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City of Redwood City (0011239.02)3-1 Woodard & Curran, Inc. Sanitary Sewer Capacity Master Plan November 2023 3.CAPACITY AND I/I ASSESSMENT The capacity performance of the system and need for capacity improvements were evaluated using the calibrated hydraulic model described in Chapter 2. This chapter discusses the criteria on which the capacity assessment was based and presents the model results and proposed capacity improvement projects. The relative amounts of I/I contributed by each flow meter basin are also presented to provide information for the City to guide future flow monitoring and I/I reduction activities. 3.1 DESIGN FLOW AND PERFORMANCE CRITERIA Sewer system capacity is assessed with respect to the system’s performance under a design flow condition. The subsections below define the design flow criteria used for the capacity assessment and the criteria for assessing system performance and identifying system capacity deficiencies. 3.1.1 Design Storm Condition The use of wet weather design events as the basis for sewer capacity evaluation is a well-accepted practice. The approach is to first calibrate a hydraulic model of the system to match wet weather flows from observed storm(s), and then apply the calibrated model to a design rainfall event to identify capacity deficiencies and size improvement projects. The design event may be synthesized from rainfall statistics or may be an actual historical rainfall event of appropriate duration and intensity. Other considerations for the design event include the spatial variation of the rainfall and the timing of the storm relative to the diurnal base wastewater flow pattern. Selection of a design rainfall event is typically based on an allowable level of risk, often expressed as the return period. It is recognized that while wet weather overflows are highly undesirable, it is not cost-effective to provide capacity for the largest possible storm event. Regulatory agencies have not adopted standard criteria for return periods, so each agency must choose a target return period based on desired level of service, potential impacts of overflows, and cost. The City has adopted a 10-year return period for analysis of wet weather capacity. Refer to the Design Flow and Hydraulic Criteria TM in Appendix B for a detailed discussion of the factors considered in the development of the design storm for this Master Plan. The 10-year design storm for Redwood City was developed based on information contained in the National Oceanic and Atmospheric Administration (NOAA) Atlas 14, Volume 61, which provides point precipitation frequency estimates for locations throughout the U.S. The design storm is a 24-hour event with a “nested” temporal distribution (that is, the rainfall for each hourly duration within the event represents the 10-year return frequency rainfall for that duration). The design storm rainfall varies spatially across the service area based on location. For central Redwood City, the design storm has the following characteristics: •Total rainfall 3.58 inches •Peak hour intensity 0.80 inches/hr. The timing of the design storm also affects the resultant peak wet weather flows. If the design storm is timed such that the peak RDI&I occurs about the same time as the peak BWF (“peak-on-peak”), the total PWWF will be higher than if the design storm occurs under average or minimum BWF conditions. Timing the storm to produce peak-on-peak results is generally thought to create a return period of the peak wastewater flow that is greater than the return period of the 1 U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, NOAA Atlas 14, Volume 6 Version 2.3: California, 2011, Revised 2014 ATTY/RESO.0077/CC RESO SEWER SYSTEM MANAGEMENT PLAN - EXHIBIT A REV: 07-14-25 LF Page 136 of 317