Dynamic interactions between cohesive sediment tracers and natural mud

a b s t r a c tKeywords: cohesive sediment settling velocity mass settling flux flocculation suspended particulate matter mud transport models New formulations are presented for the settling velocity and mass settling flux (the product of settling velocity and sediment concentration) of flocculated estuarine mud. Physics-based formulae for these are developed based on assumptions of a two-class floc population (microflocs and Macroflocs) in quasi-equilibrium with the flow. The settling velocities of microflocs and Macroflocs are related to floc size and density via the Kolmogorov microscale as a function of turbulent shear-stress and sediment concentration, including heightdependence and floc-density-dependence. Coefficients in the formulae are calibrated against an existing large data-set of in situ observations of floc size and settling velocity from Northern European estuaries. Various measures of performance show that the resulting formulae achieve an improved level of agreement with data compared with other published prediction methods. The new formulae, with the original calibration coefficients, perform well in tests against independent measurements made in two estuaries.

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“…Winterwerp et al, 2006;Baugh and Manning, 2007;Gratiot and Manning, 2007;Manning and Dyer, 2007;Spencer et al, 2010;Spearman et al, 2011).…”

Section: Assumptionsmentioning

confidence: 99%

Settling velocity and mass settling flux of flocculated estuarine sediments

et al. 2013

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“…Transmission electron microscopy images have also visually identified the presence of both clay minerals and quartz mineral fragments within natural microfloc structures (Spencer et al 2010). This all suggests that when mixed sediments flocculate, the sand particles favour the microfloc fractions, probably attributed to the closer and stronger interparticle bonding potential of the microflocs.…”

Section: Role Of Aggregate Composition and Biology In Mud: Sand Mixtumentioning

confidence: 99%

The settling dynamics of flocculating mud-sand mixtures: Part 1—Empirical algorithm development

et al. 2011

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European estuaries tend to be regarded as being either predominantly muddy or sandy. In some estuaries, the cohesive and non-cohesive fractions can become segregated. However, recent laboratory tests have revealed that mud and sand from many coastal locations can exhibit some degree of flocculation. A clear understanding of the dynamic behaviour of sediments in the nearshore region is of particular importance for estuarine management groups who want to be able to accurately predict the transportation routes and fate of the suspended sediments. To achieve this goal, numerical computer simulations are usually the chosen tools. In order to use these models with any degree of confidence, the user must be able to provide the model with a reasonable mathematical description of spatial and temporal mass settling fluxes. However, the majority of flocculation models represent purely muddy suspensions. This paper assesses the settling characteristics of flocculating mixed-sediment suspensions through the synthesis of data, which was presented as a series of algorithms. Collectively, the algorithms were referred to as the mixedsediment settling velocity (MSSV) empirical model and could estimate the mass settling flux of mixed suspensions. The MSSV was based entirely on the settling and mass distribution patterns demonstrated by experimental observations, as opposed to pure physical theory. The selection of the algorithm structure was based on the concept of macroflocs-the larger aggregate structures-and smaller microflocs, representing constituent particles of the macroflocs. The floc data was generated using annular flume simulations and the floc properties measured using the video-based LabSFLOC instrumentation. The derived algorithms are valid for suspended sediment concentrations and turbulent shear stress values ranging between 0.2-5 g l −1 and 0.06-0.9 Pa, respectively. However, the MSSV algorithms were principally derived using manufactured mixtures of Tamar Estuary mud and a fine silica sand, which means that the algorithms presented are site-specific in nature, and not fully universal in application. In terms of mass settling flux (MSF) accuracy: at the lower flux range (195-777 mg m −2 s −1 ) most MSSV predictions were within a few percent of the observations, whilst for the largest observed MSFs (1.3-21 g m −2 s −1 ), the MSSV demonstrated a close fit with the data. Even for the highest observed MSF of 33 g m −2 s −1 (produced by a 75M:25S mixed suspension), the MSSV only under-estimated the flux by 18%. The MSSV algorithms indicated a trend whereby a rise in sand content, and a subsequent decrease in mud, favours the microflocs as the dominant flux contributor. Parameter comparison testing indicated that by applying a singlesediment assumption to a mixed-sediment environment, pure mud algorithms under-predicted at each concentration by as much as 25% and did not handle sandy mud sediments particularly well. Slow constant settling velocity (0.5 and 1 mm s −1 ) parameters severely under-predicted MSF (at times dow...

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“…Recent studies reveal that natural mud and cohesive sediments (e.g., used as tracers) with similar properties, lead to flocs of completely different characteristics (e.g. settling velocity) with respect to natural muddy material (e.g., Spencer et al, 2010). Further, properties of microand macro-flocs are strictly connected to the percentage of suspended materials (sand, silt, clay) within the water column (Whitehouse and Manning, 2007;Manning et al, 2010, this being captured by recent empirical and numerical models able at reproducing the dynamics of sand-mud mixtures (Manning et al, 2011;Spearman et al, 2011).…”

Section: Introductionmentioning

confidence: 99%