Rainstorm-induced flood is one of the most challenging issues in the world. The management of flood disasters necessitates a clear understanding on associated flow characteristics and drainage mechanisms. This paper presents a field survey on the stormwater flows in a urban drainage system, focusing on the flow properties in a sewer pipe and a receiving stream. In particular, the influences of precipitation patterns, tide levels and drainage discharges from the pumping station have been analyzed. According to the results, the water level changes in the receiving stream are strongly dependent on the hyetograph. Classifying the precipitation patterns into intermittent rain, rain shower and continuous rain, the differences in the sewer flow responses are clarified. The pressurized flows, backflows from sewer pipes to open canals and backwater effects from downstream pipes and the pumping station are confirmed, which are important mechanisms of pluvial floods. The drainage discharge from the pumping station leads to an increase of water levels in the receiving stream both upstream and downstream, which may increase fluvial flood risks. The findings are of great meaning for integrated management of sewer-river systems to prevent/mitigate flood disasters.

 Recently, the river infrastructure and channel have been damaged by driftwoods flowed out during floods induced by heavy rain in Japan. The previous studies on local scouring around bridge piers have been conducted for a long time, but there is few research focusing on driftwood catchment. In this study, we aim to clarify the effect on the local scour/accumulation around the pier by changing the driftwood capture conditions (installation height and density) by conducting a small model experiment. As a result, it was found that when the height of driftwood installation is near the bottom and the density of driftwood is high, the effect of scouring is shown. Moreover, we proposed "annular riprap work" considered driftwood catchment condition as a new scour countermeasure. The construction range of the countermeasure work was narrower than that of the conventional countermeasure work, and it was confirmed that the scouring resistance was higher than that when trapping driftwood.

 The overflow breaches in historical flood events have been investigated using a process-based model (Delft3D) in order to understand levee breach and inundation characteristics related to variations in river flood magnitudes. The model consists of shallow water equations coupled with secondary flow, grain-size distribution of riverbed and levee materials, and erosion of steep banks adjacent to wet cells. The model validation is carried out against a 200-meter wide breach of the levee along the Kinugawa River (Japan) in September 2015. The model is appropriate for the estimation of final breach width, morphological changes in flood plain, inundation depth, and flow velocity. Numerical experiments on levee breach and inundation by using river discharges different from the historical flood event show that the variations in flood peak discharge and flood duration have a significant influence on breach widening and deepening.

 A coupled human fluid numerical model was applied to transportion of a water accident sufferer in a channel with a weir-like obstacle on the bed. Simulations were conducted with 80 differrent combinations of water depth, flow velocity and weir height. As a result of the simulations, two different processes for getting over the weir were found. When flow speed exceeds 1m/s, the sufferer suspends in water and gets over the weir. Otherwise, if flow speed exceeds 0.5 m/s and weir’s height is less than 50cm, the sufferer gets over the weir in rolling on weir’s surface.

 The authors have formalized the energy loss coefficient for two-way and three-way circular manholes where one or two inlet pipes are connected to an outlet pipe. However, there are many variables to consider in a four-way circular manhole at a crossroad, which makes formulation difficult. Therefore, the authors first formulated a model for a four-way circular manhole under the condition that the flow rates of two opposed lateral inflow pipes were the same. In this paper, we clarified energy loss characteristics of a four-way circular manhole by changing the planar arrangement of three inflow pipes with different pipe diameters, and further varying inflow rates to the three inflow pipes. When the flow rate in the straight pipe was small, the energy loss coefficient for the lateral inflow pipes changed more than that for the straight pipe with the change of lateral flow rates.

 An intake vortex is occurred in an intake facility of a hydroelectric power station. It causes damages on a facility due to an air hammer and a reduction of power generation output. The researches have been conducted on a vertical intake of a hydroelectric power station. However, it is difficult to apply those findings in the vertical intake to other intake types such as horizontal intake directly. In this study, the hydraulic properties of horizontal intake are discussed experimentally. Suction velocities of horizontal intake pipe are changed by changing diameter of the pipe, with other conditions, ie, a flow rate, a water depth and a submergence depth are kept constant. As a result, it is found that the occurrence of the intake vortex is related to the distance between the upper edge of the horizontal pipe and the water surface and the swirl flow of the horizontal plane of near free surface is occurred at narrow regions near the left and right side of the endwall of the intake channel.

 Transient test-based techniques (TTBTs), in which hydraulic transients are used, are expected to provide leak detection methods at lower costs and less labor than other methods in pipeline systems. This study investigates the effectiveness of a leak detection method of TTBTs by using leak-induced reflective waves as well as optimization. In numerical simulations, a three-series pipe system that has varying pipe diameters along the pipeline is considered for an example of all pipes except for a single pipe. Subsequently, a method for analyzing the transient pressure using joint-induced reflective waves is proposed for the leak detection method. In a laboratory experiment conducted on a model pipeline with a helical structure, the leak position is detected with a high accuracy of 2% or less of the total pipe length in all cases. The leak detection results demonstrate that the proposed method is effective for analyzing the transient pressure, including the high-frequency components (or reflected waves) derived from the characteristic structure of pipes.

 Air entrained phenomenon can often occur in hydraulic structures, for example, head-tank spillway of hydro power plant, stormwater systems, and so on. When the air-water mixture flow is discharged to the submerged outlet tank, there is both shock and noise due to the instantaneous release of large bubbles; that is, the air hammer phenomenon occurs. In this study, in order to contribute to the quantitative evaluation of the air hammer phenomenon using computational fluid dynamics, the influence of the calculation grid on the size and velocity of the large bubble, the energy loss, and the pressure fluctuation was investigated. Furthermore, it was clarified that the calculation results on the two-phase air-water two-phase slug flow in a horizontal pipe, 100mm ID, was compared with the experimental results and that it could be reproduced accurately.

 If a submerged hydraulic jump has a large submergence, a high velocity flow below a sluice gate is located near the bottom. When the main flow lifts to the water surface from the channel bottom, at a certain stage, the main flow is periodically deflected. In this case, it might be difficult for swimming fishes to migrate upstream through the hydraulic structure, even if the fish passage is installed at the downstream of the structure.

 In this study, the installation of baffle blocks with diagonal upstream face at the downstream of a sluice gate was proposed in order to disappear a periodically deflected flow in submerged hydraulic jump. The experiments yield that the periodically deflected flow can be disappeared by optimal installation of baffle blocks. The artificially controlled flow has been characterized by time averaged velocity and the velocity with time series.

 The authors proposed the installation of roughness slope with stacked boulders for the protection of riverbed. This paper presents a comparison of energy dissipator between roughness slope with stacked boulders and abrupt drop. The experimental investigation on the formation below an abrupt drop yields that a hight velocity flow near the bottom continues far doswnstream. Also, the protection region predicted by design manual is short from the location of main flow in the jump. While, the experimental investigation on the installation of roughness slope with stacked boulders yields that the main flow lifts to the water surface in a short distance, because the approaching Froude number can be redused by the shape effect of stacked boulders. Furthermore, the installation of protection concrete blocks below the stacked boulders may support for the bed protection.

 The characteristics of the coherent structure formed backward facing step flow were investigated by using the flow visualization method and DPTV (Dye streak pattern Particle Trackin Velocimetry), and the correlation with the flow velocity distribution characteristics was examined. As a result, it was clarified that the logitudinal vortex structure formed downstream of the backward facing step has remarkable rotation due to the high shear layer, and its size increases and its inclination angle increases in the downstreram direction. In addition, it was revealed that longitudinal vortex structures are hardly generated near the bottom wall of the flow field. Furthermore, it is inferred that the longitudinal vortex structure directly contributes to the instantaneous Reynolds stress (-uv), and their form and scale strongly influence the characteristics of the long-time average distribution of Reynolds stress.

 We conducted a study using a 3D-type numerical analysis model on inlet type driftwood capturing facility. Since the specific gravity of the target broadleaf tree exceeds 1, it moves near the river bed with effect of the bed friction. In the numerical model, a bed friction term between the bed and driftwood is introduced, and the rolling movement of the driftwood is considered. We introduced a frictional force anisotropy model to reproduce the sliding and rolling motions. For the collision of driftwoods, the collision force was evaluated with the distinct element method (DEM). As a result of applying the numerical analysis model to the previous experiment by Kato et al. it was confirmed that this model can reproduce the driftwood behavior around the capturing facility in reasonable accuracy.

 Riverbed degradation occurs in many rivers. It damages river crossing structures, such as weirs and groundsills, with backward facing steps. If a large flood occurs in such a situation, it is feared that the protection-blocks will be washed away due to local scour downstream from the structures. We conducted a vertical two-dimensional numerical simulation of local scour and the protection-blocks destruction occurring downstream from river crossing structures. These phenomena are complex phenomena due to the interaction between flow, sediment and rigid body. Therefore, a coupling method of the Explicit Moving Particle Simulation (E-MPS) method and the Discrete Element Method (DEM) has been developed. The E-MPS method is one of the MPS method. To investigate the applicability of the present method to local scour and the protection-blocks destruction occurring downstream from groundsills, we compared the numerical obtained by the coupling method with those from the movable-bed hydraulic model experiment we conducted. Although there is a problem in the reproducibility of changes over time, the results show that the present coupling method has the almost applicability to these phenomena.

 While the point-particle DNS (PP-DNS) is a promising approach for simulating particle-laden flows, its applicability to cases where the particle mass and/or volume loadings are high is still less clear, since the size of the particles is quite small and hence datasets available for verification of the method remains to be very limited. Here the applicability has been examined with the help of a reliable database of recently performed particle-resolved DNS (PR-DNS). It is revealed that the PP-DNS can predict turbulence modulation by particles only qualitatively. It is also presented by the PP-DNS that the preferenctial concentration of small particles in turbulence is less sensitive to the turbulence modulation.

 Flooding with a discharge of approximately 4200 m³/s occurred in the lower Asahi River during the Heavy Rain Event of July 2018. The flood flow over the historic diversion weir was the first time ever observed after its renewal work. For flood control tasks, hydraulic engineers have examined the diversion discharge by laboratorial experiments and two-dimensional (2-D) numerical simulations. However, the flood impacts on the subsequent changes of both vegetation and riverbed conditions have not been fully investigated yet in the earlier works. Furthermore, the 2-D flow model commonly cannot allow for reproducibility of flows around the actual river section with a complicated bed geometry and heavily vegetated conditions. Therefore, a new 3-D numerical model has been proposed herein for flood flow simulation based on a porous model considering distributed vegetation establishment using airborne laser bathymetry (ALB) data before and after the rainfall event. Simulated results demonstrated that the proposed model performed well, compared with the existing 2-D model and field observations.

 With the flood disasters that have been occurring frequently in recent years, the sophistication of discharge observation has been promoted. Since the existing river monitoring cameras can be used, interest in image measurement methods is increasing. Among the image measurement methods, the STIV method is regarded as a powerful method because of its high measurement accuracy and robustness of measurement. However, depending on the shooting conditions such as rough weather, there are cases in which the conventional automatic analysis method produces an erroneous value, which is a challenge for establishing a real-time measurement system. Therefore, in this study, we tried to solve these problems by incorporating the deep learning method, which has been successful in the field of image analysis in recent years, into the STIV method. As a result, it was clarified that STIV method using deep learning is able to obtain correct results in many cases where the conventional method outputs anomalous values.

 Using U-Net, which is one of the semantic segmentation methods, we constructed a model for distinguishing floating objects (scum) at river surface from fixed point camera images, and evaluated the performance based on RMSE, missed rate, and misrecognition rate regarding water surface coverage. The discriminant model was able to reduce the missed rate sufficiently by learning a small amount of image data (about 100 images). However, it was necessary to train a larger amount of data to reduce the false recognition rate. Although the discriminant model was applicable to the images taken at multiple sites, the discriminant accuracy was greatly affected by the angle of view of the camera and the quality of the learning data. By applying transfer learning, it was possible to construct a model with relatively high discrimination accuracy by learning less image data than normal learning.

 Land cover classification (LCC) map in rivers is one of the important fundamental materials for hydraulic research works and practical tasks in terms of both flood control and water environment conservation. Recently, massive flooding because of heavy rainfall motivates hydraulic engineers to estimate the flow capacity at local river sections accurately by re-evaluating the LCC map of the cross-sectional area, vegetation height, and vegetation density, using useful remote sensing methods such as airborne laser bathymetry (ALB). For this work, we developed a new methodology for land cover classification (water area, bare ground, herbaceous species, arborous species, and bamboo grove) in rivers. We used an image processing technique of deep convolutional neural networks (DCNNs) for aerial photographs taken during ALB measurements in conjunction with the dataset of laser point cloud and vegetation height (digital surface model data minus digital terrain model data). We applied our method to two flights of ALB measurement data (in March 2017 and November 2017) for the 4.5 km-long lower reach of the Asahi River. Results showed that our method predicts the LCC map more accurately in case of support of the ALB dataset, especially for detection of herbaceous species, compared with predictions using aerial photographs only. However, if we use different season dataset for learning and prediction processes, classification performance is degraded largely for arborous species because of defoliation.

 Field observation of the electric conductivity in the borehole adjacent to the river was performed. Results of the field observation showed that the electoric conductivity was remarkably low at the limited 2 meter width in the borehole locally. Additionally, the measured electric conductivity showed fluctuation. Experiment for the examination of the fluctuation of the electric conductivity was performed. Results of the experiment showed that the fluctuation of the electoric conductivity was caused by inflow of 2 types of water with different electric conductivity.

 The purpose of this study is to propose a flow velocity value noise removal algorithm optimized for Riverboat, which is the most used in ADCP observation. Using the ensemble deviation flow velocity Δ𝑣𝑙, sensitivity analysis performed of the reflection intensity and correlation an optimal threshold was obtained by using the following method. In particular, when Δ𝑣𝑙 is 0.4 m/s, the effectiveness flow velocity value was higher than the previous method and the bias of the frequency distribution of the effective velocity values was small. The proposed algorithm increases the effective velocity, it can be used to clarify the uncertainty of ADCP observations and we could contribute to this. Since the proposed algorithm is optimized for Riverboat, it is possible to use for the rapid river in Japan.

 Surrogate bedload monitoring system measures acoustic or vibration signals caused by bedload impact, and the grain size and sediment transport rate are estimated on the basis of the signals. Although many methods have developed to convert the signal to the targets, the process is rarely regarded as signal processing. As various signal processing techniques have proposed, one is worth using them. Another problem is that many current regression models depend on site-dependent coefficients and therefore lack generality. Singal processing generally consists of three steps, denoising, feature extraction, and analysis. It is important to choose suitable techniques in each step for the target data. In this study, the goal is set to improve the model accuracy and generality in the analysis step. Here, the Gaussian process regression was chosen as a model considering its high expressiveness and interpretability. The model proposed based on flume experiments provided high accuracy in grain size and bedload transport rate estimate for field experimental data with using only signal, flow velocity, and water depth.

 Ground water dynamics are commonly grasped with “tracer experiment.” The objective of this study is to develop a new tracer with DNA. To do this, we conducted a basic experiment on the production of the DNA tracer and its effectiveness. On DNA analyses in environment such as underwater, it was impossible to use the uncovered plasmid DNA as the tracer because the real-time quantitative PCR analysis was conducted after the DNA extraction from the filtered material. However, it is revealed that if DNA is enclosed in liposome, DNA is easy to trap using filter, and not decomposed within 96 hours. In addition, it is clarified that the liposome enclosed DNA is usable as the tracer for groundwater dynamics because it showed the similar dynamics to the salinity when we conducted the groundwater penetration experiment using the pipeline with 1-m silica sand.

 In this study, we compared the topographic data and the sediment balance data obtained by 1) the cross-section survey and 2) the UAV Photogrammetry that applied the water surface correction to the shallow water area of the Shinji Lake in Shimane Prefecture in order to examine the effectiveness of underwater topographic survey using UAV/SfM-MVS in lakes and marshes. As a result, after removing the outliers, the average difference was 0.165m when the value in the water area of the DSM was not corrected by the water surface correction coefficient. In contrast, the average difference after the correction was 0.059m. In addition, the difference in sediment balance was within the range of 2 to 7%, and the tendency of sedimentation and erosion was similar in both methods. Therefore, it was clarified that the UAV photogrammetry is effective for monitoring sediment dynamics in shallow water of lakes and marshes.

 Aquaculture farms play a vital role for food security in Japan. The cultivation potential is directly related to the quality of the marine environment and therefore needs monitoring. A new and novel automated vessel named as ”Quadmaran” is developed for carrying out environmental surveys. In this paper, we clarify the feasibility of automated control method for course keeping and evaluate the sailing performance. Real sea tests are conducted to verify the efficacy of automated control system and the sailing performance of the vessel in wind. As an example of collecting data using Quadmaran, dissolved oxygen and water temperature are automatically collected and evaluated at the oyster farm in Nanao bay, Ishikawa prefecture, Japan.

 In order to make clear the occurrence and maintenance mechanism of deep scouring in the Kiso River, we analyzed the flow measured by ADCP at multiple floods. It was confirmed that the planform vortex became obvious interacting with the progress of local scour, and the planform vortices concentrated the flow inside a deep scour hole. Next, we focused on the vorticity as a hydraulic index for evaluating the magnitude of the planform vortex that maintains the deep scour. The effects of various measures were tested by numerical simulations, and efficient methods for backfilling were devised to reduce vorticity in order to prevent the expansion of deep scour.

 In this study, we used high speed video camera having high resolution to capture a great number of driftwood images per second for measuring driftwood velocity, and applied PIV (Particle Image Velocimetry) and PTV (Particle tracking Velocimetry) to surface flow velocity. The channel gradient, drifting length, and driftwood shape given to the relative velocity between drift-wood velocity and surface flow velocity were examined based on the motion equation of driftwood. Cylindrical driftwood be-comes faster than free surface velocity with increasing channel slope and driftwood length when the driftwood angle is small, and the product of driftwood length and channel gradient is proportional to the square of the velocity difference between driftwood velocity and surface flow velocity.

 The hyper-concentrated sediment laden flows have caused flood control, river environment and irrigation problems such as aggradation and clogging. Several researchers documented the effects of large concentrations of sediments on the fluid properties such as the viscosity, the density, and the reduction of fall velocity of bed sediment particles, the increase of sediment discharge. However, such a sediment flow feature has not been investigated in detail on the effects of suspended clay concentration on large scale vortex structure. In this paper, we experimentally investigated velocity fluctuations in hyper-concentrated sediment laden-flow in an open channel with two-dimensional square ribs by using Particle Image Velocimetry(PIV). The results showed that rheological properties significantly dissipated the large scale coherent vortex identified on the second invariant of the velocity gradient tensor over the bed with square ribs by comparison with the clear water flow.

 Triangular-shaped surface waves occur with large amplitude in a river during a flood. The large amplitude surface waves are concerned to cause damage to river structures. Higashikawa et al. (2019) found that the train of triangular-shaped surface waves occurs due to soliton resonance. They assumed that the propagation direction of the soliton resonance inclines to the flow direction when the modified Miles' solution is applied. In this study, we aim to derive theoretical solutions by modifying the hypothesis. As a result, our new solutions agree with the laboratory experiment results better than Higashikawa et al. (2019). Furthermore, we confirmed that triangular-shaped surface waves occur due to soliton resonance.

 This study reproduced an accerelted open-channel flow with contaminat section in a laboratory flume, and focused on a relaminarization phenomenon. Reductions of normalized Reynolds stress and turbulence intensity were recognized by turbulence measurement. Further, we considered a physical mechanism related to a turbulence energy production. It was found that a turbulence production term expressed by acceleration of mean streamwise velocity becomes negative through the acceleration section. This phenomenon could be explained considering the turbulence energy budget of generation and dissipation.

 In this study, a new quasi-two-dimensional estimation method for estimating flow discharge, cross-sectional velocity profiles, and low channel roughness coefficients in a compound channel with vegetation by continuous observed water level is developed. Experimental data on water level and flow hydrographs and roughness coefficients were observed for a compound channel with and without vegetation along a low channel. The accuracy of the estimation method was verified by comparing the estimation results with the experimental results. It shows that the quasi-two-dimensional estimation method can reproduce flow hydrographs, low channel roughness coefficients and velocity distributions in cross-sectional experiments with sufficient accuracy.

 The combination of impermeable and permeable spur dikes can create various flow patterns behind them. We call this type of spur dike “hybrid spur dike”. In this study, hydraulic functions of a vertically hybrid spur dike that upper or lower part of dike is made permeable were investigated experimentally. Flow structures and sand deposition behind the spur dike were measured under various ratio of upper and lower permeable part. The flow corresponding to the front permeability type is altered by vertical mixing and transverse vortex, and becomes to have three-dimensional aspect. Such flow becomes uniform vertically 3 times of spur-dike length downstream, and the velocity reduction becomes in order of permeable fraction. The configuration of sand deposition well corresponds to the flow structures.

 Numerical experiments were conducted about turbulent flows over roughened open-channels with different aspect ratios (b/h = 0.36 ~ 1.38) and relative depths (h/r = 3.63 ~ 12.00). The time-averaged flow over a bottom roughness element collides with the adjoining roughness element, and the pressure increases in front of the roughness element. It is clear that the sum of pressure and gravitational term of the Reynolds equation exerts a force toward the valley in cross-section near the valley to drive the flow sinking into the valley, and exerts upward force in cross-section near the top of roughness element to drive the flow over the roughness element. Secondary flows change longitudinally and cross-sectionally when outer secondary flow and bottom secondary flow merged with these vertical flows.

 This study proposes a numerical simulation model to integrate prediction for the occurrence of landslides, runout process of soil blocks released by landslides, and erosion-deposition process of the sediment in a channel network. The simulation model was applied to the upper reaches of Souzugawa river in Hiroshima prefecture during heavy rainfall in July 2018, to investigate landslides and their runout process. The simulated result shows that immediately after the occurrence of landslides, the sediment deposition takes place in 2nd order to 4th order channels, and after the end of rainfall, the deposition takes place in the 4th order channel. The result indicates that sediment produced by the landslides is stored in the channel network and after that runout with the flood.

 Prediction of erosion and sedimentation in ungauged basins at the northern coast of Peru is a critical issue to mitigate flood disasters. In this paper we developed a spatially-distributed integrated hydrological and sediment model based on kinematic wave equations to carry out sediment transport simulations using spatially distributed satellite rainfall data. We compared our simulation results against the collected field sediment samples from the 2016 flood at Tumbes River, Peru. The results show that our integrated model performs with robust convergence at 500 m and 1,000 m resolutions and successfully reproduced a hysteresis discharge-sediment concentration relationship according to historical time series analysis. Afterwards, the influence of topographic resolution on sediment parameters was assessed. We identified the most sensitive parameters and its effects on sediment concentration and sediment yield output. For a comprehensive analysis of DEM resolution dependence, we discuss how settling velocity and particle diameter drive the results convergence.

 We conducted flume experiments to investigate the mechanism of the concentration of coarser particles at the debris flow’s front with various particle-size distribution. In our experimental results, as the particle sizes of the material became coarser, the flow distance became longer or the flume gradient became lower, the concentration of coarser particles at the flow’s front progressed more remarkably. According to these results, a falling of finer particles through the interstice between the materials in the flow’s interior may explain the sediment sorting and the concentration of coarser particles at the flow’s front. We developed a numerical model to describe the concentration of coarser particles at the flow’s front by the above manner based on the conventional one-dimensional model. Our model could explain temporal changes in the percentages of all particle-size in the flow’s interior on various particle-size distributions of the materials, several flow distances and various slope gradients.

 Methods to evaluate erosion rate of bed sediment composed of very fine materials by means of entraiment velocity are discussed in the present study. It has been revealed that entraiment velocity depends on the Richardson number. To investigate the applicability of entraiment velocity on the estimation of erosion rate of bed sediment, flume experiment is conducted using very fine sediment; pearl-cray sand. As a result of experiment to investigate kinematic viscosity of the mixture body of water and pearl-cray sand, it has been revealed that the kinematic viscosity significantly changes at the concentration around 18-20%. As a result of flume experiment to investigate entraiment velocity, though the entrainment velocity is much less than the values obtained from previous researches in the cases that cohesion is dominant, in the cases with less concentrarion, the results are consistent with previous researches.

 Presents study evalueated the formula for equilibrium suspended sediment concentration at reference level using observed results in an actual river. The three models, such as Ashida-Michiue equation, Lane & Kalenske model and the model proposed by Harada et al., were evalueated in this study. Observed results were selected from the one in Buramaptra River conducted by Authors using ADCP, sediment sampling and others. The behaviour of the suspend sediment are non-equilibrium state. On the other hand, most of the models were constructed under equilibrium state. In order to evaluate the models, the authors carefully selected the observed results, which are reached to the equilibrium state.

 With in situ sediment samplings, washload and suspended sediment were discriminated. Suspended sediment were composed with the sediments, which exist at the river bed. Washload is something which are transported from outside. With this understanding, the sediment with more than 48 μm were selected as the suspended sediment and they were used for evaluation. Concentration of 5.0×10^-4 [m³/m³] was mixture of washload and suspended sediment, and 57% was consideded as suspended sediment.

 Each model estimated the consentation as order of 10^-1 by Ashida-Michiue formura, 10^-2 by Lane & Karenske formura, and 10^-3 by Harada et al. The one with Harada et al. is closest among three, but it still overestimated.

 Present study discusses the structures of flows with bed-load, suspended load and hyper-concentrated sediment load, and formulates their velocity profiles, thicknesses of bed-load layer and sediment concentrations. The entrainment velocity, which is employed in specifying the mixing process of density stratified flow, is introduced in evaluating the sediment transportation from bed load layer to the upper turbulent layer to formulate an equilibrium sediment concentration and the relative depth in between bed load layer and the upper layer. Such formulations realize that the mud flow process, which forms flows with a hyper sediment concentration, is explained successfully, using flume data which were carried out in 1990s.

 In this paper, thought experiment and flume experiments have been taken in order to indicate the characteristics of sediment transport and flow distributions. Especially, the mechanisms that the sediment movement take influences to flow structure are focused on.

 PIV by using fluorescent pigment as tracer can measure flow velocity distribution in the condition that water flow including sediment transport. And I measure the sediment particle motion by PTV. These data give informations about characteristics of flow velocity distribution near bed. Flow velocity change at the depth of the water whole, and Reynolds stress decreases under the case that flow include suspended sediment.

 The numerical movable-bed experiments were conducted to investigate the effect of particle shapes on the bed load motion.The river-bed materials used in the numerical experiments were three kinds of particles with the same volume and different shapes (Thin, Rod, Sphere). As a result, while the effect of particle shape on the saltating motion was small, the effect of shape on the bed load incipient movement was large. It is considered that the posture and arrangement of the particles on bed surface caused the difference in the incipient movement. It is also formed that the numerical movable bed experiment is an essential technique for understanding the motion of gravel bed particles.

 In this study, we investigated the hiding effect of larger gravels on smaller sand which occurs on the riverbed containing boulders. Riverbed materials can be classified into three grain-sizes groups of sediment: boulders which cannot move at all, gravels which move on riverbed as bedload, and fine sand which move as suspended load. These sediment groups are called L-, M- and S-particle. L-particles exert hiding effect on M- and S- particles. As well, larger M-particles exert hiding effect on smaller particles. We conducted the experiments under the condition that sediment which transport on riverbed is composed of two grain sizes particles. From the results of the experiments, a formula expressing the hiding effect of larger gravels was derived. The other experiments were conducted under the condition that movable sediment is composed of three grain sizes particles. Then we investigated the applicability of the hiding effect formula of larger gravels.

 Supplying finer sediment to the gravel-bed river can cause the selective sediment transport, and the bed variation is accompanied with the porosity variation and affected by sheltering effect of the immovable sediment. When selective sediment supply such as the sediment replenishment from the reservoir is planned, the simulation model that express such phenomena is necessary to evaluate the riverbed variation with the impact on environment of downstream river. In this study, the porosity-bed variation model is expanded to express the riverbed variation with the variation of exchange layer. Furthermore, the model is applied to the field experiment of sediment supply carried out in the Yahagi River, and is confirmed the availability.

 In the actual river, the shale bed with consolidated clay is exposed and eroded, which causes problems inriver management. There are few studies on clay bed, and the change process of clay bed has not been clarified. In this study, we aimed to clarify the deformation process of clay bed based on the experiment was conductedusing the straight channel and the curved channel. Previous studies have revealed that when bed load is transported to the clay bed, a mixed layer is formed. However, there are many unclear points about the formation process of the mixed layer. Therefore, we focused on the formation process of the mixed layer.

 In September 2018, a large number of slope failures occurred in the Atsuma River basin due to the Hokkaido Iburi Eastern Earthquake. The main channel morphology is alternate sandbars in the middle and lower reaches of the Atsuma River having a gentle gradient. In this study, we conducted flume experiments to know the effects of silt on alternate sandbar morphology composed of sandy materials in the Atsuma River. Results showed that the wavelength and wave height of sandbars were affected when the rate of silt increased by more than 10% compared to the volume of equilibrium bed load. Sandbars almost disappeared when the rate of silt exceeded 30%. In addition, silt deposited locally where the boundary areas between sandbars and low flow channel. This is because silt is transported as suspended load on sandbars so that it hardly deposit there. While, at the boundary area, silt is captured by the water, flows from the front edge of sandbars to low water channel, as well as bed load material.

 At Typhoon No. 19 in 2019, at the site where the embankment was breached in Tokigawa River, it has been confirmed that a stratified soil layer was exposed around back side toe of embankment. The harderodible soil layer (here after HEL) may promote erosion in the upstream direction by preventing downward erosion, and may accelerate the time to breach. Therefore, the objective of this study is to clarify the effect of vertical position of HEL on local scouring around toe of embankment by hydraulic experiments.

 In the experiment, a total of 15 cases were carried out. By examining the residual sediment height, zi around the backside toe of embankment, the conditions under which erosion easily proceeds in the upstream direction were elucidated. As results of the experiment, it was found that the residual soil height ratio, zi/^zwo became smaller as the installation height of HEL became shallower. In the case where the height of the HEL is 1/4 ^zwo and 2/4 ^zwo, zi/^zwo became minimum when h1/H is 0.175.

 Recently, record breaking heavy rain have continued one after another due to the extreme weather conditions, causing enormous inundation damage throughout the country. Levee breach is one of the disasters that causes a great amount of flood damage, as in the case of Typhoon No. 19 that occurred in 2019. Therefore, it can be said that elucidating the mechanism is important for disaster prevention. So far, the authors have examined the simulated levee composed of clay, sand, and gravel by basic experiments for storage of water in a river. At that time, we have examined the effect of clay and gravel on the breach mechanism by changing the respective content ratios. In this study, the influence of the river flow on the levee breach process is examined by conducting a levee breach simulation.

 This paper presents a new type of check dam that has a convex steel frame to the upstream side. To examine the effects of trapping debris flow of the proposed dam, a new numerical simulation was developed that was capable of predicting the interaction of debris flow including driftwood and the steel frame. The numerical simulation of the debris flow occurred in Hiroshima in 2014 was performed by the developed model where the debris flow impacted against three dams of different types (a closed-type dam, an open dam with a straight frame and an open dam with a convex frame). The results of the simulations showed that the check dam with the convex frame trapped roughly 30% more debris flow than the colosed-type dam did. It was confirmed that the convex frame of the check dam was an effective structure for trapping debris flow.

 The rapid risk assessment of a landslide dam requires the understanding of the important factor in predicting the scale of flood caused by a landslide dam failure. The failure modes of a landslide dam are classified into three types of overtopping failure, slip failure and progressive failure. We analyzed the factors influencing the scale of flood due to a slip failure of a landslide dam by the numerical simulation. The results show that the larger reservoir water volume and the smaller erosion resistance of the dam material is, the larger peak discharge of the flood occurs. Furthermore, the results suggest that the long mass movement caused by a slip failure does not necessarily lead to an increase in the peak discharge of the flood. If a landslide dam is composed of high erosion resistance material, the long mass movement due to a slip failure tends to increase the peak discharge of the flood.

 Suspended sediment deposits around foot protecton of river levee, and promotes growth of vegetation. The vegetation does not just provide the living place for habitat, but has possibility to increase the resistance of foot protection to the running water. In this study, deposition of suspended sediment around foot protection block was analized by the hydraulic experiment and numerical simulation. And effective block arrangements which promote deposition were devised by the simulation.

 The result shows that setting block group at regular intervals cause more widely deposition of sediment than setting blocks continuously to the river flow direction. In curved section, the block arrangements influence opposite suspended sediment.

 In this study, we focused on the fact that a weir with an opening activates the fluidity of the sediment compared with conventional continuous fixed one and examined the removal function of the sediment deposited upstream of the weir. Experimental results showed that the weir with the opening increased the fluidity of the 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. In the gradual flow section, the descent of water level and river bed increase linearly as the opening depth increases. In addition, it was shown that the local scour shapes upstream and downstream of the weir were similar to each other as a result of normalization on the representative scale of vertical and transverse shapes.