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Abstract

Debris fans in low-order Appalachian Mountain drainage basins can be used to estimate the return periods between catastrophic debris flow events such as the Hurricane Camille storm of 1969 in Virginia. Debris fans in Davis Creek, Virginia, have been the sites of repeated debris flow deposition at least three times during the last 11,000 years. Debris flow frequency estimates are possible if individual events can be recognized in the fan stratigraphy. Discrimination of events is based on the recognition of paleosols, and on abrupt changes in sediment texture and in matrix composition at suspected event boundaries. Major controls on slope stability appear to include the orientation of the slope, bedrock structure, and presence of colluvial hollows at the sites prior to slope failures. Hollows are sites of between-event accumulation of colluvium, and are areas of subsurface water concentration during heavy rains. Tropical air masses seem to have been a factor in most historical Appalachian debris flows. The early Holocene initiation of debris flow activity on the central Virginia fans appears to coincide with paleoclimatic data, indicating the commencement of conditions that permitted the invasion of tropical moisture into the region at the close of Pleistocene time.

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