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June 1991 Cyp,us TCC •Analysu o~Sufxidrnrr E/jeru on Hydrology • l 1.0 INTRODUCTION This analysis of subsidence effects due to longwall mining in Panels 5 and 6 on surface and ground water hydrology in the study area is based on site speafic morphologic, hydrologic, and hydrogeologic characteris[ia. The rerults of the rubsidence analysis prexnted in Exhibit 7B, Fish Creek AVF Stream Study, Subsidence Predictions, are the bazis for the asxssment of hydrologic effects. Study area topognphy and proposed mining geometry are illustrated by Map 1, Subsidence Impact on Hydrology. 2.0 IMPACTS ON SURFACE WATER Subsidence due to longwall mining can impact surface streams or impoundments in the following ways: • Vertical settlement of [he ground surface • Localized changes in surface slopes • Vertical or sub-vertical subsidence cracks cauxd by tensile or compressive strains The degree of impact depends on site specific topographic and geologic characteris[ia az well az the mining parameters. The degree of vertical subsidence and changes in surface slope can be calculated and the nature and extent of surface fracturing can be predicted using site specific information coupled with the results of documented research. • Vertical xttlement can impact any hydraulic system, natural or man-made, which depends on gravity flow. Streams, ponds and ditches can be disturbed by significant changes in surface configuration. Subsidence can either inceax or decease local ground surface slopes. Such gradient changes in a stream channc] can impatt the stream by changing flow and material transport characteristic. Vertical xttlement of uniform magnitude over large areas typially has negligible effect on surface water bodies. Related changes in scream gradient are very minor {if any) and uniform vertical displacement has very little effect on impoundments such as stock ponds. The magnitude of vertical xttlement and surface fracturing is dependent on the materials underlying the streambed. Thex impacts would be more pronounced in bedrock and much lcss evident in unconsolidated sediments. Differential vertical xttlement over short distances will rerul[ in localized changes in ground surface slopes. If ruch changes occur along a stream channel, stream gradient will change within the affected reach. Any signifiant changes in gradient will impact stream flow and material transport characteristic. In an extreme case, major changes in gradient could disrupt flow and qux ponding or result in significant erosion and down<vtting. 2.1 IMPACTS ON FISH CREEK Fish Creek, a perennial stream with headwaters in the Dunckley Flat Tops area, is tributary to Trout Creek. The confluence of these streams is approximately 13.5 miles downstream of the County road (upstream of the study area). The watershed of Fish Creek upstream of the study area is approximately 41 square miles. ACZ /nc. • P.O. Box 774018 • Sreamtart Springb Colorado 80477 • (303J879-6260