A 3D-3D coupling model was constructed for simulating the jamming process by the driftwood around bridge piers. The model of water flow was URANS type, and the driftwood was expressed as connected spheres. The collision between driftwood was evaluated by the discrete element method. Computations were carried out under the same conditions as the previous experiment. We employed the modified driftwood Richardson number (DRI) as a dimensionless number governing the three-dimensionality of driftwood jamming. Computational results showed that the capture ratio increases and the sink ration decreases with the increase of DRI. The tendency was in qualitative agreement with previous experimental result. However, the present computation overestimated the three-dimensionality of driftwood jamming.

 In recent years, river survey with Airborne LiDAR Bathymetry (Hereafter, ALB) have been growing, and the utilization of three-dimensional data for river management is expected. This paper shows the possibility of improving the accuracy of longitudinal survey obtained only by point cloud to about ± 1cm by appropriately processing the systematic error of ALB data based on the purpose of river management and characteristics of equipment. And, this paper verifies the effect of the missing rate of ALB data on hydraulic analysis in mountainous rivers, and as an example, the mesh size of the ground height required to reduce the relative deviation of the calculated water level to about 10%

 Non-contact type of measurement methods with support of cost-efficient devices and improved algorithms for flow analysis have been actively developed for river discharge observations during flooding. These updated methods are expected to be used widely in practical works for both river channel planning and disaster reduction. We conducted flow measurements using a space-time image velocimetry (STIV) method at Nakano Observatory in the Ota River during the heavy rainfall in July 2018. Results showed that the discharge values obtained every hour using the existing H-Q rating curve were overestimated by approximately 10 percent, compared with those using the present STIV method.

 This study aims at examining the accuracy of the present STIV method for river discharge estimation during the flood, using numerical simulation results and water level observations. First, we obtained the data for both the topo-bathymetry and vegetation properties within a targeted reach by airborne laser bathymetry (ALB) measurement results. Secondly, we conducted numerical simulations of the flood flows using the discharge hydrograph of two kinds: the rating curve and the STIV method. Lastly, we compared simulated water levels with observations during the flood. Results demonstrated that the STIV method provided more accurate discharges, compared with the rating curve.

 Accurate measurement of river discharge for complex and shallow bathymetries needs sophisticated monitoring systems. On the other hand, observation site in most cases is considered as the most important challenge that must be resolved, otherwise additional uncertainties and errors will be introduced to discharge estimates. In this work, author tested a new version of the Fluvial Acoustic Tomography (FAT) system operated by high-frequency 53-KHz underwater acoustic transducers. Monitoring site, however, was very stringent and fixed. Consequently, arrival times data recorded by the FAT system was highly scattered which impaired the quality of discharge estimates and had produced imperfect results.

 In order to overcome this challenge, authors used a straightforward imagery solution, to retain only the desired data and eliminate outliers using image subtraction feature. The results showed that streamflow estimated by FAT had very good agreement with discharge estimated by the Rating Curve (RC) method. Also, it can be deduced that imagery methods are very powerful in eliminating noisy data when employed to time-series analysis.

 In recent years, high precision discharge estimation methods, such as ADCP, have been actively developed with the advancement of observation equipment in Japan. However, it is rather difficult to apply it to rivers throughout the country. Therefore, it is considered that discharge measurement by the float method will continue in the future. The float method has problems such as (1) the validity of velocity correction coefficients is low, (2) the difference between traced streamlines of floats and survey lines, (3) the resolution with the cross-sectional direction of the flow velocity is low. Therefore, improvement of discharge estimation accuracy is desired.

 In this research, we examined a discharge estimation method that combines numerical float model and DIEX (Dynamic Interpolation and Extrapolation) method, for which improvements in discharge estimation accuracy have been identified for the problems of (2) and (3) in the past research. As a result, this method shows sufficient discharge estimation accuracy of about 1% estimation error in small scale flood and large scale flood cases. This method is considered to be very useful in practice because it can be easily calculated if there are data required for general flood flow analysis and observation results by floats.

 The space-time image velocimetry (STIV), which can calculate surface flow velocity distribution from video images of river flow and calculate flow rate, has become increasingly known in Japan and abroad. To enhance the usage of STIV, the maximum entropy method which can reproduce vertical velocity distribution including velocity dip, is applied to estimate flow discharge with reasonable accuracy. The result applied to the snowmelt flood of the Shinano River showed yielded improved data over the conventional technique. In addition, streamwise turbulent velocity intensity distribution for a range of about 1km of the Shinano River was obtained successfully by analyzing local texture gradients appeared in space-time images (STIs).

 In an open channel turbulent flow, such as a river flow, its free surface deforms dynamically and shows various behavior depending on hydraulic conditions. It is known that free surface in a river flow behaves as gravity waves and coincidently shows a turbulence-like feature. However, it is not fully understood that how the dominant behavior of free surface in a river flow changes depending on flow conditions. In this study, time-depending profiles of free surface in river flows were reconstracted from imagery shot from a riverbank and Fourier spectral analyses were performed using that data. As the results, it was made clear that the free-surface behvaior in a river flow consists of superpositions of gravity waves and flow, also of stationary waves that are the results of the interacion between the flow and waves. Additionally, the contribution ratio of the flow to free-surface fluctuation are found to decrese in proportion to wave length of the stationary waves or squares of the Froude number.

 Sediment transport monitoring in mountainous regions is useful for integrated sediment management in sediment routing system and planning countermeasures for reservoir sedimentation problems. Recent monitoring devices including a hydrophone and a turbidity meter have successfully applied as indirect sediment transport monitoring techniques. Conventional hydrophone is made of a steel pipe, and in a mountain stream with heavy sediment run-off, the pipe is easily damaged by gravel or stone impacts, and it is often the case that reliable data can not be obtained for long-term observation. Therefore, some of the devices today employ a steel plate instead of a steel pipe, so that this works against a high-impact collision. The use of the steel plate type devices, however, are still in a study phase because the nature of their data and methods to convert their outputs to bedload transport rate are yet unclarified. This study aims to compare behaviors of the pipe type and plate type hydrophones by analyzing field experimental and real observation data by both devices installed in the same location, and to discuss applicability of the method to calculate bedload transport rate developed for the pipe type device, to the data obtained by the plate type one.

 In recent years, heavy rainfall disasters have occurred frequently due to the occurrence of linear precipitation zones, and the monitoring of high spatial resolution of rainfall is indispensable for understanding and predicting heavy rainfall. In this study, for the purpose of constructing a rainfall monitoring system with an on-board rain gauge using an optical rain gage, we verified the practicability of the optical rain gauge. The demonstration experiments with artificial rainfall and actual rainfall on the roof revealed that the dome-shaped optical rain gauge can be used as a quick-response rain gauge that is an alternative to the tipping bucket rain gauge. In addition, as a result of mounting the optical rain gauge and anemometer on a vehicle and simultaneously monitoring relative wind speed and rainfall, it revealed that accurate rainfall measurement is difficult under the condition of high reltive wind peed while traveling.

 Observation sensor of three dimensional flow direction and velocity was developed. The sensor has been validated for use in high-speed groundwater flow. Sensitivity of the sensor was calibrated by laboratory experiments. Observation results of the developed sensor showed The sensor was comfirmed to have good accuracy by comparison with the previous method.

 Turbulent flow measurements were conducted in a vertical cross-section for open-channel flow displaying a large water surface deformation. The channel bed roughened by sparsely distributed strip roughness and relatively steep flow conditions were examined. For turbulence measurements, optical flow techniques developed in the field of computer vision, were first used in such open-channel flows after evaluating the accuracy using particle simulation images. It was confirmed that optical flow technique is capable to use for such turbulence measurements and turbulence generation is significantly suppressed for flows with large surface deformation.

 This study examined a method of machine learning to classify a land cover state in UAV river aerial images taken at two different timings. The UAV images with different vegetation states were obtained in a 2-km section of Kurobe River in 2017 and 2018. The method combined machine learning and clustering in three stages. At the initial stage, machine learning results created by using images at one timing were used for land coverage classification of the images at the other timing. At the subsequent two stages, vegetation classification was performed by machine learning with clustering and the histograms of feature quantities, respectively. A random forest was used as a machine learning algorithm, and a K-means method was used for clustering. As a result of the method, classification accuracy finally became about 0.9 in F value. The result confirmed that the present method of combining machine learning and clustering could properly classify a land cover state in the river channel with the different vegetation states.

 In this study, we used metabolome analysis technology to investigate the intracellular dynamics of cyanobacterial groups that form blue-green algae. Specifically, we took samples of the blue-green algae formation from the water surface and at a water depth of 30cm in the morning and afternoon and at night, and conducted metabolome analysis on these samples. The analysis results showed variations in the metabolite were consistent with differences in the water depth (water surface and at a water depth of 30cm). This is also reflected in the sugar-nucleotide that forms the base of biofilm; hence, the low amount of extracellular polysaccharide is considered to be a reason why blue-green algae do not form within the water.

 In saline aquaculture ponds in the tropics, intermittent rainfall during the rainy season can cause salinity stratification, and strong solar radiation can cause thermal stratification. The combination of these factors may result in thermohaline convection in the ponds. Thermohaline convection in a closed saline water body can in turn induce heat storage in these ponds. We investigated how heat storage in experimental saline aquaculture ponds was influenced by the variation in the weather and water properties during the rainy season. If the pond stratifies into three layers with a salinity gradient in the middle layer, heat storage can occur. The salinity gradient prevents heat from escaping from the lower layer by effectively insulating it from nighttime radiative cooling. When the salinity gradient is perturbed by rainfall, solar radiation, or wind, the heat stored in the pond can begin to dissipate. We explain how the stratification stability and the heat balance in the ponds account for the occurrence of heat storage in the ponds.

 Selective withdrawal (SW) systems regulate the temperature of water released downstream of dam while vertical curtains (VC) prevent phytoplanktons from the shallow upstream reaches to disperse into the main reservoir. These water control facilities can affect the reservoir’s temperature distribution. Through long term analysis of Ogouchi’s temperature data together with numerical simulation, this paper explains the variation in temperature profiles during the three distinct periods in the reservoir’s operation namely, (1) deep penstock withdrawal (DPW), (2) purely SW and (3) combined SW and VC. Schmidt’s Stability Index (SSI) and Thermocline Strength Index (TSI) were computed to define thermal stratification. DPW promotes mixing and makes the reservoir generally warmer while SW creates a strong thermocline (high TSI) and when combined with VC, increases thermal stability (high SSI). Using 3-D hydrodynamic simulation, the conditions were successfully replicated. Results showed that outflow location and facilities largely affect the temperature and velocity profiles and the mixing and transport of river water within the reservoir.

 As the basis of design method for facility scale of bubble circulation as the countermeasure for water quality preservation, simple index is theoretically investigated. Firstly, simple dynamic model for cyanobacteria is proposed based on some simple assumptions including far field horizontal density flow induced by bubble plume. Secondly, by using some assumptions, theoretical required simple index equation is derivated under steady state condition for preventing harmful cyanobacter. Finally, we analyzed required facility scale under the some parameters corresponding to actual engineering and natural conditions. The required theoretical facility scale agree with empirical value being successfully suppressing cyanobacteria in actual reservoirs in spite of some assumptions. Furthermore, the effects of some conditions such as the countermeasure managements as well as natural conditions on the facility scale are discussed. These results can provide useful scientific basis for actual dam managements of eutrophicated reservoirs.

 In most of current conventional tsunami numerical models, the steady flow friction law is usually used for accessing the tsunami-induced bottom shear stress. However, we have been theoretically proved that the the steady flow friction law was not valid in almost the entire model domain from the tsunami source, it was the wave friction law instead. Therefore, the estimated bottom shear stress by steady friction law is inaccurated. We have proposed a new correction method for the steady tsunami-induced bottom shear stress. In this study, A numericall implementation of this correction method into the one-dimensional shallow water equation model of tsunami propagation is presented. The new correction method is called as the Method 1 and the traditional method is the Method 2 in this study. The Method 1 is calculated by the iteration process. As a result, the comparison results indicate that the simulated maximum tsunami wave height and tsunami-induced velocity by the Method 1 and Method 2 were had almost similar results. However, the bottom shear stress estimated by the Method 1 was obtained about 10 times higher than the Method 2 at the tunami source area and the difference decreases as the water depth decreases. Two methods are coincided from the location of water depth around 6m to the shoreline.

 Laboratory experiments were conducted to understand how a tsunami-like solitary wave runup can be reduced in one horizontal dimension against the effects of the slope and forest width in cross-shore direction. The wave height to depth ratio from 0.1 to 0.4 was used. The resultant transmitted wave height and maximum runup for the plane slopes of 1/4 and 1/7 were measured and compared with the forest on the same slopes. The dimensional analysis was used to pick up important parameters for comparing the maximum runup effect. The runup can be reduced to 28% by increasing forest width in cross-shore direction. The damping factor for the wave runup without the forest model can be increased from the range of 22% to 75% up to the range of 31% to 98% with the introduction of forest width for non-breaking waves. Thus, a forest on gentle beach slope helps to increase more resilience against tsunami attack even when energy reduction at the beach slope is not sufficient enough.

 After the Great East Japan tsunami, a multiple defense system like a double embankment system was proposed to mitigate tsunami damage. In this study, flume experiments on a double embankment system were conducted with reference to sea embankment structure planned in Iwate Pref., in which the structure of the landward embankment was varied. The relationship between the flow condition and the energy reduction were investigated in detail. The energy reduction rate is not much affected by the structure itself when the hydraulic jump occurs in between the embankment. When there is no hydraulic jump in between the embankment, the energy reduction rate sometimes decreases due to the structure of the landward embankment. The forest on the top of landward embankment is important for energy reduction. As the tree trunk breakage or overturning could decrease the effectiveness, the critical breaking condition was analyzed. The landward embankment planned in Iwate Pref. is found to keep the mitigation function by the second embankment with the forest on top for the Level 2 tsunami.

 Tsunami mitigation strategy has been changing from single to multiple defense systems following the 2011 Great East Japan tsunami. For enhancing the mitigation capability of tsunami defense structures, a hybrid (combination of natural and artificial method) defense system consisting an embankment model followed by a forest model was investigated in this study, in which a horizontally double-layer forest model (HDLM) was proposed to strengthen a finite width forest. To elucidate the effectiveness, a single-layer forest model (SLM) and different arrangements of HDLM were tested against the overtopping flow. Results showed that different types of hydraulic jump were formed in the hybrid defense system and energy reduction of the overtopping flow increased by 16-65% compared to the single embankment model. Moreover, the hydraulic jump position was controlled within a specific area in the combination of embankment model and HDLMs and effectively reduced the flow velocity within the defense system as well as reduced the energy downstream.

 The coastal structures and coastal forests in large area of the Tohoku and Kanto districts of Japan were destroyed by the Great East Japan Earthquake tsunami on 11 March 2011. The local scouring induced by tsunami overflow caused the destruction of coastal embankment. Mitigation of the local scouring at landward toe of embankment is required to prevent destruction of coastal embankment. The objective of this study is to investigate the effects of overflow depth, embankment height and vegetation thickness on overflow pattern and local scouring at landward toe of embankment by hydraulic model experiment.

 The boundary of incomplete overflow and submerged overflow was confirmed more clearly using vegetation thickness. The depth of local scouring at landward toe of embankment becomes small in case of submerged overflow.

 Recent research has shown the effectiveness of a vegetation patch (V) upstream of a moat (M) then embankment (E) structure in reducing the fluid force of a tsunami wave. However, according to the author's knowledge, research on the behavior of tsunami generated large driftwood (TLD) over these hybrid-designs does not exist. This paper, therefore, elucidates the behavior of TLD with relevance to these hybrid-designs in terms of the retaining function, change in debris’ velocity, and general movement pattern of a model debris. This research showed that in the case of VEM, approximately 32% reduction in the debris peak velocity could be achieved. Moreover, in the case of VME, by approximately decreasing the maximum flow depth downstream of this hybrid-defense structure by 50%, a reduction in the moment by impact force can be achieved. This reduction in impact moment can be significant in terms of, for example, inland vegetation implemented to trap debris.

 The 2011 Great East Japan Earthquake and Tsunami induced such intense devastation on many coastal regions and among them, the Naruse and Kitakami River mouths are the areas where their sandspits had not been recovered for a long period. This study, by analyzing aerial photos, has learned that the sandspit were vanished after being hit by the tsunami. Subsequently, waves were able to propagate further into the river mouth and sediment deposition was observed at the areas roughly hundred meters to 1 km upstream from the river mouth. Afterwards, the recovery of the sandspits has reflected similar process which sediment advances seawards. This phenomenon is observed in either the Abukuma River mouth in which the area was also damaged by the tsunami or the Mogami and Koyoshi River mouths where the jetties were constructed. Based on this study, the expected time for these areas to recover is evaluated as well.

 Sand spit elongation is remarkable at the mouth of Ly Hoa river, located in northern Vietnam, and sudden migration of the river mouth has sometimes been induced by sand spit breaching. In order to understand the frequency of spit breaching and subsequent spit reformation process, a series of Landsat and Google Earth images which has been accumulated since 1976 were analyzed to clarify the characteristic change of the estuary topography. In order to quantitatively investigate such characteristics, the tip coordinate and the area of the sand spit on the left of the estuary were calculated. From the satellite analysis, it is found that the river opening has been moved consistently to the down-drift direction, and its traveling speed has reached 90m/year. In particular, the area of the sandbar has increased remarkably, and the increasing rate is almost constant. Using this value, the rate of sand volume intruded into the river mouth by waves was estimated and it is almost consistent with longshore sediment transport rate along the surrounding sandy beach.

 Spatio-temporal variations of the absorption coefficient and backscatter coefficient of seawater in the Japanese open coastal sea were investigated based on the MODIS data. The average backscattering coefficients over the whole period were large in the coastal sea of Hokkaido such as Ishikari Bay and off Tokachi coast, and small in those of off Tottori and Misawa coasts. EOF analysis results have shown that the first mode is dominant (contribution ratio is over 80%) in the southwestern Pacific coastal sea, and the backscatter coefficient mainly increased around the large river estuaries during the flooding period. The relationship between the backscatter coefficient and the absorption coefficient demonstrated different characteristics in Pacific Ocean Southwest, Pacific Ocean East and Hokkaido coastal seas.

 Field studies in the DinhAn branch of the Hau River, belonging to the Vietnam Mekong Delta, were conducted at spring tides (4 March 2018 and 21 to 22 April 2019) to examine the vertical and longitudinal salinity distribution along the DinhAn branch as well as the vertical salinity distribution at two typical cross-sections. The measured data were used to classify mixing and stratification of the DinhAn estuary during a tidal cycle. The results indicate that along the Hau River (1) the salinity intrusion length at flood tide is 15 km longer than that at ebb tide and (2) the maximum salinity appears at the river mouth and irregularly reduces toward upstream during flood tide. Moreover, at the cross-section far from the river mouth 22km, (3) the peak of salinity lags one hour after the peak of water level and (4) the maximum salinity during one tidal cycle occurs at ebb tide. Finally, all three stratification parameters, including Pritchard, Froude, and Richarson numbers show that the partial mixing and moderate stratification condition prevailed in the DinhAn branch during a tidal cycle. These results are useful in operating the salinity control gates in terms of timing and duration.

 The object of this study is to predict the distribution of cesium deposited on the bottom of Tokyo Bay by means of the Hybrid Box Model linked with a diffusion model, released by Fukushima Daiichi Nuclear Power Plant. On the basis of the load fluxes from rivers, the distribution of concentration (separately for dis-solved cesium and small and large particulate cesium) in seawater and the mass of cesium sedimentation to the bottom of the bay were calculated by the models. Results were compared to the published observation data in order to examine applicability of the used models. Estimated cesium concentrations in the sediment show good agreement with the observed data.

 In recent years torrential downpour occurs frequently due to the effect of climate change. Because of this, excess of high turbidity in reservoirs and long term persistence of turbidity has recently been regarded as a serious problem. Behaviour of suspended sediment is highly influenced by characteristics of reservoirs and of its catchments, such as turn over rate and erosion severity.Hence, this study conducted future prediction of turbidity in 30 reservoirs in Japan by applying vertical 1-dimension hydraulic model.Change in air temperature and solar radiation were considered as a climate change effects. Result showed increased annual SS outflow in future scenario by 16.8%. Our result also indicates that effect of climate change might depend on the condition of thermal stratification of reservoirs. In circulation period, effect of climate change might be significant as inflow depth is unstable in the period.

 Internal solitary waves are likely to be excited when eternal forces are given, such as wind or tide. In the case where the thickness of pycnocline is very thin, the classification index is successfully proposed in the previous study. This study thus investigates whether the classification index can be applied or not even when the thickness of pycnocline becomes thicker by using a three-dimensional hydrodynamic model, Fantom. As a result, there are some differences due to the increase in the thickness of pycnocline. However, they don’t change their specific breaking forms, so it is concluded that the classification index can be applicable when the thickness of density interface changes.

 Marimos, natural monuments of the country, are in danger of extinction, but the mechanisms of the formation of Marimos has not been understood. To clarify the mechanisms of the formation of Marimos, it is needed to estimate the rotation angle of Marimos due to wind waves and the critical velocity at which Marimos start moving. Therefore, we developed the Marimo model that consists of two different phase models, liquid and solid phases, by coupling a three-dimensional hydrodynamic model (Fantom) and a Distinct Element Method (DEM). The Marimo model has improved the reproducibility from the comparisons with laboratory experiments. The critical velocity is found to be the function of the Marimo radius, density and the bed conditions. In addition, it is revealed that random wave is one of the most significant factors for the rotation of Marimos.

 Red tides occurring in the coastal area has a tremendous negative impact on the fishery industry. The occurrence of red tides is strongly affected by the coastal environment. In the Ariake Sea including Isahaya Bay, red tides occur frequently. Although continuous observation of water quality has been conducted for other years in coastal area, the forecasting model of red tides has not been constructed yet. In this study, using big data mesured in Ariake Sea and Isahaya Bay, we tried to make a phytoplankton prediction model from SVR, RF, and GBRT of machine learning. As a result, there was no problem in performance if the data was divided into learning and prediction data at random. However, when the data was divided in time series, the Chl-a fluctuation characteristic in the prediction data was not included in the learning data, so it was confirmed that the reproducibility of the time series model is poor.

 An intake vortex is entrained air from a water surface, and it causes problems in a hydropower facility due to equipment failure. In a previous research, the authors found that an increase of a velocity in a middle and lower layers of a flow field may be related to an occurrence of an intake vortex. It maybe a key to clarify a cause of an intake vortex. In this study, inner diameters of intake pipes are changed under a condition that a flow rate and water depth are kept constant, and examined how the flow field changes. As a result, it is found that an occurrence of a large vortex such as an intake vortex is related to a number of occurrences of a small vortex and so on.

 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. This study is a numerical investigation for pressure fluctuations created by the air-water two-phase flow in a horizontal pipe, 100mm ID, on submerged outlet condition. These results are compared with the experimental results. Moreover, the correlation between the pressure fluctuations and the velocity generated by the large bubble releasing is clarified.

 In this paper, which focuses on energy dissipation from a leakage point in a pipe, the mathematical model that estimates a leakage location from the damping of the pressure fluctuation is derived, and improved considering the influence of friction on the energy dissipation. The model is verified by an experiment, and leakage location can be estimetaed within an error of less than about 2% of total length of the pipeline.

 The flow condition below drop structures changes with discharge and tailwater elevation. As the flow passing over a drop structure, hydraulic jump, plunging flow (submarged jump), and wave type flow are formed. For the formation of plunging flow, river bed below drop structures should be protected from local scour and bed degradation, because a main flow continues along the bed far downstream. It is important to clearfy velocity characteristics in the plunging flow. This research focus on the plunging flow below low drop structures. If the relative drop height becomes lower, the effect of downstream drop shape on velocity characteristics is negligible, and a main flow continues along channel bottom far downstream.

 Stepped channels can be used to dissipate the energy of supercritical flow in steep channels. For the hydraulic design of stepped channels, it is important to clarify the nonaerated flow depth, the velocity characteristics, the boundary layer thickness, and the position of the inception point. This paper presents an investigation of an analytical method for the water depth and the boundary layer development in the nonaerated skimming flow on a stepped channel. A method for calculation of the water depth and the boundary layer thickness is determined on the basis of the momentum integral equation. The relation between the energy thickness and the loss of energy head is clarified by using an energy equation. The results of the analysis are validated by the present experiments.

 A simple depth-averaged flow model is proposed considering the deformation of stream-wise velocity to reproduce the characteristics of hydraulic jump. Eirstly, representing the streamwise velocity distribution using a power series of non-dimensional vertical coordinate, the dependency of coefficients in the power series on the streamwise coordinate is formulated using 2-D continuity and Reynolds equations. The water surface profile equation for a hydraulic jump is deduced using the depth-averaged continuity and momentum equation with iteration procedures. Then the analytical solutions with two and three exponential terms for water surface profiles are compared to the previous experimental data. The velocity distributions at the origin with the critical depth are also examined with the comparisons to the previous experiments to verify the simplified model proposed in this study.

 With increasing frequency of heavy rainfall events as a result of climate change, the long term assessment of sediment dynamics becomes a critical issue in maintenance and management of dam reservoirs. This paper discusses how heavy rainfall events in the upper basin affects sedimentation in a dam reservoir. A mathematical model was proposed in order to reproduce long-term changes of shallow landslide area in the test basin. The dataset of landslide area was obtained by aerial image analysis. It was assumed in the model that governing parameters of shallow landslide were intensities of the short- and long-duration precipitation, where the latter is deeply concerned with ground water content in the catchment. A threshold of shallow landslide occurrence was determined in consideration both of the short- and long-duration rainfall events. Applying the model to the Sagae Reservoir basin area in Yamagata Prefecture, the historical change of shallow landslide area was successfully reproduced by the model with higher accuracy than by the previous model⁷⁾ in which the long-duration rainfall event was not considered. In addition, it was shown that the landslide area predicted by the present model was better correlated with the bottom sediment volume in the dam reservoir than the previous model.

 In this study, we focus on the Yoshino River basin in the Shikoku region, Japan and calculate the sediment yield, dam sedimentation, riverbed fluctuation, and river mining from 1965 to 1999, estimating the amounts of sediments supplied from the rivers to the coast on the Kii Strait. The calculation indicated that the annual average values of sediment yield, dam sedimentation, riverbed fluctuation, river mining, from 1965 to 1999 were approximately 3 million m³/year, 0.7 million m³/year, −1.2 million m³/year, and 1.9 million m³/year, respectively. Using a sediment balance equation, the potential of sediment supply from the rivers to the coast was estimated to be 1.6 million m³/year. In addition, large amounts of sediments produced in the upper reaches of the Ikeda Dam, where steep mountains mostly occupy the area, were estimated to be largely deposited in the dam groups such as the Sameura Dam, the Yanase Dam and the Shingu Dam.

 Unazuki dam and Dashidaira dam, which are constructed in the Kurobe River basin with a large amount of sediment yields, conduct the coordinated sediment flushing for the purpose of maintaining the function of the dam, preventing the degradation of the riverbed and reducing coastal erosion. However, the riverbed degradation and the shortage of large bed material are still progressing in some sections. In this paper, we constructed analytical model of quasi three-dimensional flood flow and bed variations using the flood flow and sediment discharge graphs from Dashidaira dam as the boundary conditions, and examined the sediment behavior at the time of coordinated sediment flushing of the two dams. We clarified the sediment transport dynamics in the reservoir and river channel system in each time zone of the operations of Dashidaira dam and Unazuki dam.

 Accurate predictions of the reservoir inflow and sediment concentration are necessary for real-time reservoir operation. This study used multiple artificial neural networks (ANNs), namely the back propagation networks (BPN) and four types of kernel function of support vector machines (SVM), to predict inflow and sediment concentration. These ANNs were calibrated and validated based on observed data of the Shihmen reservoir for typhoon events from 2008 to 2015. To avoid the risk of selecting multiple ANNs, the switched prediction method (SPM) is proposed to select the optimal predicting module time by time. This paper compares the predictions from SPM with optimal individual predictions and the ensemble means (EM) with respect to the root mean square error. The improvements in SPM compared with optimal individual ANN and EM are 3.8% and 10%, respectively. In conclusion, the uncertainty of the predictions could be effectively reduced by applying the switched prediction method.

 Processes leading to landslide dam outburst are classified into the following three types: erosion due to overtopping, instantaneous slip failure, and progressive failure. There have been many indoor channel experiments and field experiments focusing on the overtopping failure of a landslide dam. Experimental research focusing on progressive failure has been limited to indoor channel experiments on a small scale. Hence, we carried out large-scale experiments on progressive failure of a landslide dam in a mountainous streambed of the Ashiarai Basin. The aim of these experiments was to observe processes of dam deformation and flood runoff. We obtained data on the dam deformation process, reservoir water level, water level inside the dam and outflow discharge. The results show that the water volume of a dam is the main factor affecting the peak discharge rate.

 There were floods of 4 450 m³s⁻¹ at the Seto-ishi Dam on the Kumagawa River in Japan during July 2018. The sluicing and flushing operations were performed for 7 days at Seto-ishi Dam, and suspended sediment concentration and particle size distribution were continuously measured using a flow-cell type ultrasonic attenuation spectrometer, and suspended sediment transport was observed. The main results follow. The amount of suspended sand transport due to sluicing and flushing operations was 490 × 10⁶ kg. This comprised sand content of 64.7 × 10⁶ kg(13.2%), cosrse silt of 145.0 × 10⁶ kg(29.6%), and wash load of 280.5 × 10⁶ kg(57.2%). Sediment of 149 × 10⁶ kg flowed out of the reservoir due to the flushing operation, and suspended sediment flow into reservoir was estimated as 341 × 10⁶ kg.

 Field observation and numerical analysis were performed for the debris flow which was happened in Yanohigashi, Hiroshima in July, 2018. Three debris flows were happened and the soil saving dam was filled with the sediment of the debris flow from the south valley. The second debris flow from the north valley attacked the residential area and damaged many houses. The results of the numerical analysis show that the multiple valleys must be considered to estimate sediment transport rate to residensial area. In case of Yanohigashi, the soil saving dam didn’t work to suppres the inundation area of the debris flow in residential area.

 Field observation and numerical simulation of mud flow were performed for the sediment disaster caused by the Hokkaido Iburi-east area earthquake. Effect of earthquake acceleration is considered into the numerical simulation model of mud flow. The results show that the difference of the flow characteristics of mud flow between simple uniform slope and valley type river basin is large. Hence, different sediment disaster prevention work and evacuation method must be considered for them. The effect of the earthquake acceleration is not appeared in the inundation region under the acceleration by Hokkaido Iburi-east area earthquake. However, earthquake acceleration with longer wavelength makes the inundation region smaller.

 It is common to consider the riverbed as being fully saturated in the treatment of debris flow. However, the field survey conducted by some researchers¹⁾ showed the presence of significant unsaturated deposits after its occurrence. Our study investigated such phenomena and considered the possible coexistence of both saturated and unsaturated layers during the initiation and propagation of the debris flow. The seepage was coupled to the surface flow and the erosion velocity of unsaturated layers took into account the apparant shear strength due to suction. Compared to the case of saturated bed, results showed when the bed is initially unsaturated a relatively slow development of debris flow with sediment mostly transported within an immature regime of debris flow and larger deposits were noticed at the downstream reach, inducing the bedslope to become milder.

 These days, numerical simulation technology and measurement technique have improved. However, there are still some problems about estimating how real river bed morphology change, especially calculating of amount of sediment flow. First of all, in this paper, the effects of sediment movement on flow velocity distribution are indicated in order to estimate hydraulics roughness in respect of calculating of amount of sediment flow. Some series of flume experients have been taken in order to measure flow velocity distribution including sediment flow. PIV by using fluorescent pigment as tracer can measure flow velocity distribution in the condition that water flow including sediment transport. These data give informations about characteristics of flow velocity distribution near bed. And some results indicate thickness of sediment flow is important for bed roughness condition.

 Floodplain vegetation affects morphodynamics of rivers with a relatively steep slope in two different ways. In some cases, vegetation have been known to protect river banks from erosion, but under certain circumstances it may promote bank erosion. In this study, authors conducted flume experiments to understand the effects of spatiotemporal vegetation intensity, which means either vetetation is flushed out, on morphodynamics, especially bank erosion and channel widening. The results showed that the presence of vegetation and defference in vegetation intensity clearly affects on channel morphodynamics. When the vegetation intensity was the weakest, the maximum channel width was confirmed because new flow paths leading into chute cutoff were formed on floodplain. As other cases, when the vegetation intensity was the largest, the initial channel path hardly changed and channel bed was degradated. In addition, when the vegetation intensity is in the middle of this experiment, vegetation was flushed out only at the outerbend of the meandering, leading into the development of channel widening at the bend quite locally.

 An estimation method of water level, flow discharge, bed level, and grain distribution was constructed using observed water level and one-dimensional numerical model for gravel transport. The numerical model employs relation for hiding and bedload transport of mixtures of Kovacs and Parker(1994) and links grain size distributions in the bed load, surface layer and subsurface with the gravel transfer function.

 The model was applied to experimental data of water/bed level, flow/sediment discharge, and grain size distribution on gravel-sand mixture bed. It showed that the model could reproduce the flow and bed variation, but more improvement was needed to predict reasonable accuracy.

 The present study investigates the lateral and longitudinal sediment sorting and an equilibrium channel pattern in the Seri River, Japan by means of numerical simulations. To discuss such sediment transport processes, we employ two methods. One is the bedload formula with its functional form of τ_*^1.5 as well as with a constant exchange layer thickness. The other with its functional form of τ_*^2.5 as well as with bedload layer thickness instead of constant exchange layer. Herein, τ_* is the non-dimensional bed shear stress. Results of the numerical simulations suggest that the sediment sorting is influenced by the choice of bedload formula and that sediment sorting takes place actively in the case of computation with the bedload formula with τ_*^2.5. The computed results are agreed with the data obtained from field observations.