A model for high-frequency acoustic Doppler current profiler backscatter from suspended sediment in strong currents

Acoustic current Doppler profilers (ADCP's) have been used to measure currents for over 20 years. Although ADCP's are not designed to measure acoustic backscatter intensity accurately, several researchers have (successfully) related the backscatter intensity to suspended sediment concentration (SSC) using a random phase acoustic backscatter model. However, this paper shows experimental evidence that in a tidal inlet during high tidal current velocities, a random phase acoustic backscatter model overestimates the SSC by factors up to 60, which could not be explained by a time-varying particle size distribution. An acoustic backscatter model is developed that includes the effect of acoustic backscatter enhanced by coherence in the particles’ spatial distribution as a result of turbulence-induced sediment fluctuations. The model results are compared with field measurements, showing a good correspondence between measured and modelled SSC, including the strong high current conditions for which the random phase acoustic backscatter model was shown to fail.