|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. In order to predict such hazards, the authors have developed Rainfall-sediment and driftwood runoff models. This model evaluates the flood, sediment, and driftwood runoff from a basin by combining rainfall-runoff analysis, 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. However, in order to conduct hazard predictions using this method, it is necessary to investigate reasonable ways to determine the following processes which particularly affect the results of the analysis.
1) Evaluation of sediment and driftwood inflow into a river channel, i.e., sediment and driftwood inflow into the "unit channel" may take place in the form of debris flow or in the form of bedload and suspended load.
2) Conditions of driftwood recruitment from the ground (or riverbed) to the water, and their deposition from water to the ground.
In order to employ the method for hazard prediction, this study investigates how these components affect the sediment and driftwood runoff.
In the Terauchi Dam (watershed area of approximately 51 km2), which was damaged by the 2017 Northern Kyushu Heavy Rainfall, sediment and driftwood deposition during the disaster event was observed. In this study, rainfall, sediment and driftwood runoff analyses are conducted for the Terauchi Dam basin to investigate the effects of the components above.
As a result, the relationship between the debris-fan and the unit river channel is revealed as follows;
1) Streams with steep slopes (approximately 10° or more) flow directly into the river channel. For fans with a slope of 4° to 10°, whether or not the stream flows into the river channel depends on the positional relationship between the river channel and the stream.
The analysis also showed that the flow into the river channel was approximately 4°. We also found that the upstream end of the river channel can be defined as approximately 4° to 10°, and if the mesh size is sufficiently fine, the debris flow into the river channel can be adequately evaluated.
2) As for the driftwood transport, it is found that the driftwood inflow into the dam could be well evaluated by setting a water depth to driftwood diameter ratio range of 1 to 1.5 as a limit for driftwood erosion and deposition.