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Executive Summary City of Redwood City (0011239.02)4 Woodard & Curran, Inc. Sanitary Sewer Capacity Master Plan November 2023 The overall objectives of this Master Plan are to develop up-to-date wastewater flow projections for the City’s collection area using current water use and flow monitoring data and future development information; develop an updated hydraulic model including all of the sewers in the collection system; use the model to identify existing capacity deficiencies and future capacity requirements; identify areas that may be a priority for flow monitoring and rehabilitation efforts to reduce I/I; and develop a prioritized plan, including budget estimates, for implementing the needed capacity improvements to the sanitary sewer system. The capacity improvements, along with additional projects planned by the City to rehabilitate and replace aging sanitary sewer infrastructure to improve its condition and reliability, will be incorporated into an overall sewer system Capital Improvement Program. Executive Summary is presented in two parts: ▪How the Master Plan was Prepared describes the scope and methodologies of the planning effort, including key planning and technical assumptions incorporated into the sewer system capacity analysis. ▪Recommended Capacity Improvement Program presents the recommended capacity improvement projects, priorities, and estimated costs. In addition, recommendations are presented for implementing the proposed capacity improvement program and prioritizing areas for future flow monitoring and sewer rehabilitation. HOW THE MASTER PLAN WAS PREPARED The City’s wastewater collection system includes sewer pipelines ranging in size from 4 to 60 inches in diameter. Previous master plans focused primarily on the trunk sewer system, primarily the 10-inch and larger sewer lines and a limited portion of the smaller diameter pipes. For this Master Plan, all City-owned sewer mains were included in the capacity assessment. The project team used a systematic process that incorporated water use and flow monitoring data, land use information, and design criteria for estimating wastewater flows in a computer hydraulic model of the sewer system. The model was used to assess how the system would perform under existing and future dry and wet weather flow scenarios, and identifies pipes that will not have sufficient capacity to convey the predicted flows under design storm conditions. Improvement projects were developed to provide the required capacity. The capital costs of the required projects were estimated, and the projects were prioritized based on areas with the greatest risk of sanitary sewer overflows under peak wet weather (design storm) conditions. Capacity Assessment Considers Existing and Future Planning Scenarios Two planning scenarios were evaluated for this study. The existing scenario examined the current capacity of the sewer system based on existing development and flow monitoring data collected in the winter of 2019/20. The future scenario included specific proposed and potential developments anticipated within the next 20 years, as identified by the City’s Community Development Department planning staff, including development in accordance with the City’s Downtown Precise Plan. Hydraulic Modeling Identifies Potential Capacity Deficiencies The model integrates various dry and wet weather flow parameters to determine system capacity under different flow and planning scenarios. Key flow components incorporated into the model include dry weather base wastewater flow (BWF), estimated based on winter water use data; groundwater infiltration (GWI), which occurs when water seeps into pipes under the ground through cracks and pipe joints, both during dry weather and wet weather conditions; and rainfall- dependent infiltration and inflow (RDI/I) during storm events. For both of the planning scenarios examined, projected dry and wet weather flows were simulated in the hydraulic model. To ensure that the model accurately represented system conditions during both dry and wet weather conditions, the model was calibrated to flow monitoring data ATTY/RESO.0077/CC RESO SEWER SYSTEM MANAGEMENT PLAN - EXHIBIT A REV: 07-14-25 LF Page 93 of 317