOC ~-3