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The following table summarizes the basin hydrology results used to determine the governing <br /> storm. <br /> Peak Direct Runoff Containment' Time to Peak <br /> Storm Discharge Volume Pool Peak WSEL Inflow/Discharge' <br /> (cfs) (acre-feet) (ft) (HH:MM) <br /> 100-yr ARI, 2-hr LS 5.73 0.33 10361.7 00:28 <br /> 100-yr ARI, 6-hr MEC 5.31 0.38 10361.5 01:03 <br /> Modeled with 6-inch outlet conduit outflow(see Attachment 5 for other configurations) <br /> The above table shows that the governing storm for this design is the 100-year ARI, 2-hour Local <br /> Storm, as it results in a higher peak water surface elevation in the Containment Pool. While the <br /> 100-year ARI, 6-hour MEC Storm has a higher total storm volume, the average discharge over <br /> the storm is much lower than the 2-hour storm and is not as limiting when routing flow out of <br /> the Containment Pool. A 40-foot-long conduit will route water from the Containment Pool to <br /> the wet well.This conduit must convey flow while neither limiting the flow rate into the wet <br /> well, nor overtopping the cutoff wall.The analysis shows that 6-inch diameter and larger <br /> conduits can meet both objectives at cutoff wall crest elevation of 10363.5. Increases to the <br /> cutoff wall height does allow for a smaller conduit, but an 18-inch conduit is included in the <br /> design due to concerns with clogging at the inlet. <br /> The second model used the governing storm determined from the first model (2-hour 100-year <br /> storm).The second model was iterated at specified cutoff wall crest elevations to determine the <br /> minimum pumping rate required to prevent a loss of containment.The required pumping rate is <br /> a function of three items: the peak inflow rate,the volume into the Containment Pool, and the <br /> pump activation elevation. A lower cutoff wall requires a larger pumping rate to convey inflow <br /> without a loss of containment, and the converse for a taller cutoff wall. Peak discharge from a <br /> 100-year ARI, 2-hour Local Storm occurs very quickly, requiring the pump to manage most of the <br /> storm volume immediately at smaller cutoff wall heights. A taller cutoff wall reduces the pump <br /> size by providing more detention storage to mitigate the impact of the peak discharge. <br /> Additionally, the pump activation elevation factors into how much time and/or elevation the <br /> pump has before it must route inflows out of the Secondary Containment Pool. At lower wall <br /> elevations, the activation elevation must also decrease, to provide enough time for the pump to <br /> respond to the rapid peak inflow. It was concluded that a reasonable option would be to <br /> incorporate two pumps in the design: one to manage baseflow into the Secondary Containment <br /> Pool, and another to route larger inflows from rainstorms or periods of large snowmelt.The <br /> small pump would use a variable frequency drive (VFD)to reduce the stress on the pump.The <br /> large pump would activate at a certain elevation above the normal pool.This two-pump system <br /> resolves issues with balancing the pump activation elevation for a single large pump, while <br /> reducing stress through the use of a VFD.The selected pump activation trigger elevation for <br /> Model 2 (for the larger pump) is 10359.5, a half-foot above the normal pool.This provides <br /> enough elevation so the large pump will not activate at baseflow-scale flows, but will activate <br /> soon enough to capture larger storm events.The below table presents the modeled pumping <br /> rates at various cutoff wall crest elevations. <br />