Landslide dam outburst floods can cause catastrophic damage to downstream areas. Therefore, it is necessary to predict the area affected by such flooding when large-scale landslide dams are found. There are three types of a process leading to landslide dam outburst : erosion due to overtopping, instantaneous slip failure, and progressive failure. Numerical simulation models of erosion due to overtopping and instantaneous slip failure have been previously developed. However, few studies have been conducted on a numerical simulation model of progressive failure. In this study, we conducted small-scale flume experiments on progressive failure of a landslide dam and investigate the process of dam deformation. The results showed that a partial collapse proceeded upward rapidly when groundwater level was high. Furthermore, the large-scale slip of a landslide dam increased the peak discharge rate. Based on the results of these experiments, we developed a numerical simulation model of landslide dam progressive failure. We then verified the model by comparing its results with the experimental results ; the model reasonably reproduced the dam deformation process and outflow hydrograph.

A part of the wing on the left bank of the Sugawa Daiichi sabo dam was destructed by woody debris or sediment discharge due to heavy rainfall in the northern part of Kyushu Island, July 2017. A large amount of woody debris was captured on the upstream of Sugawa Daiichi sabo dam, and the influence of woody debris has been considered to be one of the cause of the destruction. However, the load acting on the wing has not been analyzed and the causes of the destruction has not been clarified yet. In this report, each load (hydrostatic pressure, impact force of stone, impact force of woody debris, etc.) was estimated from data acquired during the field survey, several concrete cores were sampled from the sabo dam, and executing concrete strength tests. Structural calculation and stability calculation were carried out based on the estimation and concrete strength tests, and the cause of destruction was analyzed. Analysis result of the structural calculation shows that destruction did not occur only due to hydrostatic pressure and it was assumed that one of the reasons for destruction was that there was a construction joint with low strength. From the stability calculation it was confirmed that a part of the wing could fall due to the collision of multiple woody debris. The destruction of the wing was estimated to be caused by multiple woody debris colliding or multiple woody debris collided and fell after lower woody debris load acted.

Large-scale deep-seated landslides and landslide dam formation have become common, for example during the 2004 Mid-Niigata Prefecture and the 2008 Iwate-Miyagi Inland earthquakes, and Typhoon Talas of 2011. Countermeasures have been aimed at both prior and subsequent events. Most countermeasures feature standby-type erosion control dams that are expected to capture debris flow or reduce the scale thereof. Although mountain foot fixing of such dams improves slope stability, few quantitative data are available ; planning remains empirical. Here, we examine slope height, the extent of ground collapse, the microtopography of the collapsed slope, and the extent and scale of prior deep failure. We quantify the erosion control afforded by mountain foot fixation of dams aimed at controlling deep-seated landslides ; we explore whether foot-fixing stabilizes slopes subjected to rising groundwater levels. We analyzed the stability of model slopes when validating and seeking to apply our quantification method. When we examined earlier deep-seated landslides in the Kii Mountain range, we found that mountain foot fixation exerted a fixed effect in terms of scale reduction. We evaluated the slope stability afforded by such fixation using a simple numerical method ; we found that it was possible to evaluate quantitatively how foot fixation reduced the extent and frequency of instability when the landform and soil parameters were defined. In the future, it is necessary to accumulate continuous observation data of groundwater level at the time of collapse occurrence, to verify the validity of this method, and to examine the application method to actual countermeasures.

The calcium and magnesium ion concentrations of stream water are greatly affected by chemical weathering and may thus predict the risk of slope failure. The on-site measurements of these ions might enable on-site risk evaluation and improve the efficiency of the field survey. We measured calcium and magnesium ion concentrations of 22 spring water sources in the 2009 Hofu disaster region using portable water-quality analyzers. We found significant differences in calcium ion concentrations between collapsed and non-collapsed regions, but no significant difference in magnesium ion concentrations. We analyzed the terrain of sites exhibiting high calcium ion concentrations that did not in fact collapse. We confirmed that the slopes were greatly affected by the heavy rainfall of 2009, increasing the risk of collapse. Therefore, the on-site measurement of calcium ion concentration was a useful tool to assess the risk of slope failure.

The first laboratory of SABO in Japan was established at the University of Tokyo in 1900. In the incipient period of this laboratory (1901-1909), two foreign teachers were invited to lecture on SABO. In 1912, Kitao MOROTO became the first professor as Japanese in charge of this laboratory. MOROTO studied SABO under F. WANG in Vienna, Austria from 1909-1912. He not only studied in Vienna but also visited many places where SABO projects were executed in France, Germany, Switzerland, Italy, Czech, Poland, Croatia and Montenegro. Through these studies and field trips, he gained much theoretical and technological knowledge concerning SABO. He wrote many ariticles about his studies and field trips and brought back many photos of SABO facilities in European counries to Japan. After coming back to Japan, he wrote a series of technological books and introduced new technologies of SABO in Europe to Japan. In recent several years, author visited many sites maily where MOROTO had visited and remained many photos of SABO facilities constructed during several decades before and after 1900 and surveyed these SABO facilities. In European coutries, the downside slope of check dam had already been steep in those days, although in Japan it had been gentle till 1920 s. Also the way of piling stones for check dam in European countries was different from that in Japan. In this paper the present situation and features of these facilities are described too. This paper will contribute toward studying about structural maintenance and effectual utilization of old SABO facilities in Japan.

Grave impacts of sediment related disasters make it imperative to undertake evacuation behavior before outbreaks of disasters. Due to technical inability to predict precisely an outbreak of disaster on a particular spot at a particular time as well as lack of phenomenal index demonstrating immediate danger of outbreak, evacuation from sediment related disasters has to be a pre-cautionary behavior in uncertain situation. In major sediment related disasters in recent years there were many inhabitants that failed to evacuate although they were in danger of being hit by outbreaks. This “evacuation failure” was also seen in the sediment related disasters caused by heavy rain in July 2018. Authorities provide informations that are expected to orient evacuation behaviors in case of danger of outbreaks. They are progressively improving in quality and accessibility. Nonetheless, it is often difficult for each individual alone to make appropriate decision in uncertain situations. That's why in addition to further improvements of authorities' informations, mutual help in evacuation at community level is highly important. Community leaders are encouraged to take initiatives in evacuation behavior with the help of administrative authorities and sediment related disaster experts.

In areas where urbanization has progressed, there are many narrow spots where houses etc. are dense, and it is recognized that the construction of the sabo dam has not progressed due to construction restrictions. In this report, for the erosion control in the Shonai River and the Kiso River area, the characteristics of the narrow area and the problems during the construction of sabo dam are described. We propose that the solution of the construction of the sabo dam by the construction machine and the structure of sabo dam.