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Differential vertical settlement over a short distance <br />will result in local changes in ground surface slopes. If <br />such changes occur along any stream channel, its gradient <br />will change. Any gradient changes will impact the stream <br />characteristics (flow and material transport). In an extreme <br />case, the gradient changes may disrupt the flow entirely. <br />Such an event is not probable in topographic conditions of <br />the area where most stream channels are relatively steep. <br />Probable changes of stream gradients were examined at <br />several locations along Stevens Gulch and East Roatcap <br />Creek; it was found that maximum probable changes in the <br />stream gradient would be on the order of 1 to 9 percent. As <br />the existing stream gradients are on the order of at least 6 <br />to 10 percent, they would be changed to 3 to 7 percent. Such <br />a change is regarded as insignificant in topographic and <br />geologic conditions of the area and considering the <br />ephemeral character of the streams. <br />The potential occurrence of subsidence cracks is by far <br />the most important feature that could disrupt the surface <br />water bodies. The current experience from t}ie Orchard Valley <br />Mine, supported by other experience gained from the mines in <br />the Somerset - Paonia area, indicates that subsidence cracks <br />may develop in areas of low overburden. <br />• Subsidence cracks occurring across a stream channel <br />will cause the disruption of a stream and may cause it to <br />flow underground (in the worst case, into the mine). Unless <br />other remedial measures are undertaken the flow will be <br />restored only after the cracks are filled with the stream <br />sediment, healed and sealed. Such a process might take <br />several years. A disrupted surface stream could be restored <br />much faster by installing culverts over the subsidence <br />cracks or by artificial sealing (grouting). <br />The occurrence of subsidence cracks was predicted in <br />previous sections of this report to be most likely in the <br />areas where the overburden thickness is 800 feet or less. <br />Such prediction coincides reasonably with the criteria on <br />the protection of surface bodies of water recommended by <br />Babcock and Hooker (1977). Pt-ey recommend that in order to <br />protect any body of surface water, a minimum of 60 feet of <br />Solid overburden strata is necessary for each 1-foot <br />thickness of coal seam to be extracted,, i~hether the mining <br />method is longwall or pillar robbing. According to them, at <br />least 600 feet of overburden would be required to protect <br />the streams. <br />• <br />- 43 - <br />