Graduate Thesis Or Dissertation
 

Debris flow characteristics associated with forest practices in the central Oregon Coast Range

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  • Debris flows in the Pacific Northwest play a major role in routing wood and sediment stored on hillslopes and in first- through third-order channels to higher order channels and valley floors. Forest practices on steep, unstable slopes and removal of riparian trees along low-order streams can affect the frequency, magnitude, and composition of debris flows. The quantity and quality of debris flow deposits provides sediment and wood fundamental to the development of the receiving channel. Field surveys document characteristics of the initiation site, runout zone, and deposit of 53 debris flows in the Siuslaw Basin of the central Oregon Coast Range, during the winter of 1996. Landslides that initiated debris flows in clearcuts had a higher frequency, larger average volume, and runout zones that affected a greater length of stream channel than landslides from forested slopes. This difference resulted in an increase in the total volume of sediment mobilized by the debris flow, and a greater proportion of this sediment came from hillslope sources. Debris flows initiated at roads had an order of magnitude greater volume of sediment compared to non-road-related failures. Debris flows of equivalent size that traveled through a forested channel delivered only a slightly greater volume of large wood, than those through clearcuts. Size-class distributions of wood in the deposit and trees on the hillslope were not well correlated. The average diameter of wood in the deposit was greater than the diameter of trees currently present on the surrounding hillslopes. This difference reflects the legacy of large woody debris stored in low-order channels and valley floors. Large trees along the edge of the runout zone is also an important component in the recovery of these low-order channels, which were transformed into a bedrock state. Large trees along the edges of forested slopes are already supplying wood to these channels, and were the only mechanism observed for trapping large volumes of sediment. This mechanism for retaining sediment in high gradient, low-roughness channels is not available in clearcuts, which now contain the greatest proportion of bedrock channels. Forest practices, by altering the frequency, magnitude, and composition of the debris flow, may alter the long-term potential for developing complex channel morphology and high-quality aquatic habitat.
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