193 / 2024-02-27 12:06:08

Rainfall sediment runoff model with various sediment yields and wood debris and its application

wood debris,debris flows,sediment runoff modeling

A. The purpose of the work and B. The key issues or problems addressed

During heavy rainfall, if a series of landslides and debris flows occurs in mountainous areas, a large amount of sediment and driftwood can flow into river channels. If such sediment and driftwood are transported downstream by flood flows, they can cause significant damage there, and Japan have often experienced such extreme hazards such as exemplified by the 2017 torrential rain disaster in northern Kyushu. The authors have developed a rainfall-sediment-runoff (RSR) model to predict such flood disasters involving large amounts of sediment and driftwood. This study investigates characteristics and applicability of the method by comparing the computation results with the observed data to apply this method for the basin-scale sediment runoff and hazard predictions.

The RSR model analyzes the rainfall-runoff process and the following processes simultaneously: ① the rainfall-runoff process; ② the sediment and driftwood runoff entering the river channel due to landslides, debris flows, and slope erosion; and ③ the sediment and driftwood runoff process using a unit river channel model.

In this study, the authors applied the model in the Terauchi Dam basin, (the watershed area of approximately 51 km2), where a lot of sediment and wood runoff occurred in the 2017 Northern Kyushu Heavy Rainfall, to investigate the applicability of the model to the hazard predictions. In addition, we applied the model to the Abira river basin, (the watershed area of approximately 300 km2), where sediment is supplied from the hilly areas.



D. The results and conclusions

As a result of a series of computation with a different combinations of upstream end unit channel slope and sediment size distributions, the results indicated three important characteristics. First, the boundary between the area analyzed as debris flow and the area analyzed as sediment transport in the river channel should be set as a slope of about 10 degrees. Second, since there is an upper limit to the sediment transport capacity of a river channel, the amount of sediment discharge increases only up to a certain point according to the limit, even when the amount of debris flow is significantly huge. Third, the amount of driftwood runoff through the channel is almost proportional to the amount of debris flow. Under such characteristics of the methods, the model evaluates sediment runoff with various types of sediment yields and wood debris.