Explosive volcanic eruption is one of the most hazardous natural phenomena. During explosive eruptions, a mixture of volcanic ash and gases is ejected from a volcanic vent into the atmosphere. For hazard risk assessment, it is important to comprehensively explain various observed data during eruptions and to understand the dynamics of explosive eruptions and the mechanism of volcanic ash dispersal. We have developed a pseudo-gas model of eruption cloud dynamics and ash dispersal. Our model has successfully reproduced the heights of eruption cloud and the distribution of fall deposits during large eruptions such as the Pinatubo 1991 eruption and those during small eruptions such as the Shinmoe-dake 2011 eruption. For more accurate estimates of volcanic hazard risks, two-way coupled models of multiphase flow are required.

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Large-scale debris flow disasters take place due to heavy rainfall recently. The present study discusses the relationship between the trigger and the inherent factor of debris flow occurrence. Three debris flow disasters in Izuoshima, Nagiso-town and Hiroshima-city are chosen for investigation. Debris flow occurrence due to rainfall is greatly dependent on two factors: previous prolonged rainfall and the last short strong rainfall. Rainfall index R' can express with one value combining the long-term effective rainfall R_w and the short-term effective rainfall r_w. In addition, it is necessary to clarify the inherent factors of a natural place, such as topography, geology and vegetation. Especially, slope gradient has strong influences on the sediment runoff characteristics, such as flow depth, velocity, discharge and sediment concentration. We should learn more about formation in land, land-use history and the history of disasters as preparations to a natural disaster.

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East coasts of Hokkaido, facing Okhotsk Sea and Pacific Ocean, are covered by sea ice drifted from northern Okhotsk Sea in late January through early of March. When tsunami occurs during the term and arrives at the ice-covered coasts, the disaster risks are significantly increased since ice floes carried by run-up tsunami can extend the possible damages through distinct dynamic behaviors occurring in ice-floe-laden flows; that is, forming pile-ups and jams of the ice floes, and enhancing ice and hydraulic forces. In this paper, features of the previous damages to residences and infrastructures by sea ice for the 1952 Tokachi-oki tsunami and the 2011 Tohoku tsunami are firstly discussed for charactering the ice-structure interactions. The fracture mechanisms of the ice floes colliding to structures during jamming and piling-up processes are identified on the basis of model experiments as well as numerical computations using a 3D Distinct Element Method. We also find structures with openings, such as windows and space between columns, induce ice jams and pile-ups and dam up the run-up flows, resulting in downstream water level rise and thus significant increase of hydraulic pressure acting on the structure, which indicates the increased risks expected in urban area.

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Tsunami-induced local scouring at the landward toe of a coastal dike and the effectiveness of countermeasures to improve its tenacity are investigated using a numerical model that considers fluid-sediment-seabed interactions. From a comparison between experimental data and numerical results, the predictive capability of the model is demonstrated in terms of water surface elevations and final scour profiles. From the numerical results, it is found that armor blocks below the crown and berm of the landward slope of the dike receive large landward and upward force because of an increase in air pressure inside the dike and a decrease in water pressure induced by flow separation, suggesting that these blocks would be vulnerable against tsunami overflow. The numerical results also show that protection blocks covering the landward side of the dike move the scouring area landward and reduce its maximum depth, suggesting that they would be effective to extend the time before beginning the damage of the body of the dike.

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Surge-Wave-Tide coupled model (SuWAT) has been developed and improved its applicability. Effect of tidal variation on storm surge is discussed by a simplified bathymetry test and hindcast simulation in Korean west coast. Effect of wave setup is also confirmed by hindcast of storm surge in Kochi coast caused by typhoon Anita. SuWAT became enabling to use time history data of atmosphere as a boundary forcing and applied to hindcast simulation of typhoon Vera, which caused the national worst storm surge disaster. Climate change experiment result by GCM is employed for future coastal disaster projections, and the results indicate future storm surge would increase in the Ise Bay and the west Seto Inland Sea.

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