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River Islands Development at Lathrop November 2020 Final Phase 2 Sewer Technical Report Rev 1 #6520 2.2.2 Phase II Recycled Water Flushing System The construction and occupancy of River Islands is expected to be completed in separate stages lasting until final buildout in 2040. The primary conveyance sewer mains (also known as trunk lines), will be required to be designed for full buildout conditions and any tributary collection areas. As a result, these trunk lines will initially be subject to low flows which can lead to sediment/debris buildup, and potentially increase corrosion and odor issues through the formation of anaerobic conditions. It is recommended that an additional two recycled water flushing stations be included in the design Phase II's sewer infrastructure to address interim low flow conditions. . The stations are recommended to provide sufficient recycled water to scour and flush out the accumulated sediments/debris (5-6 feet per second) and to decrease the occurrence of continued low flow conditions. The sizing of flow control valves will need to consider limiting flushing flows to prevent overwhelming/surcharging the downstream collection system. Similar to the existing flushing stations, the proposed recycled water flushing stations should be designed with a remote control panel allowing operators to control flushing events at set time intervals. This situation will also require increased monitoring of sediment and odor by City staff to ensure the flushing program is adequate. 2.2.3 Interim and Emergency Bypass System — Phase II At full buildout and under normal operating conditions (see Section 6), the River Islands sewer collection system will have a northern and southern trunk lines designed to be the primary means of conveying the generated sewage offsite. As shown in Figure 2-1, northern trunk line will be responsible for conveying sewage from Collection Areas C1, C2,Al and portions of A2 and the southern trunk line will be responsible for conveying sewage from Areas B1, B2 and the southern portion of Area A2, primarily consisting of the employment centers. The remaining portions of the development will continue to be constructed in a series of separate stages, with the portions of collections areas C2, B1 and eventually C1 proposed to be constructed first. To prevent having to oversize or design the sewer lift stations to accommodate the interim conditions, bypass force mains as shown in Figure 2-1, will be constructed allowing for LS-C2 to pump to Area's B1 collection system and LS-B1 to pump into Area's Al collection system. These interim force mains should be designed as a permanent system allowing for the City's operations staff to use them as emergency bypass force mains should the need arise in the future. During the design phases of the Phase II collection system, it is recommended that the River Islands sewer collection model be re-run for future interim and emergency conditions to determine the capacity limitations for each scenario. To be discussed further in Section 7, collection area's Al and A2 sewer infrastructure was designed with additional capacity to accommodate potential tributary flows. Re-running the model for interim conditions will allow for capacity verification of the existing Al and A2 infrastructure. 11