Suspended Sediment Transport – Flocculation and Particle Characteristics

Abstract

In the history of sediment transport research within hydrological sciences, there has possibly been no greater paradigm shift than that which has been provided by the identification of flocculation as a dominant mechanism operating within, and mediating, cohesive suspended sediment transport. Flocculation (the process of aggregating smaller particles together to form larger particles) significantly modifies the hydrodynamic properties of the sediment by altering their effective size, shape, density, porosity, and composition. Such changes have a dramatic impact on the downward flux of sediments and as such influence the transportation and fate of suspended sediments. The traditional assumption of suspended sediment being inorganic individual grains for the quantification and modeling of sediment transport is likely to result in erroneous results and management decisions. Flocs settle faster than their individual constituent particles (but orders of magnitude slower than that predicted by Stokes' equation for a similar size), and, for large flocs, can have porosity close to 100% and a corresponding density close to that of water. Flocs are composed of significant living organic components (primarily bacteria) which produce an extracellular polymeric substance (fibrils) which glues particles together giving them greater strength than an electrochemically bound floc, and a pseudo plastic nature. Because of this active living component, flocs should be thought of as micro ecosystems (composed of a matrix of water, inorganic, and organic particles) with autonomous and interactive physical, chemical, and biological functions or behaviors operating within the floc matrix. A better understanding of how floc structure influences floc behavior will help to refine our knowledge and management of suspended sediment transport in a variety of aquatic ecosystems.

Modeling Sediment Flocculation in Langmuir Turbulence

“…In contrast, for cohesive sediment, which is primarily the fine-grained mixture including silt, clay, fine sand, and organic matter, the additional cohesive characteristic due to electrochemical or biochemical attraction enables individual sediment particles to stick together to form an agglomeration of mineral and/or organic particles called floc or aggregate (Maggi, 2005;Mehta, 2013;Son & Hsu, 2011;Winterwerp & Van, 2004). Flocculation processes, which include aggregation and breakup, increase the complexity in determining the settling velocity and other properties (e.g., size, shape, and density) of cohesive sediment (e.g., Droppo et al, 2004;Dyer, 1989;Droppo, 2006;Mehta, 2013;Strom & Keyvani, 2011;Winterwerp & Van, 2004). Flocculation processes are also important for suspended sediment concentration (SSC) in the water column available for horizontal transport (e.g., Droppo et al, 1998;Sherwood et al, 2018;Verney et al, 2011).…”

A model for the settling velocity of flocs; application to an aquaculture recirculation tank

“…These solids are generally <65 μm in diameter and behave as cohesive sediments [2]. These sediments form fl ocs or aggregates, made of water, inorganic particles and organic particles [3][4][5]. To obtain adequate settling models for these particles is an open fi eld of research [6].…”

Experimental observations of cohesive sediment dynamics in aquaculture recirculation tanks

“…These solids are generally less than 65 μm in diameter and behave as cohesive sediments [2]. These sediments form flocs or aggregates, made of water, inorganic particles and organic particles [3][4][5]. The method of obtaining adequate settling models for these particles is an open field of research [6].…”