The method to estimate the raindrop size distribution at altitude covering with radar beam from the one observed at the ground is discussed and by this new method, the method to estimate rainfall intensity from radar reflectivity factor is proposed. The equations to estimate the raindrop size distribution at the upper altitude from the one at the ground are derived from one dimensional simulation of raindrop collision/breakup. The new model proposed in this paper can estimate rainfall intensity observed by a radar raingauge more accurately.

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This paper introduces the microwave radiation theory to show a limitation of the soil moisture estimation method currently on use. To estimate the soil moisture profile, it then incorporates the Kalman filtering technique to utilize the multifrequency observations.

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An artificial dispersion method by spraying water drops has been considered for warm fog without any ice phase process. This method is based on the fallout of tiny fog droplets being collected by the bigger spraying drops. The processes of collection, breakup and sedimentation are considered in the present stochastic model for seeing the spray effect on the stagnant fog. Fog visibility is improved higher than 1000m, meteorological definition of fog, by spraying drops with the economical water supply. It is found all the 240m deep fogs, of which LWC (water liquid content) less than 1g/m³ (55m-300m visibility), will be dispersed within 30 seconds by spraying drops which has 10g/m³ LWC, corresponding to 1.625litre water per square meter.

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The diffusive tank model is a quasi-hydraulic model concentration on the continuity relationship among the storage, the inflow and outflow and deals with a flood wave motion as the diffusion wave. As the water level of drainage channels changes very slowly due to a back water effect in a low-lying drainage basin, the unsteady flow in the channel can be simplified to a non-uniform flow.
In this paper, runoff analysis by this diffusive tank model in a low-lying drainage area where is about 1.7km² is carried out. And, after the agricultural refinement of this drainage basin, runoff discharge will be estimated and runoff characteristics from paddies in this field is investigated.

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It is important for the precaution activity of inhabitants to know how much rain will cause the urban inundation in their city. We discussed in our previous paper the possibility of prediction in case of Ube city using Tank model and Point rain expecting method. The Tank structure, Tank parameters and critical occurrence level of residual water in Tank were determined to separate clearly the inundation occurrence cases and the non-occurrence cases in the past rains. Ube's results show that the inundation prediction is possible 10 minutes to 1 hour earlier than the actual occurrence. Same investigations are made for 4 other local cities in Yamaguchi prefecture in this paper. It is found that other city's Tank structures are same as Ube's and their Tank parameters and occurrence level are quite similar to each others.

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Turbulence measurements of partly vegetated open-channel flows were conducted accurately with PIV (Particle Image Velocimetry) and LDA (Laser Doppler Anemometer). The horizontal vortices near the free surface are generated by the shear instability which increases with an increase of the vegatation density and the Froude numbers. Secondary currents near the free surface are generated by the horizontal vortices and its strength increases with an enlargement of the vegatation density and the Froude numbers. This hydrodynamic mechanics of secondary currents is quite different from non-vagatated open-channel flows, in which the secondary currents are generated by the anisotropy of turbulence. A new eddy model is proposed to explain both the secondary currents and horizontal large-scale vorticies in partly vegetated open-channel flows.

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The numerical model of tank-model type to simulate and predict long term behaviors of the density interface in a brackish lake is proposed. From this calculation, we recognized that the outflow mechanism of the salt water in this lake, which is very sensitive to the inflowing discharge and has two types of suction and entertainment, plays important roles for the behaviors of the salinocline in Lake Abashiri. By using the outflow model, which mixed the entrainment type with the suction one, we can simulate the behaviors of the salinocline for the last decade correctly.

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3-D Large Eddy Simulation was carried out on plumes advancing over the horizontal surface. The motion of plumes is devided into three groups, immature state, transition state and stable state. The results of this calculation agreed with the experiments of gravity currents by Simpson in stable state. Turbulent heat flux on the interface obtained from Large Eddy Simulation was consistent with 0.01ΔθΔu which is derived from log-linear law by using the experimental results from the previous studies. It was found that there is the two-fold relation between the vertical diffusion coefficient and local Richardson number.

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Large Eddy Simulation using the Smagorinsky model of turbulent flow is carried out for a straight open channel with one flood plain. The Reynolds number based on hydraulic radius and cross-sectional average velocity is about 5, 300, and the total number of grid points is 79*75*220. The calculated results agree reasonably well with the experimental data by Tominaga & Nezu in mean velocities and turbulence quantities. Then, the balances of the streamwise vorticity equation and the momentum equations are analyzed to discuss the mechanism of mean secondary flow. Instantaneous flow field is also visualized. It is confirmed that the present simulation captures streak structures and that instantaneous secondary flow near the edge of flood plain has unsteady and localized features with large magnitude in velocity.

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Two representative Low-Reynolds number models from two most widely used groups of two-equation turbulence models, together with their respective original standard models, have been put to detailed examination based on comparisons with DNS and experimental data on two-dimensional open channel flows. It has been found that recent fine adjustments to reproduce the DNS data in near-wall low-Reynolds number regions do not necessarily improve prediction of general flows with free surface in complex geometry. Appropriate treatment of the free-surface effects is more important. The low-Reynolds number version of the k-ω model showed the best predictive accuracy in the present study.

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Numerical study of turbulent flow developing in a meandering open-channel with rectangular cross sections has been carried out by using an algebraic stress model. The flow of interest to the study is one periodic meandering channel of five ones. In the calculation, govering equations are transformed from the physical plane to the calculation plane by using boundary-fitted coordinate system. The calculated results show that unsteady flow is observed for this meandering channel. Therefore, time-averaged numerical results are compared with the experimental data measured by Muto et al. It was found that the present numerical simulation could predict well the characteristic features of streamwise velocity, i. e. the maximum streamwise velocity generated inner bank at bend apex and the location of the maximum velocity moved to the other side bank along the meandering channel, which is different from those of curved open-channel flow. Adding to this, the distributions of secondary flow and Reynolds stresses are reproduced by the present method. The calculated results also suggest that meandering channel flow is featured by unsteady movement of the secondary flow pattern.

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It is important to know the characteristic of water surface fluctuations in dam reservoir. Because the inflow to reservoir is estimated from an increment of hydrostatic water level. But the many kind of noises included in water level data prevent to measure the hydrostatic water level. This study reveal the existence of water surface fluctuations like some kind of seiche, beat phenomena and water surface set-up induced by wind by processing the water level data from the multiple water gauges in Jozankei Dam Reservoir. And this study suggested a new type wave equation with force term, which means an effect of wind on water surface. The result of analyzing this new type equation can explain about the observed water surface fluctuations.

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This paper proposed an energy balance equation including the effect of wave diffraction as a multi-directional random wave transformation model. The term of wave diffraction was included explicitly in the model, which was formulated based on a parabolic wave refraction-diffraction equation. The present wave model is easy to solve numerically and is stable. The wave model was employed to estimate wave transformations through the mouth between two breakwaters, and the estimated results were compared with the predictions by the existing energy balance equation without wave diffraction effect and with the Sommerfeld solutions. The comparison between the wave heights shows that the present wave model is the good predictor. The present wave model was also applied to the experiments of wave transformation due to an elliptic shoal, and it was found that the model predictions agreed well with the experimental observations excluding the cases where the wave breaking on the shoal was severe.

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As a scheme to improve the river environment in an urbanized basin, it is important to restore the hydrologic relationships between stream and aquifer. In this research, based on the hydraulic models of the stream-unconfined aquifer system, the dynamic interaction between them is analyzed to evaluate the bank storage effect of aquifer. Especially for the simple linear system, closed forms are formulated and numerical solutions are computed for the more universal nonlinear system. Through the paper, it is pointed out that the bank storage of aquifer functions to keep the stability of flow condition in the stream channel.

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