The topographic restoration method (here in after TRM) that can estimate the topographic features of pre-disaster condition by debris flow was developed. The DEM from the airborne LIDAR data and the Gaussian mixture model were applied for estimating the cross-section and central position in damaged stream lines by debris flow. TRM was applied to Mt. Umyeon catastrophically damaged by debris flows in 2011 in order to confirm how accurate to restore topography. Root Mean Square Error(RMSE) between LIDAR data before debris flow and restored data by TRM after that in Mt. Umyeon showed 0.668 m. The three kinds of the soil distribution output such as a specific purpose such as soil volume distribution, section-specific distribution and sediment transport distribution were developed. These output may be useful to estimate a spatial distribution of debris including soil, boulders and rocks within the statistical confidences. As analyzing result of the soil distribution output, we concluded that the soils in the stream-bed and bank along the streams contribute to the sediments in a downstream much more than the amount of soils in a land sliding site.

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Stability of riverbanks under three unsteady river flow conditions is simulated. First is the case of constant water level in the river, second is the case of filling the river to the riverbank top and finally the case of drawdown. Two modes of drawdown are analyzed; the rapid drawdown and the slow drawdown. A finite element model of saturated unsaturated seepage flow was de-coupled with a plain strain elastic-plastic finite element model using strength reduction technique to calculate the stability factor of safety (FOS) of bank material. The influences of location of phreatic surface, pore-water pressure, drawdown rate and ratio on the riverbank stability are discussed in details for each case. Results showed that safety factor for saturated riverbank is nearly 83% of its dry value (for the studied case). The case of filling is most likely to stabilize the riverbank. For the case of slow drawdown, the minimum FOS occurs when the water depth is about 0.25~0.3 the bank height. The case of rapid drawdown is the most critical case.

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Early-warning systems for natural disasters are important tools for disaster risk reduction. In 2005, the Japanese government initiated a new nationwide early-warning system for landslide disasters. The system uses the RBFN method to set the criterion (Critical Line: CL) primarily based on rainfall data recorded as not triggering disasters. Moreover, after large earthquakes, the thresholds (CL) are scaled down by 20–50%, considering the seismic disturbances in the slopes. In this study, we examined the validity of these ratios by analyzing disasters which occurred during the seven months after the Great East Japan Earthquake. As a result, we found that the use of interim CL improved the landslide capture rate by 18.8%, indicating that the interim standard is effective. The required minimum ratio of scaling down the interim CL is 20–30% in areas affected by more than level 5+ earthquakes without changes of landslide capture rates. The required minimum ratios are expected to reduce false predictions.

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After the 2010 eruption of Mt. Merapi in Indonesia, many instances of debris flows and flash floods occurred in the Putih River basin. The characteristics of debris flows and flash floods were analyzed using images of these events captured by an internet protocol (IP) camera installed near the PU-C11 sabo dam and rainfall records collected at the Gunung Maron observation station. The magnitude of debris flows and flash floods during the 2011–2012 rainy season was smaller than during the 2010–2011 rainy season. Additionally, the rainfall intensity that caused no debris flows or flash floods during the 2011–2012 rainy season was smaller than that observed during the early 1990s, when few debris flows and flash floods occurred. These results suggest that the hydrological regime of the catchment area was radically altered after the eruption.

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