It is recognized that the secondary current of the first kind and the outer-bank cell generated at open channel bends affect not only bank erosion but also lateral velocity distributions. These phenomena have three-dimensional (3D) flow structures, therefore, 3D model is basically needed to predict the phenomena. However, 3D computation is usually expensive in practical, thus two-dimensional (2D) model is recognized as an efficient tool for practical purposes. Nevertheless, comparisons between 2D and 3D results not well made in curved open channel flow. In this study, we carried out detailed comparison(are ) s between 2D and3D(a ) results in both mild and sharp curved open channel flows under a fixed flat bed as well as a deformed bed conditions. The numerical results showed that the depth-averaged profile on a flat bed by 2D model is more different than 3D results in case of a mild bend than the case of a sharp bend. If the bed is deformed, the difference of 2D and 3D results becomes smaller.

View full abstract

In open-channel turbulent flow, changes in flow features often appear on a water surface. Therefore, water surface is subjected to various influences but its characteristics about the source of surface variation, fluctuations and advection of surface features are not fully understood. In the present research, in order to investigate advection characteristics and source of water surface fluctuations, experiments by PIV and numerical simulations by LES are conducted for an open-channel flow with strip roughness. As a result, cross-correlation analysis of water surface fluctuation made it clear that advection characteristics of water surface fluctuation with strip roughness wall are different from the on smooth wall. Moreover, it was found that water surface fluctuation due to bursting phenomena from near bottom wall is dominant by using a Pre-Multiplied Spectra.

View full abstract

This paper presents a set of numerical flow simulations to investigate the flood control ability of the Hyakken River channel which was constructed as a floodway of the Asahi River for preventing inundation of Okayama Castle in the 17th century when the modern hydraulics had not been developed. Calculation conditions like the channel topography and the design flood discharge were estimated by comparing the old description about the channel with the numerical data obtained in recent days. The computation by using unsteady shallow water equations showed the results which correspond to the records of flood events of the day, and the results also suggested that the channel was designed very reasonably even without the modern knowledge of hydraulics.

View full abstract

Here we consider the puzzle of long-runout turbidity currents. It is well known, through direct evidence of the flows or from the morphology that they create, that turbidity currents can run out over 100 km in lakes and reservoirs, and over 1000 km in the ocean. The currents do so without dissipating themselves via the excess entrainment of ambient water. Existing layer-averaged formulations are, however, unable to capture this behavior. Here we use the formalism of a Turbidity Current with a Roof to show that the flow partitions itself into two layers. The lower driving layer approaches an asymptotic behavior with invariant flow thickness, velocity profile and suspended sediment concentration profile. The upper rarified layer continues to entrain ambient water indefinitely, but the concentration in that layer becomes ever more dilute, and the layer ultimately has little interaction with the driving layer. This partition likely allows the driving layer to run out long distances, and to follow morphology of its own creation such as leveed subaqueous channels.

View full abstract

Recently, a localized torrential downpour was happened frequently as a heavy rain, and rain-water was leaking out of gutter installed in roofed rail station. As the mechanism of drainage through drain pipe has not been clarified, the drainage capacity could not be predicted. In this paper, the mechanism of drainage has been discussed on the basis of experimental works by using 14 prototype models for the arrangement of drain pipes, and the prediction of drain capacity has been proposed. Systematical measurements yield that the length of vertical drain pipe connected to gutter is most significant to keep drainage capacity in gutter by focusing on the relationship between water depth in gutter and the existence of water level in the vertical drain pipe. The drain capacity can be predicted by applying Torricelli’s and Bernoulli’s theorems supported discharge coefficients for all experiments, loss factor due to bending, and friction factor for roughness bed.

View full abstract

Recently, many river channels tend to be densely vegetated due to regime shifts in hydrological, fluvial and ecological processes and it is a critical engineering issue to evaluate conveyance capacity of vegetated channels. Although drag force coefficient is popularly used in evaluating drag force of vegetation, it is not always a convenient parameter in determining high water level, HWL, for a design flood. In this study, an equivalent Manning’s coefficient nv, which corresponds to drag force of overgrown vegetation in alternative bars, was theoretically evaluated by the two-dimensional two-layer (2D2L) hydrodynamic model. It was found that nv monotonically increases with increasing discharge as long as vegetation emerged. On the other hand, nv slightly decrease with discharge after vegetation submerged. Findings is to provide useful information for flood management of vegetated river channel.

View full abstract

These days, 2-dimentional numerical simulation of changing bed elevation would be useful for river management. However, it is difficult to predict sediment transport at the bed including the complex surface and the substrate structure in the river. There are still some problems to be solved in estimating flow resistance considering both bed surface and vertical condition, and it lead to difficulty to estimate sediment transport. Therefore, a series of flume experiments were conducted in order to examine flow resistance and velocity distribution near bed surface. These experiments indicated the importance of both surface and substrate structure of river bed for prediction of sediment transport.

View full abstract

For flexible vegetation flow, the passage of the coherent vortex causes a local depression in the canopy and the waving motion is called Monami. It is well known that the Monami motion has significant heterogeneity and the Monami scale appears to be a function of the vortex size. However, no detail information on the Monami scale exists (streamwise and lateral direction). So, in the present study, simultaneous measurements of two-line flexible vegetation elements are conducted by using 2-color PTV. The results suggested that the flexible vegetation element in each sub-channel exhibits a periodic sequence of Sweeps and Ejections.

View full abstract

A riverside concavity tends to cause recirculation eddies and resultantly the deposition of suspended sediment. In this study, effects of spur dike placed upstream side in a main channel on flow structures in a side concavity zone were investigated. Location distance of the spur dike from the upstream end of the concavity zone and the length of spur dikes were changed under two different discharges. The flow patterns, magnitude of recirculation eddy and water exchange velocity in the concavity zone were changed by these parameters. The reattachment length of the separated wake zone behind the spur dike is important for the control of the flow structures in the concavity zone. When the reattachment point lies upstream of the downstream end of the concavity, the flow in the concavity is accelerated. The effects of spur dike are changed with water depth because the length of the separation zone behind spur dike is reduced in cases of submerged spur dike.

View full abstract

Groynes have been used for the protection of banks, stabilization of bed morphology and also for establishing navigation channels. A typical Japanese river is characterized by large and sudden variations in discharge due to big amounts of rainfalls, so that the groynes are often in submerged condition. The present study focuses on the effects of submergence on the mean flow pattern and resistance around a single groyne. The PTV method and numerical shallow flow analysis are adopted in order to calculate velocities accurately. The flow pattern and flow resistance are estimated by comparing velocities at several points between upstream and downstream of groyne. The relations between flow resistance and submerged flow depth are discussed in the present study.

View full abstract

A lot of groynes are set up in Tonegawa. Recently, the groyne of the megalith volume in the Maebashi Park and the water passage part maintained as one body is constructed to the hemicycle among those to make a close contact with water image. There is a report that the scour damage of the shore protection and the existing groyne had generated at the flood decreased after constructing them. It is understood that the large-scale separating flow is formed in the downstream part around a hexahedron groyne to which the cylinder shape was put out according to a current finding. In the present study, the control effect of the flow decided to be clarified experimentally for the hemicycle groyne. As a result, the effect of avoiding water weakens more than a hexahedron groyne by setting up the hemicycle groyne. The range of the ascending current generated in the groyne downstream decreases as for the flow velocity decrease rate of the groyne of the hemicycle large from a hexahedron groyne.

View full abstract

The behavior of freshwater inflow in the northern part of Isahaya Bay during summer season in 2013 was preliminarily examined in order to elucidate a mechanism that occurs hypoxic water. Two freshwater origins were identified; one is from Chikugo River, and the other is from the gates of Isahaya Flood Regulation Pond. Fresh water originated from Chikugo River directly flowed into the northern part of Isahaya Bay via the western part of Ariake Bay. However, the inflow current became weak when the southwestern wind was strong, and the water column in the northern part of Isahaya Bay was not stratified. During the ebb tide, the freshwater flowed into the entrance of Isahaya Bay, and then the surface water with low salinity moved into the northern part of Isahaya Bay during flood tide.

View full abstract

This study examined effects of land load on the water environment of the Ariake Sea. The land and sea areas were examined by classifying them to western, inner, and eastern parts. Impact of the inflow water on each water quality has been grasped by the correlation analysis. In the period from 1981 to 2010, NO3-N, plankton deposition, DO and DIN decreased, while PO4-P, NO2-N, COD and transparency increased in the Ariake Sea. The increasing effect of the inflow water from the land on the western part sea is estimated with respect to the nutrient after 1997. Water quality in the eastern part is considered to be characterized by inflow loads characteristics of the basin. Water quality of the inner part is considered to be strongly influenced by the inflow loads. Water quality of the western part is considered to be dependent on the loads from the Isahaya Bay regulating reservoir.

View full abstract

We applied the Green’s Function approach to estimate control parameters of a numerical model for calculation of tidal currents and physical interpolation results from the radial M2 velocity observed by the Rader. We employed a 2-D model with wet-dry module using ROMS and calculated the M2 currents in the Ariake sea with 3 cases of which horizontal resolutions are 1/60, 1/120 and 1/240°. The four control parameters; (1) the magnitude and (2) the phase lag of tidal current ellipses and sea level from a global tidal model results, (3) the horizontal viscosity and (4) the sea bottom drag coefficient, were estimated by data assimilation of the radial M2 velocities by the RADAR and tidal gauge’s data into the model results. The estimated parameters suggest that (1) the magnitude and phase lag need not to change, (2) the drag coefficient is 1.8 × 10^-3, (3) the viscosity ranges 10 — 50 m² s^-1. Moreover the magnitude, phase lag and the drag coefficient are independent on the grid resolutions. The physical interpolation from all the radar observations represents the horizontal distribution of the tidal currents and sea level in the Ariake Sea.

View full abstract

We conducted four-dimensional variational (4D-Var) data assimilation system in the Regional Ocean Modeling System (ROMS) in the Osaka Bay and modified the adjoint model to avoid numerical instability near narrow straits. While sequential assimilations with twenty-four 1-hour assimilation windows, the previous adjoint model assimilation made blowing up in 19-hour assimilation window, but the modified adjoint model was carried out stably over twenty-four windows. In addition, both models of RMSEs indicated almost the same values until 18-hour window. One 24-hour, two 12-hour, four 6-hour and twenty-four 1-hour assimilations are performed with modified model. The 24- or 12-hour window showed large RMSEs of sea surface temperature between the model and observations. The variation of salinity RMSEs was small. There are still substantial challenges we have to set up configurations of background error covariance.

View full abstract

Object of this study is to predict distribution of radioactive contamination water in front of Fukushima by means of a 3-dimensional flow model linked with a diffusion model, released by Fukushima Daiichi Nuclear Power Plant. It was confirmed that radionuclides(Cs-137,Sr-90,H-3) distributions in coastal areas can be predicted by means of nesting form the whole Pacific Ocean model. Results were compared to the published observation data in order to examine applicability of the used models. As a result, the impossibility of the influence humanities was suggested to be with in front of Fukushima.

View full abstract

Laboratory experiments were conducted to propose a microbial fuel cell-based technology for recovering electrons from the sludge deposited in estuarine regions, in order to improve the reduced environment (i.e. removal of hydrogen sulfide "H₂S" and redox potential "ORP") of the sludge and the water environment. The results suggested that the electron recovery could perfectly remove H₂S from the sludge and increase the ORP of the sludge. By electron recovery, the ORP of overlying water was kept higher and the electrical conductivity was kept lower comparing with those without the electron recovery. These ensure that the electron recovery could also confine the ion diffusion from sludge to overlying water, which lower the aggravation of water environment by the ion diffusion from the deposited sludge.

View full abstract

This study has a quantitative evaluation on the functions of Wakatsu Port training levee in the lower reach of Chikugo River. Numerical simulations of flow and sediment transport in this lower reach of Chikugo River are implemented. 1D unsteady flow numerical simulation is executed to obtain the boundary conditions for a quasi-3D unsteady flow model which is used to simulate the detailed flow features around the training levee. The movement of sediment in the water is simulated by solving the 3D diffusion equation for the sediment. As a result of the 3D flow simulation, it becomes apparent that the existence of training levee causes to accelerate the flow of left bank side of the river. The sediment transport analysis gave understanding of the deposition conditions by comparing the river bed variation distribution.

View full abstract

The elucidation of the developing process of sand bar in the river mouth is important for river management. In this study, in order to clarify the developing process of sand bars in the Yura river mouth located in Fukuchiyama city in Japan, the field measurements of sand bar form were carried out. Numerical simulation were also carried out using a series of two-dimensional model of wave and bed material load. In those results, the numerical results for the developing process of sand bar agree well with the field measurement results, and the developing process of sand bar at the Yura river mouth was evaluated by results of numerical simulation and field measurements.

View full abstract

The rip current occurs at the wave breaking area near the beach, and the accident that is related to the rip current occurs every year in the world. Some generation mechanisms of rip current are known. For example, the rip current occurs at the sand beach that has the cusp topography, and occurs near structures such as piers or groins. In Niigata, there is not many large beach, and ordinary the beach is enclosed the groins those were built to protect from the coastal erosion. This study tried to comprehend the generation mechanism of rip current at the enclosed beach by the groins. Firstly, this study did field survey and comprehended the large scale and velocity of rip current. In this method, the rip current was pigmented by the sea water colorant, and the pigmented current was recorded as the aerial photography by the multi-copter. Secondary, this study reproduced the rip current by the numerical simulation. The result was compared with the field observation data.

View full abstract

The Northern Kyushu Heavy Rain in July, 2012 caused very serious damages. The event also caused many drift woods disasters in river basins and coastal areas. This research focuses on transport of drift woods in the Ariake Sea due to the rain event. Numerical simulation of their transport by 3D hydrodynamic model (DELFT3D) combined with assumed drift woods tracking model and comparison between the calculation result and collection data by Ministry of Land, Infrastructure Transport and Tourism were conducted. From the result, the followings were clarified: 1) total drift woods volume of discharged from all rivers into the sea was estimated as 17,500m³; 2) volume of discharged drift woods from each A-class river was estimated; 3) relationship between the volume and specific maximum river discharge was evaluated.

View full abstract

To estimate the fragility of wooden buildings (houses) in relation to the tsunami water depth and hydrodynamic force during the 2011 Great East Japan tsunami, a numerical simulation was conducted in two different regions using a nonlinear long wave equation model that includes the breaking or washout condition of trees and houses, and trapping function of driftwood by coastal forest. Fragility curves as a function of water depth, fluid force and moment by fluid force were analyzed for different level of house damages. Some differences exist especially for the fragility curve of washing out houses, but the curve has been improved by considering the trapping effect by trees, and become similar to that in the region where coastal forest was thin and breaching of embankment was not occurred.

View full abstract

The serious damage of breakwaters occurred in the tsunami caused by the 2011 off the Pacific coast of Tohoku earthquake. It is important to reinforce breakwaters with the concrete blocks and to give the function of tenacious structures. Hydrodynamic experiments had been performed to study the damage of concrete blocks armoring on rubble mounds for the over flow in tsunami. In the case of breakwater, the critical values of the overflow depth above the crest of breakwater, in which the damage of concrete blocks occurs, are affected by the height of breakwater above the still water level and the water depth from still water level to the foundation of reakwater. The positions where the damage of armor concrete blocks occur on the foundation change with the overflow depth above the crest of breakwater.

View full abstract

In order to examine a water environmental improvement, human sense data on the water environment is very important criteria. However, human sense on the water environment has not been discussed in detail. This study examined the feature of human sense on dirtiness, color and smell of the water environment and the relation between human sense data and physical behaviors which are tide, rain situation and seasonal change. This study area is Hori River which flows in Nagoya City. The time and area on the worst condition are obtained with the time-space distribution of the human sense data on dirtiness, color and smell. Moreover, the difference of human sense between dirtiness and smell is shown clearly. Finally, the human sense models are developed with the Hayashi’s quantification methods. The validity of models, the difference of analysis results between dirtiness and smell, and future subjects are shown by comparison forecasting human sense data and questioner results.

View full abstract

Reservoirs occupy a large portion of water resources in Japan. Therefore, in this study, relationship between watershed conditions and nutrient concentration, loading and inorganic ratio of inflowing river to reservoirs was investigated by statistical analysis. 52 reservoirs were chosen from all over Japan. The mean concentration of total nitrogen was strongly influenced from artificial factors in basin. Phosphorus and nitrogen loading were estimated by two ways; multiple regression analysis or equations obtained by multiplying influent quantity and nutrient concentration. Those equations were capable of estimating nutrient loadings of inflowing river with high precision.

View full abstract

We have designed a hydrodynamic simulator with an idea inspired from biological-cell structure. The simulator updates status of cell and membrane objects by nerve-like objects, rather than updates values stored in index-based numeric matrices. The water body may be composed of a single cell (unicellular), or cells (multicellular) interconnected through membranes. The membrane acts as a flux-controller between cells on both sides, i.e., computes advection and diffusion fluxes. Dependent variables and source terms are all objects and embedded in a cell like organelles of a biological cell. Each model object is implemented in C++ to obtain reasonable computational speed, and used from a script language Python to increase user flexibility and extensibility. We explained the advantages of the present design against to the previous ones, and confirmed the accuracy of the present model using analytical solutions.

View full abstract

Lake Izunuma is a shallow eutrophic lake located in the northern part of Miyagi Prefecture, Japan. Lotus (Nelumbo nucifera) is the dominant species of the lacustrine vegetation, which has possibilities of accelerating the eutrophication through long-term accumulation of the dead body. In this study, field observations were conducted to understand annual cycle of nutrients (nitrogen, phosphorus, carbon) with regard to growth and mortality of lotus community. Nutrient stock was calculated from the results of the dry weight and nutrient content rate of lotus. Nutrients are stored mainly in living lotus shoots (leaves and stems) during the growing season, while they were transfered to dead body accumulated on the bottom after late autumn. Moreover, living lotus of approximately 388.5 g m-2 is supplied as dead body, and dead body of approximately 32.3 g m-2 accumulates without being decomposed in one year. From this result, possibility of organic matter storage in Lake Izunuma was suggested.

View full abstract

In order to develop an alternative denitrification system for purification of leachate from a municipal waste landfill, a laboratory experiment was carried out. Denitrification performance using alternative carbon resources in place of methanol was experimentally investigated in a laboratory tank. The carbon sources under investigation were hydrogen feeding agents, beer lees and solidified food oil. Hydrogen feeding agents and solidified food oil showed higher denitrification performance than the beer lees. The water quality model was verified by comparing the analytical solutions with the laboratory data using hydrogen feeding agents as carbon source. Time-dependent behaviors of every water quality component observed in the experiment were very well reproduced by the model. The analysis showed that the hydrogen feeding agent was found to be a very promising material for efficiently removing nitrogen from seriously contaminated leachate.

View full abstract

Oxygen deficit water near the bottom of Lake Suwa has often been measured during the summer since the 1990’s. This report is based on field measurements at one point and intermittently with a cycle of two weeks. In this study, continuous field measurements of dissolved oxygen near the bottom at several points were conducted in order to understand the temporal and spatial characteristics of the oxygen deficit water in detail. The main results indicated that: 1) Rainfall and strong winds often put an end to the oxygen deficit state, but the variation in air temperature and water temperature of the inflow from rivers after these phenomena play an important role in the regeneration of the oxygen deficit state. 2) At night on some fine weather days, strong water cooling to the surface water by cold air supplied oxygen to the bottom water due to the disappearance of daily stratification. The spatial distribution is not uniform and this phenomenon was often measured at the north-west part in a lake especially.

View full abstract

The heat budget at water surface is composed of short- and long-wave radiations and latent- and sensible-heats. In this paper we improved the estimation of short- and long-wave radiations that have major effects on the water temperature change in a reservoir by analyzing observed radiation data at Tono Reservoir and simulating water temperature in the reservoir. The results are summarized as follows: 1) we can improve short-wave albedo by an equation that takes solar zenith angle, wind speed, humidity and scattered light ratio as parameters, 2) downward long-wave radiation estimation can be improved by considering not only cloudiness but also air temperature and proper emissivity, 3) the observed emissivity for upward long-wave radiation was 0.99 and 4) the accuracy of the downward long-wave radiation has more effect for improving water temperature estimation in a reservoir than short-wave albedo.

View full abstract

Regarding sediment bypassing operation, real case study is very limited to only Miwa dam or Koshibu Dam in the Tenryu river. Therefore, further study is needed in order to assess quantitative efficiency of sediment bypassing based on reservoir geometry, hydraulic conditions and deposited sediment property. We have developed a three-dimensional sediment transport model that can calculate the bed deformation and complex flow around the diversion facility. We compared the numerical and experimental results on diversion facilities and confirmed the validity of the model. Using this model, we performed calculations by varying the particle size of sediment, the slit width of diversion facility and flow rate. Based on these calculation results, we investigated three dimensional effects on sediment deposition and discharging process from upstream of diversion weir to sediment bypass tunnel. Finally, we have discussed sediment bypassing ratio and proposed empirical equations with sediment properties and diversion facilities.

View full abstract

New method of generating the upwelling current which advances aeration of density stratification of ponds was discussed based on the numerical simulation and water tank experiments of buoyant and non-buoyant condition. Accuracy of the numerical simulation was verified by previous study on non-buoyant jet and it showed good agreement. Unsteady experiments of slanting-discharged buoyant wall jet indicated that the discharged jet formed the vertical rise upwelling current with low pressure and mixed upper and lower layer. In addition, the non-dimensional gross entrainment volume of the vertical rise upwelling current is estimated by 0.5 which is almost equal to the entrainment volume of wall jet. The presented method generates steady upwelling current which has the entrainment volume of wall jet.

View full abstract

This study describes the development of a three-layer model based on a fully-nonlinear and strongly-dispersive internal wave model and its application to a stratified lake response. The efficient numerical model for three-layer system is needed to analyze non-linear and non-hydrostatic lake response as a two-layer model often fails to illustrate realistic stratified lake responses. Therefore, this study aims to develop a three-layer model based on the fully-nonlinear and strongly-dispersive internal wave model (the three-layer FSI model). The three-layer FSI model was verified by simulating the belt-driven three-layer laboratory experiments by Stevens & Imberger (1996). The present model well predicts the measured results of the experiments and supports the classification by the Wedderburn number and Csanady’s theory. The computed results also show the importance of non-hydrostatic pressure on the generation of the internal solitary wave during upwelling process.

View full abstract

Currently, the progress of eutrophication in various closed waters such as a reservoir is causing the outbreak of algae there. As a method to mitigate this problem, is the introduction of a jet pump type flow device. This device can cause the jet of mixed water of the upper water and lower water by sending the upper water to the pipe that was placed in the lower layer. However, the flows characteristics such as suction or entrainment change by the shape of the device. Therefore, in this study we used a model of a jet pump type flow device, experimentally examined the relationship between device shape and flow characteristics. In addition, we developed the theoretical formula. Finally, we judged the device shape quantitatively by comparing a theory value with measurements. The results were as follows: (1) By a theoretical formula, straight pipe is better for suction. (2) In order to mix enough the length of the tube must be at least 5 times the diameter.

View full abstract

For operation and management of dams, it is most essential to evaluate inflow and outflow discharges in dam reservoirs. Outflow discharges have been evaluated from reservoir water elevations and gate openings. However, water surface elevations might be different by observation point at a meandering part of reservoir. In this study, a 2D depth-integrated flow model was improved in order to consider water surface elevations near gates. Moreover, estimation method of outflow discharge was proposed using charts, which were constructed employing the 2D flow model. The comparison between the numerical and observed results revealed that the proposed method could reasonably reproduce the outflow discharge even if difference in water surface elevation was generated.

View full abstract

To develop early tsunami warning system utilizing ocean radars, the evaluation of the variety of measuring coverage and data accuracy is indispensable in real oceans. The field observation was carried out at 5 minutes interval with two digital beam forming ocean radars with VHF band from 2012 to 2014 in the sea of Enshu. The high data acquisition areas are found in the extent of 17km off the coast on a hill site and of 13km on a low ground site. The measured current by the ocean radar were well correlated with that by the current-meter in the depth of 2m near the coast with the correlation coefficient of ~0.6. It is inferred that the main factor of difference in both data sets was due to the presence of wind-driven current through the multi-regression analysis with both current data and wind data. In addition, the order of the temporal current deviations as to the representative time-scale of one hour is about 5cm/s under the ordinary sea conditions, which suggest that ocean radars could sufficiently detect the current deviation due to grant tsunami.

View full abstract

Recently, instruments based on ultrasonic attenuation spectroscopy have been developed to measure the concentration and the particle size distribution of suspended sediments in running water. In this paper, we investigated each ultrasonic attenuation spectra for suspensions of sediment particles with irregular shapes and of spherical glass beads, by using an ultrasonic pulse with broad bandwidth. The measured attenuation spectra cover a wide frequency band of 5-8MHz. The experimental results demonstrated that the spectra could be brought into close agreement with the rigid movable sphere model using two equivalent sphere size parameters. The findings indicate that a methodology using equivalent spherical scatterer model is applicable to measurements of the concentration and the particle size distribution of suspended sediments in running water.

View full abstract

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.

View full abstract

In recent years, the so-called unmanned air vehicles (UAV), remotely controlled airplanes or multicopters, have become available for various civil engineering purposes. In the field of river engineering, although they have been used to investigate the area of vegetation zone or other objectives, measurement of river flow has not been conducted by using an UAV, probably due to the difficulty of image stabilization. In this research, we developed a method to measure river surface velocity distributions by using videotaped images. The specific feature of the method is the introduction of a high-accurate and efficient image stabilization in order to apply the space-time image velocimetry (STIV) to airborne images. The developed method was applied to investigate snowmelt flood of the Uono River, with success of measuring twodimensional velocity distributions for a river reach of about four hundred meters. The obtained results were used to compare with those measured by ADCP or STIV using oblique images from a riverbank.

View full abstract

In this study, we confirm accuracy of velocity and discharge evaluated by a dynamic interpolation and extrapolation (DIEX) method which has been developed by the authors. We here apply the present and conventional methods to evaluate cross-sectional velocity and discharge from point velocities of simulated float data. The results indicate that the DIEX method has high accuracy for discharge evaluation from point velocities even when spatial resolutions of velocity data was low, showing wide applicability of the DIEX method. The results also revealed that the accuracy of the conventional method for discharge evaluation was also high when spatial resolutions of velocity data was high and appropriate coefficient of float was used.

View full abstract

It is of special importance to measure mean velocity and discharge in natural rivers for preventing flood disasters. This study developed autonomous-mobile floating-robot for the purpose of automatic measurement of mean velocity in an open-channel flow. A PID control method could keep the position of the floating-robot against main stream, and then mean velocity could be evaluated reasonably by a rotational speed of screw propeller. Accurate laboratory experiments using LDA provide the calibration curve that connects the propeller speed and mean current velocity. Furthermore, it was recognized that our robot can move successfully in the horizontal plane from the start point to the target one.

View full abstract

In applying image-based techniques such as space-time image velocimetry (STIV) to river flow measurements, the images usually taken from a riverbank have to be rectified by using several ground control points (GCPs) set along the river. However, the rectification process sometimes generates erroneous result due to measurement errors of GCP or even when the measurements are correct, the rectified images are difficult to use for a far region due to the lack of enough special resolution. Furthermore, appropriate setting of GCPs becomes difficult for the flow measurement of deep valley. Therefore, we developed an efficient correction method of rectified images by using functional relation of the collinear equation. Moreover, a new method by using a digital clinometer while shooting image is proposed that can reduce the number of GCP in the actual field measurements. An evaluation method for the measurement accuracy of STIV is also proposed.

View full abstract

Bedrock erosion rate E due to collision of saltating bedload is expressed by E=βq_b (1-P_c), where q_b is the bedload transport rate, P_c is the areal fraction of bedrock covered with gravel and β is the abrasion coefficient. It is known that bedrock becomes weathered to have many cracks and voids after repeated wet-dry cycles. However, the possible impacts that weathering of bedrock has on the erosion rate have not been elucidated. In this study, laboratory experiments are conducted for understanding how weathering of bedrock would affect the rock tensile strength and the erosion rate. For this purpose, boring cores taken from natural bedrock rivers are exposed to artificial wet-dry cycles. The experimental results suggest the following: (1) the abrasion coefficient is estimated by β=1.4×10^-4σ _T^-2 (d/k_sb)^0.5, where d is the size of colliding gravel and k_sb is the bedrock hydraulic roughness height; (2) the tensile strength of the bedrock decreased exponentially due to repeated wet-dry cycles, σ _T/σ _T0 = exp (-C_dwNW_a0/σ _T0), where σ _T0 is the initial rock tensile strength, W_a0 is the initial water absorption rate, N is wet-dry cycles and C_dw is a constant.

View full abstract

Erosion process of clay riverbed was investigated. The clay riverbed is eroded by the action of water flow. It is also eroded by moving sand and gravel. In this study, the influence of moving sand and gravel on erosion of the clay is investigated. This process is affected by the amount of sand and gravel transport in the river. Movable bed experiments were conducted under the several conditions of both the flow discharge and the rate of sediment supply. As a result, we found that the relationship between the erosion rate of the clay riverbed and the rate of supplied sand has extreme value. It was also found characteristic vertical structure under the bed surface.

View full abstract

Experimental investigation was conducted on the dynamic equilibrium state of bed which was composed of extremely wide range of sediment. A void of larger stones was filled with medium gravel which moves as bed load and finer sand which moves as suspended load. Experiment was conducted with supplying gravel and sand. The flow discharge and the sediment supply discharge of gravel were set as fixed value, and the sediment discharge of sand was changed. The layer of only gravel was exposed on the bed surface under the condition that the discharge of sand was small. On the other hand, if the supplying rate of sand was large, the layer which was composed of both gravel and sand was formed below the bed surface. It was verified that this results were not changed even if the flow rate and the discharge of gravel changed.

View full abstract

Present study aims to investigate the influences of lateral bed slope and turbulent diffusion on the erosion and deposition processes in suspended sediment dominated rivers. An expression of erosion term in bank region, which is composed of mass conservation equations for sediment in fluid body and bed layer, is derived based on the mechanism of sediment transportation by turbulent diffusion. Lateral profile for the sediment concentration which yields two peaks is derived theoretically from a depth integrated convection equation in an idealized flow field. Results of numerical computation on bed evolution suggest that the ridge due to sediment deposition is formed in bank region, erosion area is produced at the inside of the ridge corresponding to the peak sediment concentration, and a flat sediment deposition area takes place due to sediment transpot by turbulent diffusion in the channel center as well. The proposed modified erosion term amplifies such lateral bed morphology.

View full abstract

This paper presents numerical simulations of tracer particle transport both in the absence and presence of free bars. The transport and diffusion of tracer particles is affected by both the probabilistic motion of sediment particles and by the river bed geometry associated with free bars. We compare results of numerical models using an equilibrium (flux-based model) and a non-equilibrium (entrainment-based model) conservation formulation for river bed variation associated with bedload transport. The results show that 1) the entrainment-based morphodynamic model successfully reproduces free bar formation, 2) the flux-based morphodynamic model fails to simulate tracer diffusion in the absence of bars, and 3) free bar formation enhances streamwise tracer diffusion.

View full abstract

This study describes a numerical model developed to simulate flow and bed deformation around river hydraulic structures. The numerical analysis for fluid solves the Reynolds-averaged Navier-Stokes (RANS) equations coupled with the Volume of Fluid (VOF) method and k-omega SST turbulence closure model. The analysis for fluid is carried out using the open source computational fluid dynamics software package OpenFOAM. The temporal change in bed topography is calculated by coupling a stochastic model for sediment pick-up, deposition, and transition using a momentum equation of sediment particles in order to account for the effect of nonequilibrium sediment transport. The numerical model was applied to the scour phenomenon upstream of a slit weir, which physically simulates sediment releasing from a dam gate. A comparison between the numerical results and experimental data indicated that the model can reproduce the scour geometry around the slit weir with sufficient accuracy.

View full abstract

This study focuses on the hysteresis of dynamic equilibrium bed composed of fixed bed and movable bed. By experimental results, the irreversibility of bed deformation between the movable and fixed beds was observed. By supplying sediment, in a rate lower than the equilibrium rate from the upstream, the bed was widely exposed. Then, we increased the sediment rate to be equal to the equilibrium rate, however the bed stay exposed without sediment layer. By increasing the sediment rate to be higher than the maximum bed load rate on the fixed bed, the bed was covered by sediment layer. In numerical analysis, the bed load layer model was applied, considering the maximum bed load rate on the fixed bed. The results of numerical analysis showed a good agreement with the experimental results. Moreover, numerical analysis showed that when the longitudinal exposed area was large, the area was hardly filled with sediment even if supplied sediment was more than the maximum bed load rate on fixed bed.

View full abstract

Detour flow around a bridge causes floods and bank erosions every year all over the world. However, the flow and the bank erosion characteristics of detour flow around a bridge are not studied well, as a result, the countermeasures for the detour flow are not taken well to design levees and bridges. In this study, the flow and bank erosion characteristics of detour flow around a bridge are discussed by using plane two dimensional bed deformation analysis. The results show that the non-dimensional shear stress at the top edge of a levee in the downstream area of the bridge is large, because the detour water flow back to the main channel there. As a result, the bank erosion is initiated there. Subsequently, the road connecting to a bridge suppresses the progress of the bank erosion to the upstream of a bridge. Additionally, the bank erosion in the downstream of the bridge is also suppressed.

View full abstract