Measurements of in situ settling velocities in the Elbe estuary

Analysis of in situ settling velocities of suspended mud flocs in estuaries is difficult to carry out because of the very fragile nature of the sediment flocs. Therefore, a number of different analytical methods have been developed. The Owen tube was developed in the late 1960s, and has been essential to the further development of more sophisticated methods to measure in situ settling velocities. There are a number of problems connected with the use of the Owen tube: possible floc breakage during sampling, flocculation in the tube during analysis caused by differential settling, and secondary flows in the tube after withdrawal of subsamples. Nevertheless, the Owen tube is one of the few instruments generally available that can be used in all kinds of meteorological and hydrographical situations, covering suspended-sediment concentrations from tens to thousands of mg-dm−3. Therefore, it is still relevant to carry out Owen tube analyses for comparison with results obtained by newly developed methods such as the in situ video technique. Another thing that makes the settling tube convenient is its inexpensiveness, making it one of the few apparatuses that one can afford to possess in duplicate. Thus, the settling tube can be tested against itself to evaluate its ability to reproduced the results when analyses are carried out simultaneously on duplicate samples. Investigations carried out with two different Owen tubes in the Danish Wadden Sea suggest that the differences between median settling velocities are typically within about 20% of one another. This is a small difference compared with those that normally occur when results of different instruments are compared.