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River Islands Development at Lathrop November 2020 Final Phase 2 Sewer Technical Report Rev 1 #6520 1. Introduction 1.1 Background River Islands, located in the Stewart Tract within the City of Lathrop, is a 4,905-acre, master planned community being constructed southwest of the San Joaquin River. The River Islands development will consist of low, medium,and high density residential,school (institutional), parks, and commercial land uses. River Islands will be constructed in two separate phases known as Phase I and Phase II. Phase I is currently being constructed over 4 separate stages and Phase II is entering the master planning stages. Currently, the generated sewage from the constructed portions of River Islands is conveyed to an interim sewer pump station (PS) located between Lakes 1 and 2. The interim sewer PS initially pumped the development's sewage to the Mossadale collection area located just west of Interstate-5 (1-5). From here it flowed to the City of Lathrop's Consolidated Treatment Facility(CTF) for treatment. The PS force mains have now been extended to connect directly to the CTF, without flowing through the Mossdale collection system. At full buildout, the River Islands sewer collection system will consist of six separate sewer collection areas based on the service area of the six sewer lift/pump stations. The distinction between lift and pump stations are that lift stations raise the hydraulic grade line (HGL)of the sewage, allowing it to flow by gravity into an adjacent collection area, while pump stations transmit the sewage under pressure. The collection areas will be designed to convey the generated sewage from each service point of connection (POC) to the corresponding sewer lift/pump station by gravity. All of the generated sewage from River Islands will eventually be conveyed to sewer pump station A2 (also known as Site C) located in the southeast of the development. The A2 pump station will pump raw sewage directly to the Lathrop CTF through the existing 12 and 18-inch force mains. River Islands sewer collection system will be required to be planned, designed, and constructed per the City of Lathrop's 2019 Design and Construction Standards (City Standards). The City Standards were created to provide the minimum requirements for all facilities and appurtenances to be turned over to the City for operation and maintenance. 1.2 Report Objectives The objectives of the River Islands Phase 2 Sewer Technical Report are as follows: • Provide updates to the previous Phase I Sewer Master Plan prepared by PACE in 2017. • Analyze and model the average dry weather sewer flows (ADWF)for Phase I and II in accordance with the City Standards under Steady State Conditions. • Analyze and model the peak wet weather sewer flows (PWWF) for Phase I and 11 in accordance with the City Standards, under Steady State Conditions. • Analyze and model the pipe velocities under PWWFs for Phase I in accordance with the City Standards, under Steady State Conditions. • Describe the River Islands sewer collection system including the conveyance networks, locations of proposed sewage lift or pump stations, and pumping between collection areas during emergency, interim, and low-flow conditions. • Provide a preliminary firm pump capacity requirement for the proposed Phase II sewer lift stations. • Present the potential impact Phase II sewer flows will have on Phase I. 6