Observing bedload/suspended load using multi-frequency-acoustic backscatter

Published alongside Gray, J.R., Laronne, J.B., Marr, J.D.G., 2010, Bedload-surrogate monitoring technologies: U.S. Geological Survey Scientific Investigations Report 2010-5091This paper describes the application of acoustic backscatter from multiple probes (1, 2 and 4 MHz) to investigate the spatial and temporal distribution of bedload and suspended load concentration in a small-scale laboratory flume experiment with a mobile sand-bed. The acoustic backscatter data are normalized to show the spatial pattern of suspended sediment concentration in relation to changes in the bed morphology as the bedforms migrate past the fixed measurement point. Each probe operates at a different acoustic frequency which results in different responses to near-bed sediment transport and suspended sediment transport. Results show a significant increase in sediment concentration upstream from each crest and reduction in sediment transport towards the trough. This pattern of sediment concentration suggests that sediment may be transported from the crest of the bedform into the main body of the flow, whilst periodic increases in transport near the base of the lee slope may reflect fluctuations in the reattachment point of the separation zone. The arrangement of multiple probes in a streamwise direction allows morphological data to be used to calculate the bedload transport rate which shows a strong correlation with the near-bed acoustic backscatter response from the highest frequency (4 MHz) probe. A single acoustic probe could be calibrated to quantify sediment concentration, but the spatial and temporal variability of the sediment transport observed at this scale means that multiple probes are unlikely to be suitable for measuring sediment size