鋼製透過型砂防堰堤の流木樹根と後続土砂の捕捉に関する実験的検討

摘要

我が国では，土石流災害が多発しており，その対策は 急務である。鋼製透過型砂防堰堤は，土石流対策の中核 となるものであるが，その機能は土石流の特性である偏 析（へんせき：流下過程で巨礫と中小礫とが分離する現 象）を利用し，巨礫を捕捉することで透過部を閉塞し， 後続土砂を堰き止めるものである。その部材間隔は，最 大礫径の1.5倍〜2.0倍に設定しておけば，開口部を閉 塞できるといわれており，確実に捕捉するために最大礫 径の1.0倍と設定されていることが多くある。%Sediment disasters cause human injuries, loss of life, as well as cultural heritage sites. Among other problems, woody debris hazards have increased due to typhoons and localized torrential rain. In particular, woody debris hazards have eroded river in the upstream region because of removal costs of woody debris from dam reservoirs and damage on dam sites. One of such counter measures is a steel open-type Sabo dams which consist of steel pipes, and they are designed to allow soil and small gravels to pass downstream through the gaps. When the debris flow has occurred, the Sabo dam can capture large rocks by trap mechanism of arch action. According to the report of the Izu-Oshima disaster case, steel open dams have captured woody drifts and sediment without rocks. Some of authors also surveyed the disaster site and noticed that steel open dams captured the woody debris which was intertwined with woody roots and sediment. However, the mechanism has not yet been clarified concerning why steel open dams have trapped the woody debris with roots and sediment without rocks. This paper presents an experimental approach on clarification of blockage of a steel open dam by woody roots and trap mechanism of following sediment by using a straight channel. The experiment sets up the steel open dam model in a straight channel and a mixed sand with driftwood model flow down from the upstream. Sand capturing mechanism and an influence of driftwood shape are observed. Finally, it is pointed out that roots of driftwood show significant effect on capturing ratio of sands.