TECHNICAL MEMORANDUM
I:\11\82323\0100\0122 TM\Feb14\11382323 TM PreFeasibilityHydrologyHydraulicsRevE 13FEB14.docx
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1.0 INTRODUCTION
Rare Element Resources (RER) is proposing a new open-pit mine for rare earth materials north of
Sundance, Wyoming referred to as the Bear Lodge Project, Bull Hill Mine. This memorandum describes
pre-feasibility level hydrology and hydraulics calculations for the site. The calculations were performed in
order to develop the framework for an initial stormwater management plan for the site, encompassing
possible stormwater diversions, runoff collection channels, haul road culverts, and sedimentation ponds.
2.0 METHODOLOGY
2.1 Hydrology Calculations
Peak stormwater flow rates contributing to, or otherwise impacting the proposed facilities at the Bear
Lodge Project were estimated for the 100-year frequency, 6-hour duration storm event in accordance with
the Wyoming Department of Environmental Quality, Land Quality Division’s Guidelines (WDEQ). Using
the site topography provided by RER, basins were delineated for the Mineable Pit, Waste Rock Facility
(WRF), Haul Roads, and Plant Site (see Figure 1). The time of concentration for each basin was then
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
frequency, 6-hour duration, and 10-year frequency, 24-hour duration storm depths were estimated for the
site from the NOAA Atlas 2 (NOAA, 1973). Hydrologic modeling was then performed using the Hydrologic
Engineering Center’s Hydrologic Modeling System (HEC-HMS) (USACE 2010) to calculate runoff
volumes and peak flow rates.
2.2 Preliminary Sediment Pond Sizing
In accordance with the Wyoming Department of Environmental Quality Water Quality Rules and
Regulations, Chapter 11, Section 31, the sediment ponds are sized to accommodate runoff from the 10-
year frequency, 24-hour duration event, which was modeled using HEC-HMS (Wyoming DEQ 2012).
Additionally, the ponds are sized to accommodate the expected sediment. In accordance with Michigan
Department of Environmental Quality guidelines, the sediment pond are sized, at a minimum, to contain
Date:
February 13, 2014
Document No.:
11382323
To:
Paul Bergstrom
Company:
Rare Element Resources
From:
Micah Richey
Email:
cc:
Tammy Rauen and Roman Popielak
RE:
BEAR LODGE PROJECT, BULL HILL MINE – PRE-FEASIBILITY HYDROLOGY AND
HYDRAULICS, REVISION E
