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The occurrence of subsidence cracks on the surface may not necessarily result in <br />direct inflows into the mine when a surface drainage is disrupted. Studies by various <br />authors such as Orchar (1969, 1975) in Great Britain and Williamson (1978) in Australia <br />indicated that the subsidence cracks may be limited in depth by the development of <br />compressive stresses replacing the tensile stresses that cause surface fracturing. <br />Changes in land surface and drainage slope may affect the run-off water quality by <br />increasing the sediment load. The increase of drainage channel slope could result in an <br />elevated flow velocity and therefore, increase the erosion potential. <br />In summary, we believe that the occurrence of surface subsidence cracks comprises <br />the most important hazard to the surface drainage regime. There occurrence is predicted <br />principally within the areas where the overburden thickness does not exceed some 800 <br />feet. Such areas include: <br />o small ephemeral drainages near the outcrop <br />o mouth of Sheep Corral Gulch <br />o mouth of Dove Gulch <br />o mouth of Freeman Gulch <br />To eliminate or reduce the hazard of surface drainage disruptions would require a <br />change of the mining plans and design of protective pillars or zones of lower recovery <br />beneath these areas as discussed in Section 8.3. Alternatively, surface modifications of <br />the surface drainage and ponds could be made in cases of any occurrence of subsidence <br />cracks. Such surface modifications would not preclude a temporary disruption of the <br />existing surface run-off regime. <br />2.3 GROUND WATER <br />Total extraction (and, to a lesser extent, partial extraction) of coal seams causes <br />collapse, fracturing, bed separation and bedding plane slip in the roof strata above the <br />seam. All of these disturbances of the overlying strata can have a substantial impact on <br />• ground water if a major aquifer is within reach of the disruption. The height of the disturbed <br />- 21 - 10/00 <br />P~-o y <br /> <br />