Devices to measure settling velocities of cohesive sediment aggregates: A review of the in situ technology

Suspended cohesive sediments in coastal water bodies occur mainly as amorphous aggregates of single particles and microflocs. The fragile nature of these aggregates demands that their settling velocities be measured under field conditions to minimise their disruption. This review presents the main devices available to measure settling velocities of cohesive sediments in situ. These include hand-operated settling tubes and automated settling columns equipped with video systems, optical and laser instruments and an underwater balance. Additionally, non-intrusive techniques can be used, such as holographic and acoustic backscatter sensors. Regarding their operational principle, these in situ devices can apply direct or indirect methods for computing the settling velocity distribution in terms of particle size, mass or concentration. These devices are briefly described emphasising some important logistical and scientific issues that arise when these instruments are used in the field, in order to facilitate the selection of an appropriate technique for a particular research application. Due to the complexity involved in the in situ settling velocity measurements, a simultaneous application of different types of instruments seems to be the more appropriate approach to understand the aggregate dynamics.