Effects of regularly arrayed square roughness on flow resistance and turbulent characteristics in an open channel have been investigated over a completely rough-bed with uniform stainless square ribs, where their heights were 5mm and 10mm. Detailed spatial measurements of streamwise and vertical velocity fluctuations were conducted using Particle Image Velocimetry(PIV)in a vertical plane along the completely rough bed surface. Experimental results indicated that flow resistance with three-dimensional square roughness is higher than that with two-dimensional square roughness, the maximum value of flow resistance appeared in transverse spacing delta/k(transverse spacing/roughness height ratio)=1 and significant degrees of spatially regular variation in the time-averaged velocities were generated along the rough elements.

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In this study, characteristics of secondary flow and coherent structure in one-sided compound open channel flows with inclined main channel side wall are investigated using PTV (Particle Trucking Velocimetry) and fluorescent dye injection method. The velocity measurement results show that intensity of secondary flow in the case of deep depth tends to be weak compared with the flow with vertical side wall and the rotation of circular secondary flow over the inclined side wall in the case of shallow depth is in the direction opposite to that one of the flow in the case of deep depth and vertical side wall. The results of flow visualization indicate that streamwise vortex is formed over the junction edge, inclined side wall and the corner of the main channel. Furthermore, the analysis of DPTV (Dye-streak pattern Particle Trucking Velocimetry) which is combined with PTV and fluorescent dye injection method shows that the streamwise vortices play important role in generating instantaneous secondary flow.

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Generally, stepped channel flows are characterized as aerated flows. For the design of stepped channels, it is important to know the aerated flow characteristics such as the aerated flow depth, the air-concentration ratio, the aerated flow velocity, and the energy head in skimming flows. Recently, the authors clarified the aerated flow characteristics for the quasi-uniform flow region of skimming flows under a wide range of channel slopes. However, for the nonuniform flow region, the aerated flow characteristics were not elucidated. In this study, an equation for the calculation of the aerated flow depth is developed in the nonuniform aerated flow region, and the aerated flow characteristics are determined for the nonuniform aerated flow region under a wide range of hydraulic conditions.

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This research detailed the interaction between the dispersive wave train and ice floe in tsunami-induced wave propagation upstream in rivers. First we conducted hydraulic experiments to investigate the effects of ice floe length and thickness using field observation data obtained when river ice breakup and scattering were caused by the 2011 Tohoku Pacific-Coast Earthquake tsunami. We measured transportation velocities of ice floes using an image analysis technique. Second, particle based numerical simulation was conducted, by which we can easily handle with violent free surface motion and ice floe movements. Direct comparisons of experimental photos and their corresponding simulation snapshots are made in terms of the reproduced waveforms and the ice floe displacement. The work highlights potential risks of ice floes when tsunami propagation upstream in a river occurs.

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Shallow landslide induced by heavy rainfall events represents one of the most disastrous hazards in mountainous regions, like Japan. This paper described a hazard assessment of rainfall-induced shallow landslides on a regional scale using a statistically-based and physically-based model. The models were applied to a case study of the heavy-rainfall event in July 2004 in Niigata Prefecture. The results were as follows. 1) In the model-training area (the Tochio area), in either of the models, the correspondence between the areas, where the model predicts a high-hazard condition, and the localization of the shallow landslide events was considered satisfactory from a qualitative point of view. 2) In the model-validation area (The Izumozaki area), the physically-based model predicted the hazard of shallow landslides better than the statistically-based model.

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When a large-scale rain event occurs, a small-scale bank collapse and sediment transportation may be caused at the junction of mountain streams. This may result in debris flow that continues downstream to join the main channel. The effect of a mountain tributary on the main river is usually negligible because its discharge volume is small compared to the main river. However, when debris flow occurs, the impact of the tributary on the main river is significant because the discharge becomes large compared to the normal situation. The difference between the discharge volumes of the tributary and the main stream, the slope angle, and the confluence junction angle all seem to have an effect. In this study, we considered the junction angle of the debris flow at the confluence of two channels. We first examined previous studies and past events of debris flow confluence to analyze the trends and to determine the average junction angle. We then conducted debris flow experiments with different channel junction angles.

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Soil erosion and runoff have led to environmental problems in Babeldaob Island, Republic of Palau, and effective measures which can reduce sediment yield in the developing bare land are necessary. This paper aims to quantify the sediment runoff from land development zone in Ngerikiil watershed at southern part of Babeldaob Island. The following three points were clarified. First, field observed results showed that severe soil erosion at the land development zone affects the behavior of sediment movement in the watershed. Second, 556 t/ha/y of soil erosion at the developing bare land was estimated by using observed data. Third, the Water Erosion Prediction Project (WEPP) model was tested to verify their applicability in predicting sediment yield at a land development zone. The sediment yield which was calculated under various landform and landuse conditions was reasonable.

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The occurrence of debris flow is strong affected by ground water flow. Heavy rain raises the degree of saturation in the riverbed deposits in steep watersheds. Ground water flow follows various parts and flows out from the soil. Therefore, the degree of saturation of a soil is not constant. But, in most studies, the riverbed deposit which is the source of debris flow is considered to be saturated. In our research, we observe the variation of water content in the riverbed deposit and rainfall in a mountainous watershed in order to evaluate the influence of unsaturated ground water flow on debris flow formation. The measurements show that most of the deposit was unsaturated when sediment movement occurred. They showed that sediment movement typically occurred during event for which the effective rainfall, computed with a half-life of 1.5 hours, exceeded 15mm.

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In this paper the authors classify the seven J-POWER dams located at the most upstream end into four groups from the viewpoint of soil production and soil transportation in each reservoir basin. And the authors also carry out correlation analysis between water inflow to reservoirs and reservoir sedimentation and analyze applicability of a sediment runoff model without soil production process.

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Suspended sediment runoff from paddy fields is neglected compared with sediment runoff from sugarcane and pineapple fields in the Ishigaki Island, Okinawa. We observed suspended sediment runoff from the double-cropping paddy fields of the island and examined the applicability of L-Q equation for the observations. Firstly, the annual suspended sediment load was 7,130 kg per ha. The runoff load during no-rainfall periods accounted for 54 percentages of the total runoff load, and the runoff load in puddling and transplanting periods accounted for 76 percentages of the total. Secondly, the different specific L-Q equations in puddling and transplanting periods and the other period improved the estimation of the annual suspended sediment runoff load.

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Viscous debris flow which is observed at Jang Jia Gully in China, is flow as lots of surges intermittently. Many intermittent debris flows have been observed in Europe mountain basin too. But these characteristics of the flow are not made to be clear. This paper is described about a wave equation and it's solutions obtained by perturbation method. This paper shows a wave equation of shallow water with sediment on inclined channel obtained by perturbation method and the solutions. It is possible to estimate the fluctuation of depth of shallow water by numerical simulation. But it is very important to obtain the wave equation and the solutions for explaining wave motion of the flow.

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This study aims to estimate sediment discharge and river bed variation based on long-term hydraulic quantities in a basin. For comprehensive sediment management throughout the river basin, it is important to elucidate sediment dynamics, such as the sediment yield from the mountains and sediment transport in the river not only during flood periods but also during normal periods. We formulated non-uniform sediment discharge from the mountainous area by using the long-term hydrologic cycle model. In addition, we performed long-term calculation of river bed variation, using the formulated equations as boundary conditions at the upstream end of the Mu River basin, Hokkaido, a river with high sediment yield. We reproduced the long-term trends of river bed variation and succeeded in estimating the sediment discharge into the estuary.

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Recent years, slit Sabo dams have been selected for their function to prevent debris flow disaster and conserve continuity of water and sediment flow simultaneously. However, once a slit Sabo dam is filled with bed load or suspended load, its pocket for debris flow prevention would be lost and sediment supply to downstream would be decreased. Though in order to resolve the problem, knowledge and investigation about characteristics of sediment transportation around a slit Sabo dam are important, not much study about slit Sabo dams have been reported. In order to clarify the effect of a slit Sabo dam on sediment transportation, the authors have researched the characteristics of rainfall-runoff and sedimentation around a slit Sabo dam in a channel in Tenjin river basin. This paper reports the results of field observation on and 1D numerical simulation of the flood event caused by Typhoon No. 12 in 2011.

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Reservoir sedimentation is one of the most important problems for securing long term achievement of hydropower plant operation in the future. We have already classified the sedimentation problem regarding the reservoir types and proposed effective sediment control measures for regulating reservoirs. On the other hand, sedimentation problem in storage reservoirs needs to be more studied. In this paper, we have evaluated the influence of sedimentation progress in storage reservoirs on power generation by analyzing long-term operation record, and predicted the future influence of reservoir sedimentation from the view points of active storage capacity and the maximum hydropower discharge.

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The characteristics of flow behavior and bed variation in a diversion of a river were investigated experimentally. The experiment was conducted on the fixed bed as well as mobile bed to examine flow mechanism and bed morphology development in a diversion of a river. Through the experiments, it was found that the characteristics of flow and bed variation in a diversion were classified into several patterns by the channel shape and relationship of water surface profile on upstream and downstream of a diversion. Numerical simulations of flows in diversion of a river were also performed by quasi-3D free-surface flow model. It shows that the model can reproduce the complex behavior of the flows with reasonable accuracy.

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Numerical simulations for flood flows and bed variation in a symmetrical river confluence were performed using a quasi-three dimensional numerical model. The vertically averaged and moment equations was used as governing equations to allow for the incorporation of pre-assumed linear distribution of horizontal velocity distributions. The model is based on finite volume method using HLL(Harten, Lax and van Leer(1983)) and HLLC(Harten, Lax and van Leer and Contact(2004)) numerical flux, which is one of a Riemann solver. The model is verified against two experimental data of flows and bed variation in symmetrical river confluence. It shows that the model can reproduce the complex behavior of the flows and bed variation with reasonable accuracy.

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Saba River passing through Hofu City, Yamaguchi Prefecture has many weirs. Recently, the planning to disuse and integrate weirs has been discussed in order to improve the conveyance ability in Saba River. In this paper, the effect of the removed and built weirs on the river bed elevation change in Saba River is discussed. Firstly, the effect of the flood in 2009 on the river bed materials is studied using the observation data. It is found from the observation data analysis that the mean sediment diameters become smaller than that before the flood in 2009. Secondary, the river bed elevation change for 60 years is studied with the numerical simulations. It is found that it take the river bed elevation at least 8 years to be stable.

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In the Lower Tone River, channel dredging and widening have been conducted for the river improvement works to respond to the flood discharge increase. However, the discharge capacity of the current river channel is still not sufficient even for the present design flood. So, it is required to discuss about the method for the appropriate river improvement works in the Lower Tone River. But, the effects of the past river improvement works on changes in river bed elevation are not sufficiently investigated. We investigated the process of the river improvement works in the Lower Tone River and clarified how the river improvement works have effects on changes in the river bed elevation by investigating the changes of channel shape and longitudinal distributions of hydraulic quantities. The results demonstrated the sediment deposition disappeared and the large bed scouring occurred and disappeared by the effect of the channel dredging and widening in each area.

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In the Agano River estuary flowing into Japan Sea, bars at the mouth are developed from both river banks. The bar causes the river discharge capacity to decrease and the water level to rise. Therefore, it is important to understand mechanism of river mouth bar flushing during a flood event. One of the authors has shown that flow and bed variation during a flood event may be explained by means of a suitable numerical analysis method using temporal changes in observed water surface profiles effected by various events in the river. The objective of this paper is to clarify flushing at river mouth bar during a large flood by temporal observed water surface profiles and the numerical analysis.

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The equipment for measuring rheological characteristics of water-fine sediment mixture is studied. The relations among wall shear stress, average velocity, velocity gradient and kinematic viscosity are derived for pipe flows assuming Bingham fluid. The equipment which is composed of a tank and several small pipes is proposed based on these theoretical relations. The diameters and the lengths of these small pipes are determined in order to obtain data on the relation between shear stress and strain rate in laminar flow conditions. Measured data on yield stress and kinematic viscosity for water and kaolin mixture suggest that the measurements are performed properly, which means we are able to measure rheological parameters of water-fine sediment mixtures fully, simply and economically.

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In this study, series of experiments were conducted in order to investigate the effect of permeability of sediment on slope erosion by artificial rainfall. In this study, the slope was mainly composed of sand whose grain size was different in each case. In addition to such experiments, another investigation was carried out to find out the effect of adhesiveness on the erosion of slope which contains some amount of clay. As a result, it was found that overland flow affects surface erosion, and therefore the slope permeability governs the slope erosion considerably. And it was also confirmed that adhesiveness affects channel network formation due to the surface erosion considerably.

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The purpose of this experiment is to understand the effect of sand transported from upstream reach of river on both deformation of bed with cohesive sediment and vertical bed structure. In this experiment, the amount of sediment feeding is equal to the amount of equilibrium bed-load transport, and experiment was continued until the deformation of the bed reaches the dynamic equilibrium state. As a result, characteristic vertical structure consisting of "sand layer", "sand-clay mixed layer" and "clay layer" were confirmed on the bed. In addition, it was seen that the existence of waves formed over the surface of cohesive sediment leads such vertical bed structure.

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Riverbed degradation occurs in many rivers in the world. It causes serious problems in river management. For example, the bedrock which had been covered with gravels is exposed due to the riverbed degradation, which destabilizes river structures, such as bank revetment works and groundsill works. In this study, hydraulic experiments were performed to determine sediment transport characteristics on bedrock channels partly covered with gravels. We obtained the following results. The bedload transport rate and the friction coefficient can be related with the coverage rate of gravels on the channel bed. A simple mathematical model describing sediment behavior on bedrock channels was proposed.

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Many bedrock channels are composed partly of alluvium and partly of bare rock, and abrasion from the influence of bed load is known to be a ubiquitous and principal mechanism of fluvial bedrock incision. The results of recent investigations have suggested that the bedrock incision rate depends on rock strength, sediment supply, grain size and the areal fraction of bedrock exposure. In this study, the authors performed laboratory experiments to evaluate the influence of rock properties on the bedrock incision rate. The results showed that rock strength and the effective porosity of bedrock have a significant effect on this rate. As the effective porosity of weathered bedrock is greater than that of fresh bedrock, these results suggest that wetting/drying cycles and freeze/thaw cycles can affect the incision rate and the resulting bedrock channel morphology.

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Since there is little influence on natural environment and flood regulation function is high, many dry dams have been planned in recent years. However, the characteristic of sediment transport in a dry dam is not sufficiently understood. The purpose of this research is to obtain the basic data for conducting hydraulic model experiments and grasping the sediment transport characteristics of a dry dam. As a result of the experiments, a sediment is flushed from the dam at the early rising stage and falling stage, and it was shown that the particle size distribution changes in time. Moreover, it was suggested that the flushing mechanisms of sediment from the dam differ in an early rising stage and falling stage. It becomes clear that a meandering talweg is formed at the just upstream of the dam. It is considered that this talweg has an important function for the sediment transport.

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Estimation of sediment transport rate and bed variation in stony-bed rivers needs to understand the effects of stones sizes and shapes in streams. However, measurements of forces acting on sediment particles in active are difficult, especially for rapid stony-bed rivers. To investigate the dynamic mechanism of stones moving in streams, we have developed a numerical movable bed channel in which three-dimensional motion of each shaped-stone is calculated, respectively. The dynamic interaction between stones motions and hydrodynamic forces on movable bed surface in numerical experiment is investigated. It is demonstrated that large stones form static stone clusters. Stones clusters have an important mechanism to stabilize stony-bed surface against hydrodynamic forces of flows.

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There are many points the dams of whose which are a river transected structure thing are unknown about the influence which it has on the physical environment of a river downstream site, specifically river channel form, stream bed material, drifting sand, and a flow. Furthermore, about the stream bed change accompanying dam withdrawal, there is no empirical research in Japan. In this research, it investigated in the Arase Dam downstream site where withdrawal is started from September, 2012. As a result, the sandbar area in the Arase Dam direct lower stream of the correlation with the dam annual maximum outflow rate was weak, and the influence from the amount of removal of the earth and sand deposited on the dam lake became clear.

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SS Flux, particle size distribution and organic matter ratio under flooding conditions were investigated in the Kumagawa River in which dam removal operating will be conducted for the first time in Japan. The results showed 99% of SS were categorized into silt or clay and median diameter of particles had a tendency to be smaller than that of other rivers. The results of non-uniform sediment rating curve (L=aQ^b) analysis indicated that the coefficient b were closely dependent on particle size. Total SS transports into the estuary were estimated 179*10⁴ ton from April, 2010 to July, 2012 and small silt (23.6~5.07µm) element had the best contribution.

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In this study, mechanisms of suspended sediment transport in the downstream region of a vegetated area that was modeled by a pile group were investigated by performing a flume experiment with the Particle Image Velocimetry (PIV) technique for particles of neutral buoyancy and relatively heavy materials. The MASCON model were additionally applied in order to reconstruct the three-dimensional information of flows in the region by using the velocity components on several horizontal planes obtained by the PIV analysis. The experimental results show that the heavy particles selectively follow the vertical motions of the relatively large-scale structures which are generated by the wakes of piles, and their behavior resemble the specific phenomenon that is called "trajectory bias."

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Sediment supply measures from the reservoir are required to reduce sedimentation and to preserve the downstream river bed environment. Then, we have been trying to develop the new sediment supply measures using the differential water head energy between upstream and downstream of the dam. We have proposed the burrowing type sediment removal suction pipe by laboratory experimental study. In this study, in order to obtain knowledge for the practical application of the pipe, we carried out the sediment discharge field test using the 200 mm diameter suction pipe at the actual very small reservoir located in the mountainous area. As a result, we understood the hydraulic characteristics such as a relationship between velocity in the pipe and the sediment concentration, a water head energy loss of the pipe and so on. It is confirmed that the burrowing type sediment removal suction pipe could be applied to remove non-cohesive sediment material without debris in a small reservoir.

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In order that Sedimentation in a dam reservoir may invade dam capacity, the countermeasure against the sedimentation is indispensable to the maintenance and improvement in the function of a dam. Authors have examined Hydro-suction Sediment Removal System (HSRS) as a countermeasure against sedimentation in dam reservoir. However, if the deposition becomes deep, soil and sand may not collapse by the effect of arch-action. Then, authors proposed Vertical multi-hole suction pipe method (VHMS) which has arranged the suction hole perpendicularly in order to promote soil and sand collapse, when deposition is deep. This research examines the performance of VMHS by model experiment.

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Authors did a movable bed and steady flow experiment at the Chiyoda Experimental Flume, a large scale flume located in the Chiyoda New Waterway of Tokachi River, and observed bed wave figure on the local bed materials. In this study, the steepness of the sand waves observed was first estimated using the Yalin-Karahan method, and found that the observed was only slightly less than the estimated value. Next, the normalized bed shear stress and normalized grain shear stress calculated from the observed values were compared, and were found to have similar tendencies as seen in the results of Kishi and Kuroki based on the hydraulic radius separation method. The bed shear stress was also estimated by calculating grain stress and form drag directly from sand wave forms with reference to the method of Kikkawa and Ishikawa etc., and the estimated values were found to be slightly smaller than the observed values.

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This paper presents a consideration of formation mechanism of tidal channel networks. We employed a simple mathematical model to explain the flow and bed evolution in intertidal zones. By assuming the regular periodic tidal wave and the simple bed geometry which has constant slope, we derived an advection - diffusion equation for the bed evolution. This equation has a Peclet type non dimensional number which expresses the ratio between advection and diffusion. We investigated this Peclet number in the experimental results conducted under the different hydraulic conditions. The results show that in the conditions with small Peclet number, which means that the diffusion effect is relatively strong, clear channel network can not develop on the bed surface. On the other hand, in large Peclet number conditions, clear and complicated channel network tends to develop.

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In September 2011, the largest discharge in recorded history was observed in Otofuke River, causing riverbank erosion in various parts of the basin. Damage was especially serious in the middle reaches, where part of a dike was completely washed out. The results of a survey suggested that the cause of the dike breakage was bank erosion associated with the development of a meandering channel. As the related development mechanism and predominant factors have not yet been clarified, this remains a priority for disaster prevention. In this study, the meandering-channel development process was reproduced using a slope failure model that linked bank erosion with bed fluctuations. The study attempted to clarify the meander development mechanism and predominant related factors based on this model.

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In order to stably maintain a river channel, the river channel form needs to suit its origin. Therefore, many researches on the origin of the plane form of a river channel or river width have been conducted. However, the relation between a channel network and meandering shape or a sandbar and the formation process of a channel network is not sufficiently clarified. In this research, in order to clarify these phenomena, a hydraulic model experiment using a wide width flume was conducted and the results were discussed using meandering theory and bar formation theory. As a result, it has been shown that the bars and braided channels may be influenced to some extent by the wave resulting from plane instability.

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This paper presents a validation of non-equilibrium bed-load model for numerical simulation of bed evolution. We applied saltation length to Nakagawa and Tsujimoto-type’s non-equilibrium bed-load model assuming that the non-equilibrium bed-load on dune bed is characterized by the salutation length scale and pick-up rate is estimated by equilibrium bed-load rate. The simulation using the presentbed-load model reasonably reproduced bed evolutions measured in experiments. We also validated values of effective shear stress, which is estimated by average flow velocity, bed shear stress and average bed-load on dune bed as the simulation results.Calculated bed-load rate in the simulations was shown to reproduce the average bed-load rate on dune bed well.

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Periodic boundary waves are sometimes observed on surfaces of large ice bodies such as glaciers and polar ice caps on Mars as well as on the Earth. These boundary waves may be formed by boundary instability between the ice surface and the fluid flowing on it. We propose a mathematical model to describe the evolution of the ice surface by the use of the Navier-Stokes equations, the heat transfer equations of flow and ice, and a heat balance equation at the boundary. Assuming that the temperature above the flow is higher than that below the ice, we perform a linear stability analysis, and obtained the results that the flow-ice boundary becomes unstable in the range of large Reynolds numbers, and the boundary waves migrate upstream in this temperature condition.

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In recent years, cyclic steps are discovered in a variety of environments, such as deep ocean floors and ice floors on planets other than the earth. Water can exists only in the form of ice in the extraterrestrial environments, where temperature is commonly by far lower than that on the earth. Therefore, cyclic steps in the extraterrestrial environments are expected to be often found on ice. In order to obtain basic knowledge on the formation of ice steps, we have performed a series of experiments in a cold room of Institute of Low Temperature Science, Hokkaido University, and reproduced ice steps migrating in the downstream direction. In this paper, we formulated the formation process of ice steps, and provide a reasonable explanation of the downstream migrating ice steps based on the experimental results.

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The spiral troughs observed on the surface of Mars' north polar ice cap show upstream-migrating structures, which indicate that those may possibly be cyclic steps formed by a density current created by cooling of the atmosphere due to the ice. It can be useful to estimate the formative process of the Mars' polar ice cap and thus the climatic history of Mars using the analogues of cyclic steps on the Earth. In this study, we have performed a series of physical experiments aimed at the formation of cyclic steps on ice by flowing fluid. Temperature distribution plays a quite important role for the formation and development of step topography on the ice surface, and was set as ice ‹ fluid ‹ ambient air in this experiment. As a result, step topography was formed on the ice except the case whose Fr is lowest, i.e., 0.76, and the steps generally developed upstream direction. The results of the present experiment agree with the mathematical model describing the evolution of the ice surface by flowing fluid.

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The objective of this study is to develop a numerical computation method based on depth integrated model for sand waves varying with hydraulic conditions. To calculate depth scale flow structures with sand dunes, we developed the general bottom velocity computation method based on a depth integrated model, in which bottom velocity and pressure acting on bed surface are calculated without taking the assumption of the shallow water flow, such as the hydrostatic pressure distribution. The flow separation behind the dune crest, which has been found to play an important role of the dune formation, is evaluated by the production terms of the depth integrated vorticity equations. Non-equilibrium sediment transport is calculated by using momentum equation of the sediment motion with the relaxation length of the particle movement. We applied the model to experimental results on sand dunes in a narrow channel and discussed the performance of the model.

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The typical bed configuration of Satsunai River, which meanders continually and is well known to be double-row bars and braided streams, can be observed in the channel. Levee construction of Satsunai River started in 1950. It has been reported that bank erosion occurred in theriverin 148 places for the 15 years from 1960 to 1975, whichwas the time just after the levee construction.In this study, numerical experiments with a full-scale channel were performedin order to realize the characteristics of bed deformation observed in Satsunai River at that time. The characteristics of bed deformation and braided streams observed in the river were found to be strongly affected by the meandering river channel.

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There are two theories about the origin of river meanders: one is based on the bed-instability theory and another one is on the bend-instability theory. According to the bed-instability theory, the meanders will be developed due to alternate bars on the bed. Although this theory was occasionally denied in the bend-instability theory because of reasons that typical wave number of developed alternate bars falls in the stable range of bend instability and so on, meanders induced by alternate bars are often seen in natural rivers in Japan. The theoretical verification of this kind of meander generation is required. In this study, the linear stability analysis is applied to the straight channel with erodible banks in order to obtain the characteristics of the meanders. The results indicate that meanders induced by alternate bars will exist and they have longer wavelength than that from previous bar-theory. And, it was found that Froude number is the one of the parameter that affects the wavelength of these meander types.

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Forested bars with large relative height between channel bed and bar surface is one of the typical figures in gravel-bed rivers. That causes strong meandering flow and bank erosion in several medium-size floods. In this study, such a morphological feature in gravel-bed rivers with double-row bars is investigated considering interaction between wood land expansion and channel formation process. A depth-averaged flow and bed-variation analysis is carried out introducing the events of invasion and destruction (wash-out) of riparian trees. Result of the simulation shows formation of a single strong meandering channel with fixed bars which can be seen as a resent channel characteristic in double-row bars with vegetation.

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The river ecosystem is evaluated based on bar morphology, because bars provide numerous places of shelter to organisms. For the dual purposes of maintaining flood protection and proper environmental functioning of the river, we need to understand bar morphology in the low-flow channel. However, there is limited knowledge of the system that bars form and of the process of change in the compound channel. Accordingly, in this paper, field observation and numerical simulation are performed to clarify the characteristics of bar formation and change. Two research sites are established in the downstream reach of the Yada River. The sites are selected such that the width of the low-flow channel is representative at one research site, and this width is wider at the other site. The change in bar morphology at each site is investigated and we succeed in clarifying the characteristics of bar morphology.

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Flood disasters caused by torrential rainfalls have recently been increasing in Japan. Representative examples are Tochigi Prefecture's Yosasa River disaster in 1998. Far exceeding capacity of flow made river widths and channel shape change because of bank erosion. But, there are few observation data such as discharge or the water level in the middle and small-scale rivers. So, we could not have discussed about appropriate river width and depth required for large flood flows. In this study, at first, we applied two-dimensional flood flow analysis for the 1998 flood occurred in Yosasa River and examine an evaluation method of the water width from the analysis and inundation situation in the floodplain areas. Then, we clarified a decision method of appropriate river width for large floods in rivers flowing valley bottom plain.

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Detailed profiles of three-dimensional flow structures and bed-forms using a new ADCP (Acoustic Doppler Current Profiler) method was developed for flow attacking on a revetment in the full scale experiment in the Jyoganji River. In the present method, individual beam data for current profiling is managed for velocity and bed profiles. This paper supplies detailed data of velocity distributions and bed forms by the field experiment. And, we proposed a numerical analysis method for the local three-dimensional flow structure around the revetment using bottom velocity computation method based on a depth integrated model. We demonstrated the accuracy of the analytical method through the comparison with velocity distributions measured by the experiment. Furthermore, the results demonstrated that combined use of the new measuring and calculating method was reliable to understand three-dimensional flow structures in flow attacking zones with steep velocity gradients and local scouring around revetments.

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Riverbed girdles are constructed to prevent bed-degradation and local scour around foundation of revetments in small and medium-sized rivers. The plane shape of riverbed girdle is basically set straight across the channel, but some kinds of upstream-pointing weir have been used for the purpose of channel stabilization in North-America. Local scour downstream of the riverbed girdle make drop and disturb the passage of aquatics at normal stage of water. In this study, effects of the plane shape of a riverbed girdle on movable-bed are experimentally examined. Three types of plane shape are examined, straight type, upstream-pointing type and split type. In cases with upstream-pointing types and split types, large scour holes made by parallel helicoidal motion of flow are observed. It is shown that apex angle have an effect on local scour at the tip of riverbed girdles.

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The method to install T-shape and L-shape groins in rivers with bars to suppress the sediment deposition and spatiotemporal change of physical environment in rivers are discussed. Subsequently, dynamic management of the shape of groins to improve bed material environment is discussed. The results show that the spatial distributions of velocity and water depth become, respectively, wide and narrow by installation of groins. This result indicates the possibility that the diversity of the physical environment is decreased by installation of groins in rivers with bars, because the bars can produce diversified physical environment originally. Furthermore, the results show that the bed material conditions can be improved by the change of the shape of groins temporally.

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Numerous spur-dikes and groins had been built for the purpose of preventing bank erosion, or stabilizing channel alignments. In each purpose, groin angle and arrangement has been studied comprehensively through past studies. It is known that horizontal angle of groin has different influence on riverbed deformation. In this study, flume experiments with movable-bed are conducted in order to discuss characteristics of bed deformation around submerged groins for different horizontal angle for the purpose of finding the fundamental shapes of the submerged groin which are suitable for channel stabilization of the small and medium-sized rivers in Japan. In results, the fundamental effect of groin angle is observed. As the angle becomes large, local scour around the tip of groin decreases. Furthermore, it is shown that scour volume is considerably reduced by devising the shape of the groin.

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In the Nakatsu river, after the dam construction in 2001, due to decrease of flood discharge, river course has been fixed, and gravel bar has been covered with vegetation. This study clarified the relationships between roughness of revetment and characteristics of morphological structures from dry riverbed to riffle-pool structures. At first, historical change of Nakatsu river is investigated. River width in the normal flow condition is stable for flow direction due to the concentration of flow, and the mechanism has been clarified through flume experiment. Secondary, through field measurement, in case the roughness of revetment is high, the stream line tend to be steep angle against the flow direction, and the morphology is different due to the difference of revetment roughness. Finally, flume experiment clarified the mechanism of how these difference appears under the controlled discharge

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