Many river disasters took place along the Yukiya River reach in Karumai-cho of Iwate Prefecture due to heavy rain in October 1999. Actual disaster conditions were observed through field surveys and the mechanisms and causes were analyzed by comparative investigations of aerial photos taken before and after the flood. Two distinctive disasters occurred along the reach. The first is the disaster at flooding areas. The surface of the ground near eroded riverbanks was scoured and sands and gravels were carried away. This debris deposited widely on downstream areas. The returning flow of inundated water to the river channel destroyed bank protection works at the downstream reach of the flooding area. The second is the flooding due to the blockage of flood flow by drift timber. Drift timber flowed down mainly from eroded side banks of low-water channels and stayed upstream of a bridge.

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Flow characteristics of 1-D dam break flows and hydrodynamic force exerted on a square pillar placed by the flow were measured by a 2-component load cell. The angle of attack of the pillar and the ratio of channel width to the pillar width (blockage-ratio) were kept 0 degree and 10, respectively. It is found that the velocity profile of the flow with no presence of a pillar follows approximately logarithmic velocity distribution on the smooth wall, and that the ratio of surface to depth-averaged velocity is about 1.12. The magnitude of drag coefficient of the pillar 1-D dam break flows is compared with that of steady uniform flows. The former increases very slowly with t_* after t_*>70.

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This study attempts to investigate the practicality of utilizing the upstream Froude number in determining the sediment flushing performance through a sluice gate within the scope of onedimensional sediment flushing model for a steep slope reservoir. Herewith, the study contemplates on the effect of the width ratio C_b, length of sub-critical zone x and sequent depth of hydraulic jump hd in determining the flushing performance. Accordingly, the flushing performance line can be visualized easily when the combinations of the above parameters are compared to the upstream Froude number F_up. The previous results indicate that the flushing performance behaves non-linearly for Froude number below 4.0 under both the fixed bed and movable bed categories. Under the current study however, as the Froude number increases more than 4.0, the flushing performance line has been discovered to become constant. Furthermore, a rebound curve could be visualized when Froude number is between 3.5-4.0.

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This study deals with the behavior of three-dimensional negative buoyant surface jets released on a sloping bottom. Flow visualization and exhaustive thermometry were carried out to investigate the streamwise variations of the flow and spreading characteristics. The jet momently plunges and reattaches on a water surface, then, the great lateral spreading originates both in the surface and bottom layers, immediately before the complete plunging. This behavior is peculiar in the three-dimensional cases and plays an important role to the flow and spreading mechanism.

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A field experiment on the heat accumulation of solar ponds was carried out using low concentration salt water. The solar pond system exhibited certain heat accumulation, by being taken the special measures, in a relatively short period (1-3 weeks). The temperature in the lower layer was about 6°C higher than that in the upper layer, in addition, maximally about 15°C higher than the air temperature in the night-time. The accumulated heat in the lower layer of the solar pond could be used for heating relatively narrow spaces, for example, rooms in general houses and greenhouses in farms.

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Motion of suspension clouds propagating along the bottom boundary, resulting from direct dumping of a mixture of fme and coarse particles from the surface of a quiescent water, is investigated. A series of laboratory experiment is conducted by varying inflow condition as well as ambient water depth. It is found that the motion of the suspension clouds can be differentiated into the transition and the gravity current regions. Parameters to adequately describe such flow characteristics as the propagation speed, height, buoyancy etc. of the front of the clouds in each region are quantified. The effects of coarse particles on the spreading of the clouds are also examined.

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In this study we investigated about annual changes ofbenthic animals and river bed environment on the Hirai river, the Aid-river and Kitaasa-river of the Tama district in the Tokyo. In the Hirai river and the Kitaasa-river, we examined about sediment volume among gravel and benthic animals in each microhabitats at riffles and pools, and investigated the relation between sediment and benthic animals. Results of the monitoring, it is observed that a velocity changing takes control of the river bed environment, and also a condition of existence for benthic animals. Therefore, it was considered that volume of mud is an index for river bed environment

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