Kiyosi Kawanisi

Characteristics of suspended sediment and turbulence in a tidal boundary layer

High-resolution measurements of three velocity components and the concentration of suspended sediment (SS) have been performed in the Ota diversion channel through a tidal cycle. Data are collected with an acoustic Doppler velocimeter at various distances from the bottom. Turbulence measurements are extended to the immediate vicinity of the bottom. Turbulent fluxes of SS concentration are directly estimated from the fluctuations of concentration and velocity. Both the mean concentration and the vertical turbulent flux increase with the Reynolds shear-stress, though the mean concentration lags the shear stress. The frequency range in which the fluctuations mainly contribute to the vertical turbulent fluxes of SS concentration is higher than that of the Reynolds shear-stress ρu*2 near the bottom. The settling velocities of SS, ws, are estimated from the transport equation of suspended sediment. The values of ws decrease during the large shear velocity. The vertical profiles of vertical eddy diffusivity are shown. The ratio of the momentum and sediment diffusivity coefficients, β = Nz/Kz, decreases with increasing values of u*/ws and the SS concentration.

Signal post-processing for acoustic velocimeters: detecting and replacing spikes

Time series recorded by acoustic velocimeters are often affected by a combination of factors, including turbulent velocity fluctuations, Doppler noise and signal aliasing. Although it is not possible to find a comprehensive threshold for identifying spurious data, the present work attempts to describe an effective technique for detecting spikes. This technique is based on transforming data into wavelet space and thresholding the wavelet basis by a consistent threshold. The universal threshold modified by a robust scale estimator such as Qn is proven to work extremely well. The suggested methods for replacing identified spikes combine times series analyses (linear time series modelling or a Kalman predictor) with a straightforward method, polynomial interpolation, to generate substitutions retaining both the trends and the fluctuations in the surrounding clean data. Then, tests were performed to reveal the influence of replacing methods on the total number of detected spikes, required iterations and physical properties of the restored signal. From the overall results, it is inferred that using the wavelet-Qn as the detecting module and integrating it with linear time series modelling/Kalman filtering as the replacement module constitutes an effective despiking algorithm. This methodology is capable of restoring the contaminated signal in such a way that its statistical and physical properties correlate well with those of the original record.

Continuous monitoring of a dam flush in a shallow river using two crossing ultrasonic transmission lines

Continuous measurements of streamflow and water temperature were carried out in a shallow gravel-bed river during dam flushing operations using a new shallow acoustic system with two crossing transmission lines. The fluvial acoustic system (FAS) was equipped with four 25 kHz broadband omnidirectional transducers. The reciprocal sound transmissions were performed between the two pairs of acoustic stations, located on both sides of the river. The four-station FAS enabled the measurement of the stream direction in addition to the depth- and range-averaged sound speed, and therefore water velocity. Discharge values computed from FAS reports were compared to those estimated by the moving-boat acoustic Doppler current profiler (ADCP) and rating curve (RC) methods. FAS estimates were in good agreement with ADCP and RC estimates over a range of 50 to 180 m3 s−1. The temperature gradients induced strong sound scattering for the 25 kHz FAS. The fine suspended sediment particles also induced a gradual decrease in the signal-to-noise ratio of the FAS. Nevertheless, the FAS has been proved to be a reliable and accurate technique for the continuous measurement of cross-sectional average velocity/water temperature.

An instantaneous 3-D analysis of turbulent flow in the wake of a hemisphere

Instantaneous three-dimensional velocity fields in the wake of a hemisphere located on an open channel bed are obtained by using multi-section flow visualization and a Mass-Consistent model. Instantaneous pictures of the organized structures are taken at a short time interval to investigate their behaviors at high Reynolds number. At high Reynolds number, the large vortex in the hemisphere wake is similar to the hairpin vortex observed at low Reynolds number. However, the long legs of the hairpin in the streamwise direction are not observed. A secondary hairpin vortex has been generated on the upstream side of the large hairpin vortex. Instantaneous spatial distributions of fluctuating velocity, vorticity production, etc. are presented and discussed.

Application of acoustic tomography to reconstruct the horizontal flow velocity field in a shallow river

A novel acoustic tomographic measurement system capable of resolving sound travel time in extremely shallow rivers is introduced and the results of an extensive field measurements campaign are presented and further discussed. Acoustic pulses were transmitted over a wide frequency band of 20–35 kHz between eight transducers for about a week in a meandering reach of theBāsen River, Hiroshima, Japan. The purpose of the field experiment was validating the concept of acoustic tomography in rivers for visualizing current fields. The particular novelty of the experiment resides in its unusual tomographic features: subbasin scale (100 m × 270 m) and shallowness (0.5–3.0 m) of the physical domain, frequency of the transmitted acoustic signals (central frequency of 30 kHz), and the use of small sampling intervals (105 s). Inverse techniques with no a priori statistical information were used to estimate the depth-average current velocity components from differential travel times. Zeroth-order Tikhonov regularization, in conjunction with L-curve method deployed to stabilize the solution and to determine the weighting factor appearing in the inverse analysis. Concurrent direct environmental measurements were provided in the form of ADCP readings close to the right and left bank. Very good agreement found between along-channel velocities larger than 0.2 m/s obtained from the two techniques. Inverted quantities were, however, underestimated, perhaps due to vicinity of the ADCPs to the banks and strong effect of river geometry on the readings. In general, comparing the visualized currents with direct nodal measurements illustrate the plausibility of the tomographically reconstructed flow structures.

Monitoring Tidal Bores using Acoustic Tomography System

ABSTRACT Kawanisi, K.; Zhu, X.-H.; Fan, X., and Nistor, I., 2017. Monitoring tidal bores using acoustic tomography system. Continuous measurements of flow velocities and suspended sediments were carried out in the Qiantang River (China) with extreme tidal bore conditions. Fluvial and coastal acoustic tomography systems (FATS/CATS) were used in the field studies simultaneously with acoustic Doppler current profilers (ADCPs) and optical backscatter (OBS) equipment. A couple of broadband transducers were installed diagonally across the river around 90 km upstream of the mouth. The length of the sound transmission line was 3050 m. Cross-sectional averaged velocities (V) collected by FATS/CATS enabled the authors to estimate important characteristics of the tidal bores (bore height and celerity). The changes in V recorded during the upstream movement of the bore ranged from 1.35 to 1.76 ms−1. The height and celerity of the bore varied from 1.0 to 1.32 m and 7.59 to 8.29 ms−1, respectively. Since a point/vertical measurement cannot represent a rivers cross section, the ADCP data for velocity and water level (pressure) near the riverbank underestimated the bore height by 22% and 16%, respectively. The maximum suspended sediment concentration (SSC) was observed to have occurred approximately 1 h after the bores arrival; the time lag between the maximum SSC and the bore front is considerably larger than the time lags in previous works. In the case of larger bores, the section mean SSCs, which were deduced from the sound attenuation of FATS/CATS due to suspended sediment, appeared to be appropriate. The time history of the velocity, measured by ADCPs, during the passage of the bore deviated from the normal velocity profile, i.e. the velocity magnitude decreased with an increasing height above the bed for the relaxation time of a few minutes following the arrival of the bore.

Turbulence characteristics in the bottom layer of a shallow tidal channel

Observation of a shallow tidal channel was executed to examine turbulent evolution in ideal condition of weak, stratified flow for a complete cycle of semi-diurnal tide. Duration of this study allows a maximum velocity variation and therefore gives comprehensive interpretation of the relevant processes such as momentum and heat transfer, turbulent kinetic energy (TKE) production and dissipation rates. Under weak stratification condition dominant through the observation period testing the parameterization of dissipation rate in the Mellor–Yamada type models revealed that close to the bed perhaps due to boundary effects experimental dissipation closure constant values were remarkably larger than commonly used value of 16.6. Also, the effect of extending critical Richardson number Ricr to infinity on parameterization of stability functions, eddy viscosity, and turbulent Prandtl number in Kantha–Carniel model was also examined. It was found that despite Kantha–Carniel model reconstructed bottom boundary layer...

Automated Real-Time Streamflow Acquisition in a Mountainous River Using Acoustic Tomography

AbstractAcquiring discharge estimates is crucial in hydrological studies, extreme event analyses, and water resources management. This study demonstrates the improved fluvial acoustic tomography sy...

Multipath propagation of sound in a shallow tidal channel and its implications on tomographic current measurements

A pilot observational experiment with Fluvial Acoustic Tomography (FAT) system was conducted in the Grand Passage, Nova Scotia, Canada, in 2014, to assess the capabilities of FAT in continuous monitoring of transport in a tidal channel. To implement the tomographic measurements, two broadband FAT transceivers operated at 7 kHz central frequency were positioned in both sides of the channel emitting a pulse every 30 s for 4 days. Three coherent arrivals were identified in the acoustic receptions. This paper investigates the influence of physical characteristics of water and current shear on multipath propagation of sound in well-mixed conditions of the channel. At the end, we report the comparison between FAT and reference velocity data collected by a moving-boat ADCP.

Application of acoustic tomography in shallow waters

This paper briefly introduces a state-of-the-art timeof- travel sensor that enables application of acoustic tomography for measuring range-averaged current velocity in extremely shallow waters. The system is called Fluvial Acoustic Tomography and has been developed in Hiroshima University. The paper presents two experiments, the first of which aims at evaluating FAT accuracy in measuring mean flow direction and velocity in an estuarine environment characterized by strong stratification. Second study investigates the applicability of FAT for long-term continuous monitoring of tidal currents in a narrow strait, where is a potential site for installation of a small tidal turbine. During each experiment a set of reference data was collected using a moving-boat ADCP. The results obtained in the first campaign indicate remarkable consistency between FAT and ADCP. It was found that the relative error in the mean flow directions reported by FAT and ADCP did not exceed 10%. The results obtained in the second experiment indicate the agreement between tomographically derived velocities with ADCP in coastal areas. The results also signify the importance of angularity error involved in recovering flow velocity when FAT with only one raypath is used.