Author(s): Yoshiya Igarashi; Norio Tanaka
Linked Author(s): Norio Tanaka
Keywords: Physical flume experiment; Diffusion coefficient; Drag force; Driftwood; Typhoon Hagibis
Abstract: In 2019, Typhoon Hagibis caused some perennial grasses and trees to be washed out and trapped around standing trees in the Arakawa River, Kanto District, Japan. Additionally, a large local scour hole was observed around the debris trapping. This presents a risk of levee breaching when a local scour hole is generated near the levee. There are many unknowns regarding the transport and trapping characteristics of floating debris produced by vegetation in a river during floods. Therefore, this study aims to clarify the transport characteristics of washout vegetation and the mechanism for trapping debris by standing trees. Two types of physical flume experiments, debris transport and trapping, were conducted in an open channel. 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. The trap rate at the first upstream row of the vegetation is especially high when the specific gravity of the debris is greater than 1.0. This is because the debris are 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 similar to the observed situations in the Arakawa River. The transport experiments show that the diffusion coefficient in cross direction of the debris becomes larger in low flow velocity regions. This is because the streamlines become complicated in low flow velocity regions, such as in front of the vegetation.
Year: 2024