Spatial Change of the Strength of Woody Debris Accompanied with Sediment Storage

Abstract

On the slope at northern part of Kyushu, many blown-over trees occured by typhoon 19, 1991. The trees, slipped down from the slope at intensive rainfall, often construct woody debris in streams. Woody debris caused sediment storage in the stream (in this case, woody debris is called a “wall”) . Spatial changes of the strength of the wall accompanied with sediment storage was investigated. Relationships between the strength of the wall and the distance from the source of trees, and between the strength of the wall and the volume of sediment storage were analyzed.
Result were as follows. The volume of sediment storage tended to increase according to increment of the strength of the wall (to 550 m³) . Momentum of debris flow stopping by the wall increased along the distance from the source of trees. These analyses lead that the wall of the lower part of stream become more stable against debris flow than the wall of the upper part of stream.