Perkins, Susan. 1989. Interactions of landslide-supplied sediment with channel morphology in forested watersheds. M.S.
Erosion rates were measured for eight 1- to 7-year-old landslide deposits in four stream channels. Once debris dams (if present) were breached, the deposits eroded rapidly, with 20 to 80% of initial volume removed in less than seven years. Erosion slowed markedly once stable channel geometries developed. A model is presented which describes the fluvial erosion of landslide deposits, and the long-term effects of landslide sediment on valley-floor and channel morphology. The diffusion-erosion model of Begin et al. (1981) is applied to the erosion of landslide deposits. Diffusion-erosion coefficients are determined, and suggestions are made for using the method to predict erosion rates of landslide deposits.
Transport of landslide-supplied sediment was studied on Salmon Creek, a fourth-order stream with gradients of 1-3% and partial bedrock control. Five landslides released a total of 7000 m3 of sediment into the lower 2 km of the creek between 1981 and 1988. Of this, 3000-4000 m3 came from 3 debris flows between 1985 and 1988, yet the volume of active fluvial sediment in the same reach was only 5000 m3 in 1988. The short residence time of sediment in Salmon Creek is due to high bedload transport rates and breakdown of gravels to suspendible sizes. Average bedload velocity is about 200m/yr.
The increased supply of sediment from landslides resulted in a shift in some reaches of the creek from a bedrock-dominated to a more alluvial channel morphology, while in other reaches little change occurred despite transport of large volumes of gravel. Aggradation and channel shifting were primarily associated with debris dams and debris-flows deposits. Morphological features such as bends, changes in gradient, and valley width also affected sediment transport rates and storage sites.
Patterns of sediment deposition in channels affected by landslides can be explained by considering the supply of sediment from upstream in combination with sediment transport capacity. This approach allows identification of reaches where aggradation will occur given sufficient sediment supply, and estimation of the effects of various sediment-supply regimes on channel morphology.