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Redwood City Sanitary Sewer Master Plan Executive Summary FINAL August 2008 2 How the Master Plan was Prepared The City’s wastewater collection system includes sewer pipelines ranging in size from 6 to 48 inches in diameter. The trunk sewer system, primarily the 10-inch and larger sewer lines and a portion of the smaller diameter pipes, was the focus of the capacity assessment in this Master Plan. These trunk sewers are indicated as “Major City Sewers” in Figure ES-1. The project team used a systematic process that incorporated population estimates, land use planning information, water use and flow monitoring data, and design criteria for estimating wastewater flows in a computer hydraulic model of the trunk 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 may not have sufficient capacity to convey the predicted flows. 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 most likely to have sanitary sewer overflows. 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 winters of 2004/2005, 2005/2006, and 2006/2007. The future scenario used the Association for Bay Area Governments (ABAG) population and employment projections for year 2030 throughout most of the City, and also assumed buildout conditions for the downtown area according to the Downtown Precise Plan. This future scenario represents over 20 percent growth in population through 2030, and 40 percent growth in number of jobs in the City. Potential Capacity Deficiencies were Identified For both of the planning scenarios examined, projected dry and wet weather flows were simulated in the hydraulic model. The model was calibrated to actual flow monitoring data to ensure that it represents an accurate depiction of system conditions during both dry and wet weather conditions. 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 base (dry weather) wastewater flow, estimated based on population and winter water use data; groundwater infiltration, which occurs when water seeps into pipes under the ground through cracks and pipe joints; and rainfall-induced infiltration and inflow (I/I) during storm events. For this Master Plan, a 5-year recurrence frequency rainfall event was selected as the “design storm”. A 5-year storm has a 20 percent chance of occurring in any given year. The Proposed Improvement Projects Address the Potential Capacity Deficiencies Model results were examined to determine where projects would be needed to alleviate capacity deficiencies. Due to the extent of surcharged pipes under the design conditions, surcharging up to 5 feet below ground level was allowed during the 5-year design event before a project was identified to alleviate a capacity limitation. These surcharge criteria allow the City to focus capital spending on areas most likely to cause sanitary sewer overflows. Where projects are identified, new pipes are sized to not exceed approximately 75 percent full under future peak wet weather conditions. Figure ES-2 shows the project locations. 6.2.A. - Page 218