For the purpose of making a stochastic model of rainfall distribution, two remarkable features of rainfall distributions were analyzed using data of a precipitation radar. One is “altitude dependence line”, which is an index of the dependence of rainfall on topography. The other is “time accumulation process”, which represents the distribution properties changing with accumulation of rainfall. As a result, the dependence properties of rainfall on topography was quantified based on the altitude dependence line. The results also indicated that the time accumulation process of rainfall in mountainous regions could be divided into three stages, and its boundary time scales and the properties of each stage were determined. Additionally, the main cause of topographic effect on rainfall distributions was also discovered through the analysis of types of rainfall events.

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The main cause of the Hiroshima disaster Junc 1999 is the heavy rain due to the cold front. This heavy rain appeared in the band with small width. It was considered that this concentration of rainfall was under the influence of the mountainous geographical features and the wind fields. A radar rain gauge was effective to know the distribution and the movement of this heavy rain, if we use the ground rain gauge together. It is considered that the radar rain gauge system is effective for short time prediction of rainfall through the movement of heavy rainfall area. We examine the feasibility of simple prediction of rainfall based on the radar rainfall gauge. As the result, it is necessary to consider the change in rainfall due to the mountainous geographical features.

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The water quality investigation of seepage water around Misogawa dam has been conducted for more than six years since the start of the initial filling of the reservoir. The investigated data show the water quality was affected by the intrusion of reservoir water to the rock mass around the dam. The electric conductivity (EC) which represent the water quality gradually evolved according to the filling of the reservoir. The evolution of EC was modeled considering the advection and water-rock reaction. The hydraulic conductivity was estimated by curve matching relevant to the measured EC using the EC model simulation. The simulated results and the measured data show good agreement and the estimated hydraulic conductivity was acceptable value. The result of applying the EC model to this site presents the availability of the model and the efficiency for estimating the hydraulic conductivity.

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Air sparging coupled with soil vapor extraction has been developed and applied to remove volatile contaminants from subsurface environment instead of groundwater extraction coupled with soil vapor extraction. In this paper, laboratory experiments, using 1mm glass beads as the porous medium, were conducted to examine air sparging performance. The purpose of these experiments was to observe the basic mechanisms of injected air and water flows in saturated porous medium during air sparging. The results obtained in this study were the following. The continuous air channels formed inside the porous medium when the grain size of the medium was 1 mm. The pulsed operation for air injection spread the region of injected air flow more than in the continuous operation. Water in the medium gradually moved outside the air flow region by pulsed operation.

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Permeability tests have been widely used to estimate infiltration capacity of field soils. This study aims to discussthe applicability of borehole falling head tests (FHT) to the evaluation of saturated hydraulic conductivity (k₀) of the soils under dry conditions. Parameters relating to soil properties are determined by preliminary analyses with the test data obtained in Tsurumi River Basin. Also, the validity of Glover's equation is examined, based on the formula proposed by ARSIT. In addition, an empirical formula specifying the initial condition of the surface soil layer is introduced on the basis of the field data observed by tensiometers, and its parameters are classified according to the season and soil depth. As a result, k₀ values evaluated from FHT carried out for the dry Kanto loam soils are in good agreement with the values obtained by constant head tests and FHT under wet conditions.

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Numerical experiments were carried out to investigate the relationship between the reduction curves of base-flow and the permeability of soil layer at hill-sloped forest. Variable parameters of the experiments were only saturated hydraulic conductivities in the direction of the hill slope and the normal direction.
In the results, the peaks of the base-flow were controlled by the magnitude of hydraulic conductivities in the both directions and arrival times in the peak base-flow were governed by the magnitude of hydraulic conductivity in the normal direction to the soil surface. The results also showed that the shapes of the base-flow recession curves could be controlled by the magnitude of hydraulic conductivities in the direction of hill-slope.

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This study explores preliminarily the effect of the two driving forces, gravity and matric potential, on the soil water movement and its regional characteristics. Three numerical experiments were conducted using a land surface model with the atmospheric forcing data at global scale. An experiment included both driving force, while the other included only a force out of the two. Our results show some important characteristics as follows. Global annual mean of water budget and that of heat one is almost same between the three experiments. Each driving force contributes to some percentile of total soil water storage but it has different characteristics at each region. Matric potential force has significant seasonal variation and contributes to some ten percentage of water content while gravity force has small variation and contirbutes to some percentage.

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Several numerical tests in this paper can be performed to check the validity of cokriging estimation of the groundwater levels in space and to show its cross-correlative effects in using the insufficient data set. Cokriging, as a type of kriging, seeks to estimate values of a certain variable at the unsampled locations. The cokriging technique makes use of the cross-correlative dependence between several variables. A groundwater level to be cokriged is evaluated as a linear combination of the available sampling data on groundwater levels and on bed elevations, which are spatially correlated with each other. Three steps are needed in the cokriging application. First, several geostatistical models, which adequately describes the spatial structure of the variables of interest, must be proposed. The second step involves using the available data to identify the unknown parameters of the models described in the first step and to select the best fitting model among proposed ones on the basis of the four information criteria. Third and finally, cokriging is employed to estimate a spatial distribution to be desired. Numerical tests are presented in this work to demonstrate the cokriging effects subject to the insufficient data. Cokriging yields reasonable and valid results because it can interpolate less sampling data due to its ability to adequately pick up the available cross-correlative data. The high applicability and reliability of the cokriging method can be reinforced with the above results.

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The flow exchange between open channel and alternate sandbar is important for instream ecosystems, water balance and water quality problems investigations. Both experimental and numerical investigations are done to get relationships between surface and subsurface flowin longitudinal and transverse direction. Under steady low flow condition, strong effects of surfaceflow conditions on subsurface flow structures in the alternate sandbars are recognized through experiments as:(1) Longitudinal flow is dominant, but the longitudinal distribution of subsurface water elevations depend on surface flow conditions.(2) Transverse gradient of subsurface flow varies by surface water discharge and bed shape. Three types of subsurface flow patterns are recognized to define flowexchange between stream and sandbar. Numerical simulations using 2-D theory shows good fit with the experimental results and field data.

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To investigate the relation between fertilizer and groundwater quality, the well water chemistry near tea plantations was studied. Well waters were classified by the difference of pH value into 3 types. A type is acidic, B type is acidic to neutral and C type is neutral. Average concentrations of NO₃-N and HCO_3^- were: A type, 33.8mg/L and 0.1mg/L, B type, 13.1mg/L and 8.2mg/L; C type, 6.9mg/L and 60.0mg/L, respectively. The decrease in pH values was associated with the increase in the concentration of NO₃-N. The concentration of carbonate increased with pH value. Therefore, the decrease of amount of nitrogen fertilization used in tea plantations and the sufficient carbonate were thought to induce rising the pH value. As concentrations of metal in well water increased with decrease in pH value, metal concentrations and pH values were influenced by the use of fertilizer and carbonate.

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The Hebei plain belongs to semi-arid climatic region in China. In this region in the future the use of the land and the demand for the groundwater will keep increasing. Water resource is a key for industrial development of the region in the 21st century. In this paper we tried an accurate estimate of recent state of groundwater resources in the plain through the detailed analysis of movement of groundwater. According to the water balance calculation, the shallow groundwater recharge was mostly from infiltration of meteoric water and estimated to be 101.14× 10⁸m³/y and the discharge was 105.2× 10⁸m³/y due mostly to pumping up, so the decreasing amount of storage was estimated to be 3.88× 10⁸m³/y resulting in lowering of groundwater level. A mathematical model of groundwater flows of the unconfined and confined aquifers was introduced to predict the future groundwater state for given groundwater usage scenarios. According to the prediction, both unconfined and confined aquifers in the plain will be confronted with very serious water shortage problems within several decades.

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A numerical model with a groundwater recharge module has been developed to predict both fresh and salt water movement at a planned new campus of Kyushu University in the western part of Fukuoka City. Emphasis is placed on how to utilize the monitored groundwater variation for the modeling of hourly groundwater recharge from rainfall.
The result of the study reveals that the model can reproduce the variations of the monitored fresh groundwater level appropriately and the depth of fresh-salt water interface. Moreover, the possible extent of both the drawdown of groundwater level and the induced saltwater intrusion are predicted.

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This paper presents the results of an exploratory study on scour occurring around submerged spur-dike. The effect of flow depth and spur-dike dimensions was studied with the help of the experimental data. It was found that the overtopping ratio and the opening ratio are significantly affected on the maximum scour depth. The larger opening ratios (α) caused relatively small scour area and the bank erosion downstream the spur-dike hardly can occur. The longer spur-dike length & the lower flow depth produced scour area wider than the shorter spur-dike length & the higher flow depth. The data collected in this investigation would be useful for the development of numerical models of scour around submerged dikes.

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Spur dikes become to be recognized as hydraulic structures for environmental functions as well as for bank protection works. To predict and control flow, sediment transport and water quality exchange by using submerged spur-dike, it is necessary to clarify three-dimensional flow structures around spur-dike zones. In this study, orientation angle of super dike is chosen as a design parameter and its effects on flow structures in spur-dike zones are investigated by using PIV method. The vertical vortices caused by the top flow and transverse vortices caused by the side flow interact with each other and produce characteristic three-dimensional vortex structures. The orientation angle changes this interaction mechanism significantly. The flow structures of downstream-oriented spur dike are very different from those of upstream-oriented and normal spur dikes.

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Groin is expected to be used for various purposes. For each purpose, groin arrangement has been studied comprehensively through observations, experiments and numerical analyses.Flow around a groin has 3-D characteristic. For practical applications, a quasi-3-D numerical model is developed with introduction of hydrodynamic force on groin to predict flow and bed topography. The hydrodynamic force is introduced theoretically by volume integration of Filler equation.
2-D numerical analysis is not effective very much for 3-D flow around a groin. But the 2-D using hydrodynamic force on groin and boundary condition of periodicity is effective in respect of workload. The 2-D numerical model is considered to be able to predict reduction of velocity due to presence of groins. In this paper, we discuss characteristics of experimental flow separation. Pressure fields around the submersible groins for different groin angles from experiment is compared with the 2-D numerical analysis having static pressure distribution. It also describes about capability and scope of application of the 2-D numerical analysis.

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The authors propose a rubble-mound-weir as a flow control structure of nature-friendly. It is expected that this type of weir would allow aquatic animals traveling along the streamwise direction, while conventional structures made of metal and concrete completely block their movement along a river. Once a discharge flow-depth relationship is identified, even such a permeable structure could have a function to impound water and control river flow. The present paper is to examine hydraulic characteristics of the rubble-mound-weir. Main focus was placed on an open channel flow running through the weir. The system is a turbulent flow running through porous media that is connected to an open channel flow at the up- and downstream direction. Water surface profiles and a discharge-water depth relationship were experimentally and theoretically investigated.

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In the prcscncc of a fixed weir, flow and bed topography in river is not only affected by the weir but also by horizontal and cross sectional channel shape around the fixed weir.
In this paper, we studied influence of a fixed weir upon flow and bed topography. At first, the fundamental influence of fixed weirs on flow was investigated by experiments of a single straight channel and a compound meandering channel with rigid bed. Secondly, the influence of a fixed weir on flow and bed topography of compound meandering channel with movable bed through experiment was discussed. Special attention was placed on the change comparison in the flow and bed topography caused by the weir position and plane shape such as perpendicular and oblique weirs.

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Characteristics of open-channel turbulent flow downstream of a falling works were experimentally investigated by using image analysis methods. Trench section with various aspect ratios was installed just downstream of the falling work in order to understand the local effect of the trench shape to the main flow. In the experiment, twodimensional flow field in a longitudinal cross-section was measured either by particle tracking velocimetry (PTV) or particle image velocimetry (PIV) depending on flow situations. The measurements were conducted by using a highresolution video camera with 12 fps (frame per second). It was made clear that open-channel flows downstream of a falling work shows a drastic change in their surface shape as the aspect ratio of trench becomes greater than a specific value. In the experiment, it was discovered that a hydraulic jump created downstream of the trench section shows quite a periodic oscillation for a small range of aspect ratios. It was also shown that the imaging techniques used in the experiments are capable of measuring largely deformed free-surface flow with a reasonable accuracy.

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The bed protection works around the hydraulic structures like dam and bridge piers are usually collapsed due to disasters of sinking into the sand and transporting of sands around its bed protection works specially during flood flow. Disasters of protection works are declined the function and the stability of important hydraulic structures, and they are the factors of utterly destruction of the hydraulic structures. So, it is very important problem in hydraulic engineering to clarify destruction mechanisms of bed protection works and to propose disaster prevention measure.
In this study, we take up the phenomenon of sand removal from bed protection works considered to be a main cause of disasters. To examine such a phenomenon, an experimental flume model was used to conduct a set of experiments subjected to water pressure variation. The experimental results found good to be able to explain the effect of water pressure variation.

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Many researchers studied the energy loss for skimming flows, in which the main flow skims on the stepped channel chute and a corner eddy is formed at each step. But a general agreement on the experimental results has not been obtained. In this paper, the energy loss for skimming flows has been investigated systematically under wide ranges of discharges, channel slopes, step heights, and dam heights. The effects of the channel slope and the step height on the energy loss for skimming flows have been clarified. The energy loss for various changes of dam height has also been made clear. Further, the energy loss for the jump formation immediately below the stepped channel end has been shown, and the results reveal that the utilization of the stepped channel is effective in dissipating the energy of a supercritical flow on a spillway or a channel chute.

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This paper presents a 2-dimensional analysis of the flow in a pool and weir type fishway using VOF method. Because of a large influence to the result by the suitability of a different scheme of advection term in the equation of motion, the appropriateness of a numerical solution is examined by using different methods. And k-ε A turbulence model are applied to coefficient of kinematic viscosity for making the precision of a solution better. It is shown that QUICK method with k-ε A turbulence model is able to reproduce a flow precisely the most for the present.

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Prediction of damaging local scour around a bridge pier is considered to be of crucial importance in bridge safety. However, it is extremely difficult to solve analytically complex flow associated with local bed deformation surrounding the pier. This paper describes development of a numerical model in simulating flow and bed deformation around a cylindrical pier. The flow was calculated using 3D Reynolds-averaged Navier-Stokes equations closed with a nonlinear k-ε A turbulence model. A moving boundary-fitted coordinate system was employed to simulate the unsteady flow field bounded by movable bed surface and free water surface. The effect of nonequilibrium sediment transport was introduced in the bed-deformation model by coupling the momentum equation of sediment with stochastic models for sediment pick-up and deposition. The proposed model was tested against laboratory data for verification. The model was found to reproduce the flow and local scour geometry with reasonable accuracy.

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In this paper, a numerical model is developed to predict flow in dam reservoirs with orifice gate. We present a high-order Godunov scheme, referred to as the CIP method, for the numerical simulation of flow in a reservoir. This method splits the integration of Navier-Stokes equations into a non-advection and pure advection phase. The solution of the non-advection phase is cubically interpolated and then advected to the solution grid points. Free water surface movement is calculated using kinetic condition of the water surface. Calculated flow and free water surface behavior is compared with an experiment conducted in a flume with an orifice gate. Good agreement between calculated and measured results are found and thus the validity of the numerical model is verified.

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The flow characteristics of undular hydraulic jumps in horizontal rectangular channels have been clarified under a wide range of experimental conditions. The flow conditions of the undular jumps are classified, and the hydraulic condition required to form each flow condition is determined by the inflow Froude number and the aspect ratio. Further, each flow condition can be characterized by the wave height, the wave length, and the formation of the shock waves which are formed from side-walls near the toe of the undular jump. Compared with recent results by other researchers, a general agreement on flow characteristics of undular jumps is obtained.

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The undertow is one of important factors which causes the disastrous erosion of beach. So far, the problems of this phenomenon have been treated both theoretically and experimentally by many investigators especially for elucidating the mechanics of transport of sediment. In order to establish the theory of undertow, it is essential to estimate quantitatively the vorticity of large-scale vortices induced on the water surface by breaking waves, because it is also one of main boundary conditions for solving the basic equation of undertow.
The present paper describes a method of theoretical predictions for estimating vorticity of large-scale vortices induced by breaking waves. In comparison of this model with other experimental and analytical results, it is found that this theoretical prediction is very useful in application for estimating vorticity of large-scale vortices induced on the water surface by breaking waves.

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In order to examine local properties of three-dimensional complex flows around a water intake with locally varying strengths of shear and vorticity with isolated vortices, a new method of flow diagnostics based on the invariants of the velocity-gradient tensor has been proposed and applied to the analysis of the laminar flows in intake bay. The results indicate that the flow topology can be effectively found by the phase plot of the second and third invariants of the pressure-gradient tensor, while vortex structure can be better identified by the three-dimensional plots of the second invariant of the rotation tensor and the vorticity number.

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Rib-like structures in rivers, such as weirs or spur-dykes, have important roles for both flood control and river environment. It is important to clarify the flow characteristics over a rib in open channels to design and maintain the river structures. In this study, two different methods, i.e. an empirical model and a 3D computation, are applied to the flow around a full-span rib with a rectangular section in open channels. In 3D computations, linear and non-linear k-ε models with quadratic terms and cubic terms are employed for the turbulence model. The results in the two methods are compared with experimental results and performances of the models are discussed. It is shown that an empirical model is effective to elucidate the fundamental aspects of the phenomena.

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Zen et al (1987) has derived a well-known equation on the dynamic behavior of pore water pressure in the study on the mechanism of wave-induced liquefaction and densification in a permeable seabed. As a result, they reveal that pore water pressure attenuates its magnitude and delays its phase during its propagation into permeable seabed and that these phenomena are highly related to the physical properties of seabed such as permeability, porosity and volume compressibility of seabed sand.
This study intends to clarify the effects of porosity on the attenuation and phase delay. A series of model tests reveals that period of the dynamic pressure loaded on the surface of seabed is a controlling factor for both attenuation and phase delay and that a firm power functional relationship exists between the attenuation rate and porosity of seabed sand although a clear effect of the porosity is not found in the case of phase drift.

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Stability of a falling water sheet is studied by measuring the displacements of water surface at the various heights of the sheet. Effects of geometry at the backward space of the sheet and disturbance generated at the upstream water surface are investigated from the power spectrum of the data. Experiment without a vertical wall at the backward of the sheet reveals that the oscillation occurs due to the shear instability of air flow. This oscillation with frequency about 10 Hz at the upstream and 20 Hz at the downstream seems to be caused by a resonance of the system, because these frequencies are amplified along the sheet as elucidated from the forced oscillation experiment.
The modulation of the oscillations observed by the vertical wall experiment, which is known as the nappe oscillation, is examined and predicted by a model derived from present study. In addition, the shear wave appearing on the sheet and leading to the sheet break-up is discussed by a linear stability analyses of the Navier-Stokes equation.

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Fluid forces acting on a circular cylinder in multiple rows of circular cylinders installed across the whole parts of channel width and partial parts of channel length in an open channel are measured by load cell with three components. Drag coefficient C_D of this circular cylinder with large arrangement density λ is larger than the drag coefficient of circular cylinder in uniform flow. Roughness coefficients known as Manning's n for this multiple rows of circular cylinders are evaluated by using these Drag coefficients C_D measured. Roughness coefficient of multiple rows of circular cylinders in open channel increases with increase of arrangement density λ and water depth. Profiles of gradually varied flow in multiple rows of circular cylinders are evaluated well by using these Manning's n.

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Aerial photographs of 1998 flood flow in the Tone River were analyzed by cross-correlation method to obtain velocity vectors over the flow surface. Characteristic features of flow downstream of bridge piers were examined by using the data. It was pointed out the discharge measurement by float estimates larger discharge than real one.
Features of velocity distribution and of the cross section profile of river bed were examined to get the guide-line on choosing the appropriate cross-section for discharge measurement.

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A longitudinal vertical cross-section of unsteady open channel flow was visualized by video camera. The 120 frames during 4 seconds in rising stage and the 120 frames during 4 seconds in falling stage were analyzed by cross-correlation method to obtain velocity vector distribution. The structure of large scale turbulence which drives longitudinal spiral motions were examined by using the obtained data.

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The laboratory tests were performed to analyse the flow in curved compound channel with small relative depth. It was found that the horizontal vortices appear at the junction of a main channel and flood plains in the outside area and the secondary flows by centrifugal forces appear in a main channel.
Next, in order to simulate flow in curved compound channel, numerical computations were performed by employing one equation model as turbulent model. The result agrees well with the experiments in the middle of a main channel or flood plains, but not with the experiments at the junction of a main channel and flood plains in the outside area. To simulate the flow more accurately it was found that the effect of horizontal organized vortex must be considered. The numerical computation has revealed that flow in curved compound channel with flood plains consists of three circulating cells induced by centrifugal forces.

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Turbulent structures along a mixing layer in an open-channel flow over a concavebed are analyzed using wavelet transforms. Experimental data of the velocity are obtained by a particle image velocimetry (PIV). At first, we evaluate the accuracy of the PIV velocity data by employing a multiresolutional analysis (MRA). The result of MRA shows that the instantaneous Reynolds stress is an appropriate signal for the coherent structure detection. Then, the distributions of the instantaneous Reynolds stress are analyzed using a two-dimensional continuous wavelet transform (2D-CWT). The spatial scales and locations of the coherent structures are detected from the maximal values of the wavelet coefficients. The characteristics of the organized motions along the mixing layer, such as an evolution of spatial scale, advection velocity, and occurrence interval, are clearly extracted in terms of the location and the time.

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In the present paper, we analyze an open-channel flow over concave bed by using a particle image velocimetry (PIV) and proper orthogonal decomposition (POD). From the experimental data of velocity obtained by PIV, flow characteristics, such as mean velocity, turbulent intensity and Reynolds stress, are discussed with respect to the different Reynolds numbers. As Reynolds number becomes larger, the recirculating flow in the concave grows to a large spatial scale. The distributions of the Reynolds stress and turbulent intensity are closely related to the scale of the recirculating flow. Then, principal components of velocity fluctuations are analyzed by using POD. From the distributions of the eigenvectors, first several modes of the component relates to the effect of the recirculating flow and a few modes in succession indicate that of the coherent structures along the mixing layer. The result shows that POD is effectively available for quantitatively detecting the dominant factors of the velocity fluctuation.

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We have proposed an image processing technique to measure simultaneously the water-surface and velocity vectors in open-channel flow. A boundary growing process is employed to extract the locations of water-surface using the gradient of luminance in the image and two threshold values of gradient. Simultaneously with the water-surface, the velocity field in the channel is measured using a particle image velocimetry.
This measuring technique is applied to the supercritical flow in the channel with a concave bed. Analysis of the instantaneously measured data of water depth and flow velocity has revealed that the fluctuation of water-surface and the velocity field interact each other. These results suggest that the present method is well applicable to rapid flows with the fluctuating water-surface.

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Stepped steep open-channels have watched with interests such as landscape effect and water quality purification function by aeration besides the energy dissipator. The authors have made some experiments to clarify the flow structure and considered mean flow characteristics by measuring flow depth fluctuations, mean flow velocity and pressure head on one step. In this study, we conducted an experimental examination on the turbulent flow characteristics obtained with hot-film anemometer to elucidate accelerating flow over one step. Experimental results showed that Reynolds normal stress and shear stress clearly decreased with downstream distance from step edge by the effects of favorable pressure gradient, and inverse cascade of energy happened.

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High-resolution videographic P. I. V. is applied to measure cross-stream velocity components at 15 Hz and 1 Hz in the lower half of a smooth-bed open channel flow at Re*=u*d /v= 300 (where d is the flow depth of 10.0 cm). We emphasize analysis of the quantity Q_x=(ω_x²-S_x²)/2 (ω_x: streamwise vorticity, S_x: rate of strain in the cross-stream plane) which expresses the relative strength of rotational and straining motion in the illuminated plane. Instantaneous flow fields, confirmed by two-point correlations, show that regions of negative Q_xlie above, below, or beside regions of strongly positive Q_x.Isosurfaces of a suitably conditioned spatio-temporal correlation of Q_xreveals a tilt and inclination of streamwise vortices (“SV”) which agrees substantially with previous results from DNS [e.g. Stretch (1990), Jeong at al.(1997)], and suggest a consistent lattice-type spatial relationship with neighboring SV.

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Downward-bursting phenomena in water beneath wind waves is important in the process of the momentum, heat and mass transfer across the air-water interface. The coherent structure is closely related to the wind-waves in specially DBBL (downward-bursting boundary layer, about 3-7 times depth of the significant wave height). The turbulent inten-sities and Reynolds stress (-uv) in closed wind water waves could be expressed by the parameters of wind-waves, the intensity of water depth fluctuation and the peak frequency.
To estimate the bursting period, the u-v quadrant threshold technique is applied to the streamwise velocity signals in the water flows. The bursting period is in a good agreement with the behavior of time series of Reynolds stress (-uv). As a result, it became clear that the bursting period has a close connection with the peak frequency of wind-waves.

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Flow measurements were undertaken in a compound channel to investigate the effect of stratification on secondary flow in two-layer density-stratified flow. Secondary flow and salinity were measured using a laser induced fluorescence technique together with a laser Doppler anemometer system. Flow visualisation was also carried out using a digital video camera to demonstrate the effect of stratification on flow. The measured data shows that the size of typical twin vortices in the compound channel reduces as the degree of stratification increases. Two generation mechanisms of secondary flow were recognized on the floodplain, namely non-isotropic turbulence driven secondary flow and density driven secondary flow. The production of turbulent kinetic energy due to buoyancy was found to be not significant in most region but is significant in the water surface region

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A directn umericals imulationo f a fully-developed turbulent flow in a two-dimensional open channel has been conducted. The effects of the instantaneously deforming free surface and the fluctuation of the vertical velocity component at the free surface are taken into account by applying the small-amplitude wave theory. The results obtained at two subcritical Froude numbers are compared with those conducted for the closed-channel flow and those for the open-channel flow with free-slipa pproximation for the free surface. Generally, the effects of the motion of the free surface on such quantities as the mean velocity and Reynolds stress distributions and their transports are found to be small except for quantities involving the vertical fluctuation near the surface. The effects on the eddy viscosity coefficient were the most significant and over a large region.

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Numerical analysis of unsteady open-channel flows was conducted by making use of low Reynolds number k-εmodel, involved with the anisotropic turbulence modeling, and VOF method. The distributions of ensemble averaged velocity and the values of turbulence have been obtained in the whole flow region from the wall to the time-depending free surface, and the values of these calculations coincide well with LDA database. In addition, the distribution of turbulent energy budget and the constituents of shear stress in depth-varying unsteady flows, which are difficult to be measured, have been calculated and the ability of this calculation about unsteady open-channel flows has been proved.

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A numerical simulation on turbulent open-channel flow, which has an aspect ratio (channel width/flow depth) of 6, was conducted by using of a large eddy simulation (LES). In this study, 1-equation model was used. The numerical results of time-averaged mean velocity, Reynolds stress and secondary currents well correspond to the experimental data which were measured by Nezu & Rodi by making use of a laser Doppler anemometer with high accuracy. The instantaneous flow fields in the cross section and horizontal planes were investigated. The instantaneous secondary currents were observed far away from the side-wall. The magnitude of the secondary currents is more than 20% of the streamwise velocity

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Fully developed three-dimensional turbulent flow over sand ribbons in an open-channel is computed to clarify the generation mechanism of secondary currents of the second kind by direct numerical simulation (DNS) using a regular grid under a generalized curvilinear coordinate system. Stable secondary currents and turbulent characteristics are reasonably reproduced. Detailed evaluation of each term in a mean vorticity transport equation revealed that the secondary currents of the second kind is generated principally by cross-planar turbulent shear stress (-v⁺w⁺), and that the anisotropy of the cross-planar turbulent normal stresses (v⁺²-w⁺²) restrains the generation of the secondary currents, where v⁺and w⁺represent vertical and spanwise velocity fluctuations normalized by friction velocity u_τ, respectively and overbar denotes the time average.

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A finite-difference method to be used in simulations of turbulence such as direct numerical simulation (DNS) and large-eddy simulation (LES) has been developed and verified in basic benchmark flows. The method is constructed using the general curvilinear coordinate system with the collocated grid arrangement of the variables so that it can be easily applied to various engineering flows including hydraulic applications involving complex geometry. The computer code is first verified by a test calculation of steady laminar flow in a curved cavity, then the same code is used in a DNS of fully-developed turbulent flow in a two-dimensional open channel. It is further used to conduct a LES simulation of the same flow. All the results indicate that the method is sound with sufficient accuracy for the intended applications.

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A fifth-order computational scheme utilizing spline interpolation has been proposed for convection terms. The physical values discretized on finite grid points are locally interpolated with a quintic spline function as a four times continuously differentiable functions. The derivatives, included in the convection terms described as non-conservative forms, are evaluated by differentiating the quintic spline function, instead of using the difference equations consisting of discretized values, as usually done in higher-order upwind schemes. Thus, the present quintic spline interpolation (QSI) method enables us to deal easily with the non-uniform grids and to treat convection terms in the near-wall regions preserving reasonable accuracy. The QSI method is applied to a onc-dimcnsional pure advection problem_ As a result, while slight phase error is included, it is shown that the numerical accuracy in the QSI method is superior to a fifth-order upwind difference in terms of the preservation of peak values in the initial step-shaped scalar distributions. In addition, two-dimensional flow fields in a cavity are solved with the QSI method. The computational results are in good agreement with the results indicated by Ghia and it is proved that the velocity distributions are predicted with the higher accuracy than the fifth-order upwind difference.

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The discharge measurement is one of the most important works for river engineer. For the present, weusually carry out two types of discharge measurement, those are the Eulerian and the Lagrangian methods. Especially, the measurement through floating device, which is typical as the Lagrange method, is adopted extensively. But most of the rivers in Japan are compound meandering channels, which have complex flow field. So, we have to check the accuracy of discharge measurements of those compound channels.
In this paper, we have investigated the accuracy of discharge measurements in a compound meandering channel by using a three dimensional numerical analysis. The accuracy of the discharge measurement is clarified from the Eulerian and the Lagrangian methods.

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Flow near riverbank erosion was examined to investigate mechanism of bank erosion through experimental study of flow field of model bank erosion shape in the authors' previous study. This paper presents prediction of flow field near bank erosion. A numerical model is developed to compute two dimensional flow velocity and water depth for different model bank erosion shapes. The purpose of this study is to reproduce measured flow field of the model bank erosion shape. Curvilinear coordinate system is used in this analysis to obtain the bank erosion shape appropriately. The model could predict the flow field inside bank erosion for small erosion surface angle (equal or less then 4 degrees) with higher accuracy. It is also able to reproduce flow field inside erosion part for larger erosion surface angle (8degrees), but absolute value of the computed velocity is larger than the measured velocity.

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Extensive development in river flood plains should be achieved in parallel with minimization of potential inundation risk of over-bank flooding. Flood risks of property damage and loss of life can be eliminated through accurate flow predictions and by proper application of mitigating measures such as flow regulation structures. This paper presents a comparison between experimental results and a 2-dimensional unsteady flow numerical model simulations of flood propagation in a compound meandering channel. The model is based on a finite volume discretization on a staggered grid with upwind scheme in flux is presented, it handles drying and wetting process for a flood plain, complex geometry and discontinuities, which are the main requirements for modeling compound channel flows. The main objective of this research is to advance the existing knowledge on flood flow phenomenon in compound channels through comparative studies of unsteady flow experiments and numerical simulations. Comparison between measured data and model simulations show generally good agreement.

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Internal waves in a bay have an effect on vertical mixing and mass transfer. Toestimate sources and roles of the internal waves, we have to carry out computation with high resolution by using a three-dimensional non-hydrostatic model. As it is hard to carry out it now, a multi-layer model is considered to be one of the most suitable model. However, internal hydrolic jumps occur frequently on an interface between layers because of small density difference and great velocity (Internal Froude number>1.0). In previous studies, a flow field including both rapid and streaming flow has been solved by CIP-m scheme in which a finite difference method is used. But it is better to use a finite difference method (FEM) because the shape of a bay are complicated and FEM can be applied to any shape. In this study our purpose is to develop CIP-m FEM. The result of an experiment was consistent with that of computation.

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