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Near and Far Field Dilution Analysis of Manteca Discharge October 2006 <br /> Chronic Simulations <br /> For the Chronic simulations, the San Joaquin River flow was set constant at 615 cfs. The <br /> downstream tide was set the same as for the Acute simulation. <br /> Under the low flow conditions of the Chronic simulation, timed treatment plant discharge <br /> is required and was computed using the method described above for the Acute <br /> simulation. <br /> For the tidally averaged dilution plots for the Chronic condition discussed below, dilution <br /> was computed using tidally averaged effluent concentration. This provides a more <br /> accurate assessment of dilution in the immediate vicinity of the discharge. However, <br /> further downstream, where the effluent plume is moving back and forth with the tide, this <br /> method may not be as accurate as if instantaneous effluent concentration were used in the <br /> dilution calculations. <br /> Tidally averaged effluent dilution was computed for each scenario and the time of lowest <br /> dilution was selected for plotting. Figure 15 shows the lowest tidally averaged dilution <br /> profiles along the centerline of the effluent plume for the three side bank discharge <br /> scenarios. Figure 16 shows the hourly average dilution profiles along the centerline of <br /> the effluent plume for the three diffuser scenarios for the same time the side bank results <br /> are plotted(also the lowest for the diffuser scenarios). <br /> At this time, the lowest dilution for the side bank outfall case occurs at the outfall where, <br /> using tidally averaged effluent concentration in the dilution calculations, there is almost <br /> no dilution for all flow scenarios. At 10,000 ft downstream of the discharge, dilution is <br /> approximately 16:1 for the 9.87 MGD simulation, 9:1 for the 17.5 MGD simulation, and <br /> 6:1 for the 27 MGD simulation. <br /> For the diffuser case, the lowest dilution occurs at approximately 4,100 feet downstream <br /> of the diffuser. Although the lowest instantaneous dilutions occur at the diffuser, on a <br /> tidally averaged basis, the lowest dilutions occur further downstream. One factor <br /> influencing this phenomenon is the initial dilution achieved by the diffuser. The other <br /> factor is that while the lower dilutions at the diffuser disappear nearly as soon as <br /> discharge stops, the effluent plume is persistent further downstream as it is moved back <br /> and forth with the tide. Therefore, when the results are averaged over the tidal cycle, <br /> times of zero concentration (when there is no discharge) are included for nearly half of <br /> the averaging period at the diffuser, whereas at around 4,100 feet downstream, lower <br /> dilutions are present over the majority of tidal cycle. The phenomenon of minimum <br /> dilution at a downstream location is not seen in the side bank discharge case because <br /> there is almost no dilution at the outfall (this is true even on a tidally averaged basis <br /> because tidally averaged effluent concentrations are used in the dilution calculations). <br /> Even though the plume is persistent over the tidal cycle at the downstream location, it <br /> does not produce dilutions as low as those seen near the side bank discharge. At 4,100 <br /> feet downstream, the side bank and diffuser dilution values are very similar. <br /> For the 9.87 MGD diffuser simulation,the lowest dilution is approximately 12:1, for the <br /> 17.5 MGD simulation 6:1 and for the 27 MGD simulation 4:1. At 10,000 ft downstream <br /> of the discharge, dilution is the same as for the side bank scenario. <br /> 12 <br />