A comparison of in situ techniques for estuarine floc settling velocity measurements

An Intercomparison Experiment carried out in the turbidity maximum of the Elbe estuary aimed to determine the relative performance of a number of methods of measuring the settling velocity of estuarine flocs. These comprised several Owen Tubes, with different sampling protocols, side withdrawal tubes, a settling box, and two in situ video systems. There were significant differences between the results which may partly relate to small-scale spatial and temporal patchiness in the turbidity field. Owen Tubes generally give settling velocities an order of magnitude smaller than the direct video measurements, which may indicate that the tubes disrupt flocs on sampling. Between Owen Tubes different methods of calculating the results may lead to different median settling velocities, especially at low concentrations. However, a well-controlled sampling protocol with the settling tubes gave consistent results. The direct video methods appear to give comparable results, and need to be evaluated further.