2009-10-30_PERMIT FILE - C1982056A (5)

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upstream of panels 4 and 5, and USGS Station 09243900, located on the eastern end of the undermined area. USGS Station 09243900 had a range of mean annual flow from 0.07 cfs in 1977 to 4.39 cfs in 1981, during a period of record from October 1975 through 1992. High flow occurs during the snowmelt, typically in April through June, and low flow occurs typically in July through September. The potential changes in the Foidel Creek gradient due to surface subsidence caused by the longwall mining in panels 9 Right through 6 Right were estimated. This estimate is based on the characteristics of Foidel Creek and results of the ground surface subsidence study presented in Exhibit 7E of this application. A stream channel profile, Figure 5i, was plotted for Foidel Creek as it meanders through the project site. The stream enters the undermined area at the edge of Panel 5 -Right and then crosses panels 6 -Right through 9- Right. The longest section of this stream will be undermined by Panel 7-Right. A total of 3.4 miles in length for Foidel Creek was undermined. The average gradient for the undermined section of the stream is 0.005 ft/ft. Based on subsidence calculations presented in Exhibit 7E (SubTerra, Inc., 1993), the predicted postmining slope of the Foidel Creek stream channel would not change by more than 12.5x10" ft/ft. The survey work done on Foidel Creek for the mining of Panel 9 -Right through 6 -Right showed no significant changes in the gradient of this creek. The creek profiles are present in TCC's semi - annual subsidence report. The average channel slope, based on the profile presented in Figure 5i, is about 0.0049 ft/ft upstream of the rib between Panels 7 Right and 8 Right and 0.0073 ft. downstream. Based on the assumption that the system is in approximate equilibrium, the subsided channel will probably adjust itself to about 0.007 ft/ft. 1. As the creek crosses over panel 6- Right, after subsidence, there will be a head -cut as much as three feet deep tapering off to zero feet about 1,000 to 2,000 feet upstream in about the same channel cross - section as exists now. This will occur as alluvial silts and clays from the streambed are eroded and re- deposited in the subsided area in the center of Panel 6 -Right and the natural streambed gradient is reestablished. No mitigation is anticipated to be required. • 2. The area of the rib between Panels 6 -Right and 7- Right, Panel 7- Right, and the Panel 7/8 rib will all subside fairly uniformly with respect to the streambed profile. Minor cuts and fills of one to two feet maximum may occur due to varying natural conditions in the channel bed and bank materials, subsidence irregularities, and channel flows. The channel will assume the same symmetry, as determined by flow conditions, as it has now. 3. A subsidence trough of as much as 42 inches may occur in the center of Panel 7- Right. However, the trough will taper out in about 350 feet on each side of the deepest portion. It will fill will alluvial sediments from the adjacent stable upstream segment in a short period, depending on flow conditions. No significant effects on the stream channel are anticipated and no mitigation is anticipated to be required. 4. The channel reach crossing Panels 6 Right and 7 Right and the ribs is predicted to subside at a channel slope very close to that of the natural channel, and no significant channel geometry changes are anticipated. 5. Panel 8 -Right has a change in slope to a flatter slope crossing the rib between panels 8 Right and 9 Right, and to a deeper subsidence trough in panel 9- Right. Channel profile equilibrium will occur as the high spot on the rib erodes about 1 to 2 feet to match pre- existing natural ground slopes. The profile will then erode slightly to match the natural grade downstream of Panel 9- Right. No mitigation measures are anticipated. The stream gradient for Foidel Creek is presented on Table 8, Exhibit 7e -3. In reference to Foidel Creek, TCC is not anticipating any significant impacts to the creek due to the mining of Panel 12- Right. TCC anticipates that the creek will react the same way it did previously. This reflects the fact that there are no sandstone outcrops intersecting the .channel. Therefore, the potential to dewater the stream is non - existent based on TCC's prior experience with Foidel Creek given this condition, i.e. no outcrop channel. Similarly, TCC does not anticipate that any significant erosional processes will be triggered by undermining and subsiding Foidel Creek. PR09 -08 2.05 -161 04/29/09