2012-12-06_REVISION - C1981008 ‎(4)‎

• Permit No: C1981008
• IBM Index Class Name: REVISION
• Doc Date: 12/6/2012
• Doc Name: Bond Release Request ‎(Part 1 of 2)‎
• From: Western Fuels Colorado
• To: DRMS
• Type & Sequence: SL14
• Email Name: MLT
• Email Name: SB1
• Media Type: D
• Archive: No

Length Slope Factor "LS"
The effects of topography on soil erosion are determined by the dimensionless "L" and
"S" factors, which account for both rill and interrill erosion. These factors have been
considerably revised since the initial Universal Soil Loss Equation. The new RUSLE
computer database for the combined "LS" factor is based on the soil's ratio of rill to
interrill erosion. For most western rangeland soils, the equations for low rill to interrill
ratio are used. Table 4 (from Predicting Soil Erosion by Water: A Guide to
Conservation Planning With the Revised Universal Soil Loss Equation - US
Department of Agriculture 1997) shows a chart based on these equations. The average
slope of the release areas in both pre -mine and post -mine is determined with an
algorithm in Carlson and AutoCAD software, using a 3D grid of the topography in both
cases. The slope length will be 1000 feet, as this is the limit of the "LS" table.
Table 3 - Pre mine vs. Post mine LS Factor
From Table 3 it is seen that the pre -mine "LS" factor for Irrigated Pasture is 1.02 based
on a length of 1000 feet and a grade of 4.74 %. Post -mine "LS" factor is 0.57 for
irrigated pasture based on 3.06% average slope. These average slopes are calculated for
both the pre -mine and post -mine conditions based on topography from pre -mine and
post -mine surveys that are then run through a Carlson software algorithm. The "LS"
factors are then interpolated from the values in Table 4.
SL -14 03DEC12.wpd 33
Calculated Avg. Land
Slope
LS Factor
Pre -Mine Irrigated
Pasture
4.74
1.02
Post -Mine Irrigated
Pasture
3.06
0.57
Length Slope Factor "LS"
The effects of topography on soil erosion are determined by the dimensionless "L" and
"S" factors, which account for both rill and interrill erosion. These factors have been
considerably revised since the initial Universal Soil Loss Equation. The new RUSLE
computer database for the combined "LS" factor is based on the soil's ratio of rill to
interrill erosion. For most western rangeland soils, the equations for low rill to interrill
ratio are used. Table 4 (from Predicting Soil Erosion by Water: A Guide to
Conservation Planning With the Revised Universal Soil Loss Equation - US
Department of Agriculture 1997) shows a chart based on these equations. The average
slope of the release areas in both pre -mine and post -mine is determined with an
algorithm in Carlson and AutoCAD software, using a 3D grid of the topography in both
cases. The slope length will be 1000 feet, as this is the limit of the "LS" table.
Table 3 - Pre mine vs. Post mine LS Factor
From Table 3 it is seen that the pre -mine "LS" factor for Irrigated Pasture is 1.02 based
on a length of 1000 feet and a grade of 4.74 %. Post -mine "LS" factor is 0.57 for
irrigated pasture based on 3.06% average slope. These average slopes are calculated for
both the pre -mine and post -mine conditions based on topography from pre -mine and
post -mine surveys that are then run through a Carlson software algorithm. The "LS"
factors are then interpolated from the values in Table 4.
SL -14 03DEC12.wpd 33