PERMFILE63825

• Permit No: C1981035A
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Doc Name: MODIFICATION A
• Section_Exhibit Name: APPENDIX 8 HYDROLOGIC DESIGNS
• Media Type: D
• Archive: Yes

• conditions. The modeling scenario consisted of the pond, initially empty, with
subsequent inflow of the 10yr and 25yr events, back to back.
To accommodate the required 10yr storage the east and west ponds are connected
by a 8" (minimum) diameter pipe which will control the flow between the ponds. The end
of the pipe in the west pond will be a vertical stand pipe with a trash rack around the
opening and at the elevation specified below. Either schedule 40 pvc or corrugated
metal pipe will be adequate. The pipe will be buried beneath the west pond
embankment and under the mine entrance road and should enter the east pond through
its embankment. A minimum slope of 0.5% should be maintained on the horizontal run
to assure it completely drains. The outlet should be covered with a trash rack. The
elevation of the inlet of the connector pipe in the west pond will be set such that the west
pond completely fills before any flow is diverted to the east pond. The spillway crest of
the west pond is at elevation 7500.0; the top of the connector pipe is at 7499.7 and the
low outlet is at 7494.5. For the east pond the spillway crest is at 7497.0 and the low
outlet is as 7492.0.
The required volume of the 10yc-24hr event for the west pond is 1.96 AF and the
annual sediment load is estimated at 0.10 AF, resulting in a total required pond volume
of 2.O6AF. The volume in the west pond at the time of the as-built surveys was 1.86 AF
below the existing spillway crest. The required volume of the 10yr-24hr event for the
• east pond is 0.19 AF and an annual sediment load of 0.01 AF, resulting in a required
pond volume of 0.20 AF. The volume in the east pond at the time of the as-built surveys
was 0.92 AF below the spillway crest. Combining the volumes for the ponds gives a
total required storage volume is 2.26 AF and an existing capacity of 2.78 AF.
Each pond will have its own outlet works. Sizing of the low level outlet is controlled
by the requirement that the 10yr volume be drained out in 48 hrs. To achieve this the
HEC-1 model was used starting with an initially full pond and no inflow. The orifice size
was varied until the drain time approximately equaled 48 hours. The outlets consists of a
3.1" (west) and 2.0°(east) diameter orifice plate connected to a culvert with a discharge
rate that varies with the elevation of the water surface in the pond. The peak stage in
the west pond was 7499.97 ft which is just below the crest elevation of 7500.0 ft. The
peak stage for the east pond was 7496.91, below the crest of 7497.0. This combination
of spillway crest elevations and low outlet elevations assures storage of the 10yr flood
volumes.
Spillway crest lengths were determined by modelling the 25yr-24hr event with the
initial water surface elevation set at the elevation of the top of the connector pipe,
7499.7fl for the west pond and at the spillway crest elevation 7497.0 for the east pond
(ponds full with 10yr event). The inflow storm hydrograph was routed through the ponds
and the spillway lengths were adjusted until they passed the 25yr peak inflow with a rise
above the crest not more than 0.5 ft. Both spillways are five feet long sharp crested
•
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January 1998