Intercomparison of in situ suspended matter (floc) size measurements

Eisma, D.; Bale, A.J.; Dearnaley, M.P.; Fennessy, M. J.; Leussen, Wim van; Maldiney, M.-A.; Pfeiffer, A.; Wells, John T.

doi:10.1016/s1385-1101(96)90765-0

Cited by 48 publications

(12 citation statements)

References 23 publications

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“…These values represented an increase of 5 to 27% with respect to the estimates established for the first crossing of the plume. Using estimate of E calculated for the ambient conditions recorded for each monitoring (8×10 -6 m 2 s -1 at CS30, 6×10 -6 m 2 s -1 at SC90), these values of E led to estimate of W s of 5×10 -5 m s -1 and 3×10 -5 m s -1 for the CS30 and SC90 sites respectively, which corresponds to typical settling velocities for fine sediment flocs (Eisma et al, 1996).…”

Section: Redeposition Of Turbid Plumesmentioning

confidence: 99%

Trawling-induced resuspension and dispersal of muddy sediments and dissolved elements in the Gulf of Lion (NW Mediterranean)

Madron¹,

Férré²,

Corre³

et al. 2005

Continental Shelf Research

125

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International audienceA dedicated trawling experiment was performed at three sites on the Gulf of Lion continental shelf, with the aim of assessing the resuspension of particulate and dissolved matter triggered by different types of trawls on muddy sediments. The different configurations were: (i) bottom trawl, with bobbin for ground rope (Rockhopper): (ii) bottom trawl, without bobbin (Medits); and (iii) pelagic trawl, towed at 1 and 10m above the seabed.;The plumes of resuspended sediment were measured using the acoustic backscattered intensity, from a towed ADCP. Concomitant profiles of particle size-distribution, light transmission and water samples were collected, outside and inside the plumes. The analysis of the data enabled derivation of the major physical and chemical characteristics of the plumes generated by the trawls; likewise, and to quantify the resuspension fluxes of sediment, particulate (PN, POC) and dissolved (nutrients) elements. The residence time and dispersal of the plumes were monitored and modelled, considering the settling velocity of the particulate matter and the near-bottom turbulence.;The results indicate that the bottom trawls produce significant resuspension, whilst the near-bottom and mid-water pelagic trawls have no impact upon the sediment. The sediment clouds at several hundreds metres astern of the bottom trawls are 3-6m high and 70-200m, wide; they were generated both by the otter doors and the net. The average suspended sediment concentrations measured in the plumes reach 50 mg l(-1). Resuspension fluxes of sediment along the path of the trawls range from 190 g m(-2) s(-1), for the coarsest sediment (clayey silt) to 800 g m(-2) s(-1) for the finest sediment (silty clay). Whilst the resuspended loads of dissolved elements (nutrients) within the plume segment suggest a release of porewater, present at least in the first few centimetres of sediment, the particulate matter load only resulted from the resuspension of less than 1 mm thickness of the sediment bed. This discrepancy shows that a very small fraction of the sediment ploughed by the trawl is effectively injected into the water column.;The monitoring of the settling of the plumes indicates a rapid decay of the sediment load, during the first hour after its generation. Some of the sediment (about 10-15% of the initial load) remains in suspension; this is due, probably, to the near-bottom turbulence that prevents the redeposition of the fine particles and aggregates. Lateral spreading of the plume is strongly dependent upon the variability of horizontal currents. (c) 2005 Elsevier Ltd. All rights reserved

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Section: Redeposition Of Turbid Plumesmentioning

confidence: 99%

Trawling-induced resuspension and dispersal of muddy sediments and dissolved elements in the Gulf of Lion (NW Mediterranean)

Madron¹,

Férré²,

Corre³

et al. 2005

Continental Shelf Research

125

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“…Several sets of observations suggest that under energetic conditions, turbulence can limit floc size in bottom boundary layers [Kranck and Milligan, 1992;Luettich et al, 1993;Eisma et al, 1996;Berhane et al, 1997], yet these studies lack the type of measurements required to evaluate the predictive ability of various proposed relationships between maximal floc size and turbulence. Specifically, previous studies have not measured waves and currents well enough to produce accurate estimates of the boundary shear stress, which is linked to e in bottom boundary layers [Heathershaw, 1979;Luettich et al, 1993] goal of this work is to use these data to conduct a rigorous assessment of the in situ effect of turbulence on floc size.…”

Section: Paper Number 2000jc900102mentioning

confidence: 99%

Controls on floc size in a continental shelf bottom boundary layer

Hill¹,

Voulgaris²,

Trowbridge³

2001

J. Geophys. Res.

110

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Abstract. Simultaneous in situ observations of floc size, waves, and currents in a continental shelf bottom boundary layer do not support generally accepted functional relationships between turbulence and floc size in the sea. In September and October 1996 and January 1997, two tripods were deployed in 70 m of water on the continental shelf south of Woods Hole, Massachusetts. On one a camera photographed particles in suspension 1.2 m above the bottom that had equivalent circular diameters larger than 250 •m, and on the other, three horizontally displaced acoustic current meters measured flow velocity 0.35 m above the bottom. The tripods were separated by ---150 m. Typically, maximal floc diameter stayed relatively constant, around 1 mm, and it showed a dependence on turbulence parameters that was significantly weaker than that predicted by any model that assumes that turbulence-induced stresses limit floc size. Occasionally, when waves and currents generated intense near-bed turbulence, flocs were destroyed. These precipitous decreases in maximal floc size also were not predicted by conventional models. The correlation in time between episodes of floc destruction and elevated combined wavecurrent stresses provides the first quantitative support for the hypothesis that floc size throughout bottom boundary layers can be controlled by breakup in the intensely sheared near-bed region. These observations demand a reassessment of the forces limiting floc size in the sea, and they indicate the potential for significant simplifying assumptions in models of floc dynamics. Repackaging of particles into flocs affects numerous industrial processes that employ gravitational settling to remove solids from suspension. Disciplines such as wastewater treatment and mineral processing therefore share a long history of research into the forces that limit floc size [Hunt, 1986]

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“…To distinguish between these two populations of particles, they introduced the terms ultimate (dispersed) and effective (flocculated) grain sizes. Since no systematic relationship exists between the ultimate and effective grain sizes (Eisma et al, 1996), ultimate grain sizes cannot be used in modelling (Droppo et al, 1998;Nicholas and Walling, 1996;Burt, 1995, 1997) and effective grain sizes have to be measured in situ.…”

Section: Introductionmentioning

confidence: 99%

In situ measurements of sediment settling characteristics in ﬂoodplains using a LISST‐ST

Thonon

Roberti

Middelkoop

et al. 2005

Earth Surf Processes Landf

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Due to a lack of data on settling velocities (w s ) and grain size distributions (GSDs) in floodplain environments, sedimentation models often use calibrated rather than measured parameters. Since the characteristics of suspended matter differ from those of deposited sediment, it is impossible to derive the w s and GSD from the latter. Therefore, one needs to measure in situ suspended sediment concentrations (SSCs), settling velocities, effective grain sizes and sedimentation fluxes. For this purpose we used the LISST-ST, a laser particle sizer combined with a settling tube.In 2002 The in situ grain size exhibited a significant positive relationship with w s , although the w s for the largest flocs showed high variability. Consequently, the variability in sedimentation fluxes was also large. In the actual sedimentation fluxes, and hence in sedimentation models, in situ grain sizes up to about 20 µ µ µ µ µm can be neglected. In floodplain sedimentation models the relation between settling velocity and in situ grain size can be used instead of Stokes's law, which is only valid for dispersed grain sizes. These models should also use adequate data on flow conditions as input, since these strongly influence the suspended sediment characteristics.

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Intercomparison of in situ suspended matter (floc) size measurements

Cited by 48 publications

References 23 publications

Trawling-induced resuspension and dispersal of muddy sediments and dissolved elements in the Gulf of Lion (NW Mediterranean)

Trawling-induced resuspension and dispersal of muddy sediments and dissolved elements in the Gulf of Lion (NW Mediterranean)

Controls on floc size in a continental shelf bottom boundary layer

In situ measurements of sediment settling characteristics in ﬂoodplains using a LISST‐ST

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