DIRECT MEASUREMENTS OF SETTLING VELOCITIES IN THE OWEN TUBE - A COMPARISON WITH GRAVIMETRIC ANALYSIS

Over the last two decades the Owen Tube or derivatives of the original design have been used extensively to determine the settling velocitie s of suspensions of cohesive material in the field. These gravimetric measurements have been used as elements of basic research programmes a nd also as important input parameters for predictions of siltation rat es. During a two year research programme at HR Wallingford undertaken between 1989 and 1991 consideration was given to an alternative techni que for analysing the settling of cohesive material. This technique is based upon obtaining high magnification video images of the processes occurring within the settling column. The Video images are then analy sed using a PC based image analysis system which enables settling velo cities and floc dimensions to be determined. The technique has been ap plied on a number of occasions over the past four years and was includ ed in the European Intercalibration Exercise that took place on the ri ver Elbe in June 1993. Analysis of the processed field data indicates flee break-up, reflocculation and the development of significant circu lations within the settling column during the withdrawal period requir ed for the gravimetric analysis (typically a duration of about an hour ). The interpretation of the visual data obtained by the video image a nalysis technique has indicated median settling Velocities significant ly greater than those obtained from the gravimetric technique. The pap er describes the field and analysis methods and results from the recen t intercalibration on the Elbe. During this exercise an experiment was undertaken whereby video image analysis was done on the results of fi lming a settling column whilst the standard gravimetric technique was applied. The paper also discusses the engineering applicability of the results obtained and raises the question as to which technique provid es the most representative rate of settlement of flocs in the natural environment?