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Knight N sold
<br /> CONSULTING
<br /> Environmental Department, Meg Burt, Senior Manager October 8, 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, 9a, 16, 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 geometry may still require an
<br /> upgraded design. 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, which is less than
<br /> the required freeboard depth. The spillway inlet weirs for EMPs 16 and 21 are not located at the low point
<br /> of the respective pond crests. Spillways do not currently exist, or no spillway data were provided, for
<br /> EMPs 8a, 9b-d, 11, 13, 17, and 18. Each of these spillways was redesigned and provided to CC&V.
<br /> 2.5.2 EMP Impoundments
<br /> The results of the hydrologic analyses to estimate 2x the runoff volume generated from the
<br /> 10-year/24-hour storm event to each EMP based on the existing layouts are presented in Tables 2.7 and
<br /> 2.9. Those tables also present the as-built storage volume capacities of the EMPs for comparison
<br /> (measured from below the spillway inlet or 2 feet below pond crest if no spillway exists). The results
<br /> 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, 22, and the ECOSA toe berm
<br /> The Colorado Office of the State Engineer(OSE, 2007) Dam Safety Program's Rules and Regulations for
<br /> Dam Safety and Dam Construction distinguishes between a "Jurisdictional Size Dam" and a "Non-
<br /> jurisdictional Size Dam" as follows:
<br /> • Rule 4.2.5.1, "Jurisdictional Size Dam"
<br /> - Reservoir capacity greater than 100 acre-feet(ac-ft)
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