Acoustical measurements and modeling of mixed-grainsize sediment under breaking waves

In 2016 and 2017, a novel field data set was collected by USACE researchers at the Field Research Facility in Duck, NC, using a combination of overhead lidar remote sensing and sub-surface acoustic backscatter intensity profiles, with instruments deployed from an amphibious vehicle (CRAB) in breaking waves with a height of 2 or more meters. A key goal of these measurements was to obtain information on the influence of wave shape and breaker type (measured by lidar) on suspended sediments (measured acoustically). The present results focus on the acoustical aspects of the study, in which frequency-dependent backscatter intensity data is used to infer sand size and concentration via an acoustic scattering model. Specifically, laboratory experiments were conducted to obtain acoustic backscatter form factors specific to the Duck NC sand, and these results are used in an inverse model to extract suspended sediment concentration and particle size from in-field backscatter intensity profiles. Pairing the results with lidar data then illuminates how sediment is suspended on a wave-by-wave basis. Results will be presented on the size-dependent form factor measured for the Duck NC sand, interpretation and validation of the sediment profile data, and sensitivities to uncertainties – including errors in the acoustic model and confounding factors such as the presence of bubbles or fine sediment in the water column.