From Fluvial to Coastal: The Challenges of Measuring the Suspended Sediment Concentration

Suspended sediments play a significant role in biogeochemical and physical processes in both fluvial and marine systems. Accurate measurements of the suspended sediment concentration (SSC) are therefore essential. Acoustic Doppler Current Profilers (ADCP) are often used to measure velocity profiles. The echo profile of the ADCP can be used to determine both acoustic attenuation and acoustic backscatter. These due variables are closely related with the SSC and empirical functions and relations that translate the SSC as function of the attenuation, or, the SSC as function of the backscatter, which are known in the literature. However, these relationships have shown to be limited in their application range, i.e., the interval of attenuation and backscatter where they are valid. To overcome these limitations, other methods have been proposed, namely methods that rely on the attenuation to backscatter ratio (ABR) to compute the SSC. In this study, a SSC measurement method based on the ratio between acoustic attenuation and acoustic backscatter, developed and validated for fluvial measurements, is applied to the coastal site in the Netherlands. In this coastal case study, different measuring stations equipped with ADCP and Optical Backscatter point Sensors (OBS) were installed. Several ADCP and OBS measured simultaneously the velocity profiles, echo profiles and point-wise measurement of the SSC in a wide range of field conditions. The aforementioned ABR method is applied to the echo profiles collected during the measurement campaigns to determine the SSC. The obtained values are then compared with the results measured with the OBS. The advantages and limitations of applying the ABR method to coastal systems are presented.