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Cutoff Wall Crest Pumping Rate' Containment Pool Residual <br /> Elevation Peak WSEL Freeboard <br /> (ft) (gpm) (cfs) (ft) (ft) <br /> G: 0.56 10362 1 0 <br /> C 0.56 10362.1 <br /> - 1 10361.6 <br /> The model results support multiple configurations of the pumping system, but it is <br /> recommended a Cutoff Wall crest elevation of 10363.5 with a total pumping rate of 250 gpm <br /> (this assumes a large pump capacity of at least 200 gpm and small pump capacity of at least 50 <br /> gpm). A"worst-case scenario" model was also evaluated,with 100%impervious soil and no <br /> initial surface storage, and the cutoff wall crest of 10363.5 provides more storage to contain the <br /> additional runoff volume, whereas 10362.5 does not (results in overtopping at crest 10362.5). <br /> While this scenario is very unlikely, it does suggest that the extra foot of concrete provides <br /> significant contingency for the containment system which is why this configuration is <br /> recommended. In the event that flows result in the overtopping of the cutoff wall crest, this <br /> water will safely spill over the rear concrete wall into the adjacent/existing 120-inch diameter <br /> CMP that conveys water under Old Highway 91 (as it did prior to construction and operation of <br /> this EPF). <br /> • Hydrology study and modeling conclusions.This section of the narrative detailed the hydrologic <br /> analysis used to size pumps and detention storage for the Secondary Containment System. <br /> Based on the pump sizing results and discussion, the following are conclusions from the <br /> evaluation: <br /> 1. The cutoff wall crest elevation should be set at 10363.5. <br /> 2. A target total pumping capacity of at least 250 gpm (-0.56 cfs) should be employed <br /> through a two-pump installation. <br /> a. The first pump should have a capacity of at least 50 gpm and use a VFD to <br /> manage baseflows and reduce stress on the pump. Wheeler intends this pump <br /> to be in near-continuous use. <br /> b. The second pump should have a capacity of at least 200 gpm.The pump should <br /> activate at elevation 10359.5 to intermittently manage heavy rainfall and <br /> periods of large snowmelt. <br /> 3. Construct an 18-inch diameter connection conduit between the Secondary Containment <br /> Pool and wet well. <br /> Secondary Containment Concrete Cutoff Construction <br /> • Drawing No. 6-807-00104 depicts a plan and section view of the secondary containment system. <br /> • Some minor vegetation clearing is expected where the cutoff structure and 18-inch diameter <br /> CHDPE intake pipe will be constructed. Limited grading and vegetation removal work in the <br /> existing drainage channel is anticipated. <br /> • Dewatering will occur during construction and is expected to involve a temporary sump (just <br /> upstream of the project work) and pumping system to pump run-on water to the Mayflower <br /> Clear Pond. <br />