Wearing down the mountains: the role of debris flows in long term
denudation of the Virginia Appalachians

The rugged topography of Virginia's western highlands is being assaulted by processes ranging from the slow creep of soil down slope to roiling debris flows of mud, rock, and water moving at speeds exceeding 40 km per hour (25 mph). Geologists call the lowering of the landscape by erosion denudation. The efficacy of moderate-magnitude, high-frequency events (such as annual floods) compared to high-magnitude, low-frequency events (rare catastrophic storms) at eroding landscapes has long been debated. Studies have demonstrated that moderate-magnitude, high-frequency floods transport a significant quantity of the overall sediment load in low-gradient, large river basins. However, recent research suggests that this may not be the case in smaller basins located in mountainous terrain. Using data from four major storms that triggered debris flows in the Virginia and West Virginia Appalachians, L. Scott Eaton, from James Madison University, and three other colleagues propose that low-frequency extraordinary storms are responsible for over half of the long-term landscape denudation in the Virginia Blue Ridge. The results of this study are published in the April 2003 issue of the journal Geology.

During the last half of the 20th century four major storms triggered debris flows and extensive erosion in the mountains of the Blue Ridge and Valley & Ridge provinces in central and northern Virginia and easternmost West Virginia. In the Blue Ridge, these storms included the August 19-20th, 1969 storm generated by the remnants of Hurricane Camille in Nelson county and the June 27th, 1995 storm centered mostly on Madison county. In the Valley & Ridge these storms included a June 1949 event caused by convective storm cells and a long duration storm in November 1985, both located along the Virginia-West Virginia border. Precipitation from these storms ranged from 200 mm (~8") to in excess of 800 mm (~32") and, in some basins, fell over a seven to eight hour period.
Mean basin denudation was estimated by dividing the drainage basin area (km2) by the total volume of sediment eroded (m3) and ranged from 0.08 to 0.22 cm (0.03" to 0.08") in Valley & Ridge basins and 0.92 to 5.07 cm (0.36" to 1.98") in Blue Ridge basins. The authors suggest that higher denudation rates in the Blue Ridge are the result of higher storm rainfall totals. Mountainous areas characterized by densely vegetated terrain, thick soil, and rock-lined streams may be little eroded by flows caused from annual floods, however catastrophic storms that trigger debris flows transport large amounts of sediment. These debris flow producing storms re-occur at intervals of approximately 3,000 years.

Reference:
Eaton, L. S., Morgan, B. A., Kochel, R. C., and Howard, A. D., 2003. The role of debris flows in long-term landscape denudation in the central Appalachians of Virginia. Geology, v. 31, p. 339-342.

C. R. Shirvell & C. M. Bailey, William & Mary Geology

 


Upstream view of channel erosion
due to debris flow caused by
storm on June 27, 1995.
Near Graves Mill, Madison County, VA.
Geologist is L. Scott Eaton.