The use of optics for the in situ determination of flocculated mud characteristics

Abstract

Morphodynamical predictive simulations of estuarine sediments require in situ mud floc data for model verification and calibration purposes. The limiting factor in many previous studies were the devices used for sampling, as flocs are very fragile. Instruments such as Niskin bottles, pipettes or the Owen tube are all very disruptive. This could be the reason that previous studies tended to show a much lower floc size range than is now known to exist. The presence of large estuarine macroflocs was observed by in situ photography and in situ laser particle size measurements, but these techniques still provided no indication of settling velocity or effective density, which are variable amongst floc populations. In contrast, the video camera based instrument developed at the University of Plymouth, INSSEV (in situ settling velocity), measures floc size, settling velocity and density all simultaneously. This operates whereby flocs are trapped in an upper decelerator chamber and then allowed to fall into a settling column located underneath. A Puffin model UTC 341 high resolution monochrome Pasecon tube video camera, fitted with a f/4 macro lens and integral low heat LED illumination, views the flocs through a window in the side of the settling column, and hence the floc characteristics can be obtained. The camera utilizes a back-illumination system (i.e. a silhouetting technique) in which particles appear dark on a light background; this reduces image smearing and makes the floc structure more clearly visible. A selection of INSSEV flocs are presented from deployments conducted in the upper Tamar Estuary during 1998. Low concentration neap tides revealed that optimum ambient flocculation conditions produced macroflocs approaching 0.75 mm in length and settling velocities of 4–5 mm s−1. These macroflocs typically resembled 'comets' or 'long stringers'. However, these stringer configuration macroflocs were in the minority and on average only represented 30–40% of the total suspended matter concentration. Throughout the more turbulent and higher concentration spring tides, INSSEV was found to be very effective at measuring floc characteristics, even within concentrated benthic suspension layers of 8 g l−1. Ideal flocculation conditions (in terms of floc size) transformed 95% of the ambient suspended particulate matter concentration present into large, fast settling, more rounded cluster-type macroflocs with settling velocities of 8–15 mm s−1 and effective densities under 50 kg m−3. A number of the smaller macroflocs had their settling characteristics significantly improved by becoming interlinked with organic matter to form stringers with a string-of-pearls configuration. Although stringers were seen to occur during both neap and spring tides, the former ambient conditions tended to produce stringers which were only a third of the size of those typically observed at springs, and thus the neap tide stringers had comparatively lower settling velocities.