PERMFILE61656

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 04e-3 Northern Mining District Subsidence Evaluation
• Media Type: D
• Archive: No

SubTerta, Inc. Twentymile Coal Company 02/I I/99
Northern Mining District Subsidence Evaluation -Final Report
• 4.1 Scoae of Work
The scope of work covered by this section of the report involves an evaluation of potential impacts
that may result from lowering the Fish Creek AVF during coal extraction in the Northern Mining
District. The following issues were investigated and resolved:
1. Will subsidence cause Fish Creek to overtop its existing high bank(s)?
2. If overtopping is likely, what remedial measures are required?
3. What will be the time phased increase in the surface area of Fish Creek due to
subsidence, and/or how much of the Fish Creek AVF will be flooded?
Several assumptions were made to simplify the analysis:
• Fish Creek elevation will rise, relative to the surrounding ground, a distance equal to
predicted ground subsidence at that point.
• The effects of spring run-off on creek elevation would be in addition to the effects of
subsidence.
4.2Aporoach
Our approach involved superimposing estimated subsidence on over 60,000 data points
representing Fish Creek and the immediate high bank adjacent to Fish Creek and comparing the
subsided elevations with existing (i.e., un-subsided) creek levels. The overFlow extent (e.g.,
• horizontal extent of flow) was also estimated where this initial calculation indicated that the creek
would overtop either its north or south high bank. The following tasks were performed:
1. Cross-sections including the north high bank (NHB), north toe (Ntoe), creek bottom
(CB), south toe (Stoe), and south high bank (SHB) of Fish Creek were developed from
a survey of the AVF containing approximately 10,000 data points (see Drawing 1).
Contours of this data were developed by Twentymile mine survey staff.
2. An excel spreadsheet was developed that contained each of the data points for
approximately 2,000 cross-sections (see Table 2).
3. Subsidence predicted for extraction of the first six panels in the northern mining district,
was subtracted from the elevation of the north and south high banks for each section.
These new, subsided elevations were then compared to the existing, un-subsided
water elevation in each section. The amount of over-topping was calculated where the
new high bank elevation fell below the original water level.
4. A table was compiled showing areas of the AVF where the creek was estimated to
over-top the high bank (see Table 3).
5. Additional section points were established north and south of the existing high banks
for sections where over-topping was predicted. The subsided elevations for these
points were established and used to estimate how far Fish Creek would flow outside
the high bank area.
6. The original and subsided areas of Fish Creek were estimated using the original and
• subsided creek widths and the calculated distance between adjacent cross-sections
(see Table 3).
PN: 98-12