Riprap protection works against local scour of sand bed are practical and effective if the riprap size is large enough and the protected bed sand is not drawn out through the riprap filter. These stability conditions were studies experimentally, and the results are applied for the design of multilayered filter protections with different riprap sizes to reduce the total thickness of the protection works. The critical ratio of riprap (D/d)_c of riprap size D to bed sand size d for phisically possible movement of sand through riprap porosity was determined, and the damping of the tractive force through the riprap filter was expressed as a function of the ratio L/D of filter thickness L to the riprap size. And examples of practical design of the multilayered filter protections were shown.

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A numerical study of turbulent flow developing in a curved open-channel flow is carried out by using an algebraic stress model. The flow configuration of interest to this study consists of a 180 degree bend of rectangular cross-section preceded and followed by straight duct sections. In the calculation, govering equations are transformed from the physical plane to the calculation plane by boundary-fitted coordinate systems. Calculated results are compared with the experimental data available. Moreover, the transitions from the secondary flow of the second kind to that of the first kind and vice versa are examined. As a result of this numerical study, it is found out that the present method can predict well the characteristic featurs of curved open-channel for secondary currents and streamwise velocity. As for the transition of secondary flow of the first kind, in a straight duct of a bend outlet, its secondary flow is accelerated downstream from bend outlet. After this acceleration, the secondary flow decays and gradually transformes into the the secondary flow of the second kind.

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An artificial air curtain has a possible applicability for the prevention of saline wedge intrusion into a river channel, together with a number of positive effects on ecological system. Salt water intrusion, arriving at the air curtain from the sea, is blown up by the bubble plume until the free surface, where the river flow forces it seawards. Experiments were conducted with different conditions of river flow rate, gas flow rate and density of sea water. The experimental results presented that high gas flow rate for a smaller volume flux and lower density of the salt water is capable of halting salt water intrusion. A k-ε model was employed to analyze the flow field. In the model, bubble motion was analyzed separately, which is then introduced into the momentum equation of water as a drag force. The simulated results achieved satisfactory agreement with experiments, therefrom providing the condition for the successful operation in terms of the volume flux ratios of the salt intrusion volume and the bubble flux, and of the salt intrusion flux and river flow rate.

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Reservoir hydrodynamics and water quality behavior are examined by performing field measurements in a reservoir: in its bottom layer, high concentration of materials brings high electric conductivity, and the water mass is stratified with inverse temperature gradient. Through the quality analyses of the reservoir water it is found that ignition losses, calcium, suspended solid, iron, manganese and other materials show high concentration to compensate density defect due to inverse temperature gradient. Based on the observed data, water quality behavior and engineering countermeasures to solve water quality troubles are discussed.

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Wave-induced pore water pressures and effective stresses are analyzed theoretically for a cross-anisotropic seabed with a vertical axis of symmetry. Analytical solutions based on the Biot's poro-elastic theory are derived in explicit form and the important parameters which governs the characteristics of the solutions are clarified. The effects of anisotropy of the Young modulus, the Poisson's ratio, and the permeability are investigated in detail for both a fully saturated and a slightly unsaturated seabed. The results show the significant influence of anisotropy on the pore water pressure and the effective stress distribution in the seabed.

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The advanced water treatment system of activated carbon filtration with ozonization was introduced to the Kanamachi Purification Plant in the Tokyo Metropolitan Government according to recent raw water pollution. This new system requires some mathematical expressions for the plant operation. We proposed some mathematical equations for water head loss in an activated carbon pond and the expansion ratio of activated carbon in a backwashing process. These equations composed of flow flux, water temperature, and a carbon size coincided with the experimental results very well. These expressions also could reduce the activated carbon cost and show the optimum size distribution for the activated carbon. Moreover, they determined the minimum amount of backwashing water.

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The methods for simulating the water quality in pipe network were studied by using finite element method. New analytical methods for estimating the flow path from a distribution tank to the observed point and retention time in every pipe path were proposed. Triharomethane (THM) and residual chlorine concentration were selected as indexes of water quality in a pipe network. The equation of THM formation rate was established based on the results of laboratory experiments. Considering the condition of joint or branch, THM and residual chlorine concentration in each pipe node were estimated. Comparing the analytical results with measured values the proposed method was shown to be available.

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An analytical solution is derived for laminar boundary layer under cnoidal wave motion. The solution is expressed in terms of Fourier series expansion of simple functions. Thus, computation of the velocity and the bottom shear stress can be made very easily. According to detailed analysis of the solution, quantities such as velocity, bottom shear stress, boundary layer thickness and phase difference show considerable difference from those under sinusoidal wave boundary layer. Comparison with present and existing experimetal data sets verifies the validity of the theory.

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Optimum backwashing conditions for activated carbon filtration ponds were examined with a series of model experiments. A backwashing process consists air scouring upward and water backwashing. Since the total head loss of the ponds was caused mainly by a layer between its surface and 50-cm-depth in activated carbon layer, this layer should be washed by air scouring. In the air scouring, the optimum water level, time, and air flow rate were determined by a series of the model experiments. On the other hand, the optimum water amount in the backwashing became a constant product of flow rate and its time theoretically and experimentally. On the optimum conditions, the operation time can be reduced by 70% and backwater can be also reduced by 70% compared with the traditional conditions.

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The percentage of impervious area is estimated from satellite remote sensing data and is used to simulate runoff discharge in an urbanized watershed. To control the water level during flooding, the rainwater storage facilities by means of house storage and public storage are examined. From the simulated results it can be seen that the rate of increase in the peak water level reduces according to the increase of percentage of impervious area and that the rainwater storage facilities are effective for decreasing the occurrence of inundation by lowering the water level, however, are not so effective to lower the water level in the inundated area.

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