Laboratory Investigation of Apparent Bedload Velocity Measured by ADCPs under Different Transport Conditions

摘要

Several studies have investigated the use of the bottom tracking (BT) mode of acoustic Doppler current profilers (ADCPs) for evaluating bedload transport. The raw apparent bedload velocity is usually noisy and contains erroneous data. This study investigates how bedload dynamics influence acoustic processes occurring at riverbeds (i.e., volume and roughness scattering). The accuracy of ADCP apparent bedload velocity measurements is analyzed in two sets of laboratory experiments using two ADCPs working at different frequencies [2 MHz RDI StreamPro (RDI Teledyne Marine, Cypress, Texas) and 3 MHz SonTek M9 (Sontek/Xylem, San Diego)], with a variety of sediment materials and different hydraulic conditions. Simultaneously, the velocity and surface concentration of the mobile sediments are measured using high-resolution cameras. Despiking and filtering are applied to the raw data, and the temporal average of the apparent bedload velocity is spatially normalized. The percentage of filtered erroneous velocity data from the ADCP time series demonstrates a strong correlation with the surface concentration of mobile particles. Velocities measured with the M9 matched the particle velocities measured by image velocimetry better than those measured with the StreamPro, which appeared to underestimate the bedload velocity by a factor of 2-4. This suggests that instruments with different acoustic frequency yield a different interpretation of the apparent velocity; instruments with lower acoustic frequency and larger acoustic sampling length are more affected by the fixed surface beneath the layer of moving particles. These results bear out the notion that filtered apparent bedload velocity can be used to estimate the spatial velocity of bedloads, but its dependence on a set of acoustic properties must be further investigated.