Preprints
https://doi.org/10.5194/egusphere-2024-524
https://doi.org/10.5194/egusphere-2024-524
27 Feb 2024
 | 27 Feb 2024

Validating floc settling velocity models in rivers and freshwater wetlands

Justin A. Nghiem, Gen K. Li, Joshua P. Harringmeyer, Gerard Salter, Cédric G. Fichot, Luca Cortese, and Michael P. Lamb

Abstract. Flocculation controls mud sedimentation and organic carbon burial rates by increasing mud settling velocity. Floc settling velocity can be predicted using a semi-empirical model that depends on turbulence, sediment concentration, and geochemical variables or an explicit Stokes law-type model that depends on floc diameter, permeability, and fractal properties. However, validation of the semi-empirical and explicit models with direct field measurements is lacking. We employed a camera, in situ particle sizing, and analysis of grain size-specific suspended sediment concentration profiles to measure flocs in the freshwater channels and wetlands of Wax Lake Delta, Louisiana. Sediment finer than ~20 to 50 μm flocculates with median floc diameter of 30 to 90 μm, bulk solid fraction of 0.05 to 0.3, and floc settling velocity of ~0.1 to 1 mm s-1, with little variation along depth. These values are consistent with the semi-empirical model, which indicates that turbulence limits variation in floc settling velocity on flood-to-seasonal time scales. In the explicit model, the effective primary particle diameter, commonly assumed to be the median primary particle diameter, differs by a factor of ~2 to 6 smaller than the median and can be better described using a simple fractal theory. Flow through the floc increases settling velocity by a factor of ~2 and can be explained by parameterizing flocs as effectively permeable clusters of primary particles. Our results provide the first full field validation of effective primary particle diameter and floc permeability theories, which improve floc settling velocity predictions of the explicit model.

Justin A. Nghiem, Gen K. Li, Joshua P. Harringmeyer, Gerard Salter, Cédric G. Fichot, Luca Cortese, and Michael P. Lamb

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-524', Anonymous Referee #1, 06 Apr 2024
  • RC2: 'Comment on egusphere-2024-524', Anonymous Referee #2, 08 Apr 2024
  • RC3: 'Comment on egusphere-2024-524', Anonymous Referee #3, 13 Apr 2024
Justin A. Nghiem, Gen K. Li, Joshua P. Harringmeyer, Gerard Salter, Cédric G. Fichot, Luca Cortese, and Michael P. Lamb
Justin A. Nghiem, Gen K. Li, Joshua P. Harringmeyer, Gerard Salter, Cédric G. Fichot, Luca Cortese, and Michael P. Lamb

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Short summary
Fine sediment grains in freshwater can cohere into faster settling particles called flocs, but floc settling velocity theory has not been fully validated. Data from the Wax Lake Delta verify a semi-empirical model relying on turbulence and geochemical factors. We showed that the representative grain diameter within flocs relies on floc structure and that floc internal flow follows a model in which flocs consist of permeable grain clusters, thus improving a physics-based settling velocity model.