RES 2025036 - Laserfiche WebLink

Element 8 – System Evaluation, Capacity Assurance, and Capital Improvements City of Pleasanton Sewer System Management Plan, July 2025 32 was calibrated to the flow monitoring data to confirm that it would predict both dry and peak wet weather flows with reasonable accuracy. Potential increases in sewer flows were also estimated and included in the hydraulic analysis based on projected future development and land use changes within the sewer service area. 8.3.2: Design and Performance Criteria The City selected a 25-year return period, 24-hour rainfall event 1 as the basis for capacity assessment and design of its sanitary sewer system. This design event is considered to provide reasonable protection from the risk of sewer spills due to infrequent, extreme wet weather events. The calibrated hydraulic model was used to simulate peak flows that would be expected in the system under both normal dry weather flows and during a design storm event, for both existing and future development conditions. Pipes with flow exceeding a flow depth to pipe diameter ratio of 0.75 under peak dry weather flow were considered deficient and in need of upgrade. Areas with excessive predicted surcharge (i.e., hydraulic gradeline reaching within 3 feet of the ground, indicating a potential risk of a sewer spill) under design storm peak wet weather flow were identified, and required capacity improvements to alleviate those capacity deficiencies were developed. However, if any surcharge was predicted to occur solely due to future development, then the City may require a capacity relief project evaluation before additional development could be connected to the system. 8.3.3: Capacity Assurance The results of the hydraulic modeling indicated that the City’s sewer system has adequate capacity to convey peak dry weather flows under both existing and future conditions. The results for design storm model simulations indicated two areas of the system that may be considered capacity deficient under existing conditions and two areas that could become capacity deficient in the future due to increased flows from future development. The capacity assessment also determined that all of the modeled system pump stations have sufficient firm capacity (capacity with largest pumping unit out of service) to convey the predicted design storm peak flow without exceeding the maximum allowable surcharge in upstream sewers. The 2024 Sewer Capacity Evaluation identified four potential capacity improvement projects required to meet the City’s capacity criteria. In several cases, alternatives were also identified, which could be evaluated as part of additional pre-design studies. Required system capacity improvements would be designed in accordance with the City’s Engineering Standards (see Element 5). The recommended projects have been incorporated into the City’s long-term CIP. At the time of this SSMP Update, the City is evaluating potential development alternatives for the East Pleasanton Specific Plan area and respective impacts to sewer capacity. Any 1 Based on NOAA Atlas 14, Volume 6, point precipitation frequency estimates and spatial variation across the Pleasanton sewer service area. Reference: https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=ca.