Evaluation of acoustic backscatter conversion to suspended sediment concentration in cohesive estuarine environments

초록

In estuaries, Acoustic Doppler Current Profilers (ADCP) are widely used for the investigation of sediment transport. Previous studies have shown that acoustic backscatter intensity is sensitive to sediment characteristics (grain size, density and concentration) and thus suspended sediment concentration (SSC) conversion with constant sediment size and density was less accurate in cohesive environments. To improve the estimation of SSC in a cohesive environment, we applied different scattering models, which are based on variable floc size and density to represent acoustic scattering properties (Thorne et al., 2014, Continental Shelf Research; Vincent and MacDonald, 2015, Continental Shelf Research). To examine the models we performed field work to measure the floc size distribution and estimated the effective density of flocs from observed volume concentration of flocs and SSC. The estimated SSC from the ADCP was compared with reference SSC obtained by water samples. The effective density of flocs was found to be temporally and spatially variable depending on the floc fractal dimension and the results showed that including floc properties into the acoustic backscatter conversion improved the estimation of SSC in the cohesive environment. Based on these results, it is expected that more accurate ADCP measurements of sediment flux can be achieved when accounting for variable floc properties.