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Subsidence cracks occurring across a stream channel could cause.the disruption of the stream. However, because <br />• of the presence of thick, fine-grained sediments in the stream bed of the study azea, the process of sealing the <br />cracks would be fast and no significant impacts on the flow are expected. <br />The occurrence of the subsidence cracks on the surface may not necessarily result in direct inflow into the mine <br />when a surface water body was disrupted. Studies by vazious authors such as Orchazd {1969, 1975) in Great <br />Britain, Williamson (1978) in Australia, and Hydro-Geo Consultants, Inc. in West Virginia (1991) and Ohio <br />(1988), indicated that the subsidence cracks aze limited in depth by the development of compressive stresses <br />replacing the tensile stresses that cause surface fracturing. The depth of surface subsidence cracks due to tensile <br />stress does not typically exceed 50 feet. Development of this type of crack would be limited in the unconsolidated <br />sediments of the Foidel and Middle Creek alluviums, which are to 30 feet thick neaz the confluence of these two <br />streams. The presence of up to 600 feet of mazine shales between the ground surface and the mined coal will also <br />limit the impacts of subsidence. <br />TCC has done considerable subsidence monitoring work at the Fish Creek AVF study azea, area immediately <br />downstream of RCR #27 culvert in Fish Creek in the WMD, to determine if subsidence would impact the creek and <br />its valley floor. One component of the work focused on making visual observations of Fish Creek to determine if <br />any cracking was occurring in the creeks' channel. The channel included sections of both sediment and exposed <br />bedrock. The visual observations, which included those made by representatives of the DMG, indicated that no <br />cracking of the channel occurred during longwall mining. <br />Predicted Impacts -Fish Creek It is anticipated that Fish Creek will respond similarly to the response <br />observed in Foidel Creek, which overlies the 7, 8, and 9 Right panels. The response observed is for a pool <br />to form in the subsidence trough for the respective panel. The extent of the pool will be dictated by the <br />• bank height. Typically, the subsidence trough lessens in magnitude as you move from the centerline of the panel <br />until it reaches zero at the angle of draw limit. As was evidenced in the EMD, i.e., no head cutting at gateroads, <br />the formation of the pool decreases erosional potential within the subsided segment of the stream. <br />The pre and post mining gradients of Fish Creek is presented on Table 4 of Exhibit 7e-3. With the formation of the <br />pools over panels 13 Right through 16 Right, TCC is not projecting any erosional problems in the stream channel <br />using the historic flow data for Fish Creek, and the visual observations along Foidel Creek which overlaid Panels 7 <br />through 9 Right. These observations did not detect any erosional problems caused by TCC's mining operation <br />The pre and post mining gradients of Fish Creek is presented on Table 4 of Exhibit 7e-6, Twentymile Coal <br />Company, AVF Subsidence Study, Final Report. With the formation of the pools over panels 17 Right and 18 <br />Right, TCC is not projecting any erosional problems in the stream channel using the historic flow data for Fish <br />Creek, and the visual observations along Fish Creek which overlaid Panels 13 through 14 Right. These <br />observations did not detect any erosional problems caused by TCC's mining operation. <br />A potential outcome from creation of the small pools is an increase in water temperature during the summer <br />months. A look at the historic data for the months of June through September indicates that the segment of Fish <br />Creek to be subsided during this five-year term already exhibits elevated temperatures during this period. The <br />historic data is presented in Table 71, Period of Record Average Temperatures, dated June through September. <br />Stations 16 and 302 are upstream of the subsided azea and will not be impacted by the proposed mining in the <br />Northern Mining District. As reflected in the data, these stations record the coolest temperatures during the period <br />of intent, although both stations reflect temperatures of greater then 18 'Celsius during July. Station 1002 is <br />• located over panel 13 Right and will be the only station impacted by the mining activities. As noted on Table 71, <br />average water temperature range from 16.3' C to 19.1 ' C during June through August and 14.2 ' C during <br />September. The temperatures during June through August aze not conducive form maintaining acold-water <br />PR 03-06 2.05-164 05/07/03 <br />