Conversion of ADCP backscatter to suspended sediment concentration in the cohesive sediment environment

초록

To improve the ADCP backscatter conversion to suspended sediment concentration (SSC) in cohesive estuarine environments, we applied a hybrid floc-scattering model, that calculates acoustic scattering characteristics based on varying floc sizes and densities (Thorne et al., 2014, Continental Shelf Research), to the backscatter conversion equation. To examine the backscatter conversion, we performed a field experiment to measure the floc size distribution, and then estimated the effective density of flocs from observed volume concentration of flocs and SSC. The estimated SSCs from the ADCP were compared with the reference SSCs obtained by water samples. The hybrid floc-scattering (HFS) model improved the acoustic conversion greatly in terms of both accuracy and precision when it is compared to the constant solid (CS) model. The slope of the regression between the estimated and reference SSCs improved to 0.88 from 0.23, while the mean relative error of the HFS model was reduced to 43% from 69% of the CS model. The improvement stems from the proper definition of the effective density of estuarine flocs. This result indicates that the more accurate ADCP backscatter converion can be achieved by accounting for variable floc properties.