Mahdi Razaz

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.

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...

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.

139. LONG-TERM OBSERVATIONS OF FLOW AND SUSPENDED SEDIMENT IN A TIDALLY-DOMINATED ESTUARY

Coastal wetlands are under a great deal of pressure from forces of human population’s intervention. It is important to measure and assess erosion and deposition rate of suspended sediment in coastal zones. To measure the suspended sediment transport along and across a tidally-dominated river, Ota River, we deployed 3 AquaDopps in two different arrangements, one in 3 stations along the river in right, left, and center of the Ota Floodway for a month, and the second was 3 stations in one cross sections at the same time. The first experiment managed to be done in the winter of 2008 and the latter in Aug. 2008. Surveying the results points out that continuous suspended sediment settlement happens along the most length of the river; in some areas in consequence of meander shape of the river, incessant transversal suspended sediment transport formed sandbars in one side of the river.