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Hydraulic Model Development and Calibration City of Redwood City (0011239.02)2-14 Woodard & Curran, Inc. Sanitary Sewer Capacity Master Plan November 2023 2.3.2.4 BWF Projections Table 2-3 summarizes the existing and future BWF for residential and non-residential land use categories. Note that based on these estimates, it appears that BWF in Redwood City could increase by over 50 percent due to potential future development and redevelopment. However, it should be noted that estimated flows from future development are based on the City’s design sewage generation factors (see Table 2-2 Error! Reference source not found.), which are generally higher than existing sewage generation rates. Table 2-3:Base Wastewater Flow Summary Type Existing BWF (mgd)Future BWF (mgd) Residential 3.49 4.69 Non-Residential 1.13 3.36 Total 4.62 8.05 Note: Includes entire Redwood City service area (including Redwood Shores and Seaport/Pacific Shores), as well as Emerald Lake Heights and smaller County SMDs tributary to City sewer system, but not including flows from Fair Oaks SMD, which are metered separately. 2.3.3 Groundwater Infiltration GWI represents a seasonal increase in wastewater flows due to infiltration of groundwater into the sewers, typically in low-lying areas or areas close to creeks or other water bodies such as Redwood Creek and/or San Francisco Bay. GWI is applied to the model as a constant flow in addition to the BWF. The amount of GWI to apply in any particular area of the sewer system is determined during model calibration by comparing the modeled dry weather flow (periods minimally impacted by RDI/I from previous rainfall) to actual observed (monitored) flows at the flow meter sites. Where modeled BWF is less than monitored dry weather flow, the difference is assumed to represent GWI. The estimated GWI for each flow meter area is then applied on a per acre basis to the subcatchments within that flow meter area. For this model, GWI was estimated based on a flow per unit of “contributing” area (acre). The contributing area includes only the acreage of parcels with sewer connections (e.g., parcels that are not vacant or open space). Note that because GWI is seasonal in nature, the modeled GWI is intended to represent a typical GWI rate during the wet weather season rather than a dry season (summertime) GWI. 2.3.4 Rainfall-Dependent I&I RDI/I results from rainfall events that produce infiltration and inflow of storm water runoff into the sewer system. RDI/I varies depending on many factors including the magnitude and intensity of the storm event, area topography, type of soil, and the condition of the sewers, manholes, and service laterals. RDI/I can be expressed as a volume or a percentage of rainfall volume (termed the “R value”) entering the sewer system from subcatchment contributing areas for each of several flow components representing different response patterns to rainfall events (e.g., fast, medium, slow). Typically, three response components are used, as illustrated in Figure 2-11. Each component is identified by a percentage of the total RDI/I volume and specific time to peak (T) and recession coefficient (K). The R component percentages and T and K values can be applied to each hour of rainfall to generate a “synthetic hydrograph” that approximates the volume and shape of the hydrograph from the actual observed event. The “fast” component of the hydrograph has the largest impact on the magnitude of the peak wet weather flow response, while the slower components can contribute significantly to the total volume of the RDI/I response. The model is used to transform rain ATTY/RESO.0077/CC RESO SEWER SYSTEM MANAGEMENT PLAN - EXHIBIT A REV: 07-14-25 LF Page 130 of 317