When a rainfall-runoff model is developed on a grid-based DEM (Digital Elevation Model), it is often assumed that water flows in the steepest descent direction at each grid point. However, this assumption cannot consider the effect of divergent topography on runoff process. This paper proposes the new numerical representation method of basin topography which can represent both concentration and divergence of water flow using a grid-based DEM. This new method is applied to the Daidogawa River basin in Shiga Prefecture, Japan, and the results shows that this method makes reasonable representation of divergent topography such as peaks and ridges.

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Characteristics of flood flow and bed topography in compound meandering channel were investigated by the result of laboratory experiments and using hydraulic and velocity vector data of flood flows, river configuration, and bed topography obtained at several natural rivers. From the data analysis of meandering compound channel flow in the laboratory natural streams, the flood flow was classified into two categories the meandering compound channel flow and meandering single channel flow, according to the relative depth above and below of 30% respectively, in case the sinuosity of the meandering compound channel flow is above 1.015. We also investigated inner bed topography through boreholes of inner bed of Gonokawa-river during flood. For both the natural river and laboratory channel, similar phenomenon of scouring occurred at the inner bed for compound meandering flows. Finally, we indicated several problems and proposed investigation method for the rational river planning of a compound meandering river.

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River channel processes with bank erosion are investigated by numerical models in comparison with field and experimental data. At first, a simplified numerical model is applied to the river morphology of the Meghna River in Bangladesh, and the relation between flow properties and the location of bank erosion is discussed. However, in the case of actual river, it is difficult to clarify main factors of bank erosion because of the complicated channel form and bank materials. Therefore, we conduct the experiments and numerical analyses of channel processes starting from initial meandering and straight plan forms, and the influence of hydraulic parameters and the behavior of sand bars on the rate of bank erosion are elucidated.

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The Boussinesq approximation for flow calculation is invalid in the case of snow drift and avalanches, in which the density of snow particles is appreciably larger than that of air. The formulation of κ-ε turbulence model for the solid-fluid two-phase flow considering the density variation is presented. The model is applied to sediment-laden streams. It is shown that the model explains well the experimental data. The model calculations for the snow drift in the cases of the slope angle of 0° and 30°, respectively, are carried out. As the bed concentration of snow particles is larger, the velocity is considerably larger in case of 30°. The Boussinesq approximation is inadequate, when the near bed concentration becomes larger than 0.03% for snow drift.

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The waving of flexible plant termed honami is caused by large scale vortices which are induced by inflectional instability of flow field. The movement of plant in turn may affect the turbulent flow field via resistance to flow, turbulence production and dissipation. The present study treats the interaction of turbulent flow and honami employing 2D LES, in which a concept of “plant grid” is used to treat the movement of plant except for the ordinary LES grid. The result indicates that honami reduces the periodicity of vortex generation and increases the temporally-averaged period of it, compared with the case for which the plant is assumed to be rigid.

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In open-channel flows with bed-load transport, mean flow properties and turbulence characteristics are affected by the bed load motions. In this study, FLDA measurements over the flat movable sand bed were conducted in open channel flows, and simultaneously the bed loads were collected and these volumes were measured. It was found that the mean velocity profile can be discribed by the log-law in the inner layer. In contrast, at the near bed region, the velocity profile deviates from the log-law. Turbulence intensity distributions, both horizontal and vertical, also change in the inner layer; the securves deviate upward from the exponential formula proposed by Nezu. The Reynolds stress profile near the bed is similar to that of a two dimensional uniform flow. The generation rate of turbulence defects in the near bed region. The phenomena are almost controlled by the bed-load motions.

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Turbulence measurements over the flat movable bed with a Laser Doppler Anemometer were conducted in open channel flows, as well as the bed loads were collected and its volume were measured. It is found that, at the near the bed, the contributions to the Reynolds stress become larger and the ratio of RS₄/RS₂ of the magnitude of sweep event to the magnitude of ejection event become smaller with the increase of the bed load transport rates. The bursting period becomes shorter as compare with the one of the non-bed load flow.

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A sediment transport formula is proposed for the each grain size of the bed load with sand-gravel mixtures, modifying the Meyer-Peter & Müller's formula proposed for the uniform size sand. Both tractive force and the critical tractive force of the each grain size are modified in the formula. Particles smaller than the mean grain size are assumed to be transported by the mean drag force after being lifted by the lift force to the top of the mean grain size level, whereas particles larger than the mean grain size are moved and transported by the drag force of the velocity at the relevant height to the larger grain size. The modifed formula is experimentally acceptable for the estimation of sediment transport rate for both larger and smaller grain size than the mean grain size.

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In this study, we investigated the relationship between the geographical and hydrologic factors andamount of deposit sand in many dams. The geographical factors include drainage area, value of undulation and are a of landslides which influence the amonut of flow-out sand. The hydrologic factors include an annual maximum amount of in-flow and tow day rain fall of maximum year. We also investigated the total amount of deposit sand and lapsing years to obtain the characteristic of depositing sand. From these results. We obtained the equation estimating the amount of deposit sand.

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Small scale experiments in a channel and large scale experiments in the air are carried out to obtain a practical formula for estimating the impulsive force on structures due to a collision of driftwoods. Expression form of the formula is determined by considerations of a simplest equation of motion for driftwoods. An analytical method is also developed to examine the validity of the formula and variation features of the impulsive force synthetically. The method is based on the simplest elasto-plastic theory taken account of the collision angle of driftwoods, the deflection of structures and so on.

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This paper examines the characteristics of wave runup on a circular pier of a bridge constructed in a shallow sea, and investigates the countermeasure for the reduction of wave runup. The experimental results showed that the wave runup of regular and random waves increase in proportion to the incident wave height at the pier. Therefore, the wave runup height can be reduced by damping incident waves. From this point of view, a submerged breakwater is employed as a countermeasure for the reduction of wave runup and the effect on reduction of wave heights and runups is investigated by hydraulic experiments and numerical simulations.

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Based on the linear long wave theory, theoretical solution is obtained for the tsunami which propagates from a tsunami source generated on a straight beach with an uniform slope. The solution indicates that the behaviors of tsunami generated on a shelf is governed by the conditions of tsunami source and that edge wave is generated considerably in the case when the tsunami source locates near a coastline. The empirical formulas are derived which evaluate the characteristics of tsunami by those of tsunami source such as the lengths of long-axis and short-axis, the position and direction of tsunami source, and so on. Further, effect of the Colioris force is also discussed.

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Seaweed forests are important as spawning, nursery and food supplying areas for fishes and shells. The natural seaweed forests have been decreasing in recent years resulted from reclamation and water quality deterioration, etc. Thus a number of trials to construct artificial seaweed forests have been done near various natural forests with concrete blocks or natural stones. This report describes the method to determine the wave force on the individual seaweed body on the basis of a series of experiments including a large wave flume experiment. The report also proposes to take this force into consideration for the design of the required weight of the cover layer blocks for an artifitial seaweed forest bed.

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A model for diffsional mass transfer of dissolved substance from bottom sediment to flowing water is formulated as a function of flow velocity and characteristics of the sediment such as water content of sediment and amount of dissolved substances contained in sediment. Laboratory experiments are carried out in order to verify the proposed model. The experimental results have shown that the mass flux increases as the flow velocity increases. The mass flux is influenced by water content of sediment as well as amount of dissolved substances contained in sediment. It is also found that the adsorption process between methylene blue and kaolinite is expressed by Langmuir isotherm equation, and it enables us to estimate a concentration of dissolved substance at the sediment-water interface. The release rate of dissolved substances estimated by the present model is in good agreement with that observed in the experiments.

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