Laboratory monitoring of bedload transport rates using hydro-acoustic techniques (ADCP)

This study aims to develop a surrogate methodology for quantification of the bedload transport in riverine environments by using acoustic devices. Bedload transport experiments were performed in laboratory conditions to test the capabilities of the acoustic current Doppler profilers (ADCP) to quantify the bedload velocity, concentration, and active layer thickness. Two ADCPs working at four frequencies (0.5MHz, 1MHz, 3MHz M9, by Sontek, and 2MHz StreamPro, by RDI) were deployed at the same time. Simultaneously, the bedload transport was monitored by high-speed cameras, and continuous bedload transport rate measurements were conducted at the end of the measurement section. The apparent bedload velocity was analyzed and compared with the velocity from the imagery data and the transport rates measured by the bedload trap. Besides the apparent bedload velocity, the ADCPs also registered the backscattered (BS) signal from the sediment bed, which appeared to be sensitive to the change of the bedload transport conditions and the type of the sediment particles. The results confirmed the capability of these acoustic instruments to measure the bedload velocity by demonstrating a strong correlation with the physical transport measurements and the velocities from the imagery data. The apparent velocities measured by the 3 MHz and 1 MHz demonstrated similar results; the 2 MHz measurements led to lower values with 2-4.5 times magnitude difference comparing with the spatially normalized image velocity. The corrected BS signal documented a clear correlation with the apparent bedload velocity, more precisely with the change of the bedload transport condition. The variation between the results from the two instruments is assigned to the different acoustic geometry of the instruments, internal processing, and availability of the instrument related parameters needed for correction of the backscattered signal. Future tests should aim towards a better understanding of the internal processing of the signal and extensive analysis of BS strength sensitivity towards a wide range of sediment types and hydraulic conditions.