Acoustic and optical backscatter from flocculating sediments (FLOCSAM)
Lead Research Organisation:
NATIONAL OCEANOGRAPHY CENTRE
Department Name: Science and Technology
Abstract
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.
Publications
MacDonald I
(2013)
Acoustic scattering from a suspension of flocculated sediments
in Journal of Geophysical Research: Oceans
Thorne P
(2014)
Modelling acoustic scattering by suspended flocculating sediments
in Continental Shelf Research
Vincent C
(2015)
A flocculi model for the acoustic scattering from flocs
in Continental Shelf Research
| Description | FLOCSAM - a short summary: In the sea, light is attenuated very rapidly (particularly in coastal areas where water can be turbid due to the presence of suspended fine material). Sound on the other hand can propagate long distances and penetrate muddy water. Additionally most instrumentation deployed in the ocean use sound to make measure, for example acoustic Dopple current meters (ADCPs), are routinely used to profile the current velocities over the water column. Increasing marine scientists have been attempting to use the strength of the acoustic signals to estimate the mass concentration of sediment suspended in the water. Because of our lack of knowledge of the way that flocculation of sediment influences the strength of the backscattered signal, empirical calibration must be used - this usually involves the collection of a large number of water samples and filtering to obtain the mass concentration. FLOCSAM was designed to make progress in the understanding of the theoretical underpinning of influence of flocculating sediments on the strength of the backscattering signal using a series of carefully controlled laboratory experiments combined with field measurements where the results of this laboratory and theoretical studies could be tested in a 'real world' environment. A novel set of laboratory experiments were conducted to systemically investigate the way that fine sediment flocculation affects the backscatter of high frequency sound. These experiments were conducted on a simple clay (kaolin) system using a commercial flocculant to control the speed and extent of the flocculation. We are pleased to report that the laboratory studies have produced excellent results which have been tested against two standard acoustic backscattering models; the elastic sphere model that is used for suspended sand, and the fluid sphere model. Our work has shown that flocs respond to sound in a way that is bounded by these two models but in a way that was not anticipated. As well as providing the first insight into the processes involved in the backscatter of sound from flocs, an heuristic fit to our laboratory data has allowed our results to be useful in a practical sense for the prediction of the mass concentration when the fine sediments are flocculated. A field experiment was also conducted in the Tamar river where there is periodic suspension of both fine flocculated material and silts/fine sands, by the tidal currents, this proved an ideal, if challenging, environment to test both our current ideas about acoustic backscatter, and the use of a combination of acoustic and optical equipment to estimate the mass concentration of suspended material (backed up by many measurements of the mass concentration in the water column to provide 'ground truth' for our estimates). Results from the FLOCSAM project have been presented at a number of international conferences including the International Coastal Symposium in Plymouth, UK, in April 2013 where Chris Vincent gave a paper on the results of the experimental laboratory work on the acoustic backscatter from kaolin clays under various degrees of flocculation (Vincent, MacDonald, Thorne and Moate, 'Does size matter? Acoustic backscatter from flocculating clay'). This work has been submitted for publication in the Journal for Geophysical Research. Sarah Bass presented a paper on the field experiments conducted within FLOCSAM using a combination of acoustic and optical instrumentation which showed how the experimental laboratory studies where able to consolidate the optical and acoustic field measurements. (Bass et al, Predicting mass concentrations of flocculating sediments using acoustic and optical backscatter). The paper by Bass et al has been accepted for publication later this year in the Journal of Coastal Research. |
| Exploitation Route | The results from FLOCSAM are the first systematic examination of the way that sound and light interacts with flocculating sediment. Previous studies that used light and or sound to measure the concentration of fine material in suspension in the marine environment were overwhelmingly empirical and included little, if any, appreciation of the physical processes occurring as sound or sound interacted with, and was backscattered from flocculating sediment. These results have provided the first controlled laboratory results and have suggested models that might be used to provide interpretation of backscattered signals based on physical processes. This will allow oceanographic instruments to make much more reliable measurements of suspended concentration in the marine environment. See above for relevance to operational oceanography In terms of the operational oceanography and sediment transport empirical relationships could be fitted to the laboratory observations which may allow the routine use of backscatter data from instruments such as the ADCP or ADV to measure the concentration of flocculated sediments, |
| Sectors | Aerospace, Defence and Marine,Communities and Social Services/Policy,Environment |
| URL | http://dx.doi.org/10.1002/jgrc.20197 |
| Description | In terms of the operational use of acoustics for measuring the concentration of flocculated sediments, we have been able to fit an empirical relationship to our observations. Our results highlight the suitability of such an approach for the routine measurement of suspensions of flocculated particles in the marine environment. From a sedimentologist's point of view, it is use of our results for the which yields estimates of concentration and particle size that is of primary interest. However, a better theoretical understanding of how sound interacts with flocculated particles is necessary, based on extension of our experiments and we believe this will inspire other scientists and engineers to move this study forward. |
| First Year Of Impact | 2013 |
| Sector | Aerospace, Defence and Marine,Environment |
| Impact Types | Societal |