Storage of Woody Debris and Sediment in a Mountain Stream, Northern Kyushu, Southwestern Japan

Abstract

Distribution of woody debris and sediment was examined in a mountain stream, northern Kyushu, southwestern Japan. A densely-welded pyroclastic flow deposit dominates the study basin, and forms steep sideslopes and a flat valley floor. The watershed was disturbed by a powerful typhoon in 1991, which leveled many trees on the hillslopes. Subsequent storm-triggered shallow slope failures deposited timber along streambeds. Channel morphology (gradient, channel width and sinuosity), location and volume of woody debris, and streambed sediment characteristics were investigated three and half years after the typhoon. Log jams of various sizes occurred in the channel ; the largest log jam was 100m long, had a volume of 3, 100m³, and was located along the middle reach of the research section. Principal component analysis of channel morphology and volume of woody debris and sediment suggests that sediment deposition is controlled by tractive force decrease owing to broad channel width and gentle gradient. The analysis demonstrates that there is no conspicuous relationship between volume of woody debris and geomorphic factors such as gradient and sinuosity. Field observations reveal that most log jams occur behind large boulders in the research section. Our results indicate that accumulation of woody debris is influenced more by topographic barriers such as large boulders than by channel morphology. We believe that the supply of boulders from sideslopes greatly influences the storage of woody debris in the investigated stream.