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. Acoustic and optical instruments are used to measure suspended sediment mass concentrations but we have very little understanding of how sound in particular interacts with complex flocs. Instruments such as Acoustic Doppler Current Profilers (ADCPs), which are frequently used to 'measure' fluxes of fine material through the water column, are usually '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 responds to muddy sediments and develop, through a combination of theory and experiment, algorithms capable of quantitatively inverting acoustic backscatter signals from cohesive sediment to predict mass concentration, and to combining these with the best features of optical sensors. The theoretical component of this study will build on the existing experience with backscatter models from non-cohesive sediment (sand) and developments will be informed by the tank, flume and field results. Small-scale (Couette-type tank) and medium-scale (flume) laboratory experiments will be conducted, where a degree of control can be exerted on the flocculation processes using simple clay suspensions (without the complexity of mixtures of sediments or of biological components) and culminating in a full-scale field campaign at a muddy estuarine site. Both the laboratory and field trials will use a variety of acoustic (ABS, ADV and ADCP) and optical instruments (OBS, LISST-100 and imaging-technology) together with pumped-sampling, velocity and turbulence measurements. The imaging technology will build on the INSSEV programme and will be especially valuable, allowing digital images of individual flocs to be processed and their fall velocity measured.