ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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Near and Far Field Dilution Analysis of Manteca Discharge October 2006 Thermal Modeling The model used for the current study was used previously to simulate temperature for the Thermal Plan Exception study(LWA, 2006). The temperature simulations were performed using the 3-D version of the model with a side bank discharge. Winter, Historical Low Flow Temperature Simulations The winter low flow condition was simulated for the Thermal Plan Exceptions study to examine the critical condition for temperature impacts. The differential between the effluent and ambient temperature is the greatest and minimum river flow results in lower dilution of the effluent plume. For this period, the difference between Manteca effluent temperatures and ambient river temperatures was approximately 17°F. The combination of low flow and strong spring tide for this simulation provide an opportunity to investigate the effects of recirculation. December 2002 stage at the Lathrop CDEC station was applied to the downstream boundary. Temperature and EC at the downstream boundary were computed within the model. Temperature and EC from the Vernalis CDEC station for this period were applied at the upstream boundary. San Joaquin River flows averaging 900 cfs, representative of historical extreme low conditions, were applied at the upstream boundary. An intermittent Manteca daily discharge rate of 9.87 MGD with a temperature of 66 °F (based on analysis of valve pit temperatures)was used. Heat exchange was computed using heat budget parameters calibrated for this period. Additional preliminary simulations were run with 28 MGD Manteca discharge. These simulations were not discussed in the Thermal Plan Exception. Historical low flow simulations with zero Manteca discharge were also performed so that incremental temperature changes resulting from the discharge could be computed. Typical model results are shown in Figure 28 through Figure 30 on December 6 at a time when the fraction of the channel cross section with temperature impacts of P F or more is at a peak. The first plot shows 3-D color contours of incremental temperature change. A plan view of surface temperature increments and profile cross sections at this time are shown in Figure 29 and Figure 30, respectively. For the time shown in these plots, the percent of the cross-sectional areas at each of these locations with temperature increments greater than 1°F are summarized in Table 2. The maximum area impacted is 22% at 1,000 ft downstream of the discharge. Similar plots from the preliminary 28 MGD simulation are shown in Figure 31 through Figure 33 for December 3, 2002. For the time shown in these plots, the percent of the cross-sectional areas at each of these locations with temperature increments greater than P F are summarized in Table 2. The maximum area impacted is 51% at 1,500 ft downstream of the discharge. 30