In December 2014, an ice jam occurred on the Osobetsu River after a snowstorm. This study investigated factors that contribute to increases in frazil slush and identified locations prone to ice jams. Previous studies using a hydraulic model experiment showed that an ice sheet's Froude number become small where ice sheets are static or accumulated. Moreover, a frazil slush calculation model that is based on the theory of heat balance in stream water accurately reproduced the measured thickness of frazil slush. However, no previous studies on ice jams have taken into account the Froude numbers of ice sheets. Also, certain weather phenomena, such as snowstorms and avalanches, are not factored into the current model for calculating frazil slush thickness. Therefore, the authors developed the model such as to take into account the effect of snowstorms. Furthermore, we used the Froude number of the ice sheet to identify potential ice jam locations on the Osobetsu River.

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 An estimation method for discharge hydrograph in rectangular channel was examined. The method, which was based on one-dimensional shallow water equations and characteristics curves equations, used the time variation of water surface profile and roughness coefficient on the boundary as input conditions of analysis and estimated the roughness coefficient. The method was verified against numerical results on uniform, non-uniform flow and experimental data on non-uniform and unsteady flows. It shows that proposed method can predict the discharge hydrograph with reasonable accuracy, and it is necessary for the method to improve in a prediction of the roughness coefficient.

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 Sediment flushing operation from large dam causes bed condition change in the downstream. It may reduce the flow resistance as the flushed sediment fills the voids among the large stones. This condition influences the flow in roughness layer which is important habitat not only for benthos, but also for swimming fishes. However, the hydraulic condition changes in roughness layer due to sediment supply is not well investigated. Furthermore, its impact to habitat is not considered neither. In this study, we investigated the hydraulic characteristics changes within the roughness layer due to bed topography changes as one of the impacts from sediment supply in natural rivers. It was conducted over immobile bed condition in experimental flume. Detail velocity measurement results within the roughness layer and the turbulence properties are presented. Besides that, suitable method to evaluate the roughness height as the bed condition changing is also discussed. The results exhibited the reducing of spatial variance of spatially averaged mean velocity, TKE, and Reynolds shear stress within the roughness layer associated with decreasing of bed roughness as the sediment fills the voids among cobbles. We also observed the decreasing of roughness layer thickness as the bed roughness reduced. This experiment helps to explain that sediment supply is not only reducing flow resistance but also limits habitat preference of benthos and swimming fishes as velocity distribution becomes homogenous within the roughness layer.

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 Flood flows analysis in the Hikosan river basin in July 2012 were performed by numerical models for the 2D unsteady free-surface flow based on HLLC(Harten-Lax-van Leer and Contact). A new procedure for a dry/wet boundary over a topography is incorporated into the model based on HLLC solver. The HLLC model was verified against the observed water level. The computation accuracy and speed were compared between HLLC model and FDS(Flux-Difference Splitting) model. It shows that (1) the HLLC model reproduce the behavior of flood flows with reasonable accuracy, and that (2) the calculation efficiency is high compared with FDS model, and that (3) the treatment of a dry/wet boundary needs to be improved for the HLLC model to conserve the mass.

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 We applied Reuse Green's Function approach²⁾ : GFA to estimate control parameters: the magnitude, the phase lag of tidal currents and sea level for open boundary conditions, the horizontal viscosity and the sea bottom drag coefficient, of a numerical model for tidal currents simulation of the radial M2 velocity observed by the Rader in the Ariake Sea. We investigated the efficiency of the approach by comparing the cost function and control parameters with that estimated by the original GFA. Reuse GFA provided almost same the magnitude of the cost function as that of the original GFA and subsequently same the control parameters, even though Reuse GFA required less the number of the times for model calculation. This results suggested that the Reuse GFA enable us to estimate the control parameters more efficiently than the original GFA. We investigated an applicability of the Reuse GFA to past sensitivity analysis of the calculations for the control parameters in order to estimate the appropriate control parameters by substituting the pseudo random numbers into the perturbation vectors for the control parameters. This results suggested that the Reuse GFA can be applied to the past sensitivity analysis, if the calculation number of sensitivity analysis for control parameters is much enough to represent independent perturbation vectors with appropriate length.

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 In this study, we conducted laboratory experiments for water purification of lake water which includes withered Microcystis by using red and blue Light Emitting Diode (LED) in order to improve hypoxia water area induced by Microcystis. As a results, concentration of dissolved oxygen decreased and concentration of dissolved nutrients increased under no light irradiation condition. On the other hand it could be conformed that hypoxia is suppressed, and it was clear that concentration of the nutrients, such as NO_3-N, NH_4-N, PO_4-P, decrease under the LED irradiation system. Photosynthesis bacterium activation system by utilizing LED irradiation system could be one of the methods by utilizing purification bacterium to improve water environment. This method can never disturb hydraulic system in water area.

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 Artificial shallow areas are being developed around shoreline of Lake Shinji since 2000. Main purpose of the development is to restore deteriorated shorelines due to the construction of concrete revetments. It also may help the decrease of the turbidity because of the wave height change. In order to reveal the effect of the development on the turbidity, field surveys and numerical simulations as to wave and turbidity were conducted in the lake. Field surveys showed that artificial shallow area decreased wave heights but turbidity did not change significantly. Wave simulations using SWAN revealed that the developed shallow area changed wave heights smaller but it also partly made the bottom shear stress larger, which might enhance the resuspension. Turbidity simulations using ELCOM-CAEDYM showed that the development of artificial shallow area did not significantly contribute to the decrease of the turbidity.

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 Lake Izunuma is a shallow eutrophic lake located in the northern part of Miyagi Prefecture. Lotus (Nelumbo nucifera) is the dominant species of the lacustrine vegetation, which has possibilities of accelerating the eutrophication through decomposition of the accumulated litter. In this study, field observations were conducted to understand annual cycle of nutrients (nitrogen, phosphorus, carbon) with regard to growth and mortality of lotus community. We modeled biomass dynamics of lotus based on the observations. Moreover, the nutrients load into the lake from dead body of lotus is considerably smaller than the inflow load. From this result, it is considered influence of the dead body on the water quality is small.

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 Lake Jusan is a brackish lake located in the down reaches of Iwaki River system and is one of the most famous fishing places of Corbicula japonica in Japan. A growth model of C. japonica was developed by applying DEB (Dynamic Energy Budget) modeling based on Kooijman (2007). The growth model was combined with the lake water quality model developed and validated by our previous studies in Lake Jusan to estimate phytoplankton biomass as food of the bivalve. Although temporal distribution of reproduction was not in a good agreement with the measured results of larvae, the total populations change of C. japonica during a year was in a reasonable range compared with the measured data.

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 We carried out water quality monitoring under the ice in brackish lake with salt-fresh water boundary. Continuous observation at constant elevations, vertical distribution observation and water sampling analysis at regular intervals were carried out. In freezing season, salinity boundary was seen broken and spreads upward due to water temperature formed reversal distribution. However, dissolved oxygen (DO) was maintained distribution with boundary. Moreover, turbidity distribution had a maximum value at an elevation of salinity 11PSU. It was cleared that elevation of turbidity peak and elevation of DO boundary ware well agreed. As a result of water sampling, water quality such as nutrient salts was revealed different behavior depend on DO boundary and not halocline. Result of comparison with flux of elution and flux of passing through DO boundary, it assumed that nutrients is stored in anaerobic layer.

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 Dissolved organic matter was measured by using Fluorescence Spectroscopy (FS) along with salinity from snowmelt season to summer at a tidal flat in Lake Komuke at the northeast of Hokkaido. Seasonalities of dissolved nitrogen and phosphorus concentrations were also examined to clarify the dynamics of nitrogen and phosphorus in sediment-water interfaces. Salinity decreases during the snowmelt season, and FS revealed that fulvic acid-like and humic acid-like fluorescence intensities were higher in sediment pore water than in surface water especially in tidepool due to accumulation of organic matter production. It is likely that NH_4-N and PO_4-P concentrations in surface sediment were mainly affected by redox condition change triggered by tidal action and infiltration flow in shore side.

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 The generic length scale (GLS) turbulence closure model has been implemented to an object-oriented 3D hydrodynamic simulator, Fantom Unstructured, to investigate geophysical flows. This paper summarizes the basics of the GLS turbulence closure model, its parameters and the discretization on an orthogonal unstructured grid. The performance of the present simulator with the GLS turbulence closure model was examined for open channel steady flow and mixed-layer deepening problems, then the computed results were compared with analytical and empirical solutions. The choice of the two-equation model and stability function affects the vertical distribution of turbulence quantities, however, the reasonable agreements are obtained for the vertical distribution of the horizontal velocity and mixed-layer depth between computed results and the solutions.

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 The flood management in dam reservoirs has been performed by using a water level observed near the dam body. However, the characteristics of the flood propagation of water level and discharge hydrograph are different in each flood flow, especially inflow of large scale flood flows. For efficient and safe dam reservoir management, it is important to clarify the mechanism of flood propagation. In this study, the hydraulic model test was carried out in order to investigate the characteristics of reservoir storage and flow mechanism in dam reservoirs. Moreover, the flood flow analysis method was developed to understand characteristics of flood flow dynamics in dam reservoirs and applied to hydraulic model tests of dam reservoirs.

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 Sediment bypass is an excellent reservoir sedimentation countermeasure which can achieve both reduction of sediment flowing into the reservoir and supply of sediment to the downstream of the dam. Sediment bypass facility has been almost completed at the Koshibu Dam in the Tenryu river, and test operation is scheduled from 2016. In order to optimize the bypass operation, further study is needed in sediment transport at the diversion facility. Then, full three-dimensional calculation depending on the operation of sediment bypass facility was performed using the improved model that can calculate the bed deformation and complex flow around the diversion facilities. Based on this calculation, time depending changes in bypassing volume of sediment, and the sediment bypassing ratio based on the river bed level was investigated. Finally, the process of inflow, deposition and discharge of sediment in the bypass facility was described, and the effective conditions that can increase the bypassing ratio was proposed.

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 A selective withdrawal system (SWS) in a dam is essential for the conservation of water quality in the reservoir. However, the operation of SWS has not been fully examined in Japan so far. On the other hand, in the United States, some dams use three or more gate levels simultaneously to get desired blended water quality. In order to reveal the intake characteristics of this operation, we conducted field observations at Tono Dam. Moreover we derived quantitative intake equations using the law of conservation of momentum and energy and applied the reservoir. Field survey and the calculated results both indicated that the amount of cold water from lower intake is greater than that of warmer water from upper intake, especially in the conditions that the density stratification is developed.

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 Field observations at the eutrophicated Haji Dam reservoir were performed to estimate the characteristics of intrusion due to a bubble plume by using uranine as a tracer. An automatic in situ fluorometer optimized for the optical characteristics of uranine enabled high resolution measurements of the temporal and spatial dynamics of the intrusion flow. The velocity and height of the intrusion obtained from the field survey agreed with existing theories under the buoyancy-inertia balance regime. On the other hand, the relationship between the intrusion volumetric flow rate and hydraulic parameter (plume number) differed from prior study. A new empirical equation is proposed for estimating the intrusion for low plume number conditions.

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 A wide shallow is in the northeastern area of Lake Ogawara. A part of the saline intruding from Takase River into the lake can spread to the shallow area and be stored in the area. To evaluate a saline-storage capacity of the shallow area quantitatively, three-dimensional numerical current simulations were conducted. A series of the simulations of the saline intrusion was conducted with various intrusion-volumes and wind conditions. The simulation results show that the saline intrusion in the shallow area can be affected by the wind speed and the storage capacity can be drastically enlarged under a strong eastern wind. An empirical equation to evaluate the storage capacity is proposed as a function of the eastern wind speed. Using the equation, the average storage capacity is estimated with all saline intrusion events in the recent decade and the capacity is near the value suggested by the previous field surveys.

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 In this paper the authors propose a practical sediment runoff model with soil production process considering slope failure as well as weathering by freezing and thawing. One of the main features of the model is to consider the actual conditions and experiences of the slope failure in the reservoir basin. The authors try to clarify applicability of the model in case of J-POWER's dams and evaluate the effects of the climate change to reservoir sedimentation. As a result, the progress of reservoir sedimentation at some of J-POWER's dam sites is estimated to be accelerated by climate change.

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 A measure to reduce the environmental impacts of dams is a dam removal which has been conducted in Arase dam, Kuma River. However we have poorly understood influences of dam removal or construction on riverbed evolution in lower reach of dam. In this study, we attempted to clarify the response of riverbed evolution after dam removal or construction by data analysis for long-term monitored riverbed and 1D analysis for riverbed evolution under various sediment supply conditions. The numerical results indicated that the propagation process of riverbed evolution to the downstream direction seemed to be a kind of diffusion process. Using this results, we can simply evaluate the riverbed evolution in lower reach after completing removal of Arase dam.

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 The use of a turbidimeter with a particle size-dependent defect is proposed for the estimation of particle size as well as continuous estimation of fine sediment flux by particle size. In this study, an artificial neural network (ANN) and a multiple regression model were built to predict the fine sediment concentration of sand (FSC_S) and of silt and clay (FSC_SiC) using observation data from the Mimikawa River, where three dams are being planned to implement an integrated sluicing operation.
 Both models, which used turbidity, discharge, and hysteresis as predictor variables, were able to generate excellent FSC_S and FSC_SiC estimates. In particular, estimation of the FSC by the ANN model, which suit for the non-linear system, may reduce the error of estimation by more than 40% of traditional sediment rating curve model. Furthermore, resulting predictions of FSC_S and FSC_SiC during storm events indicated that both fractions changed every hour. The developed methodology was used to simultaneously obtain reliable and continuous estimates of FSC with respect to particle size.

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 It is important to develop the method to estimate the river discharge at rivermouth for the forecasting of sediment behavior in the nearshore zone. However, it is difficult to develop the method because of tidal effects. In this study, water elevation data H at the Minato-ohashi Bridge which is located at slightly upstream from rivermouth and river discharge data q at the midstream of the Naka River, which are maintained to the Minato-ohashi Bridge, in the Tochigi and the Ibaraki prefectures in Japan. At the small q, H was linearly affected by the tidal elevation at Oarai η to the south of rivermouth. Then, tidal effects were removed from H data and the removed data with the time-rag consideration from the midstream to the Minato-ohashi Bridge were well shown by the function of root-q. Finaly, field observations by using the ADCP at rivermouth were conducted and q was corrected by using observation data. In conclusion, the equation of river discharge Q at rivermouth, H and η was obtained.

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 This study is concerned with the non-dimensional relationships for the bank-full channel characteristics as well as dimensionless tractive force for stable channel of gravel and sand bed alluvial rivers. The condition proposed by the author for stable channels, that satisfies rational regime relations, is verified with use of selected existing data of natural Japanese and overseas rivers, so that such relationship as α=KI^-1/3 is identified and the value of K is estimated from these data. Then, the obtained non-dimensional relationships along with the estimated K-value are tested against existing data of Japanese and overseas natural rivers, laboratory experimental data as well as three natural Japanese A-class rivers in Kyushu. The relationships are found to well predict all these data.

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 This study is based on stochastic process, and with respect to sand waves in river bed with a hydraulic consideration. The irregularity of sediment transportation on the movable bed to have the physical significance have to be dealt with stochastic process. With using the distribution function of the step-length, derive a sand wave equation and compare with another equation derived from the deterministic perspective based on principle of hydraulics in movable bed. And we can presume higher orders Fokker-Planck equation in stochastic processes and apply sediment transportation. The purpose is to look at and interpret at different perspective, and also examine the correlation with sand wave equations derived in this study.

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 The spiral troughs on Mars' polar ice caps are considered as cyclic steps formed on ice due to katabatic wind. Katabatic wind is a type of density current which flows downward pulled by gravity because of the denser air cooled by the ice floor. Due to the spatial non-uniformity of solidification of water vapor included in the air onto the ice floor, and sublimation of ice into the air, the boundary waves are formed on the ice floor. We performed laboratory experiments simulating the condition that katabatic wind blows on ice. Boundary waves were formed on the ice in the case that there was a large difference in temperature between the room air and the bottom of the ice.

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 Movable bed experiments using a wide laboratory flume without influence of fixed side walls were carried out in this study in order to observe the long-term deformation process of channel plane forms. The channel plane form deforms with bank erosion, braided streams and bed geometry in the channel. In this experiment, the initial straight channel changed to the plane shape with narrow parts (nodes) and wide parts (anti-nodes) finally. The plane shape with narrow parts and wide parts was seen several times during the deformation process. The formations of the former watercourses were also seen in the process that the channel plane shape shifts to a renewed plane shape with different phase.

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 The effect of the artificial flood to river channel disturbance is clarified quantitatively using the results of field observations and numerical simulations (iRIC Nays2D 4.2). The 1st viewpoint is maintenance of distributary leading to former watercourses which induces river channel disturbance at the time of a large flood. The 2nd viewpoint is a scale of river channel disturbance at the time of a large flood. The difference between the existence and non-existence of the artificial flood has been discussed. It was evaluated quantitatively that artificial flood is effective in maintenance of distributaries. Moreover, it was found quantitatively that the effect of mainstream alternation at bifurcations during a large flood which is expected by the maintenance of distributaries.

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 This paper discusses how to improve the existing one-dimensional river meandering model. In the existing model, the main stream shift was simply assumed to trigger the side bank erosion. On the other hand, in the new model, the influence of the intensity of secondary flow is considered in addition to that. First, the weight of effects of the two factors is determined by calculating the infinitesimal wave length, and comparing that with the data of the length of alternating bars in actual river channels. After that, the calculation in finite amplitude meandering is conducted, and compared with the satellite image analysis data of natural rivers collected from Google Earth. The simulation result basically agreed with the image data in the values of wave length and shape of meandering.

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 River improvements to the ship-bottom-shaped channel are considered as one of effective measures for both flood controls and river environments. However, study on ship-bottom shaped channel in curved channel has not been conducted. For evaluating effectiveness of ship-bottom-shaped channel in curved channel, it is important to investigate mechanisms of flow structures and bed variation around the gently sloped outer bank. From the above reasons, practical calculation model which can estimate three-dimensional flows, bed variations and outer bank erosions is required.
 This paper is positioned to a fundamental study for cross-section design of ship-bottom-shaped channel in curved channel. The quasi-three-dimensional numerical model is applied for experimental results on flows and bed variations conducted in curved channels with gently sloped outer bank.

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 This paper presents an experimental study of coupling fluvial erosion and cantilever failure with the slump block effect for cohesive banks. Two types of cohesive materials with different percentages of silt-clay content were investigated under similar hydraulic conditions using acceleration sensors to clarify failure phenomena. The experimental results showed that fluvial erosion of the lower part of the cohesive banks progressively undermines the upper part during the initial stage of a cantilever failure. Tension cracks then develop at the upper surface of the cohesive banks, and beam-type failure occurs thereafter. Furthermore, slump block failures and decompositions were observed as the new phenomena in this study. The geometries of slump block including its failure and decomposition mainly depended on the cohesive force of the cohesive banks. Therefore, a reduction of the silt-clay content leads to smaller slump block dimensions as well as faster decomposition.

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 To elucidate alternate bar profiles and bar formation under the non-uniform sediments condition, flume experiments and linear stability analyses were conducted. The experiments results show that alternate bars profiles were disturbed in the case that discharge was reduced by 30% from the case that alternate bar were formed. Those bar profiles disturbance assumed to be caused by forming fix bars covered by coarse sediments. The experiments results also show that bar formation was not formed in some cases. Linear stability analyses results show that prominent instability at 1m bar wave length, that is one third of proper alternate bar wave length, was conformed in one of the case that alternate bar was not formed. Thereby, it was assumed that the peculiar instability restrained bar formation.

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 A meandering stream is often observed in gravel-bed rivers with bars which attacks river bank and causes local scouring and bank erosion in even a medium-size flood. Therefore, to maintain the safety against flood in gravel-bed rivers, we need knowledge on mechanism of such formation process and factors which bring about serious bank erosion under a strong meandering stream. In this study, a depth-averaged flow and bed-variation analysis is carried out under the regime condition in alternate bars considering invasion of vegetation in bars and bank condition. Results of the simulation show the formation process of a single strong meandering channel caused by invasion of vegetation in bars and bank erosion effect.

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 This study aims to clarify the formation process of sediment exchange layer in riverbed through the field observation of two rivers and flume experiment, and to discuss the relationships between sediment transportation system and exchange layer. Several devices for the observation of sediment exchange layer were installed in the Asa and the Syubuto river. As a result, the observed exchange layer thickness was larger than previous studies. The flume experiment was conducted to confirm the forming process of exchange layer, and revealed that the thick exchange layer is formed due to the bed degradation of discharge increasing period and sedimentation of decreasing period. The mechanism were also confirmed in the two field rivers.

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 It is known that the vertical structure of the river bed of the Chikugo River estuary consists of complex structures of gata-soil and sand. In this study, we develop a new model of bed variation analysis in the river estuaries with complex structures of bed layers, and we clarify the mechanism of bed variation and the amount of sand supply from the Chikugo River estuaries to the Ariake Sea during a flood. In the model, when the rate of volume of gata-soil is larger than the porosity of sand, bed variation are calculated by the GBVC method and continuity equation of sediment which takes into account the sediment discharge of sand and erosion speed of the cohesive material so as to correspond to the volume of gata-soil and sand. This is because the gata-soil has characteristics of the transportation different from sand and mixture of the sand and the gata-soil has great influence on the bed variation.

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 One of the main causes of bedrock erosion is quicksand saltation on the bare bedrock. On the other hand, if bedrock is covered by gravel, bedrock erosion is restrained by decreased collision of quicksand on bedrock directly. And the topographic change by bedrock erosion cannot be calculated because we use constant value of bedrock hydraulic roughness height (k_sb). In this study, we tried to improve flume numerical calculation model (Nays 2d_bedrock) so that we can calculate the topographic change by using standard deviation of topography (σ_t). And we reproduce flumed channel experimental results using constructed model. In addition, we conducted numerical experiment imitating the “washing rock” which is one of geographic features of bedrock erosion. The results suggest the following: (1) the numerical results correspond with past experimental results; (2) we can reproduce the “washing rock” by numerical experiment using standard deviation of topography (σ_t).

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 This study focused on the alignment of spur dikes fixed on rigid-bed fields, and discussed the effect of angle of spur dikes on the characteristics of flow, sediment transport, and bed variation. The results show that acute-submerged spur dikes (30-degree) significantly produced sediment deposition areas between spur dikes, because the flow in the upstream area of acute-submerged spur dike was divided into two directions. One of the flow directs toward the main flow and transports sediment to the main flow from the spur dikes. While, another flow directs toward the joint between the spur dike and the fixed wall. The later flow transports bed load material into the upstream area of the acute-submerged spur dikes and the material deposits there or passes over the dike and deposits between spur dikes.

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 Scouring or depositing downstream of submerged weirs with an opening is a sediment phenomena resulting from the interaction of the three-dimensional turbulent flow field around the structure and the moveable sand bed. This paper presents the experimental study on the downstream channel bed due to weir with an opening, paying attention to the effects of relative overflow depth on local scouring around the structure, sand bars and three-dimensional flow patterns. The experiments were conducted under the clear-water scour condition for an equilibrium scour hole. The experimental results show that local scouring and sand bar development downstream of the submerged weirs decrease with relative overflow depth and turn out strongly paired cellular secondary currents.

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 Several rivers in Hokkaido are facing riverbed degradation by countermeasures such as Sabo dams. The riverbed widely exposed can be eroded by collision of gravel particles. Previous studies have discussed sandbar formation on the fixed-bed and mechanisms of bedrock erosion. However, bedrock erosional morphology by sandbar formation had not been carried out. The experiments are conducted in the flume into which mortar is placed to imitate bedrock. Alternate sandbars are observed during the experiments. The results suggest that both sides of the channel are eroded with sediment transportation by sandbar formation. Moreover, it is confirmed that sediment supply rate and initial alluvial thickness have a large impact on bedrock erosional morphology.

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 Three dimensional experiments are conducted to analyze the downstream scour dimensions after levee overtopping. For elucidating the difference of scour dimension under various hydraulic (overtopping flow height at the levee top, d_bank) and geometric parameters (levee height, h_bank, overtopping flow width along the levee, W_bank), 3D scour profiles are investigated. From the analysis, it has been observed that at equilibrium conditions, for the same d_bank cases scour depth (S_d) is higher for greater h_bank (10cm) compared with lower levee height (6cm). In contrast, scour length (S_l) is longer in case of lower h_bank compared to higher h_bank for same d_bank. The stronger hydraulic jump that took place in case of smaller levee height cases appears to play a significant role in developing greater scour length along with shorter scour depth. Three separate equations for S_d, S_l and S_w (scour width) have been proposed from multiple regression analysis. These results can be useful for the study of levee design.

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 In the downstream tidal basin of the Kiso River, large-scale groins of about 200 m long were constructed and diverse river environment with pools and creeks was created by scouring, sedimentation and foresting in intermediate areas of groins. The scouring zone downstream of large-scale groins played an important role on creating and maintaining embayment water zone. In this study, we examined the scours formed behind the large-scale groins in the Kiso River by field observations using ADCP and other observation equipment set downstream of the groins. Then the mechanism of creating downstream scour was investigated by 2-D and 3-D numerical analyses in considering the effects of groins' lengths on the scouring. As a result, we revealed that offshore-heading streams occur behind the groins during ebb tide and flood event and it may cause the scouring downstream of the groins.

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 River mouth sandbars produce bed slope gradients in longitudinal and lateral directions with a contracted channel between them. The bed topography around the sandbars changes vertical velocity distribution due to stream curvatures and flow accelerations and decelerations. It is important for flood flow and bed variation analysis to evaluate three dimensional flows around the sandbars. The objective of this paper is twohold. First, we derive the method to evaluate the variation in the vertical velocity distributions and discuss the appropriate calculation method for flows in the river mouth with sandbars using the computation result by the General Bottom Velocity Computation method. Second, we develop a calculation method of flows and bed variations around sandbars with the large bottom boundary gradients and investigate the effects on the computations results of flows.

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 This paper is intended to investigate the characteristics of suspended sediment transport and flow field surrounding Bandal-like structures. Bandal-like structure is a kind of groin which may be considered as a combined structure consisting of an impermeable groin at the upper part and a permeable groin at the lower part. Bandal-like structure is an indigenous structure used in the Indian Subcontinent for navigation enhancement, bank protection and channel stabilization. Bandal-like structure works well under suspended sediment conditions. It is important to clarify the hydraulic and morphological implications of it in order to understand its performances and working mechanisms. In this study, attempts have been made to seek scientific evidences for Bandal-like structures with a laboratory experimental study on the local flow, suspended load concentration and bed deposition around Bandal-like structures.

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 The main purpose of Tidal Basin Management is to get suspended sediment deposits gradually under a controlled system. When a cut is made at an appropriate point of embankment, muddy water enters the tidal basin during high tide, depositing a portion of suspended sediments on the basin before flowing back towards the ocean during low tide. An attempt has been made to study through laboratory experiments to precisely look into the suspended sediment transport at entrance of beel. With change of outflow at side basin or tidal beel and change of sediment concentration at inlet, the transportation and deposition of sediment are investigated. Transportation and deposition of suspended sediment mainly depend upon direction of flow and magnitude of outflow discharge at side basin. Better understanding of sediment transport phenomena increases the effectiveness of Tidal Basin Management.

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 The Arase Dam on the Kumagawa River in Kumamoto Pref., Japan, has been subsumed under removal project as the first high-dam removal in Japan. In March 2015, the dam crest on the right side has been removed with the first flushing and continuity of flow and sediment has been restored. In this study, we conducted elevation measurement with RTK-GNSS and SfM analysis using UAV aerial photo set. As a result of our investigation, we found significant scouring in the upper stream of the dam site caused by several times of floods (Q_max=2000-3000 m³/sec). According to our estimation, 1.5 * 10⁵ m³ of the sediment on former reservoir (26% of total) are washed to the downstream and 1.8 * 10⁵ m³ was found on the immediate downstream of the dam site and it created new sand bar.

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 This paper presents the applicability of a numerical movable-bed channel as a prediction tool for dynamic interactions of flow and particle motions. Motions of flow and particles represented in the numerical movable-bed channel provided a good explanation for vertical distributions of particle velocity and concentration measured in a flume experiment on debris flow. We investigated effects of particle shapes and coefficients of contact forces (coefficients of restitutions and resistant friction angles) in the DEM method on simulated motions of flow and particles. Simulated motions of flows and spheres in the numerical movable-bed channel by using different coefficients of contact forces were not able to represent simulated motions of flow and gravel particles. We also studied forces exerted on flow and particles. Streamwise fluid forces on particles predicted negative values in upper parts and positive values in lower parts of the debris flow due to variation of magnitude of streamwise buoyancy and drag over the depth.

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 Lahar, which is a wet mass of volcanic fragments flowing rapidly downhill, is a secondary disaster that occurs frequently during rainy season in 13 rivers of Merapi volcanic area. While the relationship between lahar and rainfall variability has been well understood, most lahar simulators used for this area tend to neglect rainfall variability as one of the inputs. HyperKANAKO, a debris flow simulator equipped with a graphical user interface (GUI), was designed to perform debris flow calculations based on its physical mechanism. The main purpose of this study is to analyze HyperKANAKO model's sensitivity when it is run on digital elevation models (DEMs) with three different resolution levels. Additionally, rainfall data from two different days measured in the Gendol catchment area were used. The results of these simulations showed that one-dimensional (1D) flow were more affected by rainfall than DEM quality, while two-dimensional (2D) lahar flow was best simulated using the finest resolution model.

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 Disasters caused by the flow including sediment on mountainous roads have been reported. Bridge type designs are typically used in highway construction to prevent debris flow. Commonly, cost-effective countermeasures against the debris flow including Sabo dams have been used. Furthermore, road banking for changing the debris direction and trapping sediment have been applied. However, banks must be designed to restrict the surface flow ingress. In this paper, a new structure is proposed as following: install drainage routes pass beneath the highway banking, and improve drainage underpass designs to separate water from debris effectively. Experimental data for the separation with steel have been presented and discussed.

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 The advantage of aerial vehicles in observing various geographic features is its wide field of view, which is different from an observation from the ground. Although the field of view depends on the height of the vehicle and the lens aperture, an area with a side length of more than one hundred meter can be observed at one time. As for measurements of a river reach, a manned helicopter or an unmanned air vehicle has been used to shoot river surface flows, from which surface velocity distributions can be measured by an aerial LSPIV or an aerial STIV techniques developed by our research group. In this paper, a new image stabilizing method was developed for video images shot from navigating aerial vehicles such as a manned helicopter or a UAV. Moreover, the relative accuracy between LSPIV and STIV was discussed for the same aerial videos. The developed method was applied to several new and old videos shot from manned helicopter or an UAV.

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 This study was conducted to evaluate the accuracy of a three-dimensional (3D) measurement method for water-surface and bed-surface profiles during the formation of sandwaves using the moving optical cutting method (MOCM). An experiment on flow through a bed with an array of uniform hemispheres was conducted, and the water surface profile and the channel bed form were measured using the MOCM. The measured results agreed with the water-surface and bed-surface profiles, confirming that MOCM can be used to measure water-surface and bed-surface profiles when sandwaves are being formed. Two movable bed flume experiments under hydraulic conditions were conducted, in which sand dunes and sandbars were formed. The measurements confirmed that water-surface and bed-surface profiles can be spatiotemporally tracked at high resolutions during the formation process of sandwaves. This suggests that MOCM is a useful technique for investigating the mechanics of sandwaves.

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 HK-curve method can rate water-discharge Q more accurately than conventional HQ-curve method by considering hydrological data not only water level H but also water-surface slope i. By the analysis using the actual measured hydrological data during flood, it was shown that the error of rated Q by HK-curve method for the measured value became less than almost 5%, although the error of rated Q by HQ-curve method sometimes exceeded 10%. In addition, it was shown that HK-curve almost same as considering whole measured data from the water-level increasing period to the decreasing period via the peak time during flood could be calculated even though only the data in the period of drawdown was used, and as the result, even when the data was measured only in the drawdown period of flood, HK-curve method could estimate highly accurate Q for the data missing period.

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 In estimating river flow discharges from water surface velocity distributions measured by non-contact methods such as radio wave velocity meters or image-based techniques, it is important to use an appropriate value of surface velocity coefficient that yields a depth-averaged velocity from a surface velocity. Conventionally, a value of 0.85 has been widely used in practical purposes but its distribution feature under various conditions has not been clarified yet. In order to investigate a fundamental aspect of the coefficient, measurement data by a tethered ADCP, a remote-controlled ADCP and surface velocities obtained by space-time image velocimetry(STIV) as well as a three-dimensional numerical simulation data were compared for the floods of the Uono River and the Niyodo River. It was made clear from measured and numerical data that the variance of the coefficient distribution tends to increase with decrease of the relative water depth and vice versa.

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