Bedload Velocity and Backscattering Strength from Mobile Sediment Bed: A Laboratory Investigation Comparing Bistatic Versus Monostatic Acoustic Configuration

Abstract

Despite the many advantages of using active ultrasound sonars, recent studies have shown that the specific acoustic geometry, signal-processing configuration, and complex surface-volume scattering process at the riverbed introduce several uncertainties in bedload estimation. This study presents a comparison of bedload velocity and bottom echo intensity measurements performed by monostatic and bistatic active ultrasound systems. The monostatic configuration is widely applied in the field to measure the apparent velocity at the riverbed with an acoustic current Doppler profiler (ADCP). Two laboratory investigations were conducted in two different hydraulic facilities deploying ADCP Stream Pro, monostatic and bistatic acoustic velocity profilers, manufactured by Ubertone. The bistatic instruments provided more accurate bedload velocity measurements and helped in understanding the acoustic sampling of the monastic systems. The bistatic profiles succeeded in measuring a profile over the active bedload layer, and the monostatic instruments resulted in different bedload velocity estimations depending on the acoustic resolution and sampling. The echo intensity increased in the cells measured within the active bedload layer with respect to the cell measuring the water column above. The cells that sampled the immobile bed surface beneath the bedload layer showed a reduction of the echo intensity compared with the cells above. The acoustic sampling, which combines the measurement volume geometry and internal processing, seems crucial for more accurate outputs. Future research about the use of monostatic instruments in the field should aim to define the best possible setting for the acoustic parameters at a given bedload condition that may be tuned by evaluating the backscattering at the river bottom together with the apparent bedload velocity.

The Assessment of Acoustic Doppler Velocimetry Profiler from a User’s Perspective

“…Broadband incoherent technologies overcome this issue (as testing of more recent acoustic probes from Ubertone has demonstrated, e.g. Conevski et al 2020). Despite these apparent limitations, the results and observations presented herein are useful both in terms of the search framework applied towards validating produced ADVP data using sound first principles, as well as a best practice guide for current users of the Vectrino II ADVP probe.…”

“…This implies that profile measurements at cells more distant from the transducer will be contaminated (due to the receiver probing velocity from adjacent cells), resulting in relatively more erroneous measurements. This is avoided from recently produced devices that have a narrower beam and wider receiving beams, resulting in more focused velocity profiles, and minimising the velocity contamination (see for example the Ubertone's ADVP, Conevski et al 2020). The above consideration may be used in an attempt to explain the "banana"-like curved shape of the velocity profiles in Fig.…”