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Near and Far Field Dilution Analysis of Manteca Discharge October 2006 <br /> If the approaching flood(rising) tide encompasses a slack tide or flow reversal (San <br /> Joaquin River flow less than 400 cfs in the downstream direction), then the discharge <br /> stops at least 30 minutes before the minimum low tide. <br /> When the approaching flood tide does not include a period with less than 400 cfs <br /> downstream river flow, then the discharge is stopped when the downstream flow at the <br /> Mossdale Bridge station drops below 1,200 cfs. <br /> Using this method, the hours per day during which discharge was allowed were summed <br /> and the respective instantaneous discharge rates for the 9.87 MGD, 17.5 MGD and 27 <br /> MGD were determined and applied during times of discharge. For example, if discharge <br /> were allowed for 12 hours during a day, the instantaneous discharge rate during those 12 <br /> hours would be 19.74 MGD or 30.5 cfs under the 9.87 MGD discharge scenario. <br /> Hourly average effluent dilution was computed for each scenario and the times of lowest <br /> dilution were selected for plotting. Figure 9 shows the lowest hourly average dilution <br /> profiles along the centerline of the effluent plume for the three side bank discharge <br /> scenarios. At this time, the lowest dilution occurs at the outfall where it is nearly pure <br /> effluent for all flow scenarios. At 10,000 ft downstream of the discharge, dilution is <br /> approximately 37:1 for the 9.87 MGD simulation, 21:1 for the 17.5 MGD simulation, and <br /> 13:1 for the 27 MGD. The highest dilution occurs at approximately 4,000 ft downstream <br /> of the discharge. This area of high dilution results from the timed discharge. Because <br /> hourly averages are shown, the water that passed by the discharge point when no <br /> discharge was occurring shows up downstream at a time when the lowest dilution occurs <br /> at the discharge. If these results were tidally averaged, the dilution peak would <br /> disappear. <br /> Figure 10 shows the hourly average dilution profiles along the centerline of the effluent <br /> plume for the three diffuser scenarios for the same time the side bank results are plotted <br /> (also among the times of lowest dilution for the diffuser scenarios). At this time, the <br /> lowest dilution occurs at approximately 700 feet downstream of the diffuser. For the 9.87 <br /> MGD simulation, the lowest dilution is approximately 11:1, for the 17.5 MGD simulation <br /> 6:1 and for the 27 MGD simulation 4:1. At 10,000 ft downstream of the discharge, <br /> dilution is the same as for the side bank scenario. <br /> In Figure 11, a plot of plume centerline dilution profiles for the 9.87 MGD cases with the <br /> side bank outfall and with the diffuser shows that the profiles only differ significantly <br /> within about 2,000 feet of the discharge point. The offset between the two results further <br /> downstream is primarily due to the slightly longer plume centerline path for the diffuser <br /> case. <br /> Plan view color contour plots of effluent dilution at the same time shown in Figure 9 <br /> through Figure 11 are shown in Figure 12 through Figure 14 for each of the discharge <br /> rates. <br /> The lowest dilution values at 45' down the plume centerline from the point of discharge <br /> are summarized in Table 1. For the side bank discharge, near the outfall, the plume <br /> centerlines follow different paths depending on the discharge rate, because the higher <br /> discharge pushes the plume further into the channel than the lower discharge. <br /> 11 <br />