2009-10-30_PERMIT FILE - C1982056 (6)

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SubTerra, Inc. Twentymile Coal Company 04/07/09 Foidel Creek Mine Permit Revision PR09 -08 Subsid Evaluation 4.3. Predicted Impacts to the AVF 4.3.1. Increase in Fish Creek's Surface Area Our evaluation indicates that subsidence would result in over - topping of either the north or south high bank 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 high bank(s) so that, in some cases, the creek level will rise up against the surrounding ground. As shown in Drawing 4, 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 2 and on Drawing 4. Under normal flow conditions, subsidence of the AVF is predicted to result in an increase of Fish Creek's surface area, after panels 15 -Left through 13 -Left have been mined out, of just under 40 acres. The subsided area of Fish Creek may expand during peak flow months (e.g., see April and May in Table 3) and during off - normal years when peak flows (e.g., see 1996 in Table 4) are experienced. However, the relative expansion is expected to 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 was estimated to be approximately 27 acres as shown in Table 5. The expanded surface area is shown on Drawing 5. 4.3.2. 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 may result in strain concentration at joints, 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 in the Northern Mining District. 5. Monitoring Requirements Visual inspection and conventional or GPS surveys of the Fish Creek areas may be used to confirm the predicted effects of mining. This methodology has previously been successfully used in the Eastern and Northern Mining District. 4 PN: 2009 -04