Fluvial Acoustic Tomography System Research Articles

Investigation of stage‐discharge model performance for streamflow estimating: A case study of the Gono River, Japan

AbstractIn water resource studies, long‐term measurements of river streamflow are essential. They allow us to observe trends and natural cycles and are prerequisites for hydraulic and hydrology models. This paper presents a new application of the stage‐discharge rating curve model introduced by Maghrebi et al. (2016) to estimate continuous streamflow along the Gono River, Japan. The proposed method, named single stage‐discharge (SSD) method, needs only one observed data to estimate the continuous streamflow. However, other similar methods require more than one observational data to fit the curve. The results of the discharge estimation by the SSD are compared with the improved fluvial acoustic tomography system (FATS), conventional rating curve (RC), and flow‐area rating curve (FARC). Some statistical indicators, such as the coefficient of determination (R2), root mean square error (RMSE), percent bias (PBAIS), mean absolute error (MAE), and Kling‐Gupta efficiency (KGE), are used to assess the performance of the proposed model. ADCP data are used as a benchmark for comparing four studied models. As a result of the comparison, the SSD method outperformed of FATS method. Also, the three studied RC methods were highly accurate at estimating streamflow if all observed data were used in calibration. However, if the observed data in calibration was reduced, the SSD method by R2 = 0.99, RMSE = 2.83 (m3/s), PBIAS = 0.715(%), MAE = 2.30 (m3/s), and KGE = 0.972 showed the best performance compared to other methods. It can be summarized that the SSD method is the feasible method in the data‐scarce region and delivers a strong potential for streamflow estimation.

Continuous measurement of flow direction and streamflow based on travel time principles using a triangular distribution of acoustic tomography systems

Monitoring flood dynamics is important to provide optimal flood mitigation practices and understand their hydrological responses. Recently, applications of travel time principles have gained a growing interest in hydrological research and river engineering. In this study, flood dynamics were monitored using the fluvial acoustic tomography system (FAT), based on travel time principles. The primary objective of this study is to continuously measure the mean cross-sectional velocity, river flow direction, and river discharge using an innovative tomographic system during two flood events. In this regard, three FAT systems were placed in a gravel bed stream forming a triangle shape to measure stream velocity along two cross-sections. By investigating the magnitude of the cross-sectional mean velocity vectors and the cross-sectional areas, we proposed new equations to evaluate the expected flow direction. The performance of flow measurement by the FAT system was verified with another reference record. Importantly, we demonstrated that the minimum number of acoustic stations to determine river flow direction in unidirectional streams can be reduced to three stations which can be more practical and easier. Further, one of the novel aspects of this study is offering new guidelines to continuously estimate flow direction using a triangular distribution of tomographic systems. In general, this study presents a promising method for monitoring flow dynamics in rivers.

Mapping tidal current and salinity at a shallow tidal channel junction using the fluvial acoustic tomography system

Applying the fluvial acoustic tomography (FAT) system to shallow tidal junctions for studying the flow division and spatiotemporal difference of velocity and salinity is vital to understand the tidal hydrodynamics in multi-channel networks. To the best of our knowledge, this is the first study that monitors continuous 2D current and salinity distributions at a shallow tidal junction using six FATs for ~34.4 days. The horizontal distribution and spatiotemporal variation of the currents and salinity were efficiently estimated by the inverse method. These results demonstrate that FAT is a potential tool for the continuous mapping of variable 2D currents and salinity at shallow tidal junctions. The reciprocating patterns of the current and salinity during the spring tide at the junction responded well to the tide. High salinity occurred around high water, whereas salinity was negligible at low water. During flood tides, significant landward currents flowed with the maximum speed of ~0.4 m/s, and significant seaward currents with the maximum speed of ~0.55 m/s occurred during ebb tides. During neap tides, the salinity pattern began to develop landward from the low water and reached a mature phase around the high water; salinity at the ebb slack remained high. Inverted FAT results indicated a counter-clockwise circulation around the low water during neap tide; some of the currents continuously flowed landward at the downstream of the junction (S4–S5), whereas others continuously flowed to the Tenma River. The behaviors of the currents varied slightly with the tide. The behaviors of the current and salinity during the neap tide mostly resulted from the density-driven current phenomenon. Furthermore, tidal harmonic analyses of the reconstructed currents were performed to clarify the river-tide interactions at the tidal junction. The results reflected the increased tidal wave deformation that occurred with the gradually increasing tidal range and demonstrated the role of the limited river flow on the tidal asymmetry at this tidal junction. This study advances the understanding of river flow dynamics in shallow tidal junction systems.

Influence of Suspended Sediment Concentration and Particle Sizes on the Sound Attenuation of the Fluvial Acoustic Tomography Technique

Fluvial Acoustic Tomography System (FATS) as an advanced technology acquires continuous streamflow data in rivers and estuaries even during floods. However, the acoustic signals are dramatically attenuated by suspending sediments which this problem is a new field of study. In this study, we propose a new equation to estimate the maximum applicable measurement distances (MAMDs). It is based on the cross-sectional suspended sediment concentration (<SSC>) and the particle sizes on the 30-kHz FATS. Our study results show that MAMD might be 2,380 m in the clear water. Moreover, the streamflow monitoring can be perfectly done while <SSC> is less than 12.67 kg/m3 with the particle radii of 3 μm, when the horizontal distance between two acoustic stations is 100 m. Also, the acoustic signals are not decayed if the particle radii equal to 20 mm and the maximum <SSC> is 6.6 kg/m3. This study highlights the performance of FATS in the presence of high <SSC> and provides a better perspective of applying FATS in different rivers with high variability of <SSC>.

Characteristics of Tidal Discharge and Phase Difference at a Tidal Channel Junction Investigated Using the Fluvial Acoustic Tomography System

This study investigates the tidal discharge division and phase difference at branches connected to a channel junction. The tidal discharge at three branches (eastern, western, and northern branches) was continuously collected using the fluvial acoustic tomography system (FATS). The discharge asymmetry index was used to quantify the flow division between two seaward branches (eastern and western branches). The cross-wavelet method was applied to calculate the phase difference between the tidal discharge and water level. The discharge asymmetry index shows that the inequality of flow division is obviously prominent during the spring tide duration, where the eastern branch has the capability to deliver greater amounts of subtidal discharge, approximately 55–63%, compared with the western branch. However, the equality of flow division between the eastern and western channels can be observed clearly during the neap tide period. The wavelet analysis shows that the phase difference at the western branch is higher than at the eastern branch, because the geometry of the western branch is more convergent than that of the eastern branch. Accordingly, the amplitude of the tidal wave at the western branch is more magnified compared with that at the eastern branch. Moreover, the phase difference at the northern branch is greater than at the two seaward branches, implying that the phase difference is slightly increased after passing through the junction into the northern branch.

Scaling characteristics of mountainous river flow fluctuations determined using a shallow-water acoustic tomography system

The aim of this study is to investigate the scaling exponent properties of mountainous river flow fluctuations by detrended fluctuation analysis (DFA). Streamflow data were collected continuously using Fluvial Acoustic Tomography System (FATS), which is a novel system for measuring continuous streamflow at high-frequency scales. The results revealed that river discharge fluctuations have two scaling regimes and scaling break. In contrast to the Ranting Curve method (RC), the small-scale exponent detected by the FATS is estimated to be 1.02 ± 0.42% less than that estimated by RC. More importantly, the crossover times evaluated from the FATS delayed approximately by 42 ± 21 hr ≈2–3 days than their counterparts estimated by RC. The power spectral density analysis assists our findings. We found that scaling characteristics information evaluated for a river using flux data obtained by RC approach might not be accurately detected, because this classical method assumes that flow in river is steady and depends on constructing a relationship between discharge and water level, while the discharge obtained by the FATS decomposes velocity and depth into two ratings according to the continuity equation. Generally, this work highlights the performance of FATS as a powerful and effective approach for continuous streamflow measurements at high-frequency levels.

Application of shallow-water acoustic tomography to measure flow direction and river discharge

High-frequency Fluvial Acoustic Tomography System (FATS) was initially used to measure flow velocity and river discharge in a mountainous river. The results showed the high-frequency FATS, not only improves the velocity resolution, but also reduces the minimum operational range from 76m to 43m in compare with the previous type of FATS. The analysis of sound wave propagation (Ray tracing) showed the bottom topography can be the reason of multi-ray paths of sound wave in the shallow freshwater rivers. A new formula based on the continuity equation introduced to estimate the variations of the angle between the flow direction and the FATS transmission line. The flow direction was measured using two crossed FATS transmission lines of 53kHz and 30kHz. The results compared to the up-looking ADCP (Acoustic Doppler Current Profiler) deployed near the intersection of the two lines which measured the changes in flow direction. The results affirmed the efficiency of the proposed method. Finally, the river discharge was estimated by both FAT systems and compared to the Rating Curve method and moving-boat ADCP estimates. The relative error of the FATS discharge measurements was less than 10%.

High‐frequency streamflow acquisition and bed level/flow angle estimates in a mountainous river using shallow‐water acoustic tomography

AbstractThe acquisition of reliable discharge estimates is crucial in hydrological studies. This study demonstrates a promising acoustic method for measuring streamflow at high sampling rate for a long period using the fluvial acoustic tomography system (FATS). The FATS recently emerged as an innovative technique for continuous measurements of streamflow. In contrast to the traditional point/transect measurements of discharge, the FATS enables the depth‐averaged and range‐averaged flow velocity along the ray path to be measured in a fraction of a second. The field test was conducted in a shallow gravel‐bed river (0.9 m deep under low‐flow conditions, 115 m wide) for 1 month. The parameters (stream direction and bottom elevation) required for calculating the streamflow were deduced by a nonlinear regression to the discharge data from the well‐established rating curve. The cross‐sectional average velocities were automatically calculated from the acoustic data, which were collected on both riverbanks every 30 s. The FATS was connected to the internet so that the real‐time flow data could be obtained. The FATS captured discharge variations at a cut‐off frequency of approximately 70 day−1. The stream exhibited temporal discharge changes at multiple time scales ranging from a few tens of minutes to days. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Variability in salt flux and water circulation in Ota River Estuary, Japan

In this study the sub-tidal and intra-tidal variations of salt fluxes in the upstream section of a shallow estuary (with a water depth of less than 3 m) were investigated. The salt fluxes were estimated based on the cross-sectional average salinity and velocity measured by the fluvial acoustic tomography system (FATS). The results indicate that the magnitude of seaward fluxes is approximately two times greater than that of landward fluxes under normal conditions. The results of short-term observation in the study area indicate that there is a phase lag of the bottom and surface salinities between the regions with the largest and smallest depths. The vertical shear flux with a peak value of −0.7 m2/s during the ebb tide indicated an important contribution to the total salt flux compared with the advective flux. A phase lag occurred between the vertical shear terms in the regions with the largest and smallest depths, which resulted from the correlation between the vertical variations of the salinity and velocity and the existence of transversal velocity circulations.

New acoustic system for continuous measurement of river discharge and water temperature

In many cases, river discharge is indirectly estimated from water level or streamflow velocity near the water surface. However, these methods have limited applicability. In this study, an innovative system, the fluvial acoustic tomography system (FATS), was used for continuous discharge measurement. Transducers with a central frequency of 30 kHz were installed diagonally across the river. The system's significant functions include accurate measurement of the travel time of the transmission signal using a GPS clock and the attainment of a high signal-to-noise ratio as a result of modulation of the signal by the 10th order M-sequence. In addition, FATS is small and lightweight, and its power consumption is low. Operating in unsteady streamflow, FATS successfully measured the cross-sectional average velocity. The agreement between FATS and acoustic Doppler current profilers (ADCPs) on water discharge was satisfactory. Moreover, the temporal variation of the cross-sectional average temperature deduced from the sound speed of FATS was similar to that measured by a temperature sensor near the bank.

Continuous measurements of flow rate in a shallow gravel‐bed river by a new acoustic system

The continuous measurement of river discharge for long periods of time is crucial in water resource studies. However, the accurate estimation of river discharge is a difficult and labor‐intensive procedure; thus, a robust and efficient method of measurement is required. Continuous measurements of flowrate have been carried out in a wide, shallow gravel bed river (water depth ≈ 0.6 m under low‐flow conditions, width ≈ 115 m) using Fluvial Acoustic Tomography System (FATS) that has 25 kHz broadband transducers with horizontally omnidirectional and vertically hemispherical beam patterns. Reciprocal sound transmissions were performed between the two acoustic stations located diagonally on both sides of the river. The horizontal distance between the transducers was 301.96 m. FATS enabled the measurement of the depth‐ and range‐averaged sound speed and flow velocity along the ray path. In contrast to traditional point/transect measurements of discharge, in a fraction of a second, FATS covers the entire cross section of river in a single measurement. The flow rates measured by FATS were compared to those estimated by moving boat Acoustic Doppler Current Profiler (ADCP) and rating curve (RC) methods. FATS estimates were in good agreement with ADCP estimates over a range of 20 to 65 m3 s−1. The RMS of residual between the two measurements was 2.41 m3 s−1. On the other hand the flowrate by RC method fairly agreed with FATS estimates for greater discharges than around 40 m3 s−1. This inconsistency arises from biased RC estimates in low flows. Thus, the flow rates derived from FATS could be considered reliable.

Long-term measurement of stream flow and salinity in a tidal river by the use of the fluvial acoustic tomography system

Summary Long-term variation of stream flow of a tidal river was measured by an innovative technology, called the fluvial acoustic tomography (FAT). The reciprocal sound transmission was performed between two acoustic stations, located on both sides of the river. Even in the tidal river with the periodic intrusion of salt wedges, the cross-sectional average velocities along the river stream axis, estimated from the travel time difference data, were consistent with the average velocities, observed by an array of moored downward-looking ADCPs. The cross-sectional average salinity was also estimated by using the mean travel time data collected from the reciprocal sound transmission, the mean values of temperature measured by the conductivity–temperature (C–T) sensors, and the ray simulation result. The derived salinity data from the FAT are comparable with that obtained by the C–T sensors. It is concluded that the fluvial acoustic tomography (FAT) is a prospective method for the continuous monitoring of tidal river discharge and temperature/salinity variations.