IMPACTS OF SURFACE MINING AND 'AOC' RECLAMATION ON SMALL STREAMS AND DRAINAGE NETWORKS

摘要

Many Appalachian drainage networks have been extensively altered by surface mining and reclamation. Mine sites reclaimed after passage of the Surface Mine Control and Reclamation Act (SMCRA) of 1977, must meet SMCRA Approximate Original Contour (AOC) requirements. However, investigations at three north-central West Virginia study show AOC is far removed from original landscape form and function. Specific drainage transformation varies greatly with reclamation style. Small stream drainage density decreased in all cases, but declines were less where sediment trenches act as low-gradient streams. Many reclaimed slopes lack small streams, relying on groundwater and overland flow to deliver water to larger streams. Thus, many headwater streams in unmined uplands are severed from the rest of the drainage system. Slopes below truncated headwater streams are prone to instability. Groundwater and sheet flow lack capacity to handle moderate- to high-magnitude runoff from intense rainfall. Steep slopes commonly showed significant gully erosion very soon after reclamation and these gullies fore-tell long-lived channel paths across reclaimed slopes. Most constructed drainage was designed for extreme runoff, and lacks the "channel within a channel" cross-sections that promote stability during formative bankfull flows that occur every year or two. Constructed channels are poor matches to natural streams with respect to gradient, sinuosity and bed materials. Oversize channels may not fail during the reclamation bond period, but inevitably most will face sedimentation or lateral erosion problems. Existing reclamation practices inadequately address the important roles of small streams in storm-water conveyance and sediment transport, and virtually eliminate ecological functions critical to the biological viability of all streams throughout a watershed. Long-term stability of reclaimed slopes and ecological sustainability of streams will require designs that accommodate both moderate bankfull flows and large infrequent floods.