Estimation of high-resolution sediment concentration profiles in bottom boundary layer using pulse-coherent acoustic Doppler current profilers

Abstract

The capability of two acoustic profilers ？SonTek's 1.5-MHz pulse-coherent acoustic Doppler profiler (PC-ADP) and RDI's 600-kHz acoustic Doppler current profiler (ADCP) with pulse-coherent mode ？was evaluated for estimating high-resolution suspended sediment concentration (SSC) profiles in bottom boundary layer. In the laboratory measurements with a PC-ADP, two types of sediments were tested to study acoustic responses to grain size. A natural sediment sample from Clay Bank, a mixture of clay and very fine sand, showed a good linear relationship between range-corrected volume scattering ( ) and backscattered strength (E) until SSC increased up to about 10 g l-1. In contrast, a commercially available kaolinite exhibited earlier signal saturation and non-unified linear regressions between and E, most likely because the particle size is much smaller than the transmitted acoustic wavelength. Using a pulse-coherent ADCP, the field measurement results from Mobile Bay, Alabama showed that the acoustically-derived SSC profiles were well matched with the optically-derived outcomes although slight discrepancies were noted. The overestimation of acoustically-derived SSC near the bed may be related to the side lobe interference near the bed and the enhanced acoustic sensitivity by coarser particles and denser aggregates eroded from the bed. Mean absolute error of acoustic estimates was within 4.1？7.3% of the optically-derived SSC range, which is attributable to the different acoustic and optical scattering responses to given sediment size spectra. Despite some error sources in acoustic inversion, the results from laboratory and field experiments suggest that the pulse-coherent acoustic profiler is able to reveal the evolution of in-situ near-bed SSC profiles with high vertical and temporal resolution.