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River Islands Development at Lathrop January 2021 Final River Islands Potable Water Technical Report—Rev 3 #6520 4.2 City of Lathrop's Potable Water Distribution System —River Islands The River Islands development is located within the southwest area of the City of Lathrop's service area and will utilize the City's PW distribution system for domestic, commercial, specific irrigation areas, and school PW demands. The distribution system will also service fire flows within the development. The City's distribution system operates as a single pressure zone, pressurized from different booster pump stations and storage sites spread throughout the City. The River Islands development will expand the City's distribution system to contain two additional City owned and operated PW booster pump stations and water storage tank sites at full buildout conditions, in addition to the SSJID Turnout#2 and 1 MG Tank currently under construction. The following sections will describe the exist/proposed infrastructure, as of the City's PW distribution system, to supply potable water supply, store, and pressurize the system within River Islands and the associated modeling assumptions under steady state conditions. 4.2.1 Potable Water Supply—City of Lathrop's Potable Water Distribution System River Islands currently has two connections to the City PW distribution system (see Figure 5-1). The first connection is the 16-inch line under Stewart Road, installed as part of first stage (Stage 1A) of the Phase I development. In June 2017, the second 20-inch connection was installed beneath River Islands Parkway which connected River Islands with the City's PW distribution network east of Bradshaw Crossing Bridge. These connections allow for the City's distribution system to supply PW directly to end users, or to on-site storage tanks. While the City's distribution has the ability to supply water directly to the end users, this operating scenario shall not be considered under normal operating conditions at full buildout, and the City's distribution system within River Islands shall be designed to be pressurized from the proposed Phase I and II BPS and Storage sites. As described in Section 4.2.3.1.1, the City's distribution system will supply potable water to the on-site Phase I and Phase II storage tanks during off-peak windows. As long as sufficient distribution pressure is maintained, a combination back pressure sustaining tank fill valve will allow for the distribution system to backfill the tanks. The two connections to the City's PW distribution system were modeled as elevated reservoirs with a hydraulic grade line(HGL)of 107.6 feet with a check valve preventing flow from entering the reservoir. The HGL of 107.6 feet was chosen as it models an approximate pressure of 40 pounds per square inch (psi) at the points of connection, which will allow for the model to determine if the Phase I and II BPS can supply (pressurize)the City's distribution system within River Islands. 17