2016-08-08_PERMIT FILE - C1981019A ‎(8)‎

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 8/8/2016
• Doc Name: Table of Contents & Intro
• Section_Exhibit Name: Volume 18 Exhibit 7 Item 23
• Media Type: D
• Archive: Yes

1.9 BASE FLOW
Base flows were not used in any pond design calculations. Treated discharge of collected water from the
Little Collom X in -pit sump will be pumped to the Little Collom X downstream pond after settling has
occurred.
2.0 SEDIMENTATION CALCULATIONS
The sedimentation calculations for each pond on the Colowyo Mine were performed using the SEDCAD4
Computer Modeling Program developed by Civil Software Design. The SEDCAD4 sediment detachment
routine is based on the Revised Universal Soil Loss Equation (RUSLE). The input parameters used in the
sedimentation analyses are presented in the following sections.
2.1 RAINFALL FACTOR. R
An annual rainfall erosivity factor of 19 was used for the Little Collom Sump (LCX -IN in -pit sump)
sedimentation pond. This factor is consistent with other recent erosion analyses performed for the
Colowyo Coal Company. However this R factor was not used in the design of Little Collom (LCX-D/S)
Pond due to the short duration of the worst case conditions for this pond. Calculations were made with
the initial sediment storage set to equal 1.57 acre-feet, which results in a sediment storage elevation of
6629.88 feet, approximately 3 months of storage at the average annual rate. Throughout the initial 6 -
month condition, Little Collom (LCX-D/S) Pond will be monitored and cleaned out to ensure the
sediment storage is maintained at or below this elevation.
2.2 SOIL ERODIBILITY FACTOR, K
A conservative soil erodibility factor of 0.31 was used for all watersheds. Future analyses will refine this
value to be less conservative and more accurate. A comparison to dominant soil types and associated K
values is provided in Table 2.
TABLE 2
SOIL ERODIBILITY FACTOR
DOMINANT SOIL TYPE
SOIL TEXTURE
SOIL ERODIBILITY
FACTOR (K)
Campspass Series
Loam
0.24
Burnette Series
Loam
0.28
Work Series
Loam
0.31
Average
0.28
2.3 LENGTH -SLOPE FACTOR. LS
Input parameters to determine the LS factor were obtained from scaled maps of the watershed areas.
Values measured from the scaled maps are shown in the SEDCAD4 printouts. Within disturbed
subwatersheds, the representative length was capped at 400 feet. Lengths of drainage paths needed for
the "LS" factor determination in the RUSLE option in SEDCAD are determined by tracing the flow path
back to the subwatershed headwaters. In most cases this distance exceeds the nominal limit of 400 feet
described in accepted references such as "Applied Hydrology and Sedimentology for Disturbed Areas",
1985, by Barfield, Warner and Haan, making reference to the original 1965 Wischmeier and Smith USDA
publication. In such cases the length of the watershed has been capped at 400 feet. This practice has been
recommended based on personal conversations with Dr. Richard Warner, one of the developers of the
SEDCAD program.
Exh. 7-23-4 Revision Date: 9/23/11
Revision No.: PR -03