Abstract
A two-dimensional debris and mud flow model that considers both laminar and turbulence flow was developed. Subsequently, the model was applied to a debris flow that occurred in Asaminami, Hiroshima, Japan in August 2014. The applicability of the model and the debris flow characteristics are discussed. The calculated horizontal distribution of sediment deposited in the Asaminami residential area was in good agreement with the horizontal distribution of the deposited large rocks and driftwood. This result indicates that the fine material in the downstream area was transported by water flow resulting from heavy rain that occurred after the debris flow. The scale of the initial debris flow was small; however, it increased with time, because eroded bed material and water were entrained to it. Therefore, it is important to reproduce the development process of debris flows to predict the amount of sediment produced, the deepest flow depth, the maximum flow velocity, and the inundation area. The averaged velocity of the simulated debris flow was about 9 m/s, and the velocity at the entrance to the residential area was about 8 m/s. This kind of information can be used to design sediment deposition dams. The travel time of the simulated debris flow from the upstream end of the main channel to the entrance of the residential area was 96 s. This kind of information can be used for making evacuation plans. Valley bed steps can suppress the deepest flow depth which is very important for the design of check dams; therefore, the high-resolution elevation data and fine numerical grids that reproduce step shapes are required to accurately calculate the deepest flow depth and maximum flow velocity.
Highlights
Sediment disasters caused by debris and mud flows that originate from surface landslides generated by high intensity rainfall events happen frequently, for example, in Japan, Miyagi in 2019, Hiroshima in 2014 and 2018, Fukuoka in 2017, Hokkaido in 2016, Kagoshima in 2015, and Izu Oshima in 2013
According to an IPCC (Intergovernmental Panel on Climate Change) report [1], because of global warming that will result in extreme weather characteristics, it is expected that the frequency of high intensity rainfall events will increase even if average precipitation does not change
The frequency of debris and mud flows caused by weather characteristics resulting from global warming are expected to increase, and effective countermeasures based on the results of numerical analysis of the debris and mud flows are required
Summary
Sediment disasters caused by debris and mud flows that originate from surface landslides generated by high intensity rainfall events happen frequently, for example, in Japan, Miyagi in 2019, Hiroshima in 2014 and 2018, Fukuoka in 2017, Hokkaido in 2016, Kagoshima in 2015, and Izu Oshima in 2013. When the solid material size is small (e.g., fine granite, volcanic ash, dry snow, etc.), a turbulence flow layer is formed on the laminar flow layer near the bed, which becomes mud flow and mixed-motion avalanche. The formation of both laminar and turbulence flows reduces the equilibrium bed slope and increases the bed erosion rate. Small scale turbulence can be formed in a laminar flow layer and must be particles, where kf =0.16, kd =0.0828, and e are the inelastic coefficients of the particles and dm is considered when estimating energy dissipation.
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