2009-09-21_PERMIT FILE - C1980007A ‎(5)‎

• Permit No: C1980007A
• IBM Index Class Name: Permit File
• Doc Date: 9/21/2009
• Doc Name: Mine Water Control Plan
• Section_Exhibit Name: Exhibit 66 Mine Water Control Plan
• Media Type: D
• Archive: Yes

West Elk Mine - Mine Water Control Plan Mountain Coal Company
• 3.3.5 Hydrograph Response Shape
Hydrograph response shapes reflect the speed at which water runs off a watershed, rated in
SEDCAD+ as fast, medium, or slow. A slow response shape would correspond to a forested
area or an area with a lot of obstructions, while a fast response shape would correspond to an
unvegetated or poorly protected area. Paved areas were assigned a "Fast' hydrograph
response shape. Other areas were assigned a "Medium" hydrograph response shape.
3.4 Subwatershed Sediment Parameters
SEDCAD+ utilizes the Revised Universal Soil Loss Equation, or RUSLE, to compute soil loss
from watersheds. RUSLE parameters, and their derivation, are explained below.
3.4.1 Soil Erodibility Factor, K
The erodibility factor of a soil refers to the susceptibility of soil particles to detachment and
transport by rainfall and runoff. Higher numbers indicate higher susceptibility to erosion. For
natural soil areas, the soil erodibility factor was obtained from the soil survey report, and
averaged in a similar fashion to the approach taken for Curve Numbers. Natural soil K-factors
varied from 0.19 to 0.37. Disturbed and revegetated areas were assigned a K of 0.30, while
paved and impermeable areas were assigned a K of 0.25. Coal product was assigned a K of
0.10, owing to its low runoff potential. In conjunction with the assigned C- and P-factors of 1.0,
the resulting K*C*P product for coal equals 0.10, yielding the same erosion potential as in
Greystone's 1994-1995 analysis of pond MB-3, which used a K of 2.0, and a CP of 0.05 for coal
product. Finally, subwatershed K-factors were computed as an area-weighted average in the
same fashion as Curare Numbers
• 3.4.2 Length-Slope Factor, LS
The length-slope factor, LS, is computed along a representative hillslope flow path and accounts
for the tendency of long, steep slopes to produce higher soil erosion rates than shorter, flatter
slopes. The representative flow length and slope were estimated from contour maps using
AutoCAD. From these, SEDCAD+ computes the LS factor. Note that the representative flow
length for an irregularly-shaped watershed is typically less than that of the time of concentration
path.
3.4.3 Control Practice Factor, CP
The RUSLE, as implemented in SEDCAD+, considers the sediment loss from an area with a
given cover and conservation practice, as compared to that of a field in continuous fallow
condition, by introduction of a Control Practice (CP) factor. The C factor accounts for the effects
of ground surface residue, canopy cover, and surface roughness. The P factor accounts for
sediment control practices such as contouring, buffer strips, and straw bales. Only the C factor
was adjusted for the West Elk site model. P was conservatively considered a constant 1.0. In
keeping with previous studies, the C-factor was set to 1.0 for coal pile (reflecting bare ground),
0.01 for impermeable and paved areas, and 0.05 for all other areas. The C = 0.05 value is
typical of both gravel-covered surfaces and grassed areas, and conservative for forested areas.
For simplicity, the subwatershed C factor was conservatively taken as the maximum of the
constituent area C-factors, rather than the weighted average.
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Tetra Tech October2008