The settling dynamics of flocculating mud and sand mixtures: part 2—numerical modelling

Abstract

Estuarine and coastal sediment transport is characterised by the transport of both sand-sized particles (of diameter greater than 63 μm) and muddy fine-grained sediments (silt, diameter less than 63 μm; clay, diameter less than 2 μm). These fractions are traditionally considered as non-cohesive and cohesive, respectively, because of the negligible physico-chemical attraction that occurs between sand grains. However, the flocculation of sediment particles is not only caused by physicochemical attraction. Cohesivity of sediment is also caused by biology, in particular the sticky extra-cellular polymeric substances secreted by diatoms, and the effect of biology in binding sediment particles can be much larger than that of physico-chemical attraction. As demonstrated by Manning (2008) and further expanded in part 1 of this paper (Manning et al., submitted), the greater binding effect of biology allows sand particles to flocculate with mud. In many estuaries, both the sand and fine sediment fractions are transported in significant quantities. Many of the more common sediment transport modelling suites now have the capability to combine mud and sand transport. However, in all of these modelling approaches, the modelling of mixed sediment transport has still essentially separated the modelling of sand and mud fractions assuming that these different fractions do not interact except at the bed. However, the use of in situ video techniques has greatly enhanced the accuracy and reliability of settling velocity measurements and has led to a re-appraisal of this widely held assumption. Measurements of settling velocity in mixed sands presented by Manning et al. (2009) have shown strong evidence for the flocculation of mixed sediments, whilst the greater understanding of the role of biology in flocculation has identified mechanisms by which this mud-sand flocculation can occur. In the first part of this paper (Manning et al., submitted), the development of an empirical flocculation model is described which represents the interaction between sand and mud particles in the flocculation process. Measurements of the settling velocity of varying mud-sand mixtures are described, and empirical algorithms governing the variation of settling velocity with turbulence, suspended sediment concentration and mud-sand content are derived. The second part of this paper continues the theme of examination of the effects of mud-sand interaction on flocculation. A 1DV mixed transport model is developed and used to reproduce the vertical transport of mixed sediment fractions. The 1DV model is used to reproduce the measured settling velocities in the laboratory experiments described in the part 1 paper and also to reproduce measurements of concentration of mixed sediments in the Outer Thames. In both modelling exercises, the model is run using the algorithms developed in part 1 and repeated using an assumption of no interaction between mud and sand in the flocculation process. The results of the modelling show a significant improvement in the abilit...

Cohesive and mixed sediment in the Regional Ocean Modeling System (ROMS v3.6) implemented in the Coupled Ocean–Atmosphere–Wave–Sediment Transport Modeling System (COAWST r1234)

“…The floc model does not explicitly account for the effects of organic matter content, pH, or salinity on flocculation rate (e.g., Mietta et al, 2009); these influences are subsumed into useradjustable parameters. It is important to note that the mass settling fluxes of mixed (sand + mud) suspensions may be overestimated if their interactions are not considered, as is the case in the approach taken here (Manning et al, 2010Spearman et al, 2011). Nonetheless, our implementation of flocculation, bed consolidation, and bed-mixing modules enhances the utility of the ROMS sediment model for interdisciplinary studies including ecosystem feedbacks (light attenuation, biogeochemistry) and contaminant transport.…”

“…Soulsby et al (2013) reviewed methods for estimating floc settling velocities and proposed a new formulation that depends primarily on turbulence shear and instantaneous suspended sediment concentration. Spearman et al (2011) noted that adjustments to settling velocity (e.g., Manning and Dyer, 2007) were able to successfully reproduce floc settling in one-dimensional estuary modeling applications. However, approaches that adjust only settling velocity do not allow for an analysis of other characteristics of the suspended particle field, such as particle size and density, which affect acoustic and optical properties, or geochemical properties (water content and surface area).…”

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

“…Spearman et al (2011) describe an example in the outer Thames, renowned for being a sandy area, where the flux of suspended sediment of mud and sand are of the same order.…”

Bedform migration in a mixed sand and cohesive clay intertidal environment and implications for bed material transport predictions

“…While reasonably accurate sediment transport predictors are available for pure sands, a knowledge gap exists for the behavior of mixed sediments composed of natural cohesive mud (clay and silt) and non-cohesive sand (Souza et al, 2010;Amoudry and Souza, 2011;Manning et al, 2011;Spearman et al, 2011;Aldridge et al, 2015).…”