Acoustic and optical backscatter from flocculating sediments (FLOCSAM)

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Fine suspended particles exert a profound influence on water quality in estuaries and coastal seas through their propensity to adsorb contaminants such as metals and organic compounds and by acting as a barrier to the penetration of light affecting growth of benthic biota; they impact on sensitive marine ecosystems and influence nutrient cycling and have considerable socio-economic impacts related to dredging and dumping activities around ports and harbours as sediment flocculates due to enhanced settling. Marine scientists, engineers and managers therefore need to be able to measure fine sediment fluxes to assess their impact on the coastal environment. However, fine suspended particles rarely exist in their primary state but form flocs which are typically aggregated, heterogeneous assemblages of mineral grains, biogenic debris, bacteria and organic material. Optical and acoustic instruments are used to measure suspended sediment mass concentrations but with almost no understanding of the effects that flocculation has on the light and sound. Instruments are 'calibrated' against water samples from which sediment mass concentration is measured. We intend to investigate this long-standing and scientifically-challenging problem of how sound and light respond to muddy sediments and develop, through both theory and experiment, algorithms capable of quantitatively inverting backscatter signals from cohesive sediment to predict mass concentration, combining the best features of optical and acoustic sensors. The theoretical results of the study will be tested and evaluated using carefully controlled small-scale (Couette-type tank) and medium-scale (flume) laboratory experiments where a degree of control can be exerted on the flocculation processes, and against a full-scale field campaign at a muddy estuarine site. Both the flume and field trials will use a variety of acoustic (ABS, ADV and ADCP) and optical instruments (OBS, LISST-100 and INSSEV) together with pumped-sampling, velocity and turbulence measurements. The INSSEV (up-graded for this project) will be especially valuable, allowing digital images of individual flocs to be processed and their fall velocity measured.