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4.6 Difference between the two hydrograph methods (ca and la) <br /> As Table 4-2below shows, the difference between the "ca" and the "la" hydrograph scaling method has a <br /> small impact on the peak water surface elevation. For lakes 14-19, the "la" method yielded up to 0.3ft <br /> higher peak (compared to "ca") and for lakes 20-26 the "ca" method yielded up to 0.3ft higher peak <br /> (compared to "la"). <br /> Table 4-2: Difference in water surface elevation between the two hydrograph scaling methods. <br /> Alt6-LAKE-V20-ca-PS-D-48-d Alt6-LAKE-V20-la-PS-D-48-d N/A <br /> Lake PEAK WSEL PEAK WSEL awSEL Lake <br /> [FEET] [FEET] [FEET] <br /> 14 7.4 7.6 0.2 14 <br /> 15 7.5 7.7 0.3 15 <br /> 16 7.4 7.7 0.3 16 <br /> 17 7.4 7.7 0.3 17 <br /> 18 7.4 7.7 0.3 18 <br /> 19 7.4 7.6 0.2 19 <br /> 20 8.0 7.8 -0.2 20 <br /> 21 8.0 7.8 -0.2 21 <br /> 22 7.9 7.8 -0.1 22 <br /> 23 8.2 8.1 -0.1 23 <br /> 24 8.2 8.1 -0.1 24 <br /> 25 8.2 8.1 -0.1 25 <br /> 26 8.3 8.0 -0.3 26 <br /> The difference between the "la" and "ca" methods is reasonable when considering how all the water was <br /> distributed throughout the lake system. Phase 2 can be divided into two sets of smaller lake systems. <br /> Lakes 14-19 all drains to the Phase 2 pump station via lake 19. Lakes 20-26 all drains to the Phase 2 <br /> pump station via lake 22. For the"la" scenarios, lakes 14-19 combined received more water than they did <br /> for the "ca" scenario and vice versa for lakes 20-26 See Section 2.4 for more details. <br /> 4.7 Outfall manifold velocity <br /> In addition to the lakes' drainage time and maximum water surface elevation for the different scenarios, <br /> the maximum velocity in the manifold pipes conveying the stormwater above the levee to the river was <br /> analyzed. Table 4-3 below shows the velocity in the three outfall pipes for the eight modelled scenarios. <br /> As the flow to the manifold are determined solely by the pumps at the Phase 2 pump station, the velocity <br /> in the outfall manifold pipes is the same for both scenarios. <br /> Table 4-3: Maximum velocities in the outlet manifold. <br /> Model configuration Manifold/outlet maximum velocity [fps] <br /> 24" 0 pipe 18" 0 pipe 24" 0 pipe <br /> AIt6-L-V20-ca- PS-D-48-d 9.2 9.6 9.2 <br /> Alt6-L-V20-la-PS-D-48-d 9.2 9.6 9.2 <br /> River Islands Phase 2 Stormwater Mitigation Rev 2 27 <br /> PACE <br />