Modeling shallow landslides and river bed variation associated with extreme rainfall-runoff events in a granitoid mountainous forested catchment in Japan

In this study, we developed an algorithm to predict the long-term dynamic behavior of shallow landslides (< 1.0 m depth) based on physical processes. Shallow landslides and rainfall-runoff processes from a mountain area were calculated using the kinematic wave method to quantify subsurface and surface water flow, calculate sediment yield, and conduct infinite-length slope stability analysis. Debris flow, bedload, suspended load, and wash load were considered in the proposed model. Moreover, we simulated the presence of dams and sand-erosion control devices in the stream channel using a non-uniform flow sub-model in which energy loss near a dam is expressed by an energy-conservation equation. Sand-erosion control is important to controlling the amount of sediment in a dam catchment and the stream channel. The proposed method was validated by applying it to the Yahagi Dam catchment in Japan. Comparison of observed and calculated landslides indicated that the model reproduces the general characteristics of sediment transport dynamics reasonably well. The results showed 7.0 million m3 of total sediment inflow to the dam, 2.6 million m3 (approximately 37% of the total sediment flux) of sediment deposition behind the dam, and 4.4 million m3 (approximately 63% of total sediment flux) of sediment discharge (mostly wash load) from the dam. A survey measured 2.4 million m3 of sediment deposition behind the dam, which compared to our calculated value of 2.6 million m3 gives an error between simulated and observed values of 7%.