ADCP’s capabilities in measuring the bed load velocity: A confirmation by means of image velocimetry under controlled conditions

The transport rate measurements and the estimation of the bedload velocity are notoriously hard and there is not a validated technique that would obtain continues data. The bottom tracking (BT) mode of an acoustic Doppler current profiler (ADCP) has been used many times to estimate the apparent velocity of the bed load. Herein is the basic idea to exploit the bias of the BT signal towards the bed load movement and to calibrate this signal with some more certain measuring techniques. These measurements are quite scarce and seldom reliable, since these are performed under not controlled conditions. So far, the ADCP apparent bed-load velocity measurements have not been confirmed by the means of image velocimetry, nor a calibration of the empirical formulas has been conducted under controlled conditions in laboratory. Therefore, this study explores several experiments under stationary conditions, where the signal of the ADCP BT mode is recorded and compared to the bed load motion measured by digital camera videography. The experiments have been performed in the hydraulic laboratories of Ottawa and Bologna, using two different ADCPs and two different cameras. In Ottawa, the set of experiments includes four runs for each of the six different sediment mixtures spanning 1 mm sand to 3 mm gravel, whereas in Bologna it includes four experimental runs for 1 mm sand, but performed in diverse hydraulic conditions. Generally, a good matching has been documented between the averaged bed load velocity measured by the ADCP and the videography. The slight deviation in single experiments can be explained by gravel particles heterogeneity, difficult in reproducing the same hydro-sedimentological conditions and the randomness of scattering processes.