ESTIMATION OF COLLUVIAL FILLING OF DEBRIS-FLOW SOURCING AREAS OF THE BLUE RIDGE AND VALLEY AND RIDGE PROVINCES IN THE CENTRAL APPALACHIANS, VIRGINIA

Page County, located in the central Appalachians of northwestern Virginia, is flanked to the west by the Massanutten Mountains and by the Blue Ridge Mountains to the east. Debris flows and other mass movements triggered by high intensity rainfall are common in the steep mountainous terrain of Virginia. In 1995, a catastrophic storm struck neighboring Madison County generating over 775 mm of rain in 16 hours in the Blue Ridge Mountains; triggering over 1000 slope failures, and scouring stream channels and debris fans. The potential for debris flow activity in Page County is likely due to the similarity of the geology, steep mountainous terrain, and the abundance of colluvium stored in debris flow initiation areas.

This project uses a quantitative procedure for calculating debris flow volumes by estimating the amount of colluvium residing in active drainage channels. Over 30 discrete drainages within Page County were examined, and approximately 200 colluvial wedges were determined to have the potential for transport as debris flow material during high magnitude rainfall events. Most of the colluvial wedges are situated within the active fluvial system of zero-to-second order tributaries. The wedges are dominantly poorly sorted, grain-to-matrix supported, and consist of sub-angular-to-angular shaped clasts. The volume of each wedge was approximated by field measurements of the dimensions of length, width, and a minimum thickness based on the estimated depth to bedrock. Median and maximum particle size of each deposit, and the slope of the wedge surface, were documented for calculations of bedload transport analyses. The extent and location of significant colluvial loading in these drainages provides knowledge to geoscientists that investigate the magnitude and frequency of past debris flow events and helps establish the percentage of drainages in this region that have the potential to yield debris flows in the future. This data also provides information to land planners and emergency responders of the potential of debris flow hazards that may occur in this region of the Appalachians in the future. Further study will include analysis of the geologic controls characterizing hazardous drainages in Page County.