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Capacity and I/I Assessment City of Redwood City (0011239.02)3-3 Woodard & Curran, Inc. Sanitary Sewer Capacity Master Plan November 2023 For Redwood City, since the design storm PWWF represents an infrequent, 10-year return period event coinciding with a conservative BWF condition, the Master Plan considers it acceptable to allow surcharging over the pipe crown, provided the hydraulic grade line (water level) remains at least 5 feet below the ground surface. (However, improvements, if needed, would be sized in accordance with the City’s design criteria, as noted below.) Under peak dry weather conditions, however, sewers should be able to convey the peak flow without surcharge. Note that the Master Plan capacity criterion of 5 feet minimum freeboard is less conservative than the maximum flow depth to diameter (d/D) of 0.75 specified in the City’s design standards. However, new development projects may still be required to provide site-specific analysis, which will be based on a capacity criterion on 0.75 d/D per City policy. Performance criteria for pump stations are based on their firm capacity, defined as pumping capacity with the largest pumping unit out of service. Force mains are considered to be capacity deficient if maximum velocity exceeds 8 feet per second (fps) under design peak wet weather flow. Maximum velocity criteria are intended to avoid excessive head losses in the force main. 3.1.3 Design Criteria for New Facilities The following criteria were used for sizing new gravity sewers for the Master Plan where needed to relieve identified capacity deficiencies. These criteria are based on the modeled design storm PWWF in the system. •Maximum allowable flow depth-to-diameter ratio (d/D) of 0.75 under peak design flow for all pipe diameters •Manning ‘n’ of 0.013 for all pipe materials •Downstream pipes at least as large as upstream pipes for new or replacement sewers where practical Criteria recommended for force mains include: •Minimum and maximum velocities of 3 and 8 fps, respectively Additional design criteria are noted in the Design Flow and Hydraulic Criteria TM in Appendix B. Pump stations should be designed to have firm capacity to handle the design PWWF. These criteria are considered appropriate, as long as the pump station design accounts for pumping efficiency under both existing and future dry weather flow conditions, “normal” wet weather conditions, and phasing in determining the number and size of pumps. 3.2 CAPACITY ANALYSIS RESULTS The calibrated model was run for existing and future conditions to identify areas of the system that fail to meet the specified performance criteria under design storm peak wet weather flows. No capacity deficiencies in the system were identified for dry weather conditions. 3.2.1 Gravity Sewer System Capacity Deficiencies The PWWF model results in the Main City Area are shown in Figure 3-2 for existing conditions and in Figure 3-3 for future conditions (see description of flow scenarios in Section 2.3.2). The figures show pipes that would be surcharged due to insufficient capacity (“throttle” conditions) as well as upstream segments that would be surcharged due to backwater, where the deficiency would result in either predicted overflows or surcharge to within less than 5 feet of the manhole rims. Most of the capacity deficiencies in the system are a result of existing wet weather flows; however, future development, particularly in the Downtown area, would increase the level of surcharge in some of the sewers. There were no gravity sewer capacity deficiencies indicated for the Seaport/Pacific Shores system, and only one small area in Redwood Shores, downstream of PS No. 09. Flow monitoring indicated extremely high peak flows in the area tributary to that pump station. ATTY/RESO.0077/CC RESO SEWER SYSTEM MANAGEMENT PLAN - EXHIBIT A REV: 07-14-25 LF Page 138 of 317