GEOLOGIC CONTROLS OF BASIN DENUDATION FROM DEBRIS FLOWS IN ROCKBRIDGE COUNTY, VIRGINIA

In 1995, a mountainous basin in the Blue Ridge Mountains of Virginia was impacted by debris flows. The basin denudation was approximately twice that of four other debris flow impacted basins in Virginia. These debris flows are characterized by deeply incised channels in zero, first and second order stream basins. In multiple failure sites, bedrock limits the lateral and vertical extent of erosion from debris flows. Colluvium tends to break into orthogonal blocks due to jointing in the quartzite. Statistical analysis shows the presence of a minimum of two joint populations that serve as bedrock controls on lateral erosion. Resistant bedding planes, synthetic to slope, control vertical erosion below a finer-grained layer of saprolite. Well-developed joints provide for both rapid infiltration of rainwater and bedrock controls for debris flow initiation. Possible mechanisms of slope failure include 1) elevated rates of fracture recharge in steep, mountainous stream valleys and 2) fissile, bedrock planes. Fracture recharge may serve to substantially increase rock pore pressure during high intensity precipitation, especially when coupled with antecedent rainfall. The combination of increased recharge, weathered bedrock and joint orientations are likely the main factors resulting in the highest measured values of basin denudation from debris flows in Virginia.