Multi-Phase flow is often related to natural disasters. Especially large flows cause drastic geomorphic changes and can lead to catastrophy in downstream areas. Debris flow is a typical two-phase flow in such phenomena. In this paper, the authors explain the behaviour of debris flow obtained by observations and experiments. A consititutive equation for debris flow is developed by considering the mechanism of energy dissipation in the flow of the mixture of water and sand particles. The consititutive equation is applied to discuss the distributions of velocity and particle concentration in uniform debris flow. The authors describe the characteristics of two-phase flow in catastrophic events by taking such examples as the Mt. St. Helens eruption, Mt. Asama's eruption and the Mt. Ontake land slide.

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Phase change of water vapour in the atmosphere produces small water particles in the atmosphere. They are usually suspended quite densely in limited space and are called clouds. When the air temperature falls below -20°C, they are replaced by ice particles. Clouds play an important role in the meteorological processes from local to global scales.
Pyroclastic flow is a complicated multiphase flow associated with volcanic eruptions. It consists of a mixture of air and hot volcanic ash and behaves like a gravity current along mountain slopes. Since the air is heated by hot ash, buoyancy in the pyroclastic flow increases like cumulus in which the air is heated by the latent heat of water.
Powder snow avalanche is a kind of turbidity currents, which occurs when a large amount of powder snow accumulates in piles along the moutain slope. Once the avalanche begins, turbulence in the avalanche takes the piled snow into the air along the path and the effective density increases. The powder snow avalanche is very destructive, because the velocity reaches about 100m/s in the developed stage.

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Hydrographic observations in the western tropical Pacific show not only the heat content variability related to ENSO but also another decadal trend related to the recent global warming issue. Since the background of SST is high in the western tropical Pacific, even weak SST anomalies may strongly affect the location of the Aleutian Low as well as the Asian monsoon in the extratropics in winter. In the off-equatorial western Pacific the ocean behaves like a dynamic slave to the winter Asian monsoon as demonstrated by the oceanic model simulation. In particular, the winter monsoon variations are responsible for the maturity or immaturity of the cold Mindanao Dome off the Philippine coast. The active (inactive) summer monsoon followed by the increasingly anomalous easterlies (westerlies) over the western tropical Pacific from summer through winter appears to be responsible for the positive (negative) SST anomalies in the western tropical Pacific. This suggests that an interesting positive feedback mechanism may exist in the Asian monsoon system, i. e., a coupled ocean-atmosphere-land system. Since vast amounts of short term natural climate variabilities ranging from several years to decades seem to originate in those SST variations in the western tropical Pacific, it will be necessary to look more closely at the Asian monsoon system.

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Buoyancy force associated with density stratification plays a key role in environmental flows. It damps or enhances vertical motions directly in the stratified flows. Under stable stratification, it induces internal gravity waves and blocking, which cause drastic changes in the flow pattern. In addition to these direct effects, buoyancy force deforms the turbulence structure in the stratified flows, which, in turn, causes drastic changes in the turbulent transport mechanisms of momentum, heat and mass, and therefore in the flow pattern. In this paper, these direct and indirect effects of buoyancy forces are reviewed, with special attention focused on the stratified atmospheric motions.

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This paper deals with the longitudinal dispersion of sediment particles in a horizontal pipe. In our theoretical work, the same method is employed as in the dispersion anaylysis of neutral matter. It is assumed that the concentration of particles is diluted and that there is no deposition of particles onto the pipe bottom. Numerical solution of a basic eqution shows that the mean velocity of sediment cloud, U_s, decreases with increase of fall velocity of particles, W, whereas the dispersion coefficient, U_L increases with W. The effect of fall velocity on these two properties, U_s and U_L, however, is not as great as in the case of open channel flow. In this paper it is also shown that a particle describes a plane-symmetric spiral trajectory when it flows downstream.

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The resistance coefficient of a particle was measured in a pumped flow with added pulsation from a piston crank mechanism. As a result of experiment, it was found that the resistance coefficient of the particle was increased by a remarkable 7Hz and was for the increased by 15Hz when the pulsation frequency of the pumping fluid was 0-25Hz. Under these conditions, the increasing amplitude of the flow rate fluctuation of the pumping fluid appeared experimentally.

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