山地流域における流木天然ダムの形成・破壊と流木の流出過程

抄録

Wood debris (WD) transport processes accompanied with the formation and failure of WD dams were studied in a small fourth-order basin with steep bedrock low-order channels in Hokkaido, Japan. The basin experienced three floods between 1992 and 2006. WD dams are wood debris accumulations that completely block channels and retain large volumes of sediment upstream. They are common in forested mountain streams with small bankfull widths, and affect transport of both WD and sediment in mountainous watersheds. Mapping and dimensional measurement of the WD dams in the study basin have revealed that many dams are distributed throughout first- and second-order channels (<10 m in width) and that these dams trap a relatively small amount of sediment, whereas fewer dams occur in third- and fourth-order channels (15-45 m in width). A dam in the fourth-order channel in the study basin was found to retain a huge amount of sediment along a 500-m reach of riverbed upstream. Half of all the WD dams in low-order (first- and second-order) channels form at the place of WD recruitment by streamside landsliding, and the other half form after WD is transported in channels by debris flows. Repeated mapping of WD dams in the low-order channels before and after flooding in 2003 and 2006 revealed that 90% erosion of dams occurs during large floods and 40% erosion occurs during middle-scale floods. Three stages of WD transport processes were identified : the first stage includes processes from WD recruitment through WD dam formation in low-order channels ; the second stage comprises failure of WD dams in low-order channels, WD transport downstream, and WD deposition in high-order channels (dam formation in some cases) ; and the third stage includes failure of stable WD dams in high-order channels, WD release, and downcutting of elevated riverbeds. In the study basin, sediment storage behind WD dams was found to account for 40-70% of all sediment stored in low-order channels and 50-60% of that in high-order channels. Therefore, formation and failure of WD dams that function as major sediment reservoirs significantly affect sediment transport in mountainous terrain