In winter seasons, snow clouds are often caused on the tip of a cold air flow by an ascending wind around Ishikari Bay. The extended distance of the cold air flow over Ishikari Bay is determined by the interfacial friction between the cold air flow and the upper wind. In order to estimate the coefficient of the interfacial friction, density current experiments of the cold air flow were carried out using the difference of the temperature, and its coefficient was derived on Re≤2×10⁵ and Re>2×10⁵, respectively. Using this coeffient and proposing the cold air flow model, we can reproduce the extended cold air flow on Feb. 18, 1996 which snow clouds occured on the tip of the cold air flow.

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A number of researches had been conducted about the flow structures in curved open channels, but they were limited to the channel with large width-to-depth ratio. This study aims to clarify the flow structure in curved rectangular open channels with small width-to-depth ratio, experimentally. The effects of the circumference angle and the radius of curvature of the curved channel were investigated. The obtained structures of secondary flow supported the conventional knowledge, but the existence of the counterrotating vortex near the outer bank was indicated. The developing process of the secondary flow was examined quantitatively with respect to the circumference angle and the radius of curvature. The structure of the primary mean velocity, the bed shear stress and the momentum transport were explained in relation to the secondary flow.

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Flow over flexible vegetation-covered bed was investigated by the flume experiment and the numerical analysis. The experimental results showed that deformation of plants reduces flow resistance and waving motion of plants increases flow resistance. The numerical model of which a k-ε turbulence model is coupled by a model of deformation of plants explanied the reduction of the flow resistance caused by the deformation of plants. The model also clarified the relationship between flow condition and quantity of deformation. The waving motion causes hetrogenety of vegetation layer and it brings an additional momentum transfer. It is supposed that the additinal momentum transfer is the causes of the increase of flow resistance. Finally, flow resistance and shear stress acting on a bed in some kinds of flood plain vegetation was estimated.

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Flows with sediment deposition in horizontal pipe are categorized into three types: flow with stationary bed, flow with bed load and flow with sliding sediment layer. Then the flow resistance and sediment transport are discussed theoretically and experimentally, in which the area in cross section of pipe is divided into three sections: clear water section, bed load section and sliding section on pipe wall.
Hydraulic radius of clear water section for both pipe wall and surface of sediment layer, which is derived with the method of hydraulic radius division, is proved to be usefull for the estimation of flow resistance and shear stress on the surface of sediment layer with which bed load can be estimated.
The critical condition for the occurrence of slip flow of sediment layer is also proved to be explained by relationship between energy gradient and sediment layer thickness.

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We experimentally investigated scouring mass of sediments by breaking wave action in a tidal river and in an experimental flume with a slope bed. Major conclusions of this study are as follows: (1) Impact of breaking wave acting on the slope is formularized by the conservation equation of momentum considering reflection; (2) The impact of breaking wave decreases as increase in wave steepness under the same wave height; (3) Cohesive sediments were scoured as piece and accumulated like marbles at the foot of the slope; (4) The mass of the sediments scoured by breaking waves, W_s, per number N of acting wave, and per unit area is formulated as (W_sg/A_s)/τ_s/N=m[(p_m/τ_s)-(p_m/τ_s)_c], where p_m and τ_s are the maximum impact of breaking wave and the shearing strength of the sediments, respectively. As a result of the experiments, m and (p_m/τ_s)_c are 0.14 and 037, respectively.

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In this paper, an improvement method has been investigated for the practical application of nearshore current analysis using Hardy-Cross method. For this purpose, applications of concentric rectangular circuit correction, initial value insertion using coarse grid, and multi-grid method are introduced.
As a result, it is clearly understood that the slow-convergance problem of Hardy-Cross method can be solved with much accurracy. In addition, the method which can be applied to The case of the presence of the non-water zones is also investigated.

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The water surface profile analysis in open channel flows is carried out to predict the water depth of rivers and artificial channels by using the steady one-dimensional flow equation. If a singular point, which is defined as the intersection of the quasi-normal depth and the critical depth, exists in a flow, the classification of a singular point is done and then the water surface profile is calculated from the control section. It is known that the singular point analysis predicts the transitional profile from the high velocity flow (Fr>1) to the normal flow (Fr<1) through a singular point, which is never realized in actual flows. Using the unsteady flow equations, the stability analysis of the water surface profile near a singular point with respect to time is carried out. It is proved that the transition from the normal flow to the high velocity flow through the saddle point is stable and the other types of transitions are unstable and are never realized.

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