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Knight R sold
<br /> CONSULTING
<br /> Environmental Department, Meg Burt, Senior Manager April 27, 2018
<br /> Cripple Creek and Victor Gold Mining Co. (Newmont)
<br /> The existing spillway results indicate the following (Tables 2.5, 2.6, and 2.9):
<br /> • The as-built inlet weir flow capacity is greater than the required 100-year/24-hour storm event peak flow
<br /> for the following EMPs:
<br /> - 6, 16, 17a, and 17b
<br /> • The as-built inlet weir flow capacity is less than the required 100-year/24-hour storm event peak flow
<br /> and thus, design modifications will be required for the following EMP spillway inlets:
<br /> - 8b, 8c, 9a,20, 21, and 22
<br /> • The as-built chute flow capacity is greater than the required 100-year/24-hour storm event peak flow for
<br /> the following EMP:
<br /> - 16
<br /> • The as-built chute flow capacity is less than the required 100-year/24-hour storm event peak flow and
<br /> thus, design modifications will be required for the following EMP spillway chutes:
<br /> - 6, 8b, 8c, 9a, 17a, 17b, 20, 21, and 22
<br /> • The as-built chute riprap safety factor is greater than the required factor of 1.20 based on the
<br /> 100-year/24-hour storm event peak flow for the following EMPs:
<br /> - 8b, 8c, and 17b
<br /> • The as-built chute riprap safety factor is less than the required factor of 1.20 based on the
<br /> 100-year/24-hour storm event peak flow and thus, design modifications will be required for the following
<br /> EMP spillway chutes:
<br /> - 6, 16, 9a, 17a, 20, 21, and 22
<br /> Where the as-built flow capacity was deemed sufficient, but the as-built riprap size was deemed
<br /> insufficient based on the 100-year/24-hour storm event, the channel may still require an upgraded design.
<br /> This is because larger riprap may cause the channel to have insufficient flow capacity.
<br /> The spillway inlet weirs for EMPs 9a and 22 were found to be less than 1.0-foot deep. Because this is
<br /> less than the required freeboard depth, these spillway inlet weirs, chutes, and riprap were not evaluated
<br /> and will be redesigned during the next stage of the project. The spillways for EMPs 16 and 21 are not
<br /> located at the low point of the respective pond crests and therefore, they will also be redesigned.
<br /> Spillways are absent, or no spillway data were provided, for EMPs 8a, 9b-d, 11, 13, 17, and 18. Spillway
<br /> designs will be provided for these structures.
<br /> 2.5.2 EMP Impoundments
<br /> The results of the hydrologic analyses to estimate 2x the runoff volume generated from the 10-year/24-
<br /> hour storm event to each EMP based on the existing layouts are presented in Tables 2.7 and 2.9. Those
<br /> tables also present the as-built storage volume capacities of the EMPs for comparison (below spillway
<br /> inlet or 2 feet below crest if no spillway). The results indicate the following:
<br /> • The as-built storage volume capacity is greater than the required 2x the 10-year/24-hour storm event
<br /> runoff volume and thus, impoundment design modifications are not required for the following EMPs:
<br /> - 8b, 16, 17/17a/17b(combined), 18,20 and 21
<br /> • The as-built storage volume capacity is less than the required 2x the 10-year/24-hour storm event runoff
<br /> volume and thus, impoundment design modifications will be required for the following EMPs:
<br /> - 6, 8a, 8c, 9a-d (combined), 11, 13, and 22
<br /> Where required, the design modifications may simply entail removing accumulated sediments within the
<br /> impoundments. Others may require an expansion of the EMP impoundment footprint.
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