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 numb er of methods of measuring the settling velocity of estuarine flocs, T hese 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 partl y relate to small-scale spatial and temporal patchiness in the turbidi ty field. Owen Tubes generally give settling velocities an order of ma gnitude smaller than the direct video measurements, which may indicate that the tubes disrupt flocs on sampling. Between Owen Tubes differen t methods of calculating the results may lead to different median sett ling velocities, especially at low concentrations. However, a well-con trolled sampling protocol with the settling tubes gave consistent resu lts. The direct video methods appear to give comparable results, and n eed to be evaluated further.