Suspended sediments in a macrotidal estuary: comparison and use of different sensors

Lunven, Michel; Gentien, Patrick

doi:10.1016/s0399-1784(00)00126-2

Cited by 9 publications

(12 citation statements)

References 25 publications

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“…In two previous studies (Gentien et al, 1995;Lunven & Gentien, 2000), we have reported cases where large organic aggregates presumably introduced biases in measurements of particle population distribution. Sampling of these aggregates being very difficult, confirmation of their occurrence can only be performed through direct video observation.…”

Section: Introductionmentioning

confidence: 91%

In situ video and diffraction analysis of marine particles

Lunven

Gentien²,

Kononen

et al. 2003

Estuarine, Coastal and Shelf Science

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A design for a new underwater video-system to detect and observe suspended particles is presented. Particles are collected and isolated in a rectangular box where they are highly illuminated by a white light plane. The total field of view is determined. The camera, equipped with a remote controlled zoom, can resolve particles sizes ranging from 25 μm to several millimetres. Real-time image analyses are therefore performed. Particle counts and size spectra are calculated and displayed. Total light intensity scattered by the illuminated particles is closely related to the back-scattering values determined by an optical back-scatter sensor. A particle size analyser using diffraction analysis is associated to this video-system on a custom profiler. Hydrological parameters are measured by a standard CTD probe associated to a chlorophyll sensor. Results are acquired and graphically presented in real time. This custom profiler presents numerous advantages in oceanographic research. Two examples of its use in different coastal areas are presented. In an estuary, temporal evolution of particle characteristics was described in relation to the tide cycle. While the video-system allows direct visualization and characterization of the largest particles, the particle-size analyser performs precise quantification of the finest ones. It was shown that the two methods were in accordance for quantification of large aggregates, which were observed around slack tide when salinity decreased. Video analyses cannot be performed above 25 mg l −1 dry weight equivalent. The system reliability, resolution and limits were also demonstrated during a cruise in the Gulf of Finland. A typical profile is presented here showing different layers, one characterized by the association of heterotrophic flagellates and detritals, and another dominated by zooplankton, the surface layer being characterized by cyanobacterial colonies. Video associated to diffraction analyses allows the study of flocculation processes in estuaries and a detailed description of thin layers.

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Section: Introductionmentioning

confidence: 91%

In situ video and diffraction analysis of marine particles

Lunven

Gentien²,

Kononen

et al. 2003

Estuarine, Coastal and Shelf Science

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“…Lunven and Gentien 2000;Lunau et al 2006;Mietta et al 2009). In estuaries, the high variability in forcing (Verney et al 2009) and the impact of biological factors make the behaviour of cohesive sediments even more complex and still not well understood (Winterwerp 2011).…”

Section: Introductionmentioning

confidence: 99%

Seasonal variability of cohesive sediment aggregation in the Bach Dang–Cam Estuary, Haiphong (Vietnam)

et al. 2012

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In the Bach Dang-Cam Estuary, northern Vietnam, mechanisms governing cohesive sediment aggregation were investigated in situ in [2008][2009]. As part of the Red River delta, this estuary exhibits a marked contrast in hydrological conditions between the monsoon and dry seasons. The impact on flocculation processes was assessed by means of surveys of water discharge, suspended particulate matter concentration and floc size distributions (FSDs) conducted during a tidal cycle at three selected sites along the estuary. A method was developed for calculating the relative volume concentration for the modes of various size classes from FSDs provided by the LISST 100X (Sequoia Scientific Inc.). It was found that all FSDs comprised four modes identified as particles/flocculi, fine and coarse microflocs, and macroflocs. Under the influence of the instantaneous turbulent kinetic energy, their proportions varied but without significant modification of their median diameters. In particular, when the turbulence level corresponded to a Kolmogorov microscale of less than ∼235 μm, a major breakup of flocs resulted in the formation of particles/flocculi and fine microflocs. Fluctuations in turbulence level were governed by seasonal variations in freshwater discharge and by the tidal cycle. During the wet season, strong freshwater input induced a high turbulent energy level that tended to generate sediment transfer from the coarser size classes (macroflocs, coarse microflocs) to finer ones (particles/flocculi and fine microflocs), and to promote a transport of sediment seawards. During the dry season, the influence of tides predominated. The turbulent energy level was then only episodically sufficiently high to generate transfer of sediment between floc size classes. At low turbulent energy, modifications in the proportions of floc size classes were due to differential settling. Tidal pumping produced a net upstream transport of sediment. Associated with the settling of sediment trapped in a near-bed layer at low turbulent energy, this causes the silting up of the waterways leading to the harbour of Haiphong.

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“…The advent of optical backscatter sensors (Bunt et al 1999;Lunven and Gentien 2000) and acoustic Doppler profilers (Land and Bray 1998;Holdaway et al 1999;Hoitink and Hoekstra 2005;Kostaschuk et al 2005) has enabled the acquisition of much longer time series of suspended sediment concentration (SSC) than would be possible by discrete sampling. Interpreted with care, long-term SSC time series provide valuable insights into the processes controlling sediment regime (Schoellhamer 1996(Schoellhamer , 2002 and, importantly, allow the evaluation of hypotheses originating from quantitative determinations of the residual flux.…”

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confidence: 99%

Tidal and Meteorological Forcing of Suspended Sediment Flux in a Muddy Mesotidal Estuary

French

Burningham

Benson

2008

Estuaries and Coasts

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Although the supply and fate of suspended sediment is of fundamental importance to the functioning and morphological evolution of muddy estuaries, reliable sediment budgets have been established in only a few cases. Especially for smaller estuaries, inadequate bathymetric surveys and a lack of intertidal sedimentation data often preclude estimation of the sediment budget from morphological change, while instrument-derived residual fluxes typically lie well within the errors associated with measurement of much larger gross tidal transports. Given suitably long-term records of continuously monitored suspended sediment concentration (SSC), however, analysis of the major scales of variation in sediment transport and their relation to hydrodynamic and meteorological forcing permits qualitative testing of hypotheses suggested by directly measured residual fluxes. This paper analyzes data from a 1-year acoustic Doppler profiler deployment in the Blyth estuary, a muddy mesotidal barrier-enclosed system on the UK east coast. Flux calculations indicate a small sediment import equivalent to just 1.5% of the gross flood tide transport. Little confidence can be assigned to either the magnitude or direction of such a small residual when considered in isolation. However, the inference that the sediment regime is finely balanced is qualitatively supported by the close similarity between floodtide and ebb-tide SSC values. Singular spectrum analysis of the SSC time series shows the expectedly large contributions to the variance in SSC at intratidal and subtidal (semimonthly and monthly) scales but also picks out intermittent variability that is initially attributed to a combination of non-tidal surge and wind stress forcing. Closer examination of the data through cross-correlograms and event-scale analysis indicates that local meteorological forcing is the major factor. Acting through the resuspension of intertidal mudflat sediments at times of strong westerlies, meteorological forcing is directly implicated in episodic sediment export from the estuary. Thresholding of tide-averaged fluxes using a range of critical wind stress values further indicates that 'tide-dominated' (i.e., low wind stress) and 'wave-dominated' (high wind stress) conditions are associated with sediment import and export. Sediment balance is potentially sensitive to the frequency of high wind stress events, since the associated sediment exports are several times larger than the average import under calm conditions. Intermittent meteorological forcing may thus exert an important control on the sedimentary balance of otherwise tidally dominated muddy estuarine systems, and the role of wind climate should not be overlooked in studies of estuary response to environmental change.

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Suspended sediments in a macrotidal estuary: comparison and use of different sensors

Cited by 9 publications

References 25 publications

In situ video and diffraction analysis of marine particles

In situ video and diffraction analysis of marine particles

Seasonal variability of cohesive sediment aggregation in the Bach Dang–Cam Estuary, Haiphong (Vietnam)

Tidal and Meteorological Forcing of Suspended Sediment Flux in a Muddy Mesotidal Estuary

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