A numerical prediction model of the flood-inundation flows with accuracy and high-speed calculation performance was developed. The governing equations applied the combined non-uniform flow equation under the river flows and linear inundation flows equation under the inundation flows in order to calculate rapidly. Inasmuch as high-speed calculation performance may decrease calculation accuracy, the optimized boundary conditions of the river flow were applied and numerical grids of the inundation flow were implemented topography-fitting grid as a new approximation method for the topography. The developed model reproduced most past floods quite accurately and can calculate very rapidly even by PC. Further more, the effects of flood-fighting activities such as levee protection activity were evaluated by the coupled the numerical model with GIS, the evaluation system can also support the levee protection activity.

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A numerical model based on Spatial Averaged Finite volume method on Unstructured grid using FDS technique for 2D Flood flows (SA-FUF-2DF model) was constructed to estimate flood control functions of flood retarding plantations. In experiments, front positions, depths and surface velocities of flood flows in flood plain with structures and flood retarding plantations as well as hydrodynamic forces acting on structures were observed. A SA-FUF-2DF model was verified against these experimental data, and flood simulations in a proto-scale urban area was also carried out. The model can reproduce the complex behavior of flood flows as well as hydrodynamic forces with reasonable accuracy. The flood simulations show that flood retarding plantations are effective in reducing flow discharge into urban area and hydrodynamic forces.

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Flood flow in Japan possesses characteristics tat the rate of water level change in rising stage is large. Cross-sectional shapes of middle and down-steam region of Japanese rivers employ compound ores consisting of main channel and flood channels. It is, therefore, required to understand the hydraulics of flood flow through various shapes of compound channels for design of safe and environmentally rich rivers. But, there are almost no reliable reference books describing such hydraulic hydraulics. We focused on unsteady hydraulic phenomenon of flood flows in compound meandering channels, and made a study on effects of channel characteristic and of flood characteristics on storage of flood flow. As a result, we clarified influences which plan and cross-section shapes of rivers, boundary condition at the downstream end and unsteadiness on propagation of flood flow and also evaluated the storage volume of flood flow in a channel.

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Heat-salt double stratification was observed in a eutrophic reservoir in which hypolimnion shows high electric conductivity and has an inverse temperature gradient. The hypolimnic water is so heavy and stable that the reservoir water has never been overturned in full-depth-scale even in mid-winter. Based on an in-situ measurement of water quality it was discussed how the heat-salt density system was constructed and related to reservoir eutrophication. The field data showed that release of dissolved materials from sediments produces salty and heavy water mass which generates an inclined plume along the bed slope of reservoir. The plume plays a major role in transporting heat and dissolved components of metals, nutrients and organic matters from the anaerobic metalimnion into the hypolimnion. This is how a relatively warm and heavy water mass with an inverse temperature gradient is concentrated there. This is new eutrophication mechanism that has to be taken into consideration in an analysis of reservoir water quality.

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Erosional processes of fluid mud in Lake Kasumigaura were examined in a test flume for the sake of practical estimations of mud suspension in the lake. The mud is characterized by its high water content, higher than 1000% around its surface. As the water content is regarded as a major factor on the entrainment of mud, the vertical distributions of water content were measured with a high precision. The displacement of mud surface caused by both erosion and compression was also measured respectively in a careful way. A simple data analysis showed that the entrainment rate of mud was expressed as a function of water content of mud and flow velocity, which expression would be useful in practical ways.

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In the transition process of dune formation, a hysteresis is observed between flow depth and discharge, which is analogous to the multiple-valued relations between the total Shields stress and the effective Shields stress found in experiments. In this study, a weakly nonlinear stability analysis is performed in order to provide a theoretical explanation for the hysteresis observed in the transition of dunes. The growth rate expansion method is used to derive a nonlinear amplitude equation, which shows that when the average bed slope is large and the effect of the local bed inclination is small, the transition of dune formation is characterized by the subcritical bifurcation. This implies that the hysteresis is caused by the subcritical bifurcation.

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A mathematical model is presented to describe the formation of river mouth terraces in this paper. The spatial distributions of velocity and suspended sediment concentration are obtained by the use of the integration method and the similarity assumption of lateral profiles of velocity and suspended sediment concentration. The inceptive topography of river mouth terraces are then obtained from the velocity and sediment concentration distributions. It is found that the sediment deposits on both sides of the jet axis when the suspended load is dominant and the effect of advection and diffusion is relatively small, while it deposits on the axis when the bedload is dominant.

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Laboratory experiments of river mouth jets were performed to investigate the profiles of velocity and suspended sediment concentration, and the inceptive topography of fluvial-dominated river mouth terraces. The experimental results were found to show reasonably good agreement with the theory proposed by some of the authors. The experiments revealed that the sediment deposits on both sides of the jet axis if the suspended load is dominant, which had been predicted by the theory. It was shown in the experiments that river mouth terraces develop into elongate deltas under the condition of dominant suspended load, which appeared to be definitely different from deltas formed under the condition of dominant bedload.

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A computational method for multiphase incompressible flows (MICS) has been proposed. The MICS is based on the CFD method for incompressible flows, in which the treatment for density and mass conservation has been improved in the collocated grid. The external forces for the particles are evaluated from the results of the fluid computations and a model of the distinct element method (DEM). It was shown that the particle drag coefficients and wake flows are successfully predicted with MICS. The MICS was applied to a falling particle near a side wall and non-uniform particles moving on the inclined bottom surface. As a result, it was demonstrated that MICS is effective to predict the particle-laden flows.

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In order to compute discharge over and through a rubble mound weir, a one-dimensional analysis and a laboratory experiment were carried out. Assuming a two-layer flow consisting of an open-channel flow over the weir and a turbulent seepage flow in the weir, one-dimensional momentum principle and continuity equation were formulated. In the analysis, interaction between the open channel flow and the seepage flow was taken into consideration and the shear stress at the two-layer interface was successfully evaluated. Under various conditions of weir length and height, porosity, rubble diameter and water depth, the analysis well reproduces longitudinal profiles of water surface and flow velocities over and inside the weir. The analysis also provides a solution of discharge as a function of relating hydraulic parameters such as weir's dimension, rubble's properties, and water depth at the upstream side. The solution was well correlated with the experimental data. The present study could be applied to practical use for making design of structure, discharge control, assessment of criteria of structure failure against flow.

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Experimental verifications for the performance of a rectangular resonator designed by the Wave Filter Theory installed at a harbor entrance and beach as a detached breakwater are shown. A modified resonator with a simpler configuration is also examined for the purpose of practical usage. It is confirmed that the installation of each of these resonators at a harbor entrance or beach can improve the wave tranquility in comparison with that in a conventional detached breakwater. The modified resonator is found to be more practical than the rectangular resonator because it effectively controls wave amplifications in the fairway of the harbor.

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Laboratory experiments were performed on beach evolution and sediment sorting around coastal structures on a initially plane 1/20 sloping beach of uniformly graded sand. One experiment was for a detached breakwater, and the other for groins. Spatial and temporal variations of beach topographies and grain size distribution of surface sediments were described. A numerical model was developed on the basis of the enhanced Boussinesq equation and a sand transport model for mixed-grain size sands under atilt waves. The validity of the model was verified with experimental data.

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A new multi-nesting approach for coastal current simulation is presented to effectively incorporate offshore influences and easily remove the difficulties in the treatment of open boundary conditions. In the present method, the computational results in a larger-scale domain are taken into account as the values to be assimilated in a smaller-scale computation. To examine the fundamental validity of the present method, we perform the computation for the propagation of a scattering wave, showing that the numerical accuracy of the present method is appreciably higher than that of a conventional method We also apply the present method to the computation of coastal currents at Shiraho reef in the Ishigaki Island The result indicates that the computational results of the horizontal velocities give good agreements with observed data, demonstrating the fundamental applicability of the present method.

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Rivers in Japan produce a lot of flow-out sand. This results in an accumulation of inflow sand at dams and a decrease in capacity of water reservoir function of dams. This also causes the decrease of level of riverbed at the lower reaches and increase of seashore erosion. Nevertheless, present sediment controls at dams are carried separately. In this paper, we discussed the present circumstances of sediment control at dams and propose a new comprehensive sediment control of dams including reservoir sediment control.

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Physical and numerical experiments are carried out to clarify several fundamental characteristics of the two-dimensional behavior of a strong bore over a sloping beach, which has been observed to be caused by a tsunami. An approximate method (referred to as “a ray theory for bores” in this paper) is presented to analyze the two-dimensional behavior of bores. The analyses with the use of the method proposed in this study and Snell's law show that the behavior of strong bores over sloping beaches is quite different from that of waves.

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