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River Islands Development at Lathrop November 2020 Final Phase 2 Sewer Technical Report Rev 1 #6520 7. River Islands Sewer Collection System Model — Phase I Results 7.1 Sewer Collection Area Al 7.1.1 Sewer Collection Area Al —Land Uses,Average Dry Weather Flows, and Peak Wet Weather Flows Sewer Collection Area Al is located in the western portion of Phase I and is comprised of development Stages 2A and 2B of Phase 1. Area Al encompasses approximately 785 acres composed of low and medium density residential units as well as parks, a K-8 school, and a high school. The calculated ADWF from collection area Al is 603,302 GPD as shown in Table 7-1. At full buildout and under normal operating conditions, the proposed LS-C2 will discharge sewage into Al's collection system along River Islands Parkway. ADWF from collection areas C1+C2 and Al produces a PWWF of 2,306,068 GPD to Al lift station (previously referred to as Stage 2A lift station). Table 7-1: Sewer Collection Area Al — Land Uses, ADWF, and PWWF Land Use Area Al Sewer Flow Average Dry Total No of DU Factor Weather Flow Land Use or Acres GPD/du or ac GPD Low Density Residential (per DU) 1,998 200 399,600 Medium Density Residential (per DU) 723 180 130,140 High Density Residential (per DU) 312 170 53,040 Commercial (per acre) 0.0 590 0 Industrial (per acre) 0.0 355 0 Parks (per acre) 88.0 55 4,842 Schools/ Institutional per acre 64.0 1 245 15,680 Total Area Al Average Dry Weather Flow 603,302 From Collection Area C1 + C2 Average Dry Weather Flow 509,409 Total Average Dry Weather Flow 1,112,710 Peaking Factor 2.07 Peak Wet Weather Flow' 2,306,068 Notes: 1. PWWF shown is based on the ADWF from areas C1+C2 and Al conveyed to the LS-A1 7.1.2 Sewer Collection Area Al — Pipe Capacity Under PWWF Figure 7-1 shows the PWWFs of all the pipes within Area Al and the corresponding pipe capacities in accordance with the City's Standards. None of the pipes exceeded the maximum capacity(Depth/diameter ratio) requirement described in Section 4.1.1. 7.1.3 Sewer Collection Area Al — Pipe Velocities Under PWWF Figure 7-2 shows the velocity of each pipe within Area Al under PWWFs. None of the pipes exceed the maximum allowable velocity list in the City Standards. As shown in Figure 7-2, 6 segments of pipe are estimated to have a velocity less than 2 feet per second with the line being at least half full under PWWF conditions. All 6 of these pipes are located along or in close proximity to the eastern trunk line. Phase I infrastructure was design in accordance with the City's 2014 Standards which utilized higher sewer generation rates than the 2019 City Standards, reflecting historical trends seen in the City. The few segments in which the model determined low sewer velocities may occur, can be attributed to the reduction in sewer generation rates as required by the 2019 City 26