An instrument has been developed to observe the settling of individual flocs in turbid water in order to to measure size and settling velocity spectra of estuarine cohesive suspended sediments. INSSEV - IN Situ SEttling Velocity instrument - is bed mounted and comprises a computer controlled decelerator chamber that collects a sample of water from which some of the suspended matter is allowed to enter the top of a settling column. The settling flocs are viewed using a miniature video system. Subsequent analysis of video tapes provides direct measurements of size and settling velocity of individual flocs down to 20 um. From this information floc effective density is estimated. The main feature of the instrument is its ability to video flocs in situ irrespective of the concentration in the estuary, with as little disturbance to their hydrodynamic environment as possible. In addition to size and settling velocity distributions, data analysis developed for the instrument produces spectra of concentration and settling flux with respect to size, settling velocity or effective density. This is the first time that these parameters have been measured in situ. Field testing in the Tamar Estuary, South West England, and the Elbe Estuary, Germany, has given useful results in flow velocities up to 0.6 m s-1 and in concentrations up to 400 mg l-1 INSSEV was used in the 1993 Elbe Intercalibration Experiment where nearly all types of instrumentation for the in situ determination of estuarine floc size and/or settling velocity were deployed over several tidal cycles. From observations in the turbidity maximum of the Tamar Estuary, INSSEV data has shown significant changes in floc population characteristics during the tidal cycle, the most important being changes in floc effective density. A strong relationship between floc effective density and ambient turbulence characteristics is shown.

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