Shunya Enomoto

Risk assessment and countermeasures for landslide dams in torrent rivers in hilly and mountainous areas

Tokuzo Hosoyamada

In rivers in hilly and mountainous areas, slope failures occur on unstable slopes due to earthquakes and heavy rains, and sediments block the river channels, forming natural dams. When a landslide dam collapses, running water containing sediment flows downstream, and there is a danger of causing disasters such as flash floods and debris flows. In the past, there is a record that a large-scale natural dam with a height of 50m in the Hidaka Horonai River in 2018 collapsed in about 2.3 hours after the formation of the natural dam in the Totsukawa flood disaster. Therefore, if a large-scale natural dam that collapses in a short period of time occurs, there is a risk of causing a catastrophe. Therefore, we thought that it was necessary to take measures in advance to reduce the damage. The purpose of this study is to simulate the behavior of rivers when a landslide dam collapses in a river in a mountainous area using a numerical model, and to investigate the effects of sabo dams according to the scale of the landslide dam.
Based on the terrain elevation distribution map (DEM) of the Imo River, where a natural dam was formed by the 2004 Chuetsu Earthquake, the discharge was given at an appropriate point and the flow of the river was reproduced using a nonlinear long-wave equation.
In this experiment, in the case without the sabo dam, the flow velocity and water depth increased as the scale of the landslide dam increased, but in some cases, the river width increased as the water depth increased, so the rate of increase in flow velocity and water depth decreased. Do you get it. Therefore, in the case of a 50m landslide dam and a 10m erosion control dam, the current velocity and water depth were exceeded. If a small-scale sabo dam is installed for a large discharge, the acceleration force due to the overflow head will exceed the effect of reducing the flow, so a half-sized sabo dam will have the opposite effect. It turned out that there was a case.