PERMFILE115749

• Permit No: C1981071
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: PR2 APPENDIX I CHANNEL DESIGN
• Media Type: D
• Archive: No

• management factor (C ); and 5) erosion control practive factor (P). In
comparing the approved and existing post mining topography the factors
R, K, C, and P will not change. The major difference between approved
and existing topography is seen as changes in slope length and gradient
(see Table 11). Therefore, reasonably conservative values were selected
for factors R, K, C, and P and these values were used in all
calculations of sediment loss.
Topographic factors were determined from measurements of slope
length and gradient scaled from topographic maps of approved and
existing topography. Calculations of soil loss for six major basins
within the Eckman Park Mine shows no change in soil loss for three
basins and a reduction in soil loss for three basins (see Maps 2 and 3
of our previous report for basin locations). The reduction in soil loss
for basins 7-1 (Slw) and 7-2 (Slw) can be attributed to a reduction in
slope length. This has been accomplished by directing slope runoff into
drainage swales (effectively increasing drainage density) as opposed to
creating a featureless planar surface. The soil loss calculations,
therefore, indicate that an increase in drainage density may have the
beneficial effect of reducing soil erosion. The reduction in soil loss
for area 41 is due to a decrease in hillslope gradient.
In most cases the existing sediment yields are equal to or less than
sediment yields for the approved topography. For area 41 and basin
9/10-1 (51E) an increase in drainage area causes existing sediment yield
to exceed the approved sediment yield. Estimates of sediment yield
• (Soil Conservation Service, 1975) suggest soil loss rates of
approximately 2 to 5 tons/acre/year for the area of the Eckman Park
Mine. The above calculations of soil loss are at or slightly below the
low end of this range suggesting that the calculations are reasonable
for the mine areas. The total sediment yield of these basins is 937
tons per year for the existing conditions as compared to 1020 tons per
year for the approved conditions; a reduction in sediment yield of
approximately 8 percent.
SUMMARY
The drawings enclosed herewith (Sheets 1-7) present a plan for
stabilization of the steep channel slopes at the Eckman Park Mine cited
in the August 8, 1986 NOV.
Use of riprap having a median diameter (D50) of 18 inches provides a
factor of safety against failure greater than 1.5 during the 10-year,
24-hour event for all locations considered. Although not specifically
addressed in this report, the proposed channel designs are based on the
assumption that only sound, durable rock will be utilized in
construction of riprap channel linings.
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