Acoustic Measurement of Suspended Sediment Flux Over Ripples and Dunes in Unidirectional Flow

In May 2015, an experiment was conducted in the main channel flume at the St. Anthony Falls Laboratory, to measure suspended- and bed-load sediment transport using a multi-frequency acoustic Doppler instrument (MFDop). The flume was operated with ~1 meter water depth over a ~50 cm thick bed of naturally-sourced medium sand. The flume uses water diverted from the Mississippi River. Therefore, the experiment closely simulated realistic field conditions, including high flow rates, turbid water, and sediment grain size variability. A series of experiments was conducted at different flow rates, ranging from the threshold of sediment transport (~1000 L/s) up to a regime with 2 meter wavelength sand ripples (~2000 L/s). At each flow rate, the MFDop was used to measure profiles of multi-frequency acoustic backscatter and 3-component velocity, from bed level to 10's of cm above-bed, in 4 mm range bins and at 30 Hz sampling rate. A rotary sonar was used to monitor the migration of bedforms under the MFDop during the data collection, until a full ripple phase had been observed. These data are used to quantify the statistics of time-resolved near-bed sediment suspension and flux at different ripple phases, via inversion of a model for multi-frequency acoustic backscatter amplitude as a function of grain size and concentration.