MEASUREMENTS OF IN-SITU SETTLING VELOCITIES IN THE ELBE ESTUARY

Analysis of in situ settling velocities of suspended mud flocs in estu aries is difficult to carry out because of the very fragile nature of the sediment flocs, Therefore, a number of different analytical method s have been developed. The Owen tube was developed in the late 1960s, and has been essential to the further development of more sophisticate d methods to measure in situ settling velocities, There are a number o f problems connected with the use of the Owen tube: possible flee brea kage during sampling, flocculation in the tube during analysis caused by differential settling, and secondary flows in the tube after withdr awal of subsamples. Nevertheless, the Owen tube is one of the few inst ruments generally available that can be used in all kinds of meteorolo gical and hydrographical situations, covering suspended-sediment conce ntrations from tens to thousands of mg . dm(-3). Therefore, it is stil l relevant to carry out Owen tube analyses for comparison with results obtained by newly developed methods such as the in situ video techniq ue, Another thing that makes the settling tube convenient is its inexp ensiveness, making it one of the few apparatuses that one can afford t o possess in duplicate. Thus, the settling tube can be tested against itself to evaluate its ability to reproduce the results when analyses are carried out simultaneously on duplicate samples. Investigations ca rried out with two different Owen tubes in the Danish Wadden Sea sugge st that the differences between median settling velocities are typical ly within about 20% of one another. This is a small difference compare d with those that normally occur when results of different instruments are compared.