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4.1 Maximum Instantaneous Peak water surface elevation and Static water surface elevation For both the "ca" and the "la" model configurations, the static and the instantaneous peak water surface elevations coincide and are 8.5 feet for all lakes. 4.2 Maximum Peak Water Surface Elevation The maximum peak water surface elevation in any of the lakes was 8.3 (lake 26, for Alt 6-L-ca-PS-D-48- d). Assuming that the planned lowest road elevations are expected to be around the same as in Phase 1 (10.5ft), the lake elevations for the analyzed scenarios would not be an issue. 4.3 Maximum stable water surface elevations As the pump stops pumping at the same elevations for both model configurations, the same stable water surface elevation is to be expected. The stable water surface elevation is 5.2 feet for all lakes and model configurations. Since the pumps stop pumping when the water surface elevation is 5.1 feet (first pump starts at 5.3 feet), 0.1-0.2 feet will evaporate naturally before the lakes reach their design water surface level of 5 feet. 4.4 Average Time to a 7 feet Water Surface Elevation The time to drain back to a 7 feet water surface elevation ranges from 0.8 days to 2.6 days. 4.5 Peak Duration and Time to Stable As section 4.4 suggests, the water surface elevation recedes down to 7 feet rather quickly with lakes 20- 26 taking the longest. It takes around 8 days for the water surface elevation to recede to its stable elevation. As Figure 4-3 and Figure 4-4 show, the draining velocity is more or less constant (constant slope in elevation vs. time graph) from the lakes' water surface elevation peak to five days after the storm after which the draining velocity is reduced. At the end of the storm when most of the stormwater runoff has been evacuated the flow between lakes and into the pump station wet well decreases, resulting in a slowdown in the drainage time for the last few inches in the lakes. The inflow to the Phase 2 pump station wet well varies with the fluctuating rainfall intensity and with the dynamic head difference between lakes. Since the pumps on-and off elevations are based on the wet well water surface elevation, there may be times where the water surface elevation in the wet well temporarily goes below the `stopping' elevation and then goes back up when the inflow to the wet well increases again. This was the case for the modelled scenarios presented in this report resulting in the last couple days' drainage only utilizing one of the pumps (25cfs) as the starting elevation for pump number two and three was not reached after a temporary low water surface elevation. For both the analyzed model configurations all pumps have stopped 8.613 days after the end of the storm. The pump flow (25cfs) at the end of the storm evacuation keeps up with the inflow to the wet well indicating that the drainage time would not be decreased significantly even if all pumps were used at the end (75cfs) as it likely would not significantly increase the flow between lakes several steps upstream of the wet well/pump station. 13 8.5 days and 8.6 days after the end of the storm event respectively for the"ca"and"la"model configurations. River Islands Phase 2 Stormwater Mitigation Rev 2 26 1� PACE