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Memo to Rick Mills <br />Feb. 15, 1996 <br />Page 2 <br />• 2) The azea of the rib between Panels 6 Right and 7 Right, Panel 7 Right, and the Panel 7/8 rib <br />will al] subside fairly uniformly with respect to the streambed profile. Minor cuts and fills <br />of one to two feet maximum may occur due to varying natural conditions in the channel bed <br />and bank materials, subsidence irregulazities, and channel flows. The channel will assume <br />the same symmetry, as determined by flow conditions, as it has now. <br />3) A subsidence trough of as much as 42 inches may occur in the center of Panel 7 Right. <br />However, the trough will taper out in about 350 feet on each side of the deepest portion. it <br />will fill with alluvial sediments from the adjacent stable upstream segment in a short period <br />of time, depending on flow conditions. No significant effects on the upstream channel are <br />anticipated and no mitigation is anticipated to be required. <br />4) The channel reach crossing Panels 6 Right and 7 Right and the ribs is predicted to subside <br />at a channel slope very close to that of the natural channel, and no significant channel <br />geometry changes are anticipated. <br />5) Panel 8 Right has a change in slope to a flatter slope crossing the rib between Panels 8 Right <br />and 9 Right ,and to a deeper subsidence trough in Panel 9 Right. Channel profile <br />equilibrium will occur as the high spot on the rib erodes about 1'h to 2 feet to match pre- <br />existing natural ground slopes. The profile will then erode slightly to match the natural grade <br />downstream of Panel 9 Right. No mitigation measures are anticipated. <br />• <br />Middle Creek fsee Figures Sj and Sj(11 <br />Slopes of the Middle Creek Channel Crossing over the Foidel Creek Mine range from 0.6745 in the <br />upstream reach to 2.0% in the central area and 1.3 °!o in the lower reach. A short reach in the central <br />area falls at 4.6% under natural conditions. Figure Sj shows the Middle Creek Profile. <br />1) Middle Creek enters the mining area at Panel 2 Right. The Middle Creek profile at the <br />center of Panel 2 Right may subside up to 46 inches. The low slope will cause the <br />subsidence channel bed slope to erode upstream as much as two feet deep, tapering off to <br />Zero feet at about 300 feet upstream. No mitigation is estimated to be required. <br />2) A low spot of about 78 inches below the adjacent profile in the subsided area of Panel 4 <br />Right wilt be filled by sediments eroded from the unsubsided area between Panels 2 Right <br />and 4 Right. The net results in this reach will be to erode about two feet in the upstream <br />reach and fill about four feet at the center of Panel 4 Right. The stream gradient will adjust <br />itself accordingly to flow conditions. No mitigation is estimated to be required. <br />3) The subsided stream channel profile between Panel 4 Right and Panel 7 Right is estimated <br />to be approximately congruent to the natural profile. The center of Panel 5 Right may <br />• subside as much 79 inches, however, the low spot will fill about two feet to maintain a <br />congruent profile. Minor erosion and deposition of one to two feet in other areas may occur <br />as the stream adjusts itself. <br />ks\c: \cyprus\twenty\suefkt. mem <br />2/15/96 <br />