Ichiro Fujita

Kobe University | Shindai · Department of Civil Engineering

The vorticity effect on solitary wave profiles has not been solved experimentally; previous studies theoretically and numerically showed that when a solitary wave progressed in the positive direction, the effective wavelength of a solitary wave with positive vorticity increased. Using laboratory experiments and fully nonlinear numerical simulations...

In the non-contact method of observing river surface velocities, the flow rate is generally estimated by an area-velocity method with a constant surface velocity coefficient. In practice, however, the surface velocity coefficient is expected to vary depending on the river geometry and other factors. A method using a vertical velocity distribution e...

In recent years, due to the frequent occurrence of floods that exceed the facility maintenance level due to climate change, non-contact flood flow measurement techniques have been paid attention and actually some measurements have been conducted by applying them instead of the conventional float method. The space-time image velocimetry (STIV) which...

Image-based river flow measurement methods have been attracting attention because of their ease of use and safety. Among the image-based methods, the space-time image velocimetry (STIV) technique is regarded as a powerful tool for measuring the streamwise flow because of its high measurement accuracy and robustness. However, depending on the image...

The surface velocity coefficient is the value that converts the surface velocity into the depth-averaged velocity when observing the discharge, and 0.85 is treated as the standard value in Japan. In recent years, surface velocity measurement by STIV is widely used, but only surface flow information can be obtained. Therefore, it is important to pro...

It has been observed that a periodic water surface oscillation ocurrs downstream of a drop structure with a trench at specific hydraulic and geometric conditions. In order to establish a relationship governing the oscillation frequency, a number of experiments using flow visualization and image analysis techniques were conducted. As a result, we pr...

This research presents an application of the sampling moiré method, a novel technique originally developed for measuring slight deformation of a static object such as a plate, to fluctuating free water surface in rough open channel flows. The method was used to capture the two-dimensional water surface deformation at a high sampling rate by dyeing...

In stratified flow, breaking of internal waves over slopes induces resuspension of bottom sediments and transport of mass. When internal waves shoal and break, flow dynamics and mass transport differ significantly according to whether the Coriolis force is included or neglected. Despite its importance, the currents generated by breaking internal Ke...

With the flood disasters that have been occurring frequently in recent years, the sophistication of discharge observation has been promoted. Since the existing river monitoring cameras can be used, interest in image measurement methods is increasing. Among the image measurement methods, the STIV method is regarded as a powerful method because of it...

Triangular-shaped surface waves occur with large amplitude in a river during a flood. The large amplitude surface waves are concerned to cause damage to river structures. Higashikawa et al. (2019) found that the train of triangular-shaped surface waves occurs due to soliton resonance. They assumed that the propagation direction of the soliton reson...

In shallow flow conditions, turbulence effects appear on a water surface as a form of irregularity of surface shape composed of a large number of fluctuating ripples. The intensity of such a fluctuation increases with the Froude number and also with the Reynolds number as can be observed in flooding river flow. In such a flow condition, surface irr...

The understanding of river flow in various flow conditions is a crucial issue in managing river environment. To satisfy the requirement of such needs, river flow velocity and discharge have been measured from low to high flow conditions. The typical way to acquire such data has been to use either a probe-type current meter or the Acoustic Doppler C...

Space–time variation of the free-surface roughness pattern, due to sub-surface turbulence, is one of the most important features of turbulent open-channel flows. Despite its importance, the free-surface dynamics is not fully understood, especially regarding the advection and propagation features of surface variations. In this study, a direct numeri...

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 visio...

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 fre...

In rapid flow rivers, it has been reported that, under high-speed flow conditions, an anti-dunes becomes three-dimensional and triangular wave sequence, in which water surface waves become steep, are generated. There are concerns about the influence of this surface wave sequence on bridges and river structures once thy are generated, but the condit...

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...

Due to the remarkable development of unmanned aerial vehicle (UAV) in recent years, its application in river engineering increases widely mainly for the measurement of ground topography such as by the technique Structure from Motion (SfM) using a series of high-resolution static images. However, although UAV usually installed a high density video c...

The heavy rain disaster in the Kinugawa River basin that occurred along with the passage of the Typhoon 18 caused the embankment destruction in the middle reach of the river on September 10, 2015. Due to the overflow, the houses in the vicinity of the embankment collapsed, causing a flood inundation spreading over a wide area. Because the embankmen...

In the unseeded image-based techniques for river surface flow measurements, advection speed of surface textures composed of surface ripples or floating objects is measured by image analysis. However, the methods would yield erroneous information when the surface texture is affected by gravity waves propagating in all directions. In order to improve...

In the present study, direct numerical simulation (DNS) in a rough-wall turbulent open-channel flow with spheres was performed. The free surface was captured by a volume of fluid (VOF) method and the spheres on the bottom were treated by immersed boundary method (IBM). Flows in different Froude number from sub-critical flow to limited flow were cal...

Due to the remarkable development of UAV in recent years, the application in river engineering increases widely mainly for the measurement of river topography. However, UAV has not been used for river flow measurements. Therefore, aerial STIV (Space-Time Image Velocimetry) technique has been developed that utilizes the stabilized video images and f...

For establishing non-intrusive techniques for river surface flow measurements, video images shot from riverbank by a river monitoring camera have been paid attention in the past decades and the image analysis techniques such as LSPIV or STIV have been developed. Although these image-based techniques assume that water surface textures are advected w...

In this study, we conducted a long-term monitoring for river water-level and discharge by using STIV-DIEX integrated system which authors have presented. Furthermore, hydraulic experiments is conducted in order to confirm the environment dependence. The results indicated that the automatic continuous monitoring was successful for nine monthes from...

Heavy rain disasters in the Kinu River occurred in September 2015 caused a major disaster such as a collapsed house near the embankment due to the embankment destruction and the overflows resulting therefrom. Since the levee breach occurred during the day, the overflowing flow from the broken levee was recorded by a media helicopter. In this study,...

Despite the remarkable improvements of flow measurement techniques and numerical models in recent years, the features of free surface fluctuations over rough wall turbulent flow have not been elucidated yet. In the present study, three dimensional measurement of time-dependent free surface profiles was performed in order to clarify characteristics...

Turbulent open channel flow with strip roughness is one of the fundamental flows representing rough wall turbulence. This phenomenon and the relation between roughness spacing and flow resistance of relatively large water depth have been discussed so far. However, surface flow features under shallow water conditions have not been investigated in de...

The Ichikawa River is a second class river that flows down south of Himeji City, Hyogo Prefecture, where various river structures are installed. Among the river structures, there is a weir, the Hanada Weir, installed at an angle of 45 degrees to the direction of the river. To investigate the effect of the weir, surface velocities were measured by a...

The space-time image velocimetry (STIV) technique is one of the image-based techniques for measuring river flow rate using video images. However, there are cases where adequate measurement accuracy is difficult to maintain depending on the shooting conditions of the river surface, and robustness has been required as a measurement system. In this re...

Measurements of flood flow have been conducted intensively at major control sections in Japan for storing reliable hydrological data for use in a long-term river planning. However, such a measurement pays attention only to flow passing through one cross-section; thus, spatial flow features are not available, while river channel changes its feature...

The space time image velocimetry (STIV) technique is presented and shown to be a useful tool for extracting river flow information non-intrusively simply by taking surface video images. This technique is applied to measure surface velocity distributions on the Uono River on Honshu Island, Japan. At the site, various measurement methods such as a ra...

Although there are many researches regarding strip roughness effects on flow resistances, researches under shallow flow conditions have been rarely conducted. In this study, we conducted shallow water experiments by varying roughness spacing, bed slope and the Reynolds number, and categorized water surface patterns into 4 patterns, that is, Flat su...

In order to examine the water surface features in a laboratory, the sampling moire method was introduced to capture time-dependent two-dimensional profiles. The target flow was a shallow open-channel flow roughened by density packed grass marbles. In order to clarify the spatial and temporal behavior of water surface fluctuations, the frequency wav...

One of the characteristic features of turbulent open-channel flows is the free-surface pattern shows time-dependent variations subject to the effect of turbulence. However, the free-surface dynamics is not fully understood especially regarding the advection feature of surface variation. Hence, Direct Numerical Simulation (DNS) that allows the defor...

In developing an efficient scheme for flood risk management, real time information of discharge as well as water level could provide more improved estimation. However, discharge information is obtained only from a stage-discharge relation developed from the past data, in which flood discharges are measured by a float method that sometime yields unr...

Recent frequently occurring floods promote the improvement of river-discharge measurement techniques for a better river management plan. Among them are ADCPs, radar velocity meters and image techniques. The advantage of image-based technique is its safety in measurements because images can be captured from a remote place. Compared with typical tech...

Kobe University provided a training program in Japan within a SATREPS project to three Ghanaian hydrologists on discharge measurement by using an image analysis software KU-STIV, developed at Kobe University for flow measurement. Then, three delegates from the Kobe University, Japan visited Ghana to actually measure stream flows at three rivers sta...

In estimating river flow discharges from water surface velocity distributions measured by non-contact methods such as radio wave velocity meters or image-based techniques, it is important to use an appropriate value of surface velocity coefficient that yields a depth-averaged velocity from a surface velocity. Conventionally, a value of 0.85 has bee...

In shallow water conditions, open-channel flows are subject to the influence of channel bottom roughness because the boundary layer or vortices generated at the bottom interact with water surface. Such an influence become more pronounced when the bottom is composed of discrete roughness such as a strip roughness. In the present study, water surface...

In a relatively small urban river such as the Togagawa River in Kobe, a torrential rain causes a sudden increase of water depth and in the worst case a drowning accident such as occurred in 2008. In order to prevent such a disaster, both structural and non-structural countermeasures are required. However, in either case accurate estimation model of...

The advantage of aerial vehicles in observing various geographic features is its wide field of view, which is different from an observation from the ground. Although the field of view depends on the height of the vehicle and the lens aperture, an area with a side length of more than one hundred meter can be observed at one time. As for measurements...

Three dimensional and unsteady flow structures generated at a river confluence are still not fully understood. In the present study, a fully-resolved LES (Large Eddy Simulation) with a VOF (Volume of Fluid) method capable of treating free surface boundary was performed to investigate three dimensional structures of turbulent flow and water surface...

Aerial photographs of a cobble bar taken by an unmanned aerial vehicle (UAV) are analyzed for the purpose of measuring spatial distribution of cobble sizes before and after a flood occurred in the Ibo River in 2013. The photographs cover the bar area spanning about four hundred meters and the images were analyzed by using a software BASEGRAIN that...

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 u...

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...

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 chara...

Image-based techniques of flow surface measurements were used to evaluate calculated results by a two-dimensional flow simulation for the 2011 flood of the Ibo River. The target site has a large cobble bar at an abrupt bend, where the flow is made complicated with a transverse step just upstream of the bend and large-sized rocks distributed in its...

Irrespective of their spatial extent, free-surface shallow flows are challenging measurement environments for most instruments due to the relatively small depths and velocities typically associated with these flows. A promising candidate for enabling measurements in such conditions is Large-Scale Particle Image Velocimetry (LSPIV). This technique u...

The Chikusa River in Hyogo Prefecture suffered from extremely heavy rains by Typhoon Number 9 (Typhoon Etau) in 2009. In the Sayo-cho area, the inundated flood claimed more than 20 lives, many of whom were trying to evacuate from the inundated rising water. To investigate the cause of such a disaster, we paid attention to people's behaviour in two...

Measurements of flood flow have been conducted intensively at major control sections in Japan for storing reliable hydrological data to use for a long-term river planning. However, such a measurement pays attention only on a flow passing through one cross-section and thus spatial flow features are not available, while river channel changes its feat...

An image-based field measurement method of river flow was applied to a large flood that occurred in the Ibo River and the measurement results were compared with those calculated by two and three dimensional numerical simulation models for evaluating the accuracy of simulations. The target river site has a large bend with a cobble bar on its right i...

In open-channel turbulence with relatively large Reynolds number, water surface is subject to various types of fluctuations depending on the strength of turbulence. The level of fluctuation varies also with the Froude number and several researches have been conducted with respect to the root-mean-square value of surface oscillation. However, the me...

In the afternoon of 28 July 2008, a flash flood occurred in the water-friendly reach of the Toga River in Kobe City, tragically drowning five people that included three children. They were among about 50 people enjoying the river environment. The flash flood was caused by sudden localized torrential rain in urbanized area of the small river basin....

Three dimensional open-channel turbulent flows with rough wall beds composed of randomly distributed gravel particles were simulated by using a large eddy simulation model for a relatively shallow water condition. The bathymetry of an actual rough bed was measured by a photogrammetric method and the digital data thus obtained was directly introduce...

A simple but accurate interpolation procedure for obtaining the three-dimensional distribution of three-component velocity data, from moving acoustic doppler current profiler (ADCP) observation data, is proposed. For understanding actual flow structure within a river with complex bathymetry, the three-dimensional mean velocity field provides a basi...

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 o...

In non-intrusive river flow measurements such as imaging or radar techniques, water surface irregularities in turbulent flow condition is assumed advected with the surface flow. The surface irregularities are usually considered to be induced by turbulence originated from boil vortices generated near the rough bed. However, the relationship between...

This paper examines the feasibility of applying a mobile, large scale particle image velocimetry (LS-PIV) system to a 300 m reach of a small river in order to estimate the discharge. Detailed velocity measurements at a number of locations were carried out using an acoustic Doppler current profiler (ADCP) and acoustic Doppler velocimetry (ADV) for i...

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 cau...

When a rectangular side cavity section is installed to a relatively narrow open-channel flow, water surface oscillation is generated within the side cavity and main channel. However detailed experimental examinations of the three-dimensional flow field for these hydraulic structures have not been conducted so far. So we measured the three-dimension...

Monitoring of river is significant both in flow management and river usage. Although river monitoring systems have been installed in many rivers in Japan, most of the existing system do not record images and utilize them for quantitative measurement of the flow. Already proposed methods for quantitative measurement include image analysis techniques...

In open-channel turbulent flows with a rough wall, water surface profiles can be affected by the relative size of roughness element to a water depth and arrangement of roughness elements. In order to find characteristics of water surface profiles under various hydraulic conditions, an experimental study by using a simple surface flow visualization...

In this study, a closed-circuit television (CCTV) system, installed for surveillance purposes, is utilized to measure the flow rate during a flood. The procedure to determine both the angle and scale-factor of the camera is described. Then, image analysis techniques, namely the direct visual measurement method, Large-Scale PIV (LSPIV) and Space-Tim...

Conventional Large-Scale Particle Image Velocimetry (LSPIV) implementation to river flow measurement is based on video recordings acquired with the camera set at a fixed location. During floods, it is of high interest to map reach of rivers that are at bankfull stage or flowing over the floodplain. In order to accommodate this interest a new approa...

Water surface fluctuations in river flows are caused by various factors such as wind effects, near-surface turbulence, or collision of boil vortices against water surface. When we view the river surface from a riverbank, such effects can be recognized as a surface image pattern moving with the flow, which can be utilized for non-contact surface flo...

The open-channel flow with a side cavity is known to exhibit various types of free surface oscillations within the cavity zone. A typical type of oscillation observed is similar to the one by a seiche generated in a closed box; however it has been reported that other types of oscillation can also be generated depending on the geometrical and hydrau...

By installing a side cavity section to a relatively narrow open-channel flow, water surface oscillation is generated within the side cavity and main channel. It has been found that there exist several modes of oscillation that depends on the aspect ratio and the Froude number by the authors; however, the three-dimensional flow structure has not bee...

At the location where a backward-facing step or, in the river engineering sense, a drop structure is installed in an open-channel, flow separates and generates a relatively large recirculating region while actively shedding separation vortices in the downstream direction. The shedding of the vortices can be reduced by installing a mild slope at the...

Inundation disasters, caused by sudden water level rise or rapid flow, occur frequently in various parts of the world. Such catastrophes strike not only in thinly populated flood plains or farmland but also in highly populated villages or urban areas. Inundation of the populated areas causes severe damage to the economy, injury, and loss of life; t...

This paper reports on an estimation method used to quantify discharge in the small irregular channel of the River Blackwater, using large-scale particle image velocimetry (LSPIV). The test reach (250 m 3 60 m) consists of relatively straight and multiple meander channels, with a bankfull depth of 0.90 m and a base width of 4.25 m. Water surface vel...

The effect of particle density on the turbulent open-channel flow carrying dilute particle suspensions is investigated using two specific gravities and three concentrations of solid particles. The particles, identical in size and similar in shape, were natural sand and a neutrally buoyant plastic. The particles were fully suspended, and formed no p...

Large-Scale Particle Image Velocimetry (LSPIV) is a non-intrusive approach to calculate velocities at the free surface of a water body. The raw LSPIV results are instantaneous water surface velocity fields, covering flow areas up to hundreds of square meters. LSPIV configurations and operational capabilities have improved due to improvements in har...