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Predicted Impacts -Middle Creek Middle Creek is a perennial stream with headwaters south of the permit <br />area. This perennial stream flows northeast for approximately 15.6 miles to its confluence with Trout Creek. The <br />.potential changes in the Middle Creek gradient due to surface subsidence caused by the longwal] mining in panels <br />2 Right through 8 Right were calculated. This calculanon is based on the characteristics of Middle Creek and <br />results of the ground surface subsidence study presented in Exhibit 7E of this application. According to SubTerra, <br />Jnc. (1996). the maximum calculated slope change would not exceed 19x ] 0' ft/ft. <br />A stream channel profile, Figure 51, was plotted for Middle Creek as it meanders through the project site. This <br />stream crosses six of the eight longwall panels in a direction almost perpendicular to the panels. The length of the <br />undermined section of this stream is less than 2.0 miles and the average gradient within this section is 0.014 fVft. <br />The hydraulic gradient of the Middle Creek stream is steeper at the crossine of pane] 7 Right. Within this section, <br />the head-cutting could be more pronounced. The gradient of 1`9iddle Creek, in the area near the confluence with <br />Foidel Creek (considered an Alluvial Valley Floor), is much flatter and no substantial head cutting or deposition is <br />anticipated. Therefore, no impacts to the water usage and to the essential hydrologic function of the AVF are <br />expected. <br />Slopes of the Middle Creek Channel crossing over the Foidel Creek Mine range from 0.0067 ft/it in the upstream <br />reach to 0.02 ft/ft in the central area and 0.013 ft/ft in the lower reach. A short reach in the central area falls at <br />0.046 ft/ft under natural conditions. Figure Sj shows the Middle Creek Profile. <br />Middle Creek enters the mining area at panel 2 Right. The Middle Creek profile at the center of pane] 2 Right <br />ma}~ subside up to 46 inches. The low slope will cause the subsidence channel bed slope to erode upstream as <br />much as two feet deep, tapering off to zero feet at about 300 feet upstream. No mitigation is estimated to be <br />required. <br />~ A ]ow spot of about 78 inches below the adjacent profile in the subsided area of panel 4 Right will be filled by <br />• sediments eroded from the unsubsided area between panels 2 Right and 4 Right. The net results in this reach <br />will be to erode about two feet in the upstream reach and fill about Tour feet at the center of panel 4 Right. <br />The stream gradient will adlust itself accordingly to flow conditions. No mitigation is estimated to be <br />required. <br />The subsided stream channel profile between panel 4 Right and panel 7 Right is estimated to be approximately <br />congruent to the natural profile. The center of panel 5 Right may subside as much as 79 inches, however, the <br />low spot will fill about two feet to maintain a congruent profile. Minor erosion and deposition of one to two <br />fee[ in other areas may occur as the stream adjusts itself. <br />4. There will be a low spot in the center of panel 8 Right of about 74 inches. The stream bed will adjust itself <br />with minor erosion upstream and downstream to achieve a profile similar to natural conditions. <br />If headcutting is greater than three feet or if incising of two feet or greater occurs more than 400 feet from the <br />point of origin of the headcut, then channel morphology should be evaluated. Channel mitigation measures should <br />be initiated, if warranted, after the evaluation. A visual examination, possibly combined with analytical results, <br />should be considered as an appropriate method for determining "damage." The character of the stream, flow, <br />sediment load, and sediment characteristics will al] influence the advent or extent of damage that may require <br />repair. <br />Subsidence Cracks The potential occurrence of subsidence cracks is another feature that could disrupt the <br />surface and shallow ground water regime. The current experience from Pennsylvania and West Virginia, <br />supported by other experience gained from mines in the Somerset, Paonta areas in Colorado, indicates that <br />subsidence cracks may develop in areas of low overburden and on bedrock outcrops. The subsidence monitoring <br />~onducted at the Fish Creek AVF study area, area immediately downstream of RCR #27 culvert in Fish Creek in <br />the WMD, did not indicate any visible cracks in Fish Creek's channel. <br />APPRO!/Et~ FEB C i3 2L00 <br />TR 99-32 2.05-]63 1/3/00 <br />