Recently, water cycle model has been applied widely and expected to be a simulation tool for conserving appropriate water cycle, and for flood control. In this paper, a new water cycle model (WCAP: Water Cycle Analysis Program) aimed to simulate not only short-term runoff but also long-term runoff was developed. A quasi-three dimensional model was adopted for flow smulation. This model divides watershed areas into optional triangular grids, which improves the accuracy of groundwater and saturation surfaceflow simulation. This model was applied to simulate water cycle of upstream area of the Kano river. Both short-term runoff and long-term runoff was simulated with satisfactory accuracy so that the validity of new model was confirmed.

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In this study, commonalities and differences of psychological processes to flood control and river environments were investigated based on the questionnaire survey which was carried out by the committee of Seki river basin. The analysis was as follows: As commonalities, both psychological processes went through five psychological steps: knowledge, interest, motive, behavioral intention, and behavior: As differences, the sense of crises against flood damage and knowledge of flood control have influenced on interest and behavior, respectively. In addition, the regional differences were more distinctive in people's attitudes of flood control. And the factor behind these differences was due to whether they were victims of water disasters or not.

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Flood control and water use regulation by dam operation play important roles in total management of a large-scale river basin. A new method of discharge control intended to increase the efficiency of flood control and water use is proposed in this paper. This method is based on the idea that there is no risk to reduce reservoir level if the amount of anticipatory release equal to the amount of inflow which flows into a dam certainly from the rainfall that has already fallen. With this method, it is possible to achieve higher storage level in reservoirs even during the flooding season, providing more opportunities for water use in addition to flood control. Furthermore, large amount of water is discharged from time to time as the anticipatory release which is useful to maintain river environment.

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It is difficult to predict flood flow in urbanized area for the complicated topographical reason. Recently, airborne laser mapping system becomes available, which gives us a very detailed topographical information. In this study, we developed a high resolution inundation analysis system using airborne laser survey results. The system enables an automatic grid generation while detecting location of buildings and streets. We demonstrated the superiority of the new analysis system by applying it to an actual inundation problem due to a tsunami. With this system, it would become possible to construct a high accurate hazard map in urban area.

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In recent years, connecting ripraps has been used at some places in Japan as a material for river improvement in rapid flow, having environmental advantage of porous structure as good as individual riprap. This material is made by connecting riprap using wires, and set on riverbed. As the wire connectors preserve stability more than using usual riprap individually, they are expected to save labor for river maintenance work and costs. Although such materials are in practical use in river channels, they have not been subjected to hydraulic analysis. We verified the effect of enhancement for limit of breakaway by experiment and theoretical estimation of its shear stress using stochastic model.

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The existing theories of channel formation by surface sheet flow assume self-preserving base state slope profiles which migrate upstream without changing their shapes because formal stability analysis becomes inpossible if base state slope profiles are in unsteady states. In this study, the theories of Izumi and Parker²⁾, and Izumi and Fujii³⁾ are extended to include the case of channelization on slopes with arbitrary shapes with the use of the "frozen time approach" and the "momentary stability concept". In the frozen time approach, time derivative terms of base state equations are dropped under the assumption that the growth of perturbations is sufficiently faster than the evolution of base states as a first approximation. The stability of unsteady base states is further studied in terms of the momentary stability concept in which base states are assumed to become unstable for perturbations if the growth of perturbations is faster than the evolution of base states.
The analysis shows that, as long as their curvatures lie in appropriate ranges, slopes become unstable to evolve into slopes incised by channels, the spacing of which is on the order of one thousand times the depth of surface sheet flow. In the erosion-dominant case such as chennelization on slopes composed of strongly cohesive soils, the spacing between incipient channels are slightly decreased with increasing slope curvatures. In the range of sufficiently large curvatures, slopes are momentarily stable as the evolution rate of base states surpasses the growth rate of perturbations. In the case that both erosion and deposition are significant such as channelization on slopes composed of weakly cohesive soils, slopes with small curvatures are found to be unstable though clear dominant channel spacing does not appear. In the range of sufficiently large curvatures, slopes tend to be momentarily stable as well as in the erosion-dominant case.

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Direct Numerical Simulation of turbulent flow in an open channel was carried out. The Reynolds number for the simulations was Re_τ=150 and the aspect ratio was 2. The coherent vortex structures were also examined using visualization techniques. The significant results were the identification of inner secondary flow which rotates toward the wall at the free surface and down away from the surface along the wall. and the basic structure consisting of a pair of secondary flows in a corner occurs not only in solid corner but also in free surface corner. Then, detailed evaluation of Reynolds stress terms and pressure terms of the momentum equations revealed that the imbalance between them causes the inner secondary flow.

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A computational method has been developed to predict the transportation of large-scale solid bodies due to 3D free-surface flows. The multiphase field including solid bodies and liquid and gas phases separated by free surfaces is assumed to be consist of the incompressible and immiscible fluids with different physical properties. The discretization of pressure equations was examined to obtain the numerically stable converged results. It was shown that the numerical method enables us to predict Rayleigh-Taylor stability problem and drag coefficient of a sphere in a uniform flow as well as the results of hydraulic experiments related to the transportation of multiple spheres by the flow accompanied by wave motions.

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In this paper, the evaluation structure of river environment is investigated based on the questionnaire survey which was carried out by the committee of Seki river basin. The results are as follows: (1) Regardless of regional differences, people judge they can come into contact with river through the open space of river, and based on the judgment, they feel familiarity to the river and evaluate the river desirable. (2) There are many regional differences between the hierarchies which are influenced by the physical environment. On the other hand, there are few regional differences between the hierarchies which are influenced by people's subjectivity.

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A sediment sorting is one of the fundamental factors of beach process. In this paper, experiments and the numerical simulation of the vertical sorting process of graded sediment in sheet flow regime under oscillatory flow have been performed. Since the key of graded sediment transport is the particle/particle interaction, the mechanics of sorting has been investigated in detail by the movable 3D bed simulator based on the distinct element method. Furthermore the internal structure of the sheet flow layer, where direct measurement of sediment-particle motion is difficult, has been examined. A sorting process of sediment is reproduced well by the present simulation.

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The effect of wave-induced mass transport in the bottom boundary layer on the net transport of sheetflow was investigated by a two-phase flow model expanded for the cross-shore direction. The net sand transport rate was found to increase significantly due to the presence of steady streaming. The increase in the local sand transport rate was successfully estimated by existing sand transport models by including the steady streaming as superimposed currents. The model was then applied to the sheet flow transport in the outer surf zone for large wave tank experiments, which improved the prediction of bar generation near the breaker zonei.

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Suction mechanism of reclaimed sand behind a rubble seawall has been investigated with hydraulic model experiments and numerical methods considering wave, structure and seabed interaction. It is consequently revealed that the Ursell parameter and the relative seawall width, namely, the ratio of the seawall width and the wavelength, have significant effects on the suction phenomenon regardless of the inclination of the seawall, and the sand suction easily occurs as vertical velocity in front of the beach and volumetric strain inside the sand increase. In addition, an increase of the sand/rubble diameter ratio results in a decrease of the aforementioned vertical velocity and the volumetric strain; hence efficiency of filter layers for the prevention of the sand suction is demonstrated.

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The density function method is one of the powerful tools for solving a complicated free surface flow problem. However, an air-water interface becomes dim due to a numerical diffusion and it leads conservation of fluid volume to fail. Tsubogo and Asai developed the volume correction method to avoid this problem. Although the long term computation for free water flow can be made by using this method, there is a room to improve their method. In this paper, the volume correction method proposed by Tsubogo and Asai is modified. The effect of the proposed method was checked by solving dam break problems with a whole closed boundary condition. It is found that the modified volume correction method is effective.

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There are few studies investigating runup heights of random waves on seawalls near shoreline. The present study proposed an estimation formula of random wave runups for seawalls in the region of shallow foreshore or above shoreline, by carrying out hydraulic experiments. In the formula, deep water wave characteristics and a revised imaginary slope, considering the actual composite beach cross section, are employed in order to determine the surf similarity parameter. Several existing formulas were also examined for their applicability; in the formulas, wave characteristics at the toe of seawall were used, and other definitions of the slope were reported for composite cross sections. It was found that the applicability of existing runup formulas were limited to small range of surf similarity parameter. The present proposed formula can be employed for a wide range corresponding to the present experimental results.

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