Method to evaluate sediment-driftwood transport processes with flood runoff in a basin during heavy rainfalls

Recently, several extreme flood hazards with active sediment and driftwood transport have occurred in mountainous areas of Japan, as typically observed in the Akatani river flood disaster in 2017. Therefore, in order to mitigate such hazards, it is required to develop methods to evaluate flood, sediment, and driftwood runoff from a basin during heavy rainfall. This research proposes integrated methods to evaluate flood, sediment and driftwood runoff from a basin during heavy rainfall using numerical methods. The proposed methods are applied to the Terauchi Dam basin, where large amounts of sediment and driftwood were discharged during the 2017 Northern Kyushu heavy rainfall event, to discuss the applicability of the methods.

The methods are composed of a distributed rainfall-runoff model, slope stability analysis, sediment and driftwood transport in the slope based on mass system equation, and sediment and driftwood transport in the river channel by unit channel model. These models are integrated as Rainfall-Sediment Runoff (RSR) model to evaluate sediment and driftwood runoff from a basin. As a result of its application to the Terauchi Dam basin, we found that whether or not the debris flow enters the river channel, i.e., the definition of the upstream end of the river channel, has a significant impact on the results. Therefore, we investigated whether the sediment supplied by debris flow enter the river channel during the actual event. The results show that the upstream end of the river channel in the computation should be defined as approximately 4% to 10% slope, and in case the mesh size is sufficiently fine, the debris flow inflow into the channel is sufficiently evaluated. In conclusion, this research proposes an integrate methods to evaluate flood, sediment and driftwood runoff from a basin, and discusses its applicability to the disasters such as the 2017 Northern Kyushu heavy rainfall event.