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(if any) and the vertical displacement would have very little effect on the impoundments <br />such as stock ponds in the area. <br />Differential vertical settlement over a short distance will result in local changes in <br />ground surface slopes. If such changes occur along any stream channel, its gradient <br />characteristics (flow and material transport) would change. In an extreme case the <br />gradient changes may disrupt the flow entirely. Such an event is unlikely in topographic <br />conditions of the area where most stream channels are relatively steep. Probable changes <br />of stream gradients were examined at several locations along Stevens Gulch and East <br />Roatcap Creek near the Bowie #1 Mine; it was found that maximum probable changes in <br />the stream gradient would be on the order of 1 to 9 percent. As the existing stream <br />gradients are on the order of at least 6 to 10 percent, they would be changed to 3 to 7 <br />percent. Such a change is regarded as insignificant in topographic and geologic conditions <br />of the area considering the ephemeral character of the streams. <br />The potential occurrence of subsidence cracks is by far the most important feature <br />that could disrupt the surface water bodies. The current experience from the Bowie #1 <br />Mine, supported by other experience gained from the mines in the Somerset - Paonia area, <br />indicates that subsidence cracks may develop in areas of low overburden. <br />Subsidence cracks occurring across a stream channel will cause the disruption of a <br />surface drainage and may cause it to flow underground (in worst case, into the mine). <br />Unless other remedial measures are undertaken the flow will be restored only after the <br />cracks are filled with the stream sediment, healed and sealed. Such a process might take <br />several years. A disrupted surface drainage could be restored much faster by installing <br />culverts over the subsidence cracks or by artificial sealing. <br />The occurrence of subsidence cracks was predicted in previous sections of this <br />report to be most likely in the areas where the overburden thickness is 800 feet or less. <br />Such prediction coincides reasonably with the criteria on the protection of surface bodies of <br />water recommended by Babcock and Hooker (1977). They recommend that in order to <br />protect any body of surface water, a minimum of 60 feet of solid overburden strata is <br />necessary for each 1 -foot thickness of coal seam to be extracted, whether the mining <br />method is longwall or pillar robbing. According to them, at least 570 feet (9.5 X 60) of <br />overburden at the Bowie #2 Mine would be required to protect any surface water bodies. <br />The impact of subsidence cracks on numerous ponds within the area is considered <br />important. Although, the probability of a subsidence crack disrupting a pond is remote. <br />PR -14 -23- 03/14 <br />