Empirical Approach Using Acoustic Doppler Current Profilers for Determining Particle Size Distributions in Estuaries with Highly Concentrated Suspended Matter

Abstract

The backscatter strength provided by multifrequency acoustic systems is frequently used to characterize suspended elements in water courses in the entire water column on a short time scale with high temporal and spatial resolution. However, in estuarine waters with highly concentrated suspended matter, sound attenuation may hinder the ability of the acoustic method to obtain grain size profiles. This article presents a novel acoustic technique to overcome this limitation by estimating the sound attenuation caused by both fine and coarse suspended matter fractions through an iterative process that starts by introducing discrete values of suspended particle concentration and size obtained from a conventional diffractometer. Results were compared with those obtained by the traditional two frequency method to highlight the substantial improvement achieved. The main advantage of the proposed methodology is the rapid detection of changes in the concentration and size profiles, which remain hidden when measurements are made with instruments that provide discrete values and require longer times. The technique was applied to obtain the particle size and concentration distributions along a cross section of an estuary characterized by zones where a massive convective process of sedimentation occurs.