2021 年 14 巻 2 号 p. 20-30
When it rains over a large area, numerous mountain streams and rivers often lead to debris flows and greater river flows. Since these flows overlap at the stream and river confluences, damaged areas around the confluences may be expanded because of a flow bottleneck formed by debris flow deposition around them. To reasonably predict the processes, we require a numerical model that considers how the deposition and flood propagation of these flows occur at the confluences based on their various characteristics. Therefore, we developed a numerical model named “the multiple inflow model with debris flows and river floods.” In the developed model, the downstream ends of several 1-D calculation areas for mountain streams are connected on a 2-D calculation area for a confluence area at any selected points. With the developed model,we performed calculations to reproduce the debris flow disaster in the Nachigawa River plain, which induced by 2011 Typhoon Talas in Wakayama Prefecture, Japan. During this event, the debris flows that simultaneously flowed into the river from several mountain streams contributed to the deposition and flooding around the streams and river confluences, and the damaged areas around them expanded. The calculated result indicates that we can estimate reasonably the deposition and flood propagation of the debris flows and river flood around the confluences and their downstream areas. This also indicates that the developed model helps us to investigate how multiple debris flows inflowing from mountain streams contribute to the disaster, and develop more efficient countermeasures for these inflows.
