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1. The Technical Memorandum describes the general storm water management plan and
estimates the size of storm water diversion and collection channels, culverts, and sedimentation
ponds. The Project Area is situated near a divide and slopes generally to the east. Whitelaw
Creek has its origins near the western side of the Project Area and flows in an east and
northeast direction to a confluence with Beaver Creek northeast of the site.
Peak storm water flow rates contributing to, or otherwise impacting proposed facilities, were
estimated for the 100-year frequency, 6-hour duration storm event in accordance with WDEQ-
LQD guidelines. Basins were delineated for the Mineable Pit, Waste Rock Facility, Haul Roads,
and PUG Facility (see Figure 1 in Appendix D). The time of concentration for each basin was
estimated using TR-55 methodology (NRCS, 1986). Curve numbers for the site were assigned
using TR-55 methodology (NRCS, 1986) in conjunction with soil survey information (NRCS,
2011). The 100-year 6-hour duration and 10-year frequency 24-hour duration storm depths were
estimated for the site from National Oceanic and Atmospheric Administration (NOAA) Atlas 2
data (NOAA, 1973). Hydrologic modeling was performed using the Hydrologic Engineering
Center’s Hydrologic Modeling System (HEC-HMS) (USACE, 2010) to calculate runoff volumes
and peak flow rates.
The sedimentation ponds are sized to accommodate runoff from the 10-year frequency, 24-hour
duration event, which was modeled using HEC-HMS (WDEQ, 2012). The sediment ponds are
sized, at a minimum, to contain the volume of sediment equal to one inch of runoff from the
entire catchment area, or 3,630 ft
3
/acre of sediment.
In addition to storage for environmental flows from the pit and Basin Pt-N, the Pit Pond is also
sized to accommodate pit dewatering flows of 500 gallons per minute (gpm). The pond water
and sediment capacity is estimated using an assumed retention time (2 days) and settling
velocity parameters based on particle size.
Channel sizing calculations were performed with an open channel flow spreadsheet using
Manning’s equation. Riprap sizing was performed using USACE methodology (USACE, 1994).
The riprap size for the water management channels varies between 6 to 18 inches (median
diameter). Culvert crossings were modeled using HY8 culvert analysis software (FHWA, 2011).
Assumptions and detailed calculations for design of the storm water diversion and collection
channels, culverts, and sedimentation ponds are provided in Appendix D.
Channel dimensions and revetment have been designed based on available topography and
preliminary facility designs. Dimensions and revetment requirements may change as the
designs become more detailed.
A berm around the upgradient side of the Mineable Pit highwall (PIT DIV N. and PIT DIV S. on
Figure 1 in Appendix D) prevents surface water flows from running into the Mineable Pit during
the design storm event and diverts the storm water north and east into the existing drainages.
Perimeter channels (as assumed) carry storm water flows along the north and west sides of the
Waste Rock Facility to a proposed sediment trapping pond on the northeast corner of the Waste
Rock Facility. Perimeter channels (as assumed) carry storm water flows along the south side of
the Waste Rock Facility to a proposed sediment trapping pond on the southeast corner of the
Waste Rock Facility. Parallel diversion channels along the south and west sides of the Waste
Rock Facility will divert the undisturbed upper reaches of Beaver Creek along the toe of the
Waste Rock Facility and terminate into an upper tributary of Beaver Creek. Perimeter channels
along the north and south sides of the PUG Plant pad convey flow to a sediment trapping pond
on the east side of the PUG Plant. The proposed diversion channels are included in Appendix
D.
