A drop structure installed in a river channel alters the riverbed configuration around it and brings both advantage and disadvantage to the aquatic life. This study investigated the effect of a drop structure on river environment through field measurements of topography and flow structure at several study sites.
  At first, we clarified that a drop structure create a riffle, a pool, and a run around it, but the pool is not created in the area where the riverbed degradation occurs. Additionally, the riffle bed in such degraded area lacks pebbles and small cobbles. The run upstream of a drop structure has two kinds of riverbed configuration in it. In a convex side, very coarse pebbles or small cobbles and sand covers the riverbed, while the uniform pebbles covers the concave side. So loose gravel bed is created in the concave side of a run. In a pool, vertical inflow from a drop structure causes the turbulent flow. When the discharge is small, fast flow appears near the surface and the water in the middle and bottom layer is stagnated. When the discharge is large, fast flow intrudes into the bottom of a pool and circulating flow is created.
  The topographic characteristics mentioned above are often changed by size distribution of riverbed material, alignment of a river channel and riverbed profile, as well as the specification of a drop structure.

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  This paper gives the results of the laboratory experiments on the DO improvement technique for the water of natural surroundings such as lake or sea. This technique forms bubble cluster on the water surface in a h-shaped pipe by aerating water to be treated introduced into the pipe. The bubble cluster has high rate of gases exchange between gas and liquid phases due to the high gradient of DO concentration in the liquid phase of water film of bubble. Evaluation of the DO improvement is done by using the equivalent DO increment devised to eliminate the influence of the DO concentration of the water before treatement. Performance of the device has been investigated by the quantity of oxygen dissolution flux evaluated by the product of the equivalent DO increment and the rate of the water treatment, and the ratio of this flux to the power required for aeration. Present experiment showed that the good condition for oxygen dissolution is accomplished at some optimum flow rate of air for aeration, that the elevation of the horizontal part of h-shaped pipe above the outer water surface should be as low as possible, and that the energy efficiency becomes high as the flow rate of air is low.

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  Vertical water infiltration in a heterogeneous-unsaturated flow field was generated through a small rainfall simulator. Solute transport characteristics with cation exchange reaction in this flow field were investigated. Temporal changes of electric conductivity and concentrations of cations and anions in the leachate accumulated at the bottom of the laboratory flow tank were observed. Macroscopic characteristics of solute transport were evaluated with a theoretical solution of the one-dimensional convective-dispersive equation. Dye tracing technique was used to investigate detailed behavior of the solute. Numerical simulation of the saturated-unsaturated infiltration analysis and convective-dispersive analysis were conducted to reproduce the lab-experiment. The heterogeneity of the water retention characteristics heavily affects solute movement. As such, power law decay of the concentration was clearly observed. Temporal change of cation concentration was difficult to reproduce using the linear adsorption model. However it can be reproduced using nonlinear adsorption isotherms of Freundlich type which considered the difference of the soil configuration.

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  To understand a mechanics of stability of armor blocks arranged at the foot of the breakwater is important issue for a prevention against damage of coastal structures. In this study, a deformation process of armor blocks, which are arranged at the foot of mound of breakwater, has been investigated from a laboratory experiment and a numerical simulation by the DEM-MPS coupling model. From the experiment, an unevenness of mound excited by wave force is found to be the factor that leads to a deformation of blocks. And an influence of the unevenness of a mound on the deformation process of armor blocks have been reproduced numerically.

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  The flood control and water use of multipurpose dams have been drawing attention as one of the adaptable measures against climate change. At the same time, there are concerns about the ability of multipurpose dams in cold, snowy regions to control heavy rain during the snowmelt season, when the impounding of service water is prioritized and the flood control capacity is small. Toward improving the flood control capacity of such dams, this study examines the possibility of releasing dam water based on weather forecasts before heavy rain. First, the forecasted cumulative rainfall was compared with the forecasted time series rainfall in terms of accuracy, and the former was found to be more accurate. Next, a method for dam water release before heavy rainfall based on the forecasted cumulative rainfall was proposed, and the method was applied to a past flood during the snowmelt season. Utilization of the forecasted cumulative rainfall was found to be effective in improving the flood control of dams during the snowmelt season. The study results further suggest that it will be possible to put emphasis on flood control in managing multipurpose dams during the snowmelt season.

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  Point sink flow of rotating stratified fluid in a reservoir of finite depth is studied numerically as an initial-value problem starting from a quiescent state. Discharge from the sink generates inertial internal gravity waves, and they propagate cylindrically in a horizontal direction. We investigate their behavior in detail to clarify the process of selectivel withdrawal formation. As time elapses more, the withdrawal-layer thickness increases gradually. The speed of such withdrawal-layer thickening becomes slower as the effect of viscosity increases. It is found that the speed of thickening is a function of the Reynolds number only and does not depend on the other parameters if the time is nondimensionalized properly using the rotating speed of the system.

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  The effect of surface salinity on the observation using the VHF ocean radar was investigated through the field observation in Ariake Bay. The receiving signal intensities were compared with in situ measurement of sea surface salinity of 1 m depth and wind velocity. The results show high attenuation in low salinity condition. This result consists with the theory of attenuation of radio wave propagating with ground wave mode. Furthermore, authors investigated the temporal variations of the distribution of receiving signal intensities after a large flood from Chikugo River. The areas of reduced receiving signal intensity spread to parallel direction with surface current and sometimes appeared without decrease in wind speed, which also agree closely with the river flow area described in literatures.

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  A one-dimensional numerical model for beach profile change was developed to predict cyclic seaward bar migrations. The cross-shore sediment transport was assumed to be composed of suspended load due to wave breaking and bed load due to velocity skewness, velocity atiltness, and beach slope. The model was calibrated with beach profile data obtained every weekday on the Hasaki coast of Japan during a 1-year period from January to December 1989. The model was able to predict cyclic bar evolutions quantitavely during a 2-year period including the calibration period and the following year of 1990, and at least qualitatively even outside the calibration period, during a 8-year period from 1991 to 1998.

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