In the mountainous rivers, a huge amount of sediment is supplied due to landslides and debris flows during heavy rainfall, which causes flood disasters with huge amount of sediment in downstream reaches. This study proposes a method to predict sediment runoff by combining a rainfall-runoff model and a unit river model to compute sediment transport. The proposed model is compared with a method to predict sediment runoff by considering the sediment erosion from the supplied sediment. As a result of computation for the Gofukuya-river flood disaster in 2019, the computational result of the proposed method underestimates the actual sediment runoff, because the sediment coarsening occurs in mountainous rivers during the flood event, which implies the importance of sediment supply settings in the mountainous rivers.

 In recent years, the exposure of clay layers has been regarded as a problem in Japanese rivers, and a plan to create a sand bed is being considered. Therefore, it is necessary to elucidate the characteristics of the "mixed layer of clay and sand” formed by bedload. In this study, we focused on the exfoliation of the mixed layer. As a result, the gravel pick-up process on the mixed layer was clarified. Furthermore, it was clarified that the "depth of gravel bite" affects this process. We also considered the relationship between bite depth and adhesive strength.

 Many experimental studies about sediment transport have been carried out until 1990's. These days, research with numerical simulation for bed changing have been taken with knowledge from experimental research results. However, there are some issues of estimation of sediment transport rate. Advanced measurement technologies, camera and PIV system, and so on enable measurement of flow distribution with sediment flow to be carried out. Therefore, some series of flume experiments have been taken in order to indicate relationships between sediment movement and water flow strictures with PIV and PTV system. I could measure sediment particle velocity and flow velocity with PIV/PTV, so that I succeeded in indicating effects of both suspended load and bed load transport on flow velocity.

 The numerical movable-bed experiment was conducted to understand sediment transport mechanism in gravel bed rivers.The riverbed material used in the numerical experiment was spherical particles and followed Talbot grain size distribution.The whole grain size particles of the riverbed surface moved and vertical sorting occurred. And, it was found that the larger the particle size, the more they were at the above location in the river bed. In addition, it was confirmed from the trajectory of the particle motion that the small particles fall between larger particles of the river bed.

 While the active layer concept has been applied to sediment transport problems in mixed-grained river beds, the problems of porosity and exchange layer thickness remain unsolved. In this study, the Eulerian sediment deposition model is proposed, focusing on the fact that the active layer is not a problem in the Descrete Element Method that solves the motions of individual particles. First, based on the basic equations used for DEM and the available porosity on the riverbed deposition conditions of sediment particles, the equations for temporal variations in particles deposition heights and particles proportions were derived and compared with the conventional equations with the active layer concept. Sedimentation experiments were conducted under static conditions, and we found that the sedimentation process depend on the ratio of coarse to fine particles. The applicability of the Eulerian sediment deposition model to the sedimentation process of mixed-grained sediment was validated through comparisons with experimental results.

 Mountain rivers are composed of a wide range of particle sizes. In particular, the movement of boulders with a size of about 1.0m has an effect on flow resistance and sediment transport. In this study, we surveyed the existence and loss of boulders and gravels, and investigated the judgement method of boulder movement due to the flood flow in the Takiyama River. Using quasi-three-dimensional flood flow analysis by Q3D-FEBS based on the observed water surface profiles and detailed topographic survey data, we attempted to evaluate the movement of boulders by flood flows. The future issues on this matter are discussed.

 In the riffle of a gravel-bed river with a wide grain size, sediment transport corresponding to the magnitude of the flow rate occurs. The purpose of this study is to obtain basic knowledge for sediment transport modeling in riffle under the ordinary flow rate. The six field surveys were conducted on the Nagara River, and three cases of experiments were conducted in the experimental flumes. For the observed hydraulic values, the magnitude of flow resistance, the balance between the movement limit of surface sediment mixture and the tractive force were examined. As a result, Hey's and VPE with the relative water depth as a parameter were able to evaluate the observed values of U/U^* relatively well. In the process of continuous water flow, partial sediment movement occurred and the process of coarsening the surface layer of the riverbed was observed in the flume. However, it was suggested that the balance of the actual river riffle would be unstable due to various fluctuation factors.

 Levee breaching occurs recently during floods in torrential rain and causes severe damages in landside area. For disaster mitigation, it is of great importance to understand the mechanism of flows and sediment transport in the breaching process and consider countermeasures. In this study, to obtain spatial distribution of saturation and examine the breaching process, hydraulic experiment of levee breaching due to overtopping flows is carried out, using transparent soil. Then, conducting image analysis, visualization of saturation in the embankment body is shown, which cannot be obtained in the case of sand.

 Severe damages caused by embankment failures during floods have been recently reported. For disaster mitigation, it is of great importance to understand its failure process, evaluate the emkankment stability by simulating flows and bed deformation accurately and consider countermeasures. The objectives in this study are to develop a numerical model and predict the embankment failure process at the back side toe due to seepage flows. By coupling the 3D flow model to simulate simultaneously overtopping and seepage flows, and the soil deformation model based on GIMP method, a numerical model is developed. Then, applying the model to the hydraulic experiment, it is shown that continuous embankment failure at the back side can be reasonably simulated.

 In recent years, local heavy rainfall has increased due to the extreme weather in Japan, causing enormous inundation damage in various places. The river levee breach is one of the disasters that causes enormous inundation damage, and a typical example is Typhoon No. 19 that occurred in October 2019. It can be said that elucidation of the levee breach mechanism is important for disaster prevention and mitigation in the midst of frequent damage. The authors have conducted fundamental experiments in which water is stored in river space and experiments that reproduce river flow, targeting simulated embankments composed of clay, sand, and gravel. We have examined the effects on river flow. In this study, we are conducting experiments under different conditions of levee material and riverbed width, and by adding new measurement items such as streamlines and overflow rates, we are investigating a more detailed overflow breach process.

 When a landslide dam collapses, the overflow discharge increases sharply. In addition, it is known that high-concentration sediment flows down due to rapid vertical erosion and side bank collapse in landslide dam. However, when there are a large number of wood debris in the flooded area of a landslide dam, there are many unexplained points about the collapse characteristics of landslide dams.In this study, we experimentally investigated the effects of wood debris deposited in the flooded area when the landslide dam collapsed. As a result, it was confirmed that in the situation where the wood debris block stopped, the wood debris block was caught in the sediment on the downstream slope of the landslide dam, and the sediment on bank downstream of the landslide dam collapsed and was covered. Also, as the number of wood debris increased, the maximum overflow discharge decreased. Further, when the wood debris block is accumulated and stopped, the possibility of erosion of the bottom surface near the eroded channel by the flow of internal wood debris block is accelerated causing collapse was suggested.

 Coastal management around estuaries with highly morphological sandspits or barriers is one of the complex engineering tasks coastal researchers and engineers encounter. Insufficient studies or understanding of the morpho-dynamics of such areas results in poorly designed protection structures and hence exacerbating the existing problem being remedied. Therefore, this study aims at quantifying the sandspit growth and longshore sediment transport rates at the Bouche du Roi inlet in Benin, West Africa. The inlet has a constantly elongating updrift sandspit which results in narrowing and closure of the inlet. During rainy seasons, this poses flooding risks to surrounding communities. The study employed the use of remotely sensed images acquired from 1984 to 2020 to perform long-term analysis. Furthermore, the depth of closure and berm elevation for the study area were calculated using well known empirical formulas. Results from the sandspit analysis revealed an average updrift sandspit growth rate of about 700 m/year and an average longshore sediment transport rate of about 1.2 × 10⁶ m³/year along the Bight of Benin. The results obtained from the method utilized in this study were similar to results from past studies in which different approaches were used.

 Hydro-morphological processes are complex in the river estuary area, since certain intensity of a flood induces sandbar flushing under fluvial systems and subsequent reformation occurs due to coastal waves, and they are processed in a short period and repeated by each flood. Therefore, it is important to establish a methodology in order to both efficiently and effectively manage riverbed at the estuary area under the dynamic complexity. This study revealed this dynamic sediment mechanisms and discussed a methodology for estuary riverbed management in the Sagami River, Japan. Firstly, dynamic processes of the sandbars are analyzed. We used 3D geomorphic survey data and numerical simulations, in order to develop 3D geomorphic data. Secondly, several geomorphic features of the sandbar are set and analyzed in combination of dredging width and flood intensity. Further, reformation processes were numerically analyzed for desirable geomorphic features of the sandbar.

 Predicting woody debris behavior in rivers is a very important task for designing a debris retention facility. This study developed 3D numerical models for simulating woody debris motion and discussed the difference in simulation accuracy based on hydraulic experiments with a debris retention facility at the channel bend. The applicability of three numerical models to the experimental data was shown to clarify the model performances and the issues associated with the models. We concluded that a numerical model using the third-order upwind difference schemes has the best applicability to reproduce the experiment observations regarding the flow structure resulting from vortex formation, the woody debris behavior from conveyance to accumulation, and the rate of debris accumulation by the retention facility.

 This study examines sediment run-off and particle-size segregation for sediment replenishment containing fine particle through laboratory experiment and 3D sediment transport simulation. The simulation is based on a Lagrangian method and the probabilistic Exner equation (PEE) for non-uniform sediment behaving as bed load and suspended load. The simulation newly introduced the average Step-length model which considers the difference of particle size between bed load and stationary bed. Experiment and simulation agreed fairly well. In addition, the average setp-length model revealed a well known phenomenon that existence of finer bed particle can increase the bed load rate of a coarser particle. The output of the present study provides a insight into sediment run-off and particle-size segregation fron the Lagrangian view. In addition, the present study indicates an important research subject for Lagrangian non-equilibrium sediment transport simulation to be resolved in the future.

 In this study, a riverbed with exposed large gravel that does not move due to water flow and a flat riverbed without boulders were reproduced in an open channel. PIV analysis was performed on each riverbed condition, and the structure of the turbulence was investigated. As a result, it was confirmed that vortex is generated behind the boulders of the riverbed where the boulders are exposed, and stronger turbulence is generated in the void space among the boulders than in the flat riverbed. In addition, it was suggested that the turbulence structure depending on the presence or absence of exposed boulders causes a difference in the structure of vertical sorting. Furthermore, the motion of a suspended load taken by a high-speed camera and the PIV analysis result at the same time were superimposed. It was confirmed that the motion of the particles changed due to the influence of the surrounding fluid motion with a slight delay by the experimental video.

 Video image analyses and numerical simulations were conducted about motions of non-spherical particles in a turbulent flow over a rough-bed channel, to reveal the effect of the particle shape on saltation motions.

 First, video image analyses revealed the effect of the shape on the transport manner of non-spherical particles. Secondaly, it was shown that numerical simulations using the APM method explain the transport manner and the movement velocity of non-spherical particles in a turbulent flow over a rough-bed channel.

 It is reported that levee breaching due to overtopping flows recently causes severe damage in landside area and, for disaster mitigation, it is of great importance to understand the mechanism of breaching process and the characteristics of flows and sediment transport, and consider countermeasures. In this study, hydraulic experiments of levee breaching are carried out under the different seriment materials. Then, numerical simulation is conducted, by coupling the 3D flow model which solves simultaneously overtopping and seepage flows and the equilibrium sediment transport model with bed load and suspended sediments. It is shown that erosion at the toe of embamkment is slightly simulated by considering the effect of suspended sediments.

 This study experimentally investigated the effects of hyper-concentrated sediment on flow resistance, flow velocity field, and large-scale vortex structure in an open channel with two-dimensional cylindrical roughness. Furthermore, the spatial distribution of the main flow velocity with highly viscosity in the rough surface flow was theoretically examined. Experimental results showed that in the high-concentration sediment flow, the increase in the viscosity coefficient weakens the Reynolds stress and the strength of the large-scale vortex, and suppresses the generation of the separation vortex at the top of the roughness. Furthermore, the theoretical formula on the vertical distribution of the main flow velocity with highly viscosity in the open channel flow could be reproduced well by the Power-law Model assuming the flow as a laminar flow.

 It has been pointed out that the deposition of suspended sediment within and around vegetation patches in a river channel by floodwater causes reduction in maximum discharge, expansion of river vegetation, and some impacts on ecosystems. In this study, to understand sediment transport processes, PIV measurements and sediment deposition experiments were conducted for flow fields in and behind a vegetation patch. As we focused on vegetation height as a parameter that affects the characteristics of the flow field around vegetation, four types of vegetation models with different heights were considered. By comparing the results of the two sets of experiments, the effects of the vegetation height on the turbulent flow structure and deposition of suspended sediment inside and behind a patch were investigated. The results indicated that changes in flow structure may determine sediment deposition areas around vegetation.

 In order to grasp the effects of vegetation such as shallow-rooted willow overgrown in gravel rivers on channel morphology and river planform, we conducted hydraulic experiments using bentgrass which has short roots. In the experiments, the process in which the meander channel maintained was reproduced. At this time, it was shown that the effect of vegetation on suppressing bank erosion was small near the apex of the curved channel, which tends to be relatively deep, and the effect of vegetation on suppressing bank erosion was large at the shallow riverbed on the downstream side of the apex. The experiment results indicate that the degree to which vegetation suppresses bank erosion varies depending on the height of the bank or the depth of the channel, which affects the characteristics of channel morphology and river planform.

 In 1997, two artificial cutoffs were performed in the Ichilo River in order to save the usability of a port from the menace of a natural cutoff. In here, we want to analyze the implications of those two artificial cutoffs in the planform of the Ichilo River using 2D numerical simulations. The numerical model used for the simulations is a modified version of the model of Asahi et al. where we expanded a module for the treatment of neck cutoffs in large scale rivers of the Bolivian Amazon such as the Ichilo. We tested the capabilities of the numerical model to simulate large-scale meanders in the short to medium term. The initial planform shape corresponded to 12 meander bends based on the planform shape of the Ichilo river in 1994, with different geometric characteristics, variable channel width and variable discharge. We observed that the model is sensitive to smoothing techniques of the planform, which is a common technique to obtain stable numerical simulations but results in unrealistic planform shapes. The numerical simulations present us the planform evolution of the Ichilo River without the two artificial cutoffs performed in 1997. The results showed that the development of the natural cutoff would have left the port located in an oxbow lake. The migration trends also indicate that the river would have moved toward the northeastern side of the city of Puerto Villarroel.

 Gravel beds formation in rivers downstream of dams may cause several issues such as a bed degradation and a channel immobilization. Many works have been conducted in order to make restoration of such rivers. On the ohter hand, when fine sediment (i.e., sand) is present in streambeds composed of gravels, the gravel can be more easily removed as compared with gravel beds without fine sediment. This phenomenon may indicate that the sediment augmentation to gravel beds using fine sediment is a viable alternative to mobilizing coarse surface layers. In this study, we investigated the effects of sand-covering to gravel beds with low-flow channel on gravel mobilization and channel evolution. The temporal variations in low-flow channel were examined by means of flume experiments. And the effects of sand-covering thickness on the channel width and depth was discusssed.

 There are many examples that, although a river has been widened to improve its discharge capacity, the channel cross-sectional area has reduced due to the subsequent deposition of fine sand. The reduction of cross-sectional areas is a severe problem for maintaining an essential function of rivers to discharge floodwaters safely. In this study, we applied the theory of self-formed channels of gravel-bed sandbank rivers by Izumi and Parker to theoretically study the reduction of channel cross-sectional area due to fine sand deposition as described above and the formation of equilibrium cross-sectional shapes by fine sand deposition on gravel beds.

 In mountain-river, the development of bedrock meanders due to incision and lateral erosion are commonly observed. The dominant factor in bedrock erosion is the collision-based abrasion by bedload. And it is also caused by weathering of the bedrock surface by mechanical effects such as wet and dry, freezing and thawing. In this study, we attempted to clarify the effects of wet–dry weathering in a channel bend with bedrock using a numerical model. The numerical model considers lateral bedrock erosion based on the amount of sediment transport in the downstream direction and the lateral slope of the riverbank. Numerical experiments were conducted by varying the amount of sediment supplied. The results of numerical experiments showed that the formation of fixed bars on the inner bank due to the influence of secondary flows, local scouring at the boundary between sediment and bedrock, the lateral erosion due to weathering was observed on the outer bank. The weathering affected the entire river channel as bed degradation due to abrasion erosion progressed. In addition, in the case of bedrock with low sediment supply, the erosion by weathering progressed on the inner bank upstream of the bend because the entire river channel is exposed.

 In this study, we estimated the flow discharge, roughness coefficient, riverbed level, and sediment discharge with bedform transitions based on the temporal variations of the water surface profile. The estimation method uses the 1D shallow water and continuity equation for the sediment discharge as governing equations. The method was verified against experimental data on bedform transition such as transition – flatbed - dune II. The verification showed that the present estimation method could reproduce the flow discharge, roughness coefficient, average bed level, and sediment discharge. We also examined the difference between the estimation results assuming the river bed as a fixed bed. It showed that an estimation result based on the river bed after the flood events had the highest reproducibility of the flow discharge.

 The braided stream of the Abe River is considered to be formed by the non-equilibrium motion of the bed load and suspended load, which change significantly in time and space due to the complex riverbed topography of the braided channels. This paper presents the bed variation mechanism of braided bars in the Abe River by calculating the three-dimensional flows and non-equilibrium motion of the bed load and suspended load based on the time series data of the observed water surface profiles of the large flood in 2014. The calculation results could well explain the changes in mainstream positions due to the movement of the braided bars.

 An extremely heavy rainfall caused huge flood disaster including the collapse of river bridges. Most of the factors are local scouring around the piers, and future maintenance of the piers and appropriate scouring measures are major issues. This study aims to clarify the effect on the reduction effect of scouring around the pier. For this, we conducted small-scale model experiments using various scour protection like block method, bag-shaped riprap method, and steel sheet pile method. As a result, the bag-shaped riprap method can have high scouring resistance compared to other scouring measures. It is also found that the scouring countermeasures work can be sufficiently effective only by constructing the scouring countermeasures on the upstream side of the pier.

 Pile-groups are one of the hydraulic structures used for riverbank protection. Controlling the local scour around a pile-group is a desirable attempt for the structural stability. In this study, bed deformations around different pile-groups were investigated experimentally. This research was aimed to depict the effects of pile-group layout parameters on the bed characteristics. The maximum scour depth was well expressed by the modified blockage ratio with an identical trend in all the cases. Increasing the modified blockage ratio resulted in a larger scour and smaller deposition around all types of the pile-groups. Total scour volume in the channel was shown to be almost proportional to the maximum scour depth in all the cases.

 Check dams are extremely effective structures from the viewpoint of preventing sediment-related disasters. However, they cause discontinuities in riverbed longitudinal shapes and sediment transportation, and countermeasures are required from the viewpoint of river environment. In this study, hydraulic experiments based on the check dam of the Iwaubetsu River located on the Shiretoko Peninsula were conducted for examined the response of the river channel due to slitting of check dam. As a result, the continuity of the bed elevation is restored by the slitting of the dam, and in the case of small-scale flooding, the alternate sandbar shape before dam construction is regenerated. However, in subsequent large-scale floods, both the wavelength and wave height of the sandbars increase, although alternating sandbars are maintained.

 In this study, we conducted laboratory-scale experiments to investigate the optimal position of in-channel sediment supply so that the supplied sediment is widely transported in the river channel. A set of experiments were conducted where we changed the relative position between sandbars and the supplied sand. We used colored particles for the supplied sediment to clearly understand its transport. Our results show that the supplied sediment is most likely to be transported into the river channel when it is placed at a location so that the riverbed elevation rises from the scour. We also found that the increase in local sediment supply leads to a shorter wavelength of the sandbar placed of the supplied sediment.

 In this study, the depth of the opening of the oblique weir was systematically changed, and the sand removal function of the sediment deposited upstream of the weir and the riverbed fluctuation downstream of the weir were investigated. The oblique weirs with openings increased the fluidity of sediment deposited upstream of the weir and lowered the riverbed. The scouring section upstream of the weir was divided into a gradually varied flow section and a local flow section based on the bed topography. The experimental results showed that in the gradually varied flow change section, the amount of water level and riverbed degradation increased linearly as the depth of the opening increased, and the river bed degradation of the oblique weir was higher than that of the orthogonal weir. In the local flow section, the scouring holes downstream of the oblique weir were found to be similar in shape by making them dimensionless on a typical length scale, and the effect of opening depth on the representative length scale was clarified.

 The field investigations were performed 3 times at June, September and December in 2019. The shape of riverbed in this test zone is changed by the riverflow conditions both flood and dry. Using by UAV analyses, the 3-dimensional riverbed shape were made clear by used DMS. Judjing from the cross-sectional direction, the sandbank materials of this test zone are composed of 1) the sands at near place of the riverflow line, 2) the gravel materials at the central zone and 3) both sands and gravels at the begitation zone near the rightside bank. As shown the characteristic of the riverbed, at the meandering point from flat to swift current, the new sand materials sediment to the outside bend. As shown the result of numerical analyses for the small-scale flood less than about 70m³/sec, the sandy-particals of diameter less than 2.3mm are washed down over the tractive force of the flood.

 The erosion process of natural riverbank consists of three parts: bank toe erosion, bank failure and removal of the deposited material on the bank toe. Erosion of the riverbank occurs intermittently during flood events by repeating this process. However, it has not been considered enough in the flood flow and bed variation analysis in rivers. This paper proposes an analysis method of the bank erosion process in alluvial fans by considering the effects of the negative pore water pressure (the matrix suction), three-dimensional flows, and non-equilibrium sediment transports at near-bank area. Moreover, it is shown that the analysis method can explain the bank erosion process of the field experiment in the Joganji River.

 The authors have been devising formulas for calculating the energy loss coefficient in a four-way circular manhole where three inflow pipes and an outflow pipe are connected at a crossroads. On the first step, a formulation was performed under the condition that the flow rates of the two opposing lateral inflow pipes were the same, and then we devised a calculation formula that could calculate the energy loss coefficient even when the flow rates of all the inflow pipes were different. In this paper, we experimentally obtained the energy loss characteristics of the four-way circular manhole with a drop height on one of the lateral inflow pipes, and developed a new formula that can calculate the effect of the drop height of the lateral inflow pipe with the conventional formula. The calculated values of the energy loss coefficient with this formula can almost reproduce experimentally obtained values within the range of this experimental condition.

 The link sleeve valve is originally designed for waterworks. This study examine the possibility of the link sleeve valve to be used as discharge valves in dam spillways. Experiments were conducted to clarify water flow rate, air flow rate and the wall pressure distribution inside the valve and in the downstream pipe of the valve, in addition to flow observation using a high-speed video camera. We should note that, in the present system, air aspiration into the downstream pipe is needed against cavitation, noise and vibration. As a result, by an analogy to the jet pump, we determined both the entrainment coefficient of air into water jet and the returning air flow coefficient due to the reverse pressure gradient in the downstream pipe. The pressure inside the valve is in proportion to dynamic head. This suggests that, even at high flow velocities exceeding the present experimental range, the pressure inside the valve increases. Therefore, the permissible velocity below which the pressure inside the valve becomes negative is presumed to be unnecessary in actual aspects.

 A low water level operation is being carried out as one of effective utilization of existing hydroelectric power stations. In a low water level operation, a water level of a reservoir is lower than a conventional water level of a reservoir. So a submergence depth of an intake pipe is not enough, it causes an intake vortex in an intake facility of a hydroelectric power station. However, there is lack of knowledge about relationship between a submergence depth of an intake pipe and occurrence of an intake vortex in the horizontal intake facility. In this study, the hydraulic properties in horizontal intake are studied experimentally. Submergence depths of horizontal intake pipe are changed every diameter of the horizontal pipe, with other conditions, ie, a flow rate, a water depth are kept constant. As a result, it is found that the distance between the upper edge of the horizontal pipe and the water surface has a greater effect on the occurrence of vortices than the submergence depth when the upper edge of the horizontal pipe becomes shallower than half of the water depth in the intake channel.

 It is reported that submerged woods in a dam lake induce gate blockage accidents. Hence the present study focuses on quantitative evaluation of critical tractive force. We measured the critical tractive force of the submerged wood in the laboratory flume varying systematically the size, density and initial yaw angle of the wood model. Also, performing movable bed experiments, an influence of initial burial depth was examined. It results in that the normalized critical tractive force could be expressed by universal formulae using Reynolds number and a correction function. Further, considering a stopping condition of the submerged wood moving downstream, we explained the distribution property of the submerged woods observed in the dam reservoir.

 In this paper, a numerical model was developed that introduced a protection block movement analysis using the Discrete Element Method (DEM) into an Eulerian two-dimensional flow and bed deformation model in order to predict the destruction process of the blocks below a groundsill induced by local scour. Concerning the block movement analysis by DEM, the particles were placed only the surfaces of the blocks and the riverbed to reduce the computational load. Now, hydrodynamic force acting the blocks was not considered in this model. The model applied to movable bed experiments with or without the protection blocks below a groundsill. As a result of the simulation without the blocks, the model could reproduce the local scour phenomenon in the experiment well. Moreover, in the simulation with the protection blocks, the model was found to reproduce the destruction process of the protection blocks induced by the local scour progresses although the number of blocks that flowed out was smaller than the experimental data.

 For the nonaerated skimming flows in the stepped channels, an experimental equation of the local skin friction coefficient is proposed with the effect of the relative step heights under a given the channel angle. The relation between the velocity distribution and the local skin friction for the nonaerated skimming flows is shown. For a wide range of the relative step heights, the analytical method for calculating the water depth and the boundary layer thickness is developed with experimentally obtained the local skin friction, determining the position of the section where the boundary layer thickness is equal to the water depth.

 The evaluation of the hydrodynamic forces acting on vegetation and structures in rivers is important for river management. In this study, we focused on undisturbed flows to discuss (i) the changes in pressure and water-surface distributions induced by installing emergent cylinders in open channel flows and (ii) the hydrodynamic force acting on them. We proposed an evaluation method of the hydrodynamic force by clarifying the force being composed of four forces whose physical origins are different, i.e., the buoyant force in the flow direction, the force in the free stream flow, the force due to water surface deformation, and the force exerted by the pressure gradient. Finally, we examined the validity of the evaluation method by applying it to a water-surface profile calculation.

 The flow structure in the rectangular cavity on the side wall of the straight open channel was examined using the flow velocity measurement method and the flow visualization method. The velocity inside the cavity becomes relatively low, and large-scale circulating flows and small-scale circulating flows in opposite directions around the vertical direction are formed. A region with a large horizontal Reynolds stress (-uw) is formed at the boundary between the cavity and the open channel. Its maximum value is below the water surface, and its value is three times as large as that near the side wall of the straight open channel. A group of longitudinal vortex structures formed on the upstream side wall stably flows down to the boundary region between the open channel and the cavity. This longitudinal vortex structure group develops along with the flow, and its fluid motion plays an important role in the formation of a high Reynolds stress region at the boundary between the swirling flow in the cavity and the open channel.

 The characteristics of the turbulent structure of a compound open channel with a discontinuous flood plain were examined using a flow velocity measurement method and a flow visualization method. The inside of discontinuous flood plain (cavity region) becomes a relatively low speed region, and large and small circulating flows centered on the vertical direction are formed. A region with a large horizontal Reynolds stress (-uw) is formed at the boundary between the cavity region and the main channel. In addition, a region with a large vertical Reynolds stress (-uv) is formed at the boundary between the cavity and the upper surface of the flood plain. Longitudinal vortex structures formed at the tip and upper surface of the upstream flood plain flow down stably into the cavity region, and they contribute to the generation of Reynolds stress by the vortex motion of the separation shear layer.

 Since the bridge piers in the river channel generate local flows, they may cause disasters. In addition, it is necessary to clarify the flow mechanism of high-concentration currents with piers in consideration of the occurrence of high-sand-containing rivers such as the Yellow River and flows containing a large amount of fine sediment during heavy rains. Therefore, in this study, we investigated the average flow characteristics and momentum transport characteristics when the viscosity was changed using an aqueous solution of sodium polyacrylate (PSA) in an open channel flow with oval bridge piers. As a result, it was clarified that the detour flow near the tip of the bridge pier and the flow toward the center of the channel near the downstream of the bridge pier are suppressed as the viscosity increases. Regarding the momentum transport characteristics, it was clarified that the momentum transport due to turbulence is extremely suppressed compared to advection due to the increased viscosity.

 Understanding flow resistance is crucial for river engineers to determine the discharge capacity and to control bank erosion and the distribution of shear stress around the boundary. Flow resistance of a channel is well known to increase with the bend, or meander, due to the development of secondary flow. Although the effects of secondary flow on flow resistance are widely recognized, the characteristics of the effect itself are not yet clear. In this study, the effects of secondary flow on flow resistance were investigated quantittively by comparing various numerical calculation methods with the experimental datasets of water surface profiles in straight and meandering channels. The results show that the applied numerical calculation method is effective to evaluate the effect of secondary flow on increasing channel resistance.

 It has been observed that a periodic water surface oscillation ocurrs downstream of a drop structure with a trench at specific hydraulic and geometric conditions. In order to establish a relationship governing the oscillation frequency, a number of experiments using flow visualization and image analysis techniques were conducted. As a result, we proposed a Strauhal number specific to the oscillating hydraulic jump. In addition, a numerical simulations based on SST 𝑘-𝜔 model was executed to successfully simulate time varying oscillating flow comparable to the experiment.

 The surface velocity coefficient is the value that converts the surface velocity into the depth-averaged velocity when observing the discharge, and 0.85 is treated as the standard value in Japan. In recent years, surface velocity measurement by STIV is widely used, but only surface flow information can be obtained. Therefore, it is important to properly determine the coefficient when estimating the discharge. In this paper, we focused on the situation where the water depth gradually changes due to the formation of sandbars. We conducted flume experiments to examine the response characteristics of the surface velocity and the surface velocity coefficient caused by changes in the riverbed slope. As a result, it was shown that the change of the correction coefficient is not symmetrical with respect to the riverbed topography, and that the coefficient decreases sharply on the downstream side of the sandbar because the influence of negative acceleration is large. The above characteristics are verified through numerical simulations as well.

 The quadmaran automated vessel was applied to the water quality observation in Hichirippu Lagoon, Hokkaido, Japan, and simultaneously, mooring array was conducted. We developed propeller guard to achieve automatic navigation in the shallow lagoon at a depth of 0.7 m under the condition of luxuriant water plants. Then, detailed water quality distribution was observed by the mooring arrays and wide-area observation conducted by the vessel. The low-salinity water was confirmed in the lagoon head during the long time due to the freshwater supply and heat exchange by solar radiation on the sea surface. In contrast, the low-salinity water at the lagoon mouth was affected by the water exchange of low-temperature and high-salinity water from the open sea. Our observation suggested that the distribution of water temperature and salinity dynamically fluctuates in the areas from the lagoon mouth to the central part.

 To investigate sediment transport in rivers, the authors measured suspended sediment by two methods: water sampling and continuous measurement with a flow-cell ultrasonic attenuation spectrometer. Two one-liter samples were taken to measure sediment concentration and particle size distribution, for calibration of the spectrometer. In the Inagawa River, sand particles could not be measured either by sample analysis or by the flow-cell ultrasonic attenuation spectrometer, but the sand bars that formed in the river channel due to sand movement during floods could be observed. To solve this problem, the sample volume was increased from one liter to 18 liters, and the concentration and particle size distribution of the samples were measured by dividing the sample into sand particles and wash load. It is necessary to improve the flow cell so that sand particles pass within the measurement area of an ultrasonic transducer.

 Continuous monitoring using fixed-point cameras is effective for early detection and understanding of the behavior of scum in urban tidal rivers. In previous studies, scum detection techniques using U-Net have been developed. However, it is difficult to apply the method to multiple locations because it requires a lot of effort and time to create the label images necessary for training. In this study, we developed a new learning method using dummy images, and evaluated its usefulness by comparing it with conventional methods using the following evaluation indicators: precision, recall, F-measure, and mIoU. As a result, we succeeded in detecting scum with higher accuracy than conventional methods, while significantly reducing the effort required to create labels, which is a bottleneck in conventional training methods. This method makes it possible to understand a wide range of spatio-temporal behavior of scum. Furthermore, by applying this method to suspended solids other than scum, it is expected to be used as a general-purpose technique for continuous monitoring of river debris.