Field measurements of suspended sediment transport with acoustic multifrequencies techniques

Our research aim is to investigate velocity, sediment transport and bed forms of a Po river reach in order to better understand and predict river channel processes. In particular this contribute shows techniques and results used to investigate grain size and concentration profiles of suspended sediment transport. An innovative method using 2 ADCPs operating at different frequencies is employed, with the ability to assess mean diameter and concentration in the water column. A simple sound absorption equation, coming from the more complex sonar equation, relating backscatter to medium features, is inverted. To do that echoes of two frequencies are utilized, knowing acoustic absorption and form coefficient functions depending not only on ADCP features but also on backscattering particle size. In order to test the acoustic method for suspension characterization (concentration and mean diameter), the 1200 kHz and 600 kHz RDI ADCPs were used in two campaigns, each one some days long, with different water stage conditions. Resulting maps and profiles of concentration and mean sediment size are in good agreement with known sediment grain size and with the related river channel morphology and bed forms.