PERMFILE41593

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 5/21/2004
• Section_Exhibit Name: Exhibit 07e-6 AVF Subsidence Study Final Report
• Media Type: D
• Archive: No

SubTerra, Inc. Twentymile Coal Company 05/28/03
PR6 AVF Subsidence Evaluation
• Maximum stream bottom gradient ranging from -2.3% to +2.3% prior to subsidence and
from -2.9% to +2.9% after subsidence.
• Maximum gradient change of 0.8% along the creek bottom.
The estimated impacts of this level of predicted subsidence are provided in the following
section.
3.4 Predicted Impacts to the AVF
3.4.1 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. 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 3 and
4. Under low-flow conditions, subsidence of the AVF is predicted to result in an increase of Fish
Creek's surface area, after panels 17 and 18 have been mined out, of just under 5 acres. Most of
the increase is associated with Panel 17-Right and, for the most part, Fish Creek is not expected
• to over-top its banks above 18-Right where it occupies a more deeply incised channel. No
impacts are predicted to be associated with the extraction of 19-Right.
The subsided area of Fish Creek will expand during April and May when peak flows (see Table 4)
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
associated, estimated increase in creek elevation of 2 feet, using the 3-D modeling software. The
additional flooded area, for high water conditions following extraction of Panel 18, was estimated
to be approximately 5 acres. The expanded surface area is shown on Drawing 5.
3.4.2 Potential for Fissure Development and Water Loss (or Stream Pirating)
Surface cracking or fissuring has previously been confined to areas where the Twentymile
sandstone either outcrops or is located relatively close to the surface. In these areas, the blocky
nature of the Twentymile sandstone results in strain concentration causing abnormal
displacement between adjacent blocks and surface cracking.
The combination of a maximum predicted tensile strain of +0.005 and relatively thick cover of
marine shale over the Twentymile sandstone in the study area will decrease the potential for
surface cracking or fissuring. The potential for water loss under these conditions is extremely low.
In addition, no evidence of "stream pirating" has been observed during extensive undermining of
. Fish Creek and its AVF by Panels 13 and 14-Right.
4 PN: 2002-06