Prefeasibility Access Road Design-Revised
18
flow. All sediment traps installed will have a controlled discharge method to avoid
embankment failure.
Re-vegetation is a very important method for permanent erosion control. Proper
techniques will be applied to ensure the mulching and seed is held in place. Re-
vegetation will be conducted as soon as construction activities and weather permit
to minimize the risk of erosion occurring.
b.
Climatological Data and Design Storm Events
Climatological Data collected from the Western Regional Climate Center for the
Sundance and Upton areas is summarized and included in Appendix F.
The design storm event used in the design of the road must allow for proper sizing of
the drainage structures to reasonably mitigate the risk damage due to flooding.
According to the 2011 Wyoming County Road Fund Manual, for new construction of
roads classified as collectors, the 15 year flood is the minimum that can be used for
design. The BLM Gold Book recommends that the 25 year storm event be used. The
structures for the access road have been designed for the 25 year, 24 hour storm
event. The drainage will be designed such that the water level does not rise above
the bottom of the crushed base for the design storm event. Considerations also
have to be made for blockage and silt accumulation. A 50% percent blocking factor
will be used to calculate overtopping depths if the culvert is less than 4 ft in
diameter. Culverts will be designed to maintain a minimum of 2 fps velocity during
the Minor 2-year storm event to minimize silt accumulation, and allow culverts to be
self-cleaning during low flows.
Design storm events were found using the NOAA Atlas 2, Precipitation-Frequency
Atlas of the Western United States. The 25 year, 24 hour storm event for the area
north of Sundance which would include the mine site is 3.4 inches. The storm event
is lower for areas of lower elevation and further to the south towards Upton. The
value for these areas is 3.0 inches.
The 2 year, 24 hour storm event used for evaluating velocities for culvert cleaning is
2.2 inches near the mine site and decreases to 1.6 inches in the Upton area.
c.
Topsoil and Closure Objectives
Topsoil needed for areas which will be required to be reclaimed at the closing of the
mine will need to be stockpiled and protected. It is assumed that the only areas
which will be required to be reclaimed are those which are on public land and not
currently an improved road. These areas would be on the Miller Creek Road from
approximately station 450+00 to 535+73. The road width that will be required to be
reclaimed is 44 ft. This will require approximately 7,000 CY of topsoil to be stored.
The area to be seeded will be 8.65 acres. The seed mix used must comply with
