257 / 2024-02-28 17:55:49

Experimental study on the transport and trapping of floating debris by standing trees during a flood event

physical flume experiment,diffusion coefficient,drag force,driftwood,Typhoon Hagibis

A post-flood survey of the 2019 Typhoon Hagibis revealed that some perennial grasses and trees were washed out and the produced floating debris (including driftwood) were trapped at fringe and inside of the remained vegetation in the Arakawa River, Kanto District in Japan. The trapping debris could increase water level locally due to the additional drag force and increase the risks of local scour around the vegetation due to the horseshoe vortices. There are many unknowns on the transport and trapping characteristics of floatings produced by vegetation in a river during floods. Therefore, the objective of this study is to clarify the transport characteristics of washout vegetation (including driftwood) and the mechanism for trapping debris by the remained vegetation.

Two types of physical flume experiments, debris transport and trapping, were conducted in an open channel with the dimensions of 0.7 m wide, 5.0 m long, and 0.5 m deep (with a constant bed slope of 1/500). Five floating debris models with different specific gravity, shape, and rigidity were used, assuming the driftwood and perennial grasses observed in the post flood survey. In the debris trapping experiment, the ratio of debris length L and the spacing of standing trees W were focused on, and the trap rate and location were investigated for the five types of debris model. In the debris transport experiments, the diffusions of the debris under a different flow velocity distribution were also investigated for the selected two debris models.

The trapping experiments show that the trap rate at the first upstream row of the vegetation is especially high when the specific gravity of the debris is larger than 1.0. This is because the debris is caught in the horseshoe vortex in front of the vegetation. The trap rate inside the vegetation is relatively high when the rigidity of the debris is low. This phenomenon is like the observed situations in the Arakawa River. The transport experiments shows that the diffusion coefficient in cross direction of the debris becomes larger in the low flow velocity region. This is because the streamlines become complicated in low flow velocity regions such as in front of the vegetation.