This study examined the effect of site characteristics on landsliding rates for clearcut sites, clearcut areas, and forested drainage basins. A landslide inventory of 1337 forested and clearcut debris slides and debris flows was taken, along with measurements of watershed physiography for forested basins and a 100-m grid sample of clearcut site characteristics.
Slope dominated other factors in controlling landsliding, though for different reasons in open slope, gully headwall, and gully sidewall terrain. Open slope debris sliding was primarily controlled by the portion of the area exceeding 35°. This control was modified by slope shape and position. Gully headwall failure rates were also strongly related to average slope and the range of slopes. Slope shape and position exerted a strong influence suggesting the importance of moisture to these failures. Rates of gully sidewall failures, however, were independent of slope - not surprising in an environment where average slopes exceed typical internal angles of friction for non-cohesive soils.
Bedrock formation (Sutherland Brown 1968), physiographic region, and aspect (when adjusted for steepland area and slope differences) have no effect on landsliding.
The volume of material entering streams depends on the factors that produce debris slides and flows, as well as on the character of the terrain between the steepland initiation points and a stream. In forested drainage basins, both flows and slides provide material to streams. The volume entering streams is controlled by the frequency of gully failures and the proportion of steeplands along the basin. In clearcut areas, the situation is simpler: most of the volume is delivered by flows. The probability of stream entry is controlled by the gradient of the footslope between the bottom of clearcut steepland and the stream. Angles near 15° are critical: above this most flows enter; below, few do.
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Updated November 27, 2008