Do cohesive sediment flocs contain sand particles and does it matter?

Spearman, J. and Manning, A.J. (2023) Do cohesive sediment flocs contain sand particles and does it matter? In: INTERCOH 2023 (17th International Conference on Cohesive Sediment Transport Processes), 18-22 September 2023, Incheon, Republic of Korea.

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The extent to which sand grains can be trapped within the fine sediment and organic matrix of a floc is still a contested scientific topic. Evidence for the interaction of sand and fine sediment was presented in measurements of floc settling velocities from different sand-mud proportions (Manning et al., 2011). These results were used to develop equations for settling velocity for different sand-mud percentages and concentrations and shear. Those equations were then used with numerical modelling to successfully reproduce time series of sand-mud concentration profiles from highly detailed field experiments (Spearman et al., 2011), providing further evidence of the interaction between sand and cohesive sediment flocs. Most recently those equations were also included in the Regional Ocean Modeling System implemented in the Coupled Ocean–Atmosphere–Wave–Sediment Transport Modeling System by Sherwood et al. (2018). In the last decade the use of X-ray micro-tomography and electron microscopy on floc structure has demonstrated how organic material (extracellular polymeric substances, algae, bacteria, etc.) dominates the cohesion and structure of fine sediment flocs (e.g., Wheatland et al., 2020; Spencer et al., 2021, 2022). These techniques have also shown how clay and organic matrices can include sand particles in muddy sediments (Figure 1), though to date such techniques have not been applied to capture the structure of flocs including sand particles. One of the questions governing the interaction of sand particles with muddy flocs is whether, and how, this interaction changes the deposition flux of mud and sand. Most sediment transport modelling codes in use today assume segregation - i.e., there is little direct interaction between mud and sand in the water column, and the only interaction occurring at, and within, the sediment bed (e.g. as postulated by Van Ledden, 2003). The evidence from the earlier work by Manning et al. (2011) and Spearman et al. (2011) suggests this may not be true. However, the generally accepted assumption of independent settling of sand particles and mud flocs with the latter settling at a speed in the region of 1 mm/s, is normally a practical and successful approach to sediment modelling, and this could imply that any interaction between mud flocs and sand particles in the water column is not significant. We examine this issue through the careful analysis of suspended settling data of sediment flocs resulting from the suspension of varying proportions of mud and sand, measured as part of the TKI - MUSA research project ( The measurements of floc size and settling speed were undertaken using the high resolution LabsFLOC2 video system (e.g., Manning et al., 2006, 2017) using sediment samples collected both from the seabed and from the water column. These were used, together with the information about the mud and sand concentrations of the samples used, to infer how mud and sand particles are distributed within the floc population. A 1DV model was then used to examine whether the distribution of mud and sand within the floc population aligns with the observed field information. The results of the study were used to evaluate whether the interaction of sand and mud particles is important in a practical sense for sediment transport modelling.

Item Type: Conference or Workshop Item (Paper)
Subjects: Maritime > General
Divisions: Maritime
Depositing User: Helen Stevenson
Date Deposited: 14 Dec 2023 11:21
Last Modified: 14 Dec 2023 11:21

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