Threshold response of small streams to surface coal mining,bituminous coal fields,central Pennsylvania |
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| Authors: | Kimball C N Touysinhthiphonexay Thomas W Gardner |
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| Abstract: | Stream response to surface coal mining and reclamation was studied in 29 small (0·13 to 5·72 km2) watersheds located in the bituminous coal fields of Central Pennsylvania. These basins, up to 82 per cent mined, were selected from 176 first-order tributaries of Beech Creek with similar vegetation, soil, lithology, and basin characteristics. Measurements were made at 262 cross-sections (an average of nine cross-sections per stream) of channel cross-section area, bankfull width, mean bankfull depth, dimensions of the largest moving blocks, stream slope, valley-side slope, basin area, and mined area. Observed differences in channel morphology were related to differences in extent of mining by means of scatter plots, correlation, cluster analysis, and bivariate regression. Stream response to increased peak discharge and channel shear stress produced by increased surface runoff from regraded mine spoil takes the form of enlarged channels and increases in the size of moving blocks. Large basin areas appear to dampen the effect of mining, resulting in limited channel enlargement with greater extent of mining. In contrast, where peak discharges and associated shear stresses exceed the combined erosional resistance of floodplain vegetation, colluvial blocks, and channel banks, streams adjust extensively to higher levels of mining, causing an abrupt increase in the size of transported blocks and eroded channels. In the first-order basins studied, this stepped response occurs at approximately 0·45 km2 mined area and 50 per cent of the total basin area mined. For streams that have exceeded both threshold levels, disequilibrium is demonstrated by a strong, positive correlation between local stream slope and basin area. Where both threshold levels of mining are exceeded, steep channel slopes reinforce the tendency of stream cross-sections to increase with greater disturbance by mining, necessitating that these streams rapidly adjust their morphology in order to attain a new equilibrium which is compatible with the conditions imposed by mining and reclamation. |
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| Keywords: | Channel cross-sections Small catchments Strip mining Equilibrium |
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