An experimental study was conducted to clarify sediment particle motion and its relation to the dune migration. High-definition action video cameras were utilized to record the images of particle motion in a domain covering dune scale. Image processing was conducted to homogenize the illumination heterogeneity due to bed form and light reflection at the water surface. Then, Particle Image Velocimetry was conducted to quantify the instantaneous velocity distribution of sediment particles. Unsteady sediment motion and its relation to the shape, deformation of the shape and migration of dunes were discussed.

A procedure for estimating flood damage to buildings, roads, and railways located in mountainous flood-plains due to probable maximum precipitation events is proposed. Probable maximum precipitation and consequent flood are estimated using the method proposed by Kadoya and Nagamori, 1979. For case study simulations, two areas, both located in the Chugoku District and damaged by recent large flood events, are selected. The characteristics of inundation flow in two areas under flood conditions caused by probable maximum precipitation are estimated using a two-dimensional flow model with fine-resolution calculation grids. Damage to humans and structures caused by probable maximum flood events is estimated based on the relationship between recorded damage due to recent large floods and inundation flow characteristics estimated using our numerical model. Effective measures to increase resilience in communities in mountainous regions are also discussed.

This paper describes a method of measuring flow rate and size distribution of sediment transported in gravel-bed river during small flood. The device developed is inexpensive, compact and can be installed in the river bed by hand. A steel shell covers the device and impact sound when sediment hits the steel shell is recorded by a digital sound recorder stored in the steel shell. The relationship between sound wave profile and a diameter of the impinged sediment particle was validated based on the result of flume experiment. The proposed method was used to measure processes of sediment transport during a flushing flow released from a dam, then the size distribution and amount of bed-load were successfully analyzed.

The increase of heavy rainfall events with global climate change highlights the importance of effective schemes to prevent or mitigate the loss of life and property due to inundation and landslides. The advancement in numerical prediction of heavy rainfall is one of the important measures to give suitable forecast and warning for flood prevention and evacuation activities. This study aims at clarifying the accuracy of a numerical model WRF (Weather and Research Forecasting) coupled with a data assimilation method LETKF (Local Ensemble Transform Kalman Filter). We carried out numerical simulation of heavy rainfall in western Shimane Prefecture, Japan, which occurred from August in 2013. Comparison between the calculated total rainfall and that measured by a radar system shows that the numerical model can reasonably reproduce heavy rainfall in the Hamada city. It also shows that the prediction without data assimilation is dfference in caluculation with assimilation.

With the increase of extreme climate events due to the global climate change, urban areas have been becoming more prone to flooding. This trend highlights the importance of effective schemes to mitigate the damage and loss due to inundation. The advancement of numerical prediction method for heavy rainfall is a key technology for early warning. The objectives of this study are twofold. One is to clarify the applicability of the combination of WRF (Weather and Research Forecasting) model and a data assimilation method LETKF (Local Ensemble Transform Kalman Filter). The other is to discuss the evolution of rainfall system that caused heavy rainfall based on the numerical results. We carried out numerical simulation of heavy rainfall in western Shimane prefecture in August, 2013. Comparison between the calculated results and those measured by a radar-AMeDAS shows that the numerical results fail to reproduce the intensity and the location of the heavy rainfall with small success during the latter half of the rainfall event. It is found that the ensemble mean rainfall provides a slightly better results about the peak value and peak time of rainfall compared to those by the deterministic approach without assimilation, showing the effectiveness of LETKF. The heavy rainfall event over Shimane prefecture is considered to have occurred by the southwest moisture flux converge near the front with the orographic effect of the Chugoku distract.

In this paper, an inundation .ow in the Matsugase district, Sanyo-Onoda City, during the 2010 July torrential rainfall event was analyzed. Topographical data of this area were measured by aerial LiDAR survey. The DEM obtained by the LiDAR survey was veri.ed by .eld survey data measured by RTK-GPS. A shock-capturing shallow water .ow model with a .ne calculation grid (two to 5 meters in resolution) was utilized to represent inundation .ow in the inundated area. The calculated water depth was compared with the water records. The characteristics of inundation .ow, especially the e.ect of the embankments in the inundated area, and those relation to local damage were discussed. E.ects of embankment rise and reduction of energy loss due to a bridge were evaluated.

To evaluate the detachment process of attached algae in rivers, a series of flume experiments were carried out. Time-series analysis of NDVI (Normalized Difference Vegetation Index) image of cobbles covered by attached algae was conducted to estimate the change in Chl-a or the decrease in attached algae density. A new method to convert NDVI to Chl-a was developed to measure the attached algae density on cobbles in a nondestructive and continuous way. Quantitative analysis of the detached rate of attached algae has shown that the detachment ratio can be closely correlated with the work done by sediment impinging on the cobbles.

Abnormal growth of submerged plants in rivers not only interferes with fishery and navigation but also blocks sunlight from arriving at river bed materials, leading to the growth failure of attached algae that living creatures there are fed on. It is important for river engineers to clarify how submerged plans can grow and be washed out because they can propagate in such a way that their lobed leaves survive in downstream reaches. This study aims at making clear the conditions how some submerge plant is washed away by the floods. The field surveys were conducted before and after an artificial flood from reservoir to measure the distribution of Egeria densa and to quantify its wash-out rate through cross-sections. These results have revealed the wash-out condition of the submerged plant.

Growth and detachment of the attached-algae in river bed is one of the key factors in ecological system in the river. The detachment process of the algae can be categorized into three types, namely 1 Natural detachment due to aging, 2 Detachment due to flow and bed material load and 3 Detachment caused by rolling of the bed material. To evaluate the detachment processes during flood, we conducted a series of field survey in a site in the Johge River. In the period this survey had been conducted, a flushing discharge and a flood due to typhoon were observed. The chlorophyll-a densities on the bed material before and after floods were measured. Based on the field survey results, the detachment processes of algae are evaluated, and a new model to estimate an amount of detachment during the flood is proposed.

An estimation method of flood discharge hydrograph is developed that couples a 2-D unsteady flow model considering the effect of vegetation toppling and a 2-D model for bed elevation change due to bed load transport. The method is applied to a flush flood from the Haizuka dam in the Jouge River where vegetation became submerged with deformation in large areas. Measurements of water level hydrographs at several points, the discharge with ADCP at a weir and bed elevation change at a point were carried out during the flood. It is shown that the estimated discharge hydrograph gives reasonable agreement with the measured result and that the reliable data of bed elevation is crucial to accurate estimation of discharge hydrograph in a steep slope river.

Railway and road embankments on a flood plain affect the propagation of inundation flow. In inundation simulation, the embankment in the flooded area is treated as the solid wall that inundation flow is able to overflow. However, the embankment involves the potential to be damaged during a severe flood event, and the breakage of the embankments causes not only physical damage but also a reduction of transportation function. In this study, the damage to the embankment of the Kishin line during the flood event in August 2009 is analyzed. The topographic and hydraulic conditions of the embankment failure are discussed.

Field measurement and numerical simulation are carried out for an artificial flush flood in 2010 in the downstream reach of Haizuka dam located at the Jouge River to quantify the effect of flush flood on the exfoliation of attached algae. Measured items are water level, chlorophyll a (Chl-a), SS and attached algae at several locations. The relation between Chl-a attached to riverbed stones and velocity at the same location before the flood and the relation between detached rate of the algae and accumulated bed shear stress during the flood are obtained based on the field observation and 2-D numerical simulation. Based on these relations the total amount of supplied algae to water over the measurement reach during the flood is estimated to around 15.0kg in which the total amount of exfoliated algae is estimated to around 9.4kg.

Floods accompanied by bed elevation change were observed in a steep reach of the Minochi River in 2009 and 2010. The flood discharge hydrograph in 2009 is estimated by three methods, namely the conventional float measurement, Large-scale PIV method and 2-D unsteady flow simulation. The 2-D flow simulation considers vegetation toppling during flood and bed elevation change due to bed load transport. In the numerical simulation, water level hydrographs observed at various points are used for boundary conditions at upstream and downstream ends and the evaluation of the vegetation drag parameter. The flood discharges obtained by three methods are compared to find that the 2-D unsteady flow model yields good agreement with the discharge by Large-scale PIV method. The selection method of the location of suitable upstream boundary in a step reach is also discussed

Using a surface video record of a large-scale flood in Yoshino River, Shikoku, Japan, some problems related to application method of Large Scale Particle Image Velocimetry (LSPIV) are examined in detail. First, from a hydraulic point of view, a new method is proposed for exclusion of incorrect velocity data and calculation of time-averaged velocity. Second, decision standards are examined with respect to the primary LSPIV model parameters. As a result, it was confirmed that the appropriate time-interval, template size and detection area size for PIV analysis procedure range between 1/3 to 1/2 seconds, between 1.5 to 2 times the maximum size of brightness pattern and about 2 times the estimated displacement distance of templates, respectively.

Open-channel flows with a relatively large-scale side cavity zone exhibits water surface oscillations in spanwise or streamwise directions. Although the side cavity zone is installed for improving the water-friendly environment, such oscillations, once occurred in the actual river in the urban areas, may cause flooding, resulting in a large damage in the residential area. In order to investigate the flow features associated with such surface oscillation, the authors have developed a 3D LES model that utilizes a density function for detecting air-water interface. With the developed model, it becomes possible to simulate water surface oscillation observed in the experiments. In this paper, the flow features related to the water surface oscillation as well as the local flow features along the interface between the main channel and the side cavity zone are investigated for the subcritical flows with relatively large Froude number.

The procedure of extraction of three-dimensional distribution of velocity field from ADCP measurement data is presented in this study. Cross-sectional velocity distributions have been generally used to indicate three-dimensional feature of flow. However complex flow fields, generated by combination of a complex bathymetry, curve of the channel and river structure, are difficult to investigate only by using the cross sectional velocity distribution. In this study, anisotropic averaging and interpolation method is introduced to remove random component caused by turbulence of flow and random error of measurement. The continuity correction using pressure equation is also used to reduce both random and system errors. The validity of our procedure is confirmed by using three-dimensional numerical simulation result. Finally, by using our procedure, the complex flow around spur-dikes is explored.

In Recent years, flood disasters are frequently taking place where relatively steep slope rivers meet in the mountainous area. The cause of the disasters is mainly due to the sudden rise of water level at a confluence, which sometimes accompanies a hydraulic jump in the upstream region. In this research, a numerical simulation based on the shallow water equations and the unstructured grid system is performed to investigate the effect of the discharge ratio to the flow patterns and the riverbed change at a confluence. In order to simulate a river confluence in the mountains, the main channel slope is set at 1/80 with a tributary channel slope of 1/40. Laboratory experiments are also conducted to provide data with respect to water surface profiles and bed variation. Water surface velocity distributions are obtained by an imaging technique, LSPIV, using a high vision video camera. It is made clear that general flow features calculated by the simulation model agree fairly well with the experiments including the locations of hydraulic jumps and local scour.

In July 2020, the Kyushu region experienced record-breaking heavy rains from July 4-7, causing extreme floods in the Kuma and Chikugo Rivers. This was followed by atmospheric instability over a wide area from western Japan to the Tohoku region, resulting in heavy rainfall on July 13-14 in the Chugoku region, and on July 27-28 in the Tohoku region, and flooding of large rivers including the Go River and the Mogami River. In recent years, record-breaking torrential rainfall disasters have been occurring every year, and the heavy precipitation scale as well as the rainfall intensity has been increasing in space and time, resulting in spatio-temporal expansion of the damage. Furthermore, in 2020, the disaster occurred while the social activities had been restricted due to COVID-19 pandemic. The compilation and dissemination of disaster survey data and lessons are essential toward the sustainable development of society. Therefore, JSCE has planned a special issue on the July 2020 torrential rain disaster in order to share and disseminate disaster information and to contribute to the advancement of technology and science related to disaster prevention and mitigation.

The float method is a standard method for flood discharge measurement but safeness and efficiency are limited. LSPIV (Large-scale Particle Image Velocimetry), as a flow visualization method, can avoid these problems. Johnson and Cowen (2016) suggested a method to calculate the local flow depth based on LSPIV results and proofed the method through laboratory experiments. This study utilizes the method to the Uji river for predicting the local flow depth. Large-scale velocity fluctuation modes in the river disturb the calculation of the integral-length scale. Proper Orthogonal Decomposition has applied to remove the large-scale velocity fluctuation.

The growth of aquatic mosses attached to cobbles composes bed material in riffles was confirmed in a river in Japan. Covering bed material with aquatic mosses reduces the biomass of attached algae that is a source of energy for commercial fish. This study focuses on how much and how the aquatic mosses keep their coverage on the river bed in riffles after suffering flood disturbances. Resistance to disturbances due to high-speed water flow and sediment impingement on mosses and the rolling of the cobble attached with mosses was investigated by a laboratory experiment. We observed that intermediate disturbance seems to remove silt in the biofilm but the biomass of mosses had tend to be maintained. Based on visual inspection of moss cover following the experiment, we found that the remaining moss consists of short fresh stems. The relationship between the rate of moss detachment and work due to sediment impingement has investigated and the existence of the optimal work per sediment impingement has been clarified.

Application of image-based method for flood flow measurements has become popular in recent years. There are several issues reducing the accuracy and applicability of the image-based flood measurements, and some of them may have been overlooked. Based on our experience, this paper discusses the issues related to video format and orthorectification, and measures to manage the issues are also presented.

Dunes were formed in sandy rivers during floods and wield significant effects on the flow resistance and movement of bed materials. Although many studies have been carried out to clarify the evolution process of bedform, few studies have directly tracked the migration of bed materials in particle basis. In this study successive images of fluorescent-colored sand particles are analyzed to reveal the characteristics of the particle migration on the bedform changing its form from a flatbed, 2D dunes and 3D dunes. Mean migrating velocity of bed material and near-bed turbulence including turbulent kinetic energy were reduced with the development of the 2D dunes. When the bedform change from 2D to 3D form, the near-bed turbulence became almost zero temporarily. After that, with development of 3D dunes, the increase of the mean migrating velocity of bed material was observed.

Better understanding of the process of bank erosion with vegetation destruction is an important issue for river dynamics. In this study, six cases experiments were conducted by changing the existence of vegetation, the density of vegetation and water level, in order to clarify how the bank erosion with vegetation destruction effect on river channel evolution. Particularly, we have applied the original making for vegetation model which destruct by river flow and topographic measurement of the laser displacement meter. In the result, it is resolved that the vegetation stops the bank erosion of upper side temporarily, and promotes a scour of underside bank. And after vegetation destruction, bank erosion restarts and widens channel intermittently.

Embankments on a floodplain, usually treated as solid walls in inundation simulations can cause considerable damage during severe flood events. The damage to road or railway embankment system not only changes the propagation of inundation flow but also destroys the transportation infrastructure. In this study, the failure of the embankment system of the Kishin Railway Line during the flood event in August, 2009 is analyzed. The factors in causing embankment failure are analyzed from the geographic and hydraulic view points. A framework to estimate the risk of the embankment failure, which is based on the analysis of the structures of the geography and inundation flow, is proposed in this work.

The confluence region downstream of the Katsura River, the Kizu River and the Uji River in Kyoto displays complicated flow patterns depending on which watershed undergoes larger precipitations. The flow characteristics at this location have not been clarified so far due to the complexity of the combining streams. In the present study, in order to obtain the flow features in normal flow conditions, surface flow measurement by using a helicopter is performed. The surface flow is visualized by biodegradable tracers and they are videotaped from a helicopter hovering at about 300 meter height following the cloud of tracers. A high-vision video camera is used for recording the images and the velocity of the tracers are analyzed by applying PIV with subtracting background movement. At the same time, distributions of temperature, pH, and other parameters related to water quality are measured by multi-purpose instrument installed on a boat. It was found that water quality of the Ktsura River is quite different from the other rivers and the longitudinal dispersion of tracers is evident at the location.

In Recent years, flood disasters are frequently taking place where relatively steep slope rivers meet in the mountainous area. The cause of the disasters is mainly due to the sudden rise of water level at a confluence, which sometimes accompanies a hydraulic jump in the upstream region. In this research, a numerical simulation based on the shallow water equations and the unstructured grid system is performed to investigate the effect of the discharge ratio to the flow patterns at a confluence. In order to simulate a river confluence in the mountains, the main channel slope is set at 1/80 with a tributary channel slope of 1/40. Laboratory experiments are also conducted to provide data with respect to water surface profiles. Water surface velocity distributions are obtained by an imaging technique, LSPIV, using a high vision video camera. It is made clear that general flow features calculated by the simulation model agree fairly well with the experiments including the location of hydraulic jumps.

It has been investigated that an open-channel flow with a relatively large-scale side cavity exhibits water surface oscillations in spanwise or streamwise directions. Such oscillations, once occurred in actual river installing a side cavity zone as a water-friendly facilities in the urban areas, may cause flooding, resulting in a large damage in the residential area. In order to investigate the flow features associated with such surface oscillation, the authors have developed a 3D LES model that utilizes a density function for detecting air-water interface. With the developed model, it becomes possible to simulate time-dependent water surface variation with a reasonable accuracy as compared with the experiments. In this paper, local flow features along the interface between main channel and side cavity zone are investigated by distributing virtual tracers following the 3D flow pattern.

When a portion of a straight open-channel is replaced by a channel with a side cavity zone, water surface in the main channel in front of the cavity as well as that within the side cavity zone begins to show significant surface oscillation for specific experimental conditions. The mode of the oscillation changes with hydraulic condition and aspect ratio of the side cavity shape. When the aspect ratio is small, only a seiche-like oscillation within the cavity zone can be observed. However, as the streamwise length of the cavity increases, i.e. for a large aspect ratio, the oscillation in the cavity zone propagates into the main channel and a longitudinal oscillation becomes evident. In order to understand the mechanism of such mode-change, we performed a stereoscopic measurement of the water surface flow oscillation by gradually changing the aspect ratio of the side cavity.

There occurred a levee break of the Izushi River near the district of Torii in Izushi-chou, Hyogo Pref. on Oct. 20, 2004, which is caused by the heavy rainfall induced by Typhoon No. 23 (TOKAGE). In order to perform a high-resolution inundation simulation of this area, detailed topographical data is measured by using a kinematic GPS system. Two dimensional shallow water equation based on the unstructured grid system is used for the simulation. Since the grid size is adjusted so that respective houses can be resolved in the simulation, it became possible to estimate hydrodynamic forces exerted on the houses, which is found to have a close relation to the actual house damages.

Field measurements using an image analysis technique, the large-scale particle image velocimetry (LSPIV), are conducted for a river reach having a series of groins in the Uji River. The measurements are performed using a digital video camera from a height of 27m of the nearby tower for meteorological survey. The river reach with a streamwise length of about 150m and the width of about 60m is cover by three-angle recordings. The measurement results from each angle are successfully connected with each other, by which the general flow feature of the region can be revealed with a reasonable accuracy. The measurement by an ADCP is also performed to obtain the internal three-dimensional flow structure. In addition, a numerical simulation based on the unstructured grid system of the shallow water equation is conducted to compare with the field measurement results and to evaluate the effect of the groins to the main flow.

Open-channel flow with a side cavity is investigated with the aid of a high-vision video camera having 1440 by 1080 pixel resolution. The experiments are performed by fixing the aspect ratio of the side cavity at one to five and by changing the Froude number of the incoming flow. At the same time, longitudinal side slope with several angles are installed within the side cavity to simulate the actual river situation and its effect on the surface flow is examined in detail. It is made clear that the introduction of a new video camera improves the measurement accuracy of instantaneous velocity fields with a smaller template size in PIV. Installation of a side slope is found to have large effects on the recirculating flow pattern at the surface.

When a relatively large side cavity zone is installed in an open channel, the influence of separated vortices and recirculation flow generated at the zone is sometimes exerted to the main flow as water surface oscillation. The mechanism of the oscillation is still not clearly understood because of the complexity of the phenomena. In this research, by using a stereoscopic surface measurement system developed by the authors, time-dependent water surface configurations are successfully measured at a sampling rate of 12Hz, from which general features of the surface oscillation is made clear for a wide range of the Froude number. We then applied the proper orthogonal decomposition (POD) method to the measured data and found out that the surface oscillation can be clearly decomposed into a Seiche-like major mode and sub-modes with smaller wavelengths.

This paper compares the accuracy of the novel PIV technique based on the optical flow concept and the conventional PIV with the Gaussian sub-pixel fitting. In the new technique, we take into account of image gradient terms with three consecutive frames in the optical flow treatment in order to improve the accuracy for complex flows. This paper includes: i) the fundamental concept and formula of the new method, ii) experiments with standard PIV images, video sequences of the meandering channel flow in a Japanese old garden and flow images of the Uji river, and iii) the discussion on the measurement accuracy. The result shows that the presented method is superior to the conventional PIV especially in the measurement of instantaneous flow field with rotational motion.

A ceremony that a few poets sitting alternately on the both banks of a meandering channel are expected to make poems during the limited time when drink-filled cups flow down and finishing poem making, poets pick up cups to drink is called Kyokusui-no-En. This sort of ceremonious game has its origin in ancient China and was introduced to Japan where it prospered in 8th through 11th centuries.
In this paper, the channel flow of Kyokusui-no-En is observed from the view point of fluid mechanics. The actual flow was studied using PIV measurement and numerical simulation. According to these results, it was made clear that the meandering channel creates the separate flow, reverse flow and vortices to make the wine cups movement elegant.

Field observations on the surface flow were carried out in a straight reach of the Uji River. Velocity distributions on the water surface were obtained based on the captured video images by means of large-scale particle image velocimetry (LSPIV). Flow structures around spur dykes are studied for both non-submerged and submerged cases.
It is clearly seen that, for both cases, the dykes are working as an additional roughness. Their effect on channel conveyance capacity is estimated in terms of the effective channel width calculated from the velocity distributions. It is also of interest that the maximum velocity filament appears at a different position as the water depth varies in case of the channel with single-side dykes such as studied here. In addition, large-scale boils shedding from the submerged dykes are clearly observed and their origin and advection mechanisms are also evaluated.

A new two-dimensional simulation model for urban rivers applicable to complicated sidewall geometry was developed. The proposed model is based on the finite volume method (FVM) with flux differcncing splitting (FDS) under unstructured grid system. Firstly, the performance of the model was evaluated by the comparison with experimental results for a straight open-channel with a long side concavity having an aspect ratio of ten. The calculated results showed fairly good agreements with the experiments, indicating the good performance of the model. Secondly, the model was used to examine the effects of various sidewall geometries on the local water-level increase at the side concavity section. It was shown that water depth variations within the side concavity section can be controlled by a separation wall and the optimum arrangement of the separation wall for the present side concavity shape was proposed.