HR: 1340h
AN: H13F-0473    [Abstracts]
TI: The Role of Debris Flows in Long-term Denudation and Landscape Evolution in the central Appalachians
AU: * Eaton, L S
EM: eatonls@jmu.edu
AF: James Madison University, Department of Geology and Environmental Science, Harrisonburg, VA 22807 United States
AB: Four major storms spanning a 46 year period from 1949 to 1995 that triggered debris flows in the Virginia-West Virginia Appalachians provided new insights into the role of high-magnitude, low-frequency storm events in long-term denudation and landscape evolution in mountainous terrain. Storm denudation measured in five Blue Ridge Mountain drainage basins (mean=3.7 cm) was approximately an order of magnitude greater compared to four basins located in the mountains of the Valley and Ridge province (mean=0.2cm). This difference is probably the result of higher storm rainfall from the Blue Ridge storms. Long-term (10$^{3}$ yrs) denudation rates were estimated using several lines of evidence, including 1) studies of the volume of sediment deposited in, or offshore of, the Atlantic Coastal Plain; 2) findings of parallel rates of continental uplift and denudation; and 3) historic sediment-load data. Using these estimates and subtracting the denudation attributed to chemical load, the mechanical denudation rate of the central Blue Ridge is approximated as 2.4 cm/k.y. Whereas debris flows recur at a frequency of approximately one event each three years somewhere in the unglaciated terrain of the Appalachians, the return interval is much greater when only individual mountainous basins are considered. Radiocarbon dating of debris-flow deposits in mountainous first- and second-order river basins of the Blue Ridge indicates a debris-flow return interval of not more than 2000 to 4000 yr. These data on debris flow frequency, combined with measurements of storm-induced upland basin denudation, suggests that approximately half of the long-term denudation from mechanical load occurs episodically by debris-flows. Although floods of moderate magnitude are largely responsible for mobilizing sediment in low-gradient streams, the data suggest that high-magnitude, low-frequency events are the most significant component in delivering coarse-grained colluvium from mountainous hollows and channels to the lowland floodplains. In the Appalachians, and probably other mountainous terrains located in humid-temperate climates, the role of high-magnitude events on geomorphic effectiveness and landscape evolution arguably has been underestimated. The presence of coarse bedload stored in upland channels, porous regolith that mantles the slopes, and densely-vegetated terrain marginalizes the effectiveness of frequent, low magnitude storms in mobilizing sediment. In contrast, high magnitude events trigger debris flows, which incise streams, export sediment from the uplands, and deposit colluvium onto debris fans or into lowland stream channels and floodplains. In the Blue Ridge, numerous upland channels impacted by debris flows in the study areas have been slow to recover; and they continue to maintain a greater hydraulic geometry than required for frequent, low magnitude storms. Throughout much of the Appalachians, the ubiquity of specific landforms and deposits; including debris fans and levees, boulder bars and terraces, remarkably wide alluvial valleys that originate at the terminus of debris fans, and single-channel floodplains that become braided during catastrophic flooding all suggest that geomorphic work and effectiveness in mountainous terrain is achieved largely by high-magnitude, infrequent events.
DE: 1815 Erosion and sedimentation
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting