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Capacity and I/I Assessment City of Redwood City (0011239.02)3-6 Woodard & Curran, Inc. Sanitary Sewer Capacity Master Plan November 2023 3.2.2 Gravity Sewer Capacity Relief Projects Predicted surcharge in a particular pipe does not necessarily indicate a capacity deficiency at that particular location, as flows can back up due to a downstream capacity deficiency and cause extensive surcharging or even overflows upstream due to backwater effects. Relieving upstream deficiencies can also create additional or more severe capacity deficiencies downstream of the relieved pipe. Therefore, developing potential solutions to capacity deficiencies is an iterative process that involves progressively upsizing throttled sewers in the model in order to achieve a situation in which no sewers are in violation of the capacity deficiency criteria (water level higher than 5 feet below ground). As noted above, most of the predicted gravity sewer capacity deficiencies are located in the Main City Area collection system. The proposed improvement projects to provide relief for the identified capacity deficiencies are listed in Table 3-1 and shown in Figure 3-4 and Figure 3-5. For each identified gravity sewer capacity deficiency, a project was developed to replace the existing pipe with a larger pipe (or in some cases, a pipe in a slightly different alignment). Replacement pipes were sized to convey the future design storm peak wet weather flow (PWWF) at no more than 75 percent full. Existing pipe slopes and depths were preserved when upsizing sewers in-place. Model runs with all capacity projects in place were made to determine the impact of increased capacity from upstream projects on downstream peak flows to verify that no additional collection system capacity deficiencies would result. As noted in the table, 8 of the 25 gravity sewer capacity relief projects were also identified as being needed in the City’s previous master plans (although recommended pipe sizes or the exact extent of the projects may have differed slightly). The other projects are primarily on small diameter sewers that were not previously included in the trunk system model used in the previous studies. Note also that all or a portion of five of the capacity improvement projects identified in the City’s previous master plan have been completed since that study; however, a number of others were found to be no longer needed based on the updated modeling conducted for this study. 3.2.3 Pump Station Capacity The City operates 31 sewer pump stations. The pump stations were modeled simplistically (flow in equals flow out) to minimize the impact of pump cycling at upstream pump stations on flow reaching downstream stations. The pump stations were evaluated to determine if they had adequate firm capacity (with the largest pump assumed to be out of operation) to convey design storm peak wet weather flows. Note that except for PS 10, all the stations have two identical pumps. Firm capacity therefore represents the capacity of the station with one pump in operation. Table 3-2 compares the design storm PWWF to rated pump capacity for each of the City’s pump stations. The table indicates that all but one of the City’s pump stations have sufficient capacity to convey future design storm peak wet weather flows. Seaport #1 pump station is predicted to exceed firm capacity in the future, primarily due to the assumed potential development of two very large parcels in that area. However, because development of those sites is highly uncertain at this time, a capacity improvement is not included in this Master Plan. PS 09 in Redwood Shores is predicted to be at or very close to its pump capacity. The City has already designed and bid a project to upgrade the pump station and replace the station’s 4-inch force main (identified as Project C27). Note that the solution proposed in this Master Plan to address the gravity sewer capacity deficiency downstream of the PS 09 force main is to extend the force main to PS10 (Project C26) rather than construct a deeper gravity sewer replacement in this area of high groundwater and poor soils. ATTY/RESO.0077/CC RESO SEWER SYSTEM MANAGEMENT PLAN - EXHIBIT A REV: 07-14-25 LF Page 141 of 317