Many traditional researches on the debris flow are based on the continuum mechanics, which handled with water and particles as one body. But this method can not describe particle/particle interaction based on dynamical mechanism of particle motions. This study takes an approach method for considering particles motion and motion of water current individually by using the distinct element method (DEM) coupled with a model for water-flow. Present method is applied to collect the basic information of collision and repulsion process in the equilibrium state on debris flow. Calculated velocity profiles and the concentration profiles are compared with the reported experimental results. And the role of particle/particle interaction in constitutive relation have been investigated from the present numerical model.

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In gorges of Mountain Rivers, banks with weak geological configuration and landslide-prone, formation of natural dams is common and such kinds of dams exist every year in Nepal, chiefly, in rainy season.
The present study discusses on mechanism of natural dam collapse and makes an effort to simulate collapse mechanism of the dams in a steep gradient flume made up with two kinds of mixtures. Firstly, using fine uniform particles; and secondly, a mixture of coarse and fine particles, i.e., non-uniform mixture. Natural dams, trapezoidal in shape, with downstream frontal slope nearly equal to the angle of repose of dam forming materials, were made in the channel for the experiment. One-dimensional momentum and continuity equations for unsteady flow in open channel and Janbu method of slope stability were used to verify the experimental results.

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In this paper, a 1-D governing equations proposed by Egashira et al. is put forward first to describe the mechanics of debris-flow. Debris-flow simulations were conducted in order to understand what happens when rainfall occurs to cause numerous landslides on steep river basin, supposing several realistic occurrences in several tributaries. Multiple debris-flow surges were observed in 1999 San Julian debris-flow event, which was simulated in different contexts. The simulated results suggest that the different possible sequences can produce different amounts of sediment volume. Maximum sediment-volume difference between independent and dependent predictions was 0.12×10⁶m³, which is very large and cannot be neglected. This shows that the order of debris flow occurrences should be taken into account for simulating debris-flow behavior at the fan-head.

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The present study deals with deposition-erosion processes of debris flow with transition between fully dispersed debris flow and sediment sheet flow. A two-layer flow model proposed by the authors was applied to twodimensional numerical simulation of debris flows, and simple numerical experiments were carried out. It was indicated that a two-dimensional numerical simulation using the two-layer flow model can analyze phenomena in which velocity directions of upper-layers of water flows are different form that of lower layers of sediment-water mixture flow.

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A severe debris flow disaster occurred at the Atsumari river, Minamata city, in July2003. The origin of the debris flow is considered as a large scale landslide caused by heavy rainfall. The soil block produced by a landslide run out to the downstream area with liquification.
I apply 2-D hybrid debris flow model to the debris flow disaster. It was proposed to explain the runoff process of the soil block including the fluidization phenomenon, which is composed of the usual two-dimensional debris flow model and a tracking model for soil block movement as solid.
The model can reproduce the debris flow disaster. The rate of the fluidization strongly affects to the runoff process. The peak discharge becomes big due to the concentration of soil blocks at the front part of the debris flow. If the unsaturated part in the soil block becomes larger, the block can not continue to flow, and it forms a natural dam.

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Study of total sediment load transport is an important consideration of the fluvial process in many nowaday rivers. In order to improve the prediction of sediment transport and morphological behavior of the channel bed, a mathematical model taking bed load, suspended load and wash load into account has been developed. In contrast to previous researches, wash load has been especially accentuated from the perspective of cohesive sediment transport. For a better simulation of the interrelationship between different sediment loads and the river bed, cohesive and non-cohesive regimes were distinguished for the bed depending on the critical clay content. Two cases of with and without wash load have been performed using a 1D numerical modeling approach. The comparison between the computational results and field data exhibited its advantages over the conventional models.

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Erosion process of soft cohesive material was investigated experimentally in the present paper. Detailed discussion on the mechanism of this process was conducted here on the basis of enormous number of experimental data. It was found that the sediment waves form on the surface of clay sample, and they migrate downstream. And the erosion process of this sample is closely correlated with this wave propagation. An other investigation was started here to reveal the effect of dry-wet history on the erosion rate. Interesting feature of the dried clay samples were obvious that the erosion rate does not affect considerably by the period of such a dry history. This mans that the erosion rate formula derived in the previous work by the authors can be effective approximately to the dried clay samples.

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The Yellow River basin has serious problems of flood and irrigation due to hyperconcentrated flow from the tributaries of its middle reach. In order to solve these problems, it is important to estimate sediment transport and bed variation in the rivers. The Kuye and Dali River are selected as the study area. First, we investigated sediment characteristics in the Kuye and Dali River. It is found that the Kuye River bed sediment is silt to coarse sand and the Dali River bed sediment is silt to very fine sand. Second, we made experiments of hyperconcentrated flow in an open channel, using silt or sand as flowing materials. It is found that hyperconcentrated flow behavior becomes quite different with grain size and concentration.

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Recently, the Sendai Coast, especially near the sedimentary leeside of the Abukuma River mouth shows significant coastal erosion, which suggests decrease of sediment supply from the river. In order to know the sedimentary environment of the river suspended sediment transport observation was conducted during the flood in 2002 at several stations. Additionally, to estimate suspended sediment during floods we used the data provided from the observation stations that discharge, grain size of bed material, slope, etc. The estimated suspended sediment transport is compared with the observed results, and suspended sediment transport rate during floods is evaluated and validated. The evaluated sediment transport near the dam reservoirs of Hourai and Shinobu in the Abukuma Rive explains major decrease of the sedimentation in the two dams.

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Recent problems facing river management include a decrease in the water storage capacity of dam reservoirs due to sedimentation and a lack of continuity in sediment moment from the upper to lower reaches. To clarify the characteristics of substance transported during flooding, we made observations of and collected samples from the Saru River in Hokkaido, which has a reservoir in its middle reaches. In this area, a massive flood occurred between August 9 and 11, 2003. We also observed this particular flood. Since the scale of this flood was very large, we were unable to make observations throughout the duration of the flood. We were, however, able to conduct observations during the first half of the flood and at the end of the flood end. Here we report our observation results.

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Erosion process of bare hillside was investigated experimentally and numerically in the present study. Experiments were conducted on the model hillside with the artificial rainfall in the experimental flume. The numerical simulation model has been developed to predict the erosion process and to understand it more deeply. The numerical model was applied to the phenomenon which occurred on the slope in the experimental flume. Based on the comparison between the experimental results and the numerical ones, the validity of the present numerical model was verified. The process of channel network evolution and the head migration of the gully were investigated on the basis of this numerical analysis. Temporal variation of sediment yield from the hillside was also studied.

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Steep channels with purely erosional beds are characterized by a series of steps formed on beds. In this paper, experiments were performed in order to study the formation and the evolution of the steps. In the experiments, the wavelength of steps were found to be of the order of ten to sixty times the initial flow depth before the formation of steps. It was observed that the wavelengths reduced over time. In order to investigate the inception of steps, a linear stability analysis is performed with the use of the spectral method with the Chebyshev polynomial. The experimental results were compared with the theoretical results derived from a linear stability analysis. The agreement between theory and experiments were found to be good.

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A flow resistance in a river changes with development or attenuation of sand waves. Therefore, it is necessary to know how the flow resistance varies with transformation of sand bed, in order to predict the water surface variations. In this study, the numerical model to reproduce the formation processes of dunes was developed. A depth averaged flow model considering the effects of vertical acceleration and a nonequilibrium sediment transport model is combined. Calculated results on the temporal development processes, the shape characteristics of dunes and flow resistance are compared with previous experimental studies.

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Field observation of bed configuration was conducted during the 2003 August flood at the Tokachi River. Radio controlled boat was used in this measurement. The aim of this observation is to clarify the change of bed topography during flood. It is found that the bar formation becomes unclear and the flow becomes straight along the channel at the neighborhood of peak water level in which the hydraulic condition becomes no bar region. The changing of observed bed configuration during the flood is compared with the result of stability analysis under unsteady flow condition. The general tendencies of bar formation change by the theoretical results agree with field data. However, the theoretical results cannot reproduce the observed bar wavelength.

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This paper describes the relation among the flood, topographical features and the vegetated region on the floodplain at Honmura in the Kita River. The past changes in topographical features and the vegetated region were measured from the aerial photographs. The grain sizes of the gravels were investigated at the several points on the floodplain. As a result, the followings have been understood. 1) Gravels piled up due to the flood in 1997, fine sand mixing clay piled up during 1983-1997 and gravels piled up due to the flood in 1982 are accumulated in the vicinity of the 11.7km section. 2) Gravels on the bare ground of floodplain moved for the flood of 2, 000m³/s in discharge. However, the large scale movement of bed materials did not occur for the flood of 3, 500m³/s in discharge if the floodplain was covered by the thick vegetation. 3) The forest zone in the river bend had the effect that the topographical feature of floodplain was stabilized. 4) The state that the herbaceous vegetation covered the whole area of the floodplain, recovered from the state of the bare ground when the condition of 1, 600m³/s or less in discharge continued for two and half years. 5) The state with thick vegetation recovered when the condition of 1, 800m³/s or less in discharge continued for five years.

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In the Kita River, several large-scale widening of low channel have been carried out from 1998 to 2003. To understand the impact of the widening on river ecosystem, field surveys are conducted at two investigation sites: Honmura and Matono districts. At Honmura district, where the widening of low channel has been carried out before 1982, variation of riverbed, plane distribution of vegetation and the thickness of surface soil layer were investigated. On the other hand, the sedimentation structure of the flood channel which have been formed in several hundreds years was surveyed at Matono District. It is inferred from these results that there would occur the extension of wetland covered with willow and phragmites community and the reduction of bare bars in the future after the widening of low channel in the Kita River.

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Hydraulic experiments were conducted in order to clarify the mechanisms of mode reduction process of sand bars. Hydraulic conditions of these experiments were set large width-depth ratio. The following facts became clear by these experiments. Starting time of the mode reduction becomes late and time to spend on the mode reduction has the tendency which becomes long, when a width-depth ratio becomes large. One of two scoured area near side walls in double-row bars is buried and single-row bars are formed.
Furthermore, the prominence wave number of the bed form is analyzed. It is possible that the single row bars formed by mode reduction from double row bars are the same with the single row bars which are formed from the early stages of experiments on the bed form characteristics. Time changes of amplification of (α₂₁+α₄₀) and (α₁₁+α₂₀) have contributed greatly to the mode reduction process.

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The characteristics of river morphology are often influenced by human impact on river basin, and it strongly reflect to the dynamics of bar particularly in the segment of fluvial fan river. Change in bar dynamics is caused by various aspects: degradation, armoring, vegetation etc. Though they multiply influence the actual bar dynamics, each role should be clarified separately. This paper focus on the role of the bed-material composition on the bar formation, particularly on the mechanism to determine the number of mode. We employ the numerical method specially prepared to describe the change of mode number in bar-development process and linear instability analysis for a bed with graded materials, and clarify how the mode number changes with the bed-materials composition.

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Fluvial bars are large-scale topography with a planimetric scale of the order of channel width, which appears on riverbeds when the aspect ratio is sufficiently large. In order to describe the formation of such multimodal fluvial bars, a weakly nonlinear analysis is performed in this paper with the use of the amplitude expansion method. In the analysis, the fundamental perturbations with four different harmonic modes are imposed on an initially flat riverbed. The equilibrium bar profiles obtained by the present multimodal analysis are comparable to those obtained from the uni-modal analysis and the numerical analysis when the aspect ratio is small. An irregularity in the time variation of bar amplitude that has not appeared in the uni-modal bar analysis, is promoted with increasing aspect ratio.

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The purpose of this research is clarifying response of degradation, aggradation, and water level when a flow characterized by sharp-peaked hydrograph runs in the place where the bed gradient is steep andalternate bars are formed Attention is also paid to the hysteretic effect of hydrograph repetition of floods. Examinations are made by the flume experiments and numerical computations.
It is found that the variation in riverbed and water level is greatly related to flow change and alternate-bar formation. The shallow water flow in a steep river is characterized by meandering flow, which occurs during decaying and early rising stages of discharge hydrograph during which the degradation tends to go on. The water level is almost in phase with the phase of altemate bar.
It becomes clear that wavelength, wave height, and scouring depth show the characteristichange with time by the hydrograph repetition.

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Re-formation process of alternate bars caused by the deformation of meandering streams which developed from alternate bars and effects of water discharge on it are investigated by means of flume tests using the uniform and the non-uniform sediment. Experimental results show that widening of meandering streams is caused by the erosion of emerged bars when the sediment supply rate is same as the sediment transport rate, whereas it is caused by both the erosion and the riverbed aggradation when the former is larger than the latter. Alternate bars are formed easily in the non-uniform sediment bed because the emerged bars in the bed are eroded easily than those in the uniform sediment bed. The bed with meandering streams is transformed into large-scale alternate bar bed by riverbed aggradation and low water discharge.

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As recent channel characteristics of gravel-bed river, bed degradation, immobilization of the lowwater course, local scour near river bank, extension of relative elevation, and overgrowth of vegetation are mentioned. These figures are produced as results of the morphological change under the medium-size floods in the low-water course affected by alternate bars. In this study, we discuss the low-water channel formation where recent river channel characteristics of the gravel-bed river are reflected, and the process of morphological change was examined from numerical calculation. First, a channel with alternate bar was made, afterwards, the flow discharge was lowered and the meandering low-water channel, which brings about the bank erosion and local scour, was formed. These processes were investigated from the results of present numerical calculation.

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It is of interest to investigate how the behavior of alternate bars leads to channel development by taking bank strength into consideration, which is affected, in turn, by bank vegetation in natural rivers, in order to design stable channels when straightening rivers or building navigation canals. This study examined the behavior of alternate bars in a channel with erodile banks numerically, by considering the bank strength, which is influenced by bank vegetation in natural rivers. The lateral widening rate of the channel, bar migration speed, and wavelength were studied numerically when the channel remained nearly straight. The bars migrated more slowly as the aspect ratio increased. In a channel with strong banks, the bars migrated with speed, and the dimensionless bar was higher. Our numerical experiments showed that the behavior of bars differed with bank strength under the same hydraulic conditions.

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Channel evolution of braided stream was studied numerically in the present study. Slopecollapse model and modified bedload function were included in the numerical model whose validity had been almost verified. The formation and developing mechanism of each braided channels is the one of the target to be understood physically. The interaction between a bed deformation and a bank erosion in this process is the other main point to be investigated. On the basis of the enormous number of data obtained by the numerical experiments under the different conditions, the important and interesting remarks on the above two points were found and discussed here.

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Channel deformation with bank erosion induced by the interaction between the flow and the vegetation along a water margin are investigated in the paper by numerical analysis. Some kind of LES model for flow field and the slope collapse model for movable bed evolution were applied. It can be realized that the river channel migrates laterally just upstream of the vegetation zones, if they set diagonally along the both sides of water margins. Comparison between the numerical result and the experimental one was conducted, and then the validity of the numerical model was verified.

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A project for river-restoration on the Shibetsu River was launched in 2000. For meander restoration, test flow of water started in March 2002 in a bifurcated channel formed by connecting the pre-improved channel (oxbow) with the present straight channel. In the eighteen months that have elapsed, the riverbanks and riverbed in the test section have greatly changed.
To analyze hydraulic phenomena under various conditions, this study aimed to develop a numerical calculation method that enables estimation of flow and riverbed evolution. We made calculations that reproduce riverbed evolution for the test section. The reproducibility was verified by comparing calculation results and measurements. We clarified the applicability and applicable range of the calculation method.

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This paper presents field data for channel change, bed material, water surface elevation and hydrograph in Tan Chau reach in Vietnam to understand mechanism of the river change. These show that bank shifting tends to occur toward anti-clock wise and sand bars migrate downstream, resulting in a much larger bar, which is influenced by co-presence of non-cohesive and cohesive sediment and by artificial protection works. In addition, bed material loads are originated by bank erosion. A 2-D numerical model is proposed to treat such river changes by introducing erosion rate formula for cohesive material and formula for estimating the thickness of sediment transport layer into a usual method. The new method is applied to the study reach which is 12km long and is supposed to be applicable for predicting channel change of natural river with fine sediment.

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In the present study, maximum scour depth around the bridge pier occurred during floods, were measured by means of brick installation under the bed surface near the piers. Field observation proved that it is possible to measure the actual and maximum scour depth by counting the bricks flushed by the floods. The target spot in the present study is Gudo point near the Shimanto-bridge at 9.5km upstream from the river mouth of Shimanto River of Shikoku-island in Japan. There were four main floods occurred for these three years from 2001-2003 and bricks have been flushed, so that the maximum scour depths were recognized for each flood and the scour hole and bed surface shows the significant variation. In this paper, the results of surveys for all these three years and characteristics of scour around Shimanto-river bride are discussed in detail.

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An explorative physical model study reported herein was conducted in a mildly meandering like laboratory flume with spurs as river training structure in order to observe the bed variation near the spurs aswell as in further down stream region. Study was performed under the clear water condition using two different grain sizes (0.28mm & 1.2mm). In this study, temporal bed variation near series of spurs in different part of bend was determined simultaneously using long run experiments under various hydraulic and experimental conditions. Moreover, bed form induced by spurs in further down-stream region was also observed. It has been shown that temporal variation of relative scour depth can be generalized using a similarity number referred to as Strouhal number. An attempt was made to observe vegetation impact on local bed evolution having planted living vegetation (alfalfa) near the spur region under laboratory condition.

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Characteristics of bank shape along approach channels at upstream reaches of spur-dikes in alluvial floodplain rivers of Bangladesh are investigated. The shape of river bank at approach channel sections are found to be non-rectangular that can be approximated as nearly trapezoidal. Available methods for the prediction of maximum local scour depth around spur-dikes are not capable to handle such problems where cross sectional shapes in approach flows are different from rectangular. Therefore, an analytical model is developed incorporating the variation of cross sectional shape of approach channels and applied to some of the spur-dikes along the alluvial floodplain rivers.

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The purpose of this study is to show the effects of the sediment movement due to the weir reconstruction.
Nikaryou-syukugawara weir in Tama River was reconstructed and the weir crest level was dropped about two meters in order to increase the flow sectional area. After the weir completion in 1999, a lot of sediment in the upper reaches of the weir ran off with two floods. It was found through the monitoring data that the flow sectional area in the upper reaches increased due to the sediment run-off. In the lower reaches of the weir, it was found that the harden-silt bed was gradually covered with gravel and that some scour holes were to be shallower. The sediment movement mentioned above is favorable for the river maintenance. A 2D numerical model was applied to the prediction of the sediment movement and it was found that the model is useful.

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In the downstream basin, risk of inundation is lower. Bank erosion often occurs in the upstream basin of rivers due to rapid flood flow. Predicting weak points of bank erosion is also important to improve river efficiently. At a present stage, It is important that machanism of bank erosion is investigated by the use of field data, model experiments, numerical analysis.
In this paper, it is made clear that causes of bank erosion and bed topography near national highway No.4 bridge by the 1998 flood in Yosasa river.

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The water level of a river in the monsoon region decreases largely in the dry season. It is known that riverbank erosion occurs in the low water level, and it is considered that the subsurface flow influences it. We obtained particle diameters and hydraulic conductivities from some samples by field survey in the Mekong River side. Critical velocity and circular slip surface are used for analysis of riverbank erosion process. It is found that some places have effects on subsurface flow to riverbank erosion according to particle kinds distribution. This distribution is estimated by numerical simulation of subsurface flow. Finally, we defect the influence areas from river mouth.

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Risk assessment and water resources management are required to get a reliable estimation not only the sub-catchment scale, but also the catchment scale with a few and/or no rain gauge station. A simple 1-D model considering slope runoff and channel flow with soil eroded, sediment transport, floodplain sedimentation and riverbank erosion is presented to assist in predicting the consequences of extreme flood event. Typhoon no.10 were used to validate the proposed numerical model. It was found that the values ε for calculated period were greater than 0.88, indicating well fit between measured data and this proposed model. In addition, the proposed model was exploited to simulate sediment transport problems. The results indicate that calculated sediment yields compared well with measured data. However, a more precisely sediment transport study is needed.

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The behavior of sediment and other substances in the entire basin is an important factor in maintaining the river channel as well as conserving the aquatic ecosystem at these places. It is especially important to understand transport during flooding, when large amounts of substances are transported. The most recent large-scale flood in Hokkaido, Japan, occurred in August 2003. It struck Hidaka district, causing many injuries and fatalities. The sediment runoff from that flood is still of concern, and examination on its potential effects is awaited. We surveyed dam basins in Hokkaido to study the relationship between the volumes of sediment and water quality components and the geological and vegetation conditions. Sediment transport during flooding is known to have significant impacts on the water environment. To analyze the flood-related runoff characteristics of sediment, the Saru River basin was chosen for its large sediment runoff. This paper summarizes the substance balance of the basin and analyzes the causal relationship between that balance and the conditions of a basin that tends to experience particularly severe damages.

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Recently in Okinawa, red soil runoff becomes a remarkable problem, and sediment movements (erosion, transportation, and sedimentation) have to be grasped for assessments of new land development plans or sustainable managements of agriculture. A purpose of this study is to grasp the sediment movements at a watershed scalefrom the results of multi-point simultaneous observation in the small agricultural basin including the erosion observation. The time series of discharge, suspended sediment concentration were measured on several rainfall events andat proper intervals of time. The amount of sediment runoff from farm land and the sediment balance were calculated from observed data. It was substantiated that the low canopy covered farm lands and agricultural managements affect sediment yield, the structure and the arrangement of settling basins affect sedimentation. After the observation, USLE (Universal Soil Loss Equation) was applied and evaluated by the accuracy and the practicality as the first step to establish the prediction method of sediment runoff.

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T Controlling the sediment inflow to Three Gorges Dam reservoir is indispensable to managing its performances and water environments. Since it is indispensable for this purpose to grasp sediment runoff in Jianlinjiang river watershed that is one of the main sources of sediment yield upstream of Changjiang river, we studied which types of erosion and yield models can be used for the future modeling of sediment runoff. In this study firstly we selected three types of surface erosion models; modified USLE model, surface flow erosion model and riverbank erosion model. Secondly, we applied them to Jianlinjiang river watershed in 1987, and examined their features, applicability and limitations. Though this watershed is so wide that the sediment runoff is thought to be much influenced by multi-sales of space and time, the surface erosion type of three models can explain the annual and monthly sediment yields to some extent.

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At first, we estimate the distribution of discharge using BTOP/MC model in the Middle Mekong river basin. Secondly, the amount of sedimentation runoff is calculated by Itakura and Kishi's equation with discharge data. These results can express the distribution of sedimentation movement, and can make the discussion. The region from Luang Praban to Nong Khai tends to produce enormous amount of sedimentation, the area between Nong Khai and Mukdahan has a deposition tendency, and the basin between Mukdahan and Pakse flows much sediment. Two flood peaks have different sediment runoff phenomenon.

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A numerical method has been constructed that computes the flow and the sediment transport in a sediment bypass tunnel that is being planned for installation at various dam sites. In these tunnels, sediment loads change abruptly due to the mechanism at the intake area. Existing methods of predicting sediments in rivers cannot be applied directly and unsteadiness and variation along the tunnel must be taken into account. The present method that considers these aspects can predict the flow and the sediment transport well compared with the model experiments. In the calculation, the transport is somewhat slower near the upstream end of the sediment deposition area. This, however, is due to the low friction on the smooth bottom wall of the model tunnel and will not be a problem in the application to the real sediment bypass tunnel, which will have a rough bed.

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The invert of concrete channels for removing deposited sediment in a reservoir receives severe abrasion from flowing gravels and boulders. Therefore, it is importanto reveal abrasion mechanism. In this paper abrasion by supplying gravels and boulders in an experimental channel and motion of gravel are studied by two large flumes. Result of surveying abrasion depth for every section which is different from the rate of coarse aggregate to total aggregate reveals the relation between the rate of coarse aggregate to total aggregate and abrasion resistance of the invert. Wavy bed is formed on the concrete invert and the wave length is approximately equal to step length of gravels that have average particle diameter of supplying gravels. By supplying gravels and boulders, roughness coefficient of the channel increases about forty percent compared to non-sediment laden flow.

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We are proposing a new sediment removal system using existing headraces as a countermeasure against reservoir sedimentation. This is a method in which sediments which flowed into the headrace are removed and discharged from the large density sediment layer at the bottom of the pipe flow before they reach the turbine. In order to rationally design the bypass system, vertical sediment concentration distribution and diversion ratios of sediment at the diversion pipe have been studied experimentally. In this paper the formula of vertical sediment concentration distribution is expressed under the conditions in which the experiment was carried out and also it is shown that the shape of the diversion point we proposed efficiently separates sediment from turbid water.

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Sedimentation management of reservoirs is one of crucial issues to realize sustainable development of water resources for the next generations. Recently, sediment-flushing techniques have been applied to Dashidaira dam and Unazuki dam in the Kurobe River, but erosion-deposition process of sediment in reservoirs and its effect on success or failure of flushing operation have been rarely discussed.
This paper studies how to measure actual sediment erosion-deposition process during flushing operations. A field measurement with 3D Laser scanner in Unazuki dam reservoir shows that 3D Laser scanner can be applicable to measure both water and sediment profiles and those temporal changes.

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The initial total storage capacity of 327 Mm³ of Sakuma dam reservoir in 1957 has been reduced to 213 Mm³ in 2002 due to sedimentation of 114 Mm³. In order to reduce flood damage in the upstream area caused by the sedimentation, attempts to lower the riverbed is being continued by dredging and flushing with river water. However, riverbed has not been lowered sufficiently, hence more efficient measures are required. So, accumulated reservoir data of flow discharge, cross sectional surveying and grain size distribution were examined to clarify actual state of the sedimentation. Results of the examination show that sediment concentration is decreasing clearly implying that a flood cannot convey the same amount of sediment as it could before. Another results of sediment movement were compared with those evaluated from 1-D analyses of non-uniform flows, indicating the efficiency of the sediment flushing. Accumulation curves of volumetric changes of cross sections were obtained for individual grain sizes, which can express sediment movement in the reservoir properly.

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The falling motion of particle clouds, produced by direct dumping of particles from water surface, into a finite water depth as well as the motion of horizontally propagating particle clouds, subsequently formed after impinging of the falling particle cloud on the bottom, along the boundary, are investigated experimentally. The study shows from the particle thermal theory and the present experimental results that the falling motion of the clouds is significantly affected by the depth of ambient water, and that the spreading distance of the horizontally propagating particle cloud become larger as the ambient water depth increases. It results in the sediments, carried by the cloud, deposit in wider area. The effects of resuspension of particles, due to impingement of the falling cloud, is also quantified in terms of the velocity, maximum width and time scale to govern resuspension process.

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Numerical simulations of 2D suspension thermal as well as axisymmetric suspension thermal on horizontal bed are performed using 3D numerical model. The thermal consists of mixture of nearly uniform sized particles water. The numerical model consists Simplified Marker and Cell (SMAC) method, Monotone Upstream-centered Schemes for Conservation Laws (MUSCL) technique and the treatment of the partilce deposition. The simulated resutls are compared with experimental data of maximum height, propagation speed, buoyancy of the particle thermal as well as deposit profile of the particles. The numerical model shows promissing resutls for simulation of particle thermal on horizontal bed and deposit profile.

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The scale effects on the conservative and non-conservative inclined thermals are examined by the numerical analysis using the κ-ε turbulence model. The saline inclined thermals are analyzed as an example of the conservative case. The inclined turbidity thermals in which fine solid particles are suspended in water are analyzed as an example of the non-conservative case. The scale effects on the flow charactersitics of the small scale and large scale themals are discussed. The variation of travel speed, the maximum height, the maximum concentration arc compared. The scale effects on the entrainment coefficient of ambient water are discussed. The contours of concentration and velocity vectors of the small scale and the large scale themals are discussed. While the flow characteristics of conservative saline thermals are similar despite of different flow scales, those of non-conservative turbidity thermals vary according to flow scales.

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A three-dimensional Large Eddy Simulation (LES) model investigating the motion of turbulent axisymmetrical as well as line thermals is presented. The model governing equations arespatially filtered 3-dimensional Navier-Stokes equations and Mass Transport equation. Space derivatives are calculated by Compact Finite Difference Scheme (CFDS) and time derivatives are calculated using Crank-Nicholson method. The turbulence generated is taken care of by eddy viscosity calculated using the modified Smagorinsky model, which includes buoyancy term. The comparison of simulated thermal characteristics like shape, size, buoyancy and mass center velocity for axisymmetrical as well as line thermals, with reported experimental results are found in reasonably good agreement. The behavior of thermal with different length to width ratio (ζ) of initial conditions, is also investigated using this model. It is observed that if the value of ζ is morc than 5, the thermal's behavior is closer to a line thermal of infinite length.

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The objective of this study was to resolve pressure loss of transitional slug flow relating to two phase flow composed of air and liquid in horizontal long pipeline. Therefore, the model experiments were conducted to get the pressure data and imaged intermittent flow using a transparcnt pipc for the observation.
This paper describes investigated result about pressure loss and variations of slug velocity that are taken remarkable effect by pipe length. As the results, the pressure loss and variation of slug flowper length of long pipeline is uneven in each measuring sections.

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The new sand capping method had been proposed in later years. The method wells up buried sands by a water jet to cover the contaminated sludge on the bottom surface. Hydraulic model tests were carried out in order to find the parameters which control the up-welling sand flux, as the first step in establishing the estimation method of the upwelling-sand volume by the new sand capping method. As a result of model tests, the followings were revealed. The excavated hole was filled with the liquefactive sand which showed complex stream with vortices. The jet discharge rate and the grain size had larger influence on the up-welling sand flux than the jet velocity and the descending speed of jet pipe. To represent the phenomena from the hydrodynamic viewpoint, the numerical simulation with the Lagrangian model of solid-liquid two phase flow was carried out for the initial stage of the upwelling. The results were in good agreement with the phenomenon observed by the model tests.

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On the basis of the results of field measurements taken by the authors in 2001, numerical analysis for a suspended solid was carried out by two-dimensional numerical model that includes the aggregation process. The main objectives are to obtain the fundamental data for the effects of turbidity maximum performance of suspended solid on the environment of water quality and the bottom sediment in an estuary. Turbidity maximum occurs in the high water phase at a 6.6km point from the river mouth and the duration of the high concentration is about 1-2 hours. These results agree well with the field measurements. At a 5.5-6.7km point from the river mouth where the aggregation is active, the diameter of the floc is about 100-1, 000 times the diameter of suspended solid at the upper boundary and the effective density of the floc is approximately 13-56kg/m³. It is approximately the same as that of fluid.

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