109 Journal of Sea Research 36 (I/2): 109-113 (1996) MEASUREMENTS OF IN SITU SETTLING VELOCITIES IN THE ELBE ESTUARY MORTEN PEJRUP and KAREN EDELVANG Ins~tute of Geography, University of Copenhagen, ~ster Voldgade 10, DK 1350 Copenhagen K, Denmark ABSTRACT 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 essen- tial 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: posaible floc breakage dur- ing 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 Instru- ments generally available that can be used in all kinds of meteorological and hydrographicel situa- tions, 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 devel- oped methods such as the in situ video technique. Another thing that makes the settling tube conven- ient 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 reproduce 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. Keywords: flocculation, settling velocity, settling tube, Elbe 1. INTRODUCTION From 9 to 11 June 1993, in situ settling velocities of suspended flocs were measured from a pontoon moored in the Elbe estuary near Brunsb~ttel. The water depth at the sampling position was approxi- mately 18 m. Samples of suspended sediment were taken with two Owen settling tubes. The time interval between successive samples was 1 hour; sampling depth was 9 m below surface, which was close to the middle of the water column throughout the tidal period. 2. METHODS The settling tubes used in this study were Owen Tubes of the Braystoke SK 110 type. This is a 1 m long perspex tube with a diameter of 5 cm giving it a total sample volume of 2 dm 3. A tail fin is mounted on the back of the tube causing it to line up in the flow direction when lowered into the water. Thus, the water can flow unhindered through the tube until it is closed at both ends simultaneously with sealing caps controlled by a release messenger operated from the surface along the suspension wire. A more detailed description of the apparatus is given by Pejrup (1986, 1988) and Edelvang et al. (1992). After the sealing caps have been closed, the tube is pulled out of the water as quickly as possible and placed in a vertical position in a tripod stand. Time between sampling and the beginning of the analysis varied between 25 and 40 seconds. It is important to keep the handling time down at an absolute minimum in order to minimize floc formation in the tube before the analysis is started. Therefore, no rotation of the tube is made before placing it in the tripod stand. Besides taking time, such rotation may create weak secondary flows in the tube. When the settling tube is placed in its vertical posi- tion, settling of the suspended flocs begins and a stopwatch is started. Subsequently, subsamples are withdrawn from the bottom of the tube. A total of 10 subsamples are withdrawn with a volume of 200 cm 3 each. Temperature and salinity were measured in the first subsample, and temperature again in the last sample.