2009 Montana Mining - Montana Mining Association

Hydrologic control for
current mine and lease by
modifying surface coal mines
in southeastern Montana
As mining operations change and continue
to expand, the need for hydrologic
control becomes an integral part of
mining operations. The purpose of hydrologic
control is the safe and uninterrupted operation
of mining activities and the ability to
contain sediment transported in storm water
runoff. Surface water control includes flood
control upstream of mining activities, and
sediment control downstream of mining
activities.
HYDROLOGIC METHODS
The design of hydrologic structures is
dependant on the level of protection that
management desires for the planned mining
areas. Flood waters entering the mining area
cause several potential concerns to the mine
operation including personnel safety, equipment
damage, the inability to mine while
Reclaimed postmining channel, southeastern Montana.
20 Montana Mining 2009
water is being pumped out, and unstable
highwalls where the flow of water has
entered the pit. Typically, flood control structures
should be designed to provide protection
for at least a 100-year, 24-hour precipitation
event. This is the amount of precipitation
occurring over a 24-hour period that statistically
may be exceeded once every 100
years, or has a one in 100 chance that it will
be exceeded during any given year.
The MDEQ requires that sediment control
structures have the capacity to contain runoff
from the 10-year, 24-hour storm as well as
the three year sediment volume, which is
0.02 acre-feet/acre for the disturbed, contributing
areas. The sediment control structure
must also be capable of passing the 25year,
24-hour flood through the spillway with
minimal erosion. If a sediment control structure
is greater than 20 acre-feet, then the
spillway is required to pass the 100-year, 24hour
peak discharge.
Hydrologic Analysis
The SEDCAD model uses a form of the
Natural Resources Conservation Service (formerly
SCS) Triangular Hydrograph Method,
and is a parametric method of estimating
flood peaks and runoff volumes from sitespecific
data. The program uses the watershed
area, time of concentration, SCS Curve
Number and a unit hydrograph response
shape to estimate the corresponding hydrograph.
Watershed routing is accomplished
utilizing the Muskingum method.
Sediment Analysis
The SEDCAD model calculates the
expected sediment transport within the
watershed utilizing a modified form of the
Revised Uniform Soil Loss Equation (RUSLE).
The model develops a sedimentgraph utilizing
eroded soil particle size distribution, soil
erodibility factor (K), representative slope
length, representative slope, type of soil
cover and control practices.
HYDROLOGIC
CONTROL METHODS
Containment options provide flood control
by establishing reservoirs upstream from
the mining area. Sediment control is provided
by establishing the reservoir downstream
of the mining area. The size of the reservoir
is based on a hydrologic analysis. Reservoirs
fall into two size classifications: Mine Safety
and Health Administration (MSHA) or Non-
MSHA. MSHA-regulated reservoirs meet at
least one of the following criteria:
Greater than 20 acre-feet (ac-ft) of capacity
and impound water to an elevation of five