A study for development of a new type of driftwood trap using steep flume experiments

Hironori MUTO, Daizo TSUTSUMI, Mayuko FURUKAWA, Taro UCHIDA, Takuro SUZUKI and Toshiyuki HORIGUCHI


Driftwood disasters, which often occur at the same time as sediment‐related disasters, have been widely reported in recent years. To prevent and mitigate driftwood disasters effectively, we developed a uniquely shaped driftwood trap that differs from typical driftwood countermeasures. The driftwood trap proposed in this study consists of horizontal rods aligned in the flow direction. When driftwood reaches the trap, it is efficiently scooped up by the horizontal rods and separated from the water and sediment. The driftwood trapping mechanism was verified by flume experiments. River channels are generally distinguished into debris flow and bedload sections by the type of sediment transport, which changes depending on the slope of the riverbed. Channels with slopes greater than two degrees are defined as debris flow sections, while channels with slopes less than two degrees are defined as bedload sections. We expect that this driftwood trap is effective for both debris flow and bedload sections but, in this study, we focused on a flume slope of five degrees to verify the trapping function in debris flow sections. In the flume experiment, water was circulated at a constant flow discharge, driftwood models were supplied into the flow at various conditions, and the trapping ratio of the driftwood trap was measured. In the analysis of the experimental results, we focused on the relative length of the driftwood models with respect to the horizontal rod spacing, the number and density of the supplied driftwood models, and the surface roughness of the horizontal rods. We found that the longer the relative length of the driftwood model, the higher the trapping ratio. The trapping ratio was also high when the number or the supply rate of the driftwood models was high. With straight driftwood models, the trapping ratio did not change with the surface roughness of the horizontal rods. However, in the case of non‐uniform driftwood models using natural trees with bends and nodes, the trapping ratio increased with rod surface roughness.

Key words

driftwood, driftwood trap, debris flow section, flume experiment, trapping ratio