Resolution 8245 - Laserfiche WebLink

City of Everett • 2024 Comprehensive Sewer Plan • October 2025 Executive Summary • ES-11 Sewer System Evaluation The system evaluation of the collection and conveyance system for the existing and 20-year planning horizon is summarized in Chapter 7. The system was evaluated for conveyance capacity, condition, and risk and resiliency. The conveyance capacity of the system to convey flows to the WPCF was evaluated using the City H/H model. The model contains the City’s combined and separated systems and is set up to capture both systems’ response to rainfall. Two model scenarios were created to evaluate deficiencies – existing and future (i.e. 20-year planning horizon). Simulation results for peak flow and peak hydraulic grade line at system components were compared to a defined set of capacity criteria. New capital projects to mitigate the deficiencies were determined for each scenario. A rainfall timeseries, referred to as the 15-storm timeseries, was developed and used for model simulations. A version of the 15-storm timeseries that incorporates climate change was developed for use in the future model scenario. A pipe is deemed deficient if the simulated hydraulic grade line at the connecting manholes is within 5-feet of the ground surface in the separated system or 2-feet from the ground surface in the combined system more than five times during the simulation using the 15-storm timeseries. The storm series contains 25- years of historic rainfall condensed to include major storms; therefore, more than five exceedances roughly equate to less than a 5-year level-of-service. Lift stations are deemed deficient if peak hourly flow into the station exceeds the station’s firm capacity or the modeled capacity of the station is insufficient to convey simulated flows thereby causing system backups. Pipeline and lift station conditions were evaluated through site visits, desktop analysis, and through conversations with City Maintenance and Operations (M&O) staff. Lift station condition was evaluated through site visits. To better inform the City’s prioritization of future asset upgrades and replacements, lift station “Asset Health” score was developed that synthesizes each lift station’s existing likelihood of failure (condition) and consequence of failure (CoF). Each criterion is rated on a 1 to 5 scale where higher numbers indicate worse conditions and high criticality, then the scores are multiplied together to get the overall Asset Health score. The resulting scores ranged from 3.1 to 10.5, with two pump stations rating higher than 10, 26 pumps stations rated between five and 10 and seven pump stations rated below five. The City directed that lift stations with asset health scores greater than 11 be deemed in poor condition, therefore, none of the lift stations are considered to be in poor condition at this time. Pipe and manhole conditions were evaluated through desktop analysis. The City regularly conducted closed circuit television (CCTV) and inspected pipes using CCTV videos and Granite inspection and scoring reports. The City provided the existing pipe data and directed that all existing 4- and 6-inch diameter sewer mains require replacement and upsizing, and pipes that are over 80 years old are considered as deteriorated or aging and require replacement or rehabilitation within the 20-year planning horizon. Costs were developed for the pipeline rehabilitation and replacement program assuming approximately 30 percent of the pipes will be replaced, and 70 percent will be rehabilitated. A Risk and Resiliency Analysis (RRA) was also conducted using the United States Environmental Protection Agency’s (EPA) risk and resilience assessment framework. This process was used to determine the likelihood of failure and summarize the existing system resiliency for the collection system. As a result, 130 prioritized assets were identified, and 22 threats were selected. Each asset and threat were paired, referred to as an “asset-threat pair”, which indicates that threat has the potential to negatively impact the ability of the asset to function and the City’s ability to continue service. Each critical asset-threat pair was rated with