Snake line plots for 52 major sediment movements, which have caused human and property damage in Japan, were created with rainfall indexes. The snake line plots were evaluated using rainfall indexes : 1) 60 minutes accumulated rainfall vs soil water index ; 2) 60 minutes accumulated rainfall vs effective rainfall (half-life 72 hours) ; 3) probabilistic 60 minutes accumulated rainfall vs probabilistic soil water index. We quantitatively identified the range of critical rainfall indexes of each sediment movement (debris flow, slope failure, sediment-laden flood, and deep catastrophic rapid landslide).

In this study, we developed and evaluated a new countermeasure construction method through experiments and analysis in anticipation of water permeability through gaps between the interlocking blocks used to construct block sabo dams. In the water channel experiment, deformation of a non-permeable sabo dam model occurred due to significant overflow and water pressure, such that its trapping function could not be maintained, whereas a block sabo dam model with permeable walls maintained its trapping function even after it was filled with sand. We applied an existing analytical model to explore this novel sediment trapping effect due to sabo dam drainage. The results of our simple model experiments based on ideal conditions suggested that a bottom drain filter would effectively capture debris flow as it passed through the unsaturated sedimentation surface ; block sabo dams require further study to ensure their effective development.

These days, nearly 700,000 sediment-related disaster hazard areas are designated in Japan. Although the population may decline rapidly in the near future, the number of dangerous areas which need countermeasures would not decrease likewise. Also it is concerned that installing disaster prevention facilities would be difficult to proceed, especially in mountainous areas. Non-structural measures such as warning evacuation and land use restrictions have also been carried out, but they have not functioned so sufficiently. To avoid a crisis, promoting to move from hazardous areas is surely the most secure policy, but it has not been applied as much for various reasons. On the other hand, infestations of COVID-19 has accelerated remote work, and lifestyles that do not fix places of residence in one place have spread at a stretch. Considering these changes, it might be possible to promote a lifestyle of living in two bases, one to live in general, and another as a safe place to avoid disasters. This might be a better solution rather than completely relocating from dangerous areas. In this paper, the required conditions for managing such a policy to arrange this lifestyle are pointed out. It is needed to extract areas where these measures are highly effective. Also, ensuring profitability by collaborating with local governments' redevelopment projects might be effective.

Sabo-related facilities are planned and designed chiefly for peak sediment discharges and for the volume of sliding debris mass. Amounts of rainfall and river or slope (sub-) surface flow discharges do not always determine their plans and designs, despite their triggering impacts. Sediment production and landslide movement are less frequent events than floods or severe ground water flows. Sabo-related facilities are expected to be instrumental in river control as well as in hillside urban drainage systems, never to be of any harm. Harmonization is indispensable. Also, with the evolving climate change, extreme rainfall seems to happen more often than before. Basic rainfall intensities are set locally in different formats. In this paper, therefore, their present states are collected and shown nationwide in terms of both 24 hour rain and supplementary 60 min rain, which vary from one area to the others. In a worst area, current planned rainfall is 40% that of the estimation (1/100) suggested in this study. The result is expected to be a basis for further detailed studies, especially for risk assessment.

Nowadays, hazard maps are commonly made available to the public. However, while public awareness of hazard maps is increasing, few people appropriately understand their meaning. And the fact that the evacuation rate in areas affected by debris flows remains low is seen as a problem. Therefore, the objective of this study was to help residents correctly understand the risk of debris flows and improve their awareness of disaster prevention. To achieve this goal, the 3D model and the virtual space environment were created by using basic survey data. We also studied a method to easily create 3D models of multiple buildings in areas where 3D city models have not been developed. As a result, it became possible to intuitively grasp the risk by comparing the height of the buildings and the height of the debris flow.