2008-12-11_PERMIT FILE - C1980007A ‎(16)‎

• Permit No: C1980007A
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2008
• Section_Exhibit Name: Exhibit 66 Mine Water Control Plan
• Media Type: D
• Archive: Yes

West Elk Mine - Mine Water Control Plan Mountain Coal
3.3 Subwatershed Hydrologic Parameters
3.3.1 Hydrologic Soil Group
For natural soil areas, the Hydrologic Soil Group was obtained from the soil survey report, and
included. Disturbed areas were modeled as bare, Group C soil, and rock outcrops were
considered to be bare, Group D soil.
3.3.2 Curve Number
The Curve Number (CN) reflects the runoff potential of an area, and is a function of the land use
and soil hydrologic classification. First, a Curve Number was assigned to each soil type above,
according to the percentage breakdown of each subgroup documented in the soil report for that
soil type, including rock outcrops. Rock outcrops were assigned a CN of 94, reflective of bare D
soils, while other soils were assigned CN's reflective of pasture in good condition (61, 74, 80 for
B, C, and D soils) and oak-aspen in fair condition (48, 57, 63) for their respective soil types.
Group A soils were conservatively assigned CN's appropriate to B soils. Owing to the variable
percentage of rock outcrop within each soil complex, resulting average Curve Numbers ranged
from 48 to 75 on oak-aspen land covers, and from 53 to 85 pasture land covers.
Disturbed areas were assigned Curve Numbers as follows:
• Coal product was assigned CN = 80, reflective of well-drained conditions, and identical
to previously approved reports at this site.
• Disturbed areas were assigned a CN of 91, reflective of bare C soils.
• Impermeable land cover was assigned a CN of 100.
. Paved areas were assigned a CN of 98.
• Revegetated areas were assigned a CN of 79, reflective of pasture in fair condition on
Group C soils.
Subwatershed Curve Numbers were assigned based on an area-weighted average of the soil
type-land cover complexes present with each subwatershed. Areas were measured in
AutoCAD.
3.3.3 Time of Concentration
The time of concentration influences the peak flow produced from a given runoff volume. The
time of concentration path was delineated in AutoCAD for each subwatershed, and the path
length and slope measured. The SEDCAD Land Flow Condition was selected to be slightly
conservative, resulting in generally short concentration times. Actual times of concentration
were computed within SEDCAD.
3.3.4 Muskingum Routing Parameters..
For the main mine site, subwatershed Muskingum routing was generally not utilized, as the
ditches were modeled explicitly as structures,.and overland flow times were generally accounted
for in the time of concentration path. Therefore, Muskingum parameters were assigned at the
structure level rather than the watershed level. For Sylvester Gulch, where natural stream
channels were not explicitly modeled, subwatershed Muskingum routing lengths and slopes
were determined in AutoCAD, and entered into SEDCAD subwatersheds where appropriate.
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