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

SubTerra, Inc. Twentymile Coal Company 02/I I/99
Northern Mining District Subsidence Evaluation -Final Report
• 7. 3-D Geologic Modeling was also used to confirm the results of the spreadsheet based
evaluation. Digitized topographic data was imported to GEMCOM 3-D geologic
modeling software along with the coordinate data for Fish Creek that is shown in Table
2. Topographic data, representing the ground surface over which Fish Creek flows,
were re-contoured in GEMCOM and exported to an Excel97 spreadsheet. This
surface was numerically subsided in Excel and re-loaded to GEMCOM. The 3-D
plane, representing the un-subsided water surface of Fish Creek, was then expanded
to meet the subsided ground surface and the new area of this surface was determined.
This exercise was repeated for panels 13 through 16-Right (Drawings 2-5).
4.3AVF Subsidence Predictions
The predicted initial and final elevations of the bottom of Fish Creek are shown in Table 4 and
Figure 5. Summary data include:
• Subsidence ranging from 0 to 67 inches.
• Maximum stream bottom gradient ranging from -0.16 to +0.18 prior to subsidence and
from -0.17 to +0.17 after subsidence.
• Maximum gradient change of 0.025 along the creek bottom.
The estimated impacts of this level of predicted subsidence are provided in the following
section.
•
4.4 Predicted Impacts to the AVF
4.4.1 Increase in Fish Creek's Surface Area
Our evaluation indicates that subsidence will result in over-topping of either the north or south
highbank over portions of the affected creek length (Table 3). Over-topping will cause Fish Creek
to spread out over the AVF and link up with oxbow lakes that were previously part of the creek.
The elevation of these areas is typically within about one foot of Fish Creek's highbank(s) so that,
in some cases, the creek level will rise up against the surrounding ground. There is no indication ~
that the creek will occupy areas outside of its AVF.
The estimated increase in Fish Creek's surface area is shown in Table 5 and on Drawings 2-5.
Differences between GEMCOM and Excel output are associated with the greater precision
associated with the 3-D modeling. Under low=flow con~tions, subsidence of the AVF is predicted
to result in an increase of Fish Creek's surface area, after panels 12 and 13 have been mined out,
of about 39 acres. This area is predicted to expand, as subsequent panels are extracted, by an
aitditional 45 acres.
The subsided area of Fish Creek will expand during April and May when peak flows (see Table 6)
are experienced. However, the relative expansion will be less than previously experienced by the
un-subsided AVF as stream velocities, and hence gradients, will decrease in flows over the
undermined areas. This may in turn result in sediment deposition over the mined out areas and
scouring in areas where subsidence has been limited by the presence of the rigid-yield pillar pair.
The net increase in Fish Creek surface area during peak flow has been evaluated, assuming an
PN ~. 98-12