Fluvial response to debris associated with mass wasting during extreme floods

Evolution of channels and bottomlands in mountain valleys of the central Appalachians is strongly influenced by debris supplied to stream channels from mass wasting during extreme storms. The type of change observed varies with basin scale and storm characteristics. Along channels receiving coarse sediment from debris avalanches or debris flows during Hurricane Camille in 1969, pure scour occurred in drainage areas less than 1 km² and gradients steeper than 0.1; in Hurricane Camille and in the June 1949 storm, mixed erosion and deposition with continuous reworking of the valley floor was observed along streams with drainage areas up to 65 km². In basins larger than 100 km², valley-floor reworking associated with influx of debris during both storms was localized and discontinuous.

In the South Branch Potomac River basin in West Virginia, intense precipitation within a small contributing area generated scores of debris slides and avalanches in June 1949; debris transported by tributaries to main valleys exceeded the competence of the larger channels and formed new bottomland. Long-duration moderate-intensity precipitation in November 1985 generated fewer debris avalanches. Flood peaks associated with a larger contributing area along the main valleys were 80% to 190% larger than in 1949 and caused extensive channel and flood-plain erosion, including truncation and removal of 1949 deposits. At some locations relict debris deposits may have influenced hydraulic conditions and affected patterns of erosion and deposition during the 1985 storm. Sequential occurrence of extreme storms with different hydrologic characteristics creates a bottomland mosaic of surfaces with varying elevations and textures.