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

Abstract

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...

Sediment resuspension, flocculation, and settling in a macrotidal estuary

“…[47] A notable relationship between settling velocity and turbulence dissipation parameter (see Figure 11c), as also suggested by Manning et al [2011] and Spearman et al [2011] is found in the data. This relationship indicates that settling velocities increase with increasing turbulent dissipation as follows (Figure 11c):…”

“…A notable relationship between settling velocity and turbulence dissipation parameter (see Figure c), as also suggested by Manning et al . [] and Spearman et al . [] is found in the data.…”

Wave Ripple Development on Mixed Clay‐Sand Substrates: Effects of Clay Winnowing and Armoring

“…In nature, however, wave ripple development is also controlled by sediment erodibility related to geochemical and biological properties of sediments (Grabowski et al, ), for example, through the cohesive properties of EPSs of microphytobenthic origin (e.g., Malarkey et al, ; Parsons et al, ). Moreover, the suspension settling of clay particles in the form of mixed‐sediment floccules has been found to affect the physical properties of the bed sediment (Manning et al, , ), which, together with mixing of this cohesive sediment into the bed by benthic organisms, is also expected to influence bed form evolution. Therefore, further study is needed to fully quantify the effect of wave forcing and bed material properties on wave ripple development and stability.…”

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).…”