Laserfiche WebLink
The predicted ground surface subsidence for the Middle Creek alluvium would reach up to 84 inches during <br />mining in panel 8 Right, where the overburden is 530 l0 670 feet thick (SubTerra, lnc.,1993). This rate of <br />• subsidence could cause short-term change of the Middle Creek streambed gradient. The maximum calculated <br />slope change (SubTerra, Inc., 1943) of 20 x 10-3 ft/ft for Middle Creek is associated with a lower overburden <br />thickness of 580 feet in the area of panel 8 Right. This change of slope would not have any significant long-term <br />impact on the stream flow and/or water quality. The meandering character of Middle Creek, cutting through fine- <br />grained sediments, would facilitate the fast-healing process of any temporary changes in the streambed gradient. <br />The above predictions were verified by Mr. Ramsay M. McDetmid, P.E.; his report is presented in Exhibit 7i, <br />Review Cyprus Twentymile Subsidence Effect on Streams. <br />It should be noted that coal is mined beneath significant bodies of surface water, including the sea, in many nations <br />(Great Britain, Canada, Australia, Japan, Chile, and Turkey). The minimum cover to seabed for total extraction <br />(longwall mining) ranges in various nations from 60 meters (197 feet) in Japan to 213.4 meters (700 feet) in Nova <br />Scotia. The guidelines recommended by the U.S. Bureau of Mines (Wardell, 1976) state that, "any single seam of <br />coal beneath or in the vicinity of any body of surface water may be totally extracted, whether by longwall mining <br />or by pillar drawing, provided that for each 1 foot thickness of coal seam extracted a thickness of 60 feet of solid <br />strata cover exists between the proposed workings and the bed of the body of surface water." This shows that at <br />the Foidel Creek Mine the longwall operation extracting 9.5 feet of coal would be safe 570 feet under a seabed. <br />The conclusion of our studies of subsidence and the potential impacts of subsidence of the Foidel and Middle <br />Creek alluviums Alluvial Valley Floor is [hat the mining in panels 2 Right through 9 Right will not have any <br />detrimental impacts on the water quantity and quality and on the essential hydrologic function of the AVF. <br />Additionally, Twentymile Coal Company conducted an exhaustive monitoring program on a selected segment of <br />• Fish Creek and its alluvial valley floor during the period of 1992 through 1994. The program was required <br />because of the Division approving Twentymile's request to undermine and subside this selected section of Fish <br />Creek and its alluvial valley floor, this area is immediately downstream of RCR #27 culvert in Fish Creek in the <br />WMD. The results of the program are summarized in Exhibit 7j, Fish Creek Alluvial Valley Floor Study. The <br />results have been submitted previously to the Division in the semi-annual subsidence reports for 1993 and 1994, <br />which are incorporated herein by reference. <br />The setting of both the Fish Creek AVF study and the mining proposed in the Eastern Mining District are similar <br />in that the valley floors are relatively narrow and the streams deeply incised. However, Fish Creek typically <br />discharges a larger volume of water than both Middle and Foidel Creeks. Historically, both Middle and Foidel <br />Creeks do not flow during the late summer and through the fall and winter months. ]t is only until recently , i.e. <br />after the initiation of mining and reclamation activities in Eckman Park, that Foidel Creek flows yearly. This is due <br />to the discharge of spoil springs into Foidel Creek. In addition, Twentymile Coal Company discharges mine water <br />into the creek, but this will cease upon completion of mining. In reference to Middle Creek, it typically goes dry <br />during the summer, fall, and winter months. 1 t^--~ ~^~ ~'^~ ' { ~Y . <br />There is a slight difference in the geologic setting between the Fish Creek AVF study area, area immediately <br />downstream of RCR #27 culvert in Fish Creek in the WMD, and the Foidel and Middle Creek areas. The <br />difference centers on the amount of cover over the Wadge seam for these respective areas. The cover for the Fish <br />Creek AVF study area was a roximatel 1 100 feet, while the cover for the Foidel Creek ranges from <br />approximately 600 eet to ],150 feet. The cover for the Middle Creek areas ranges from 600 to <br />1 <br />J <br />TR 99-32 2.05-167 APIA®~~~ F[P 0 3 2000 1/3/00 <br />