A simple model for turbulence induced flocculation of cohesive sediment

The transport and fate of fine grained cohesive sediments in estuarine and coastal waters is influenced by the settling velocity of the sediment, which in turn is affected by flocculation effects. The flocculation processes depend on the physico-chemical properties of the sediment and the water, and on several physical mechanisms, of which turbulence is a major one. In the present paper the effects of turbulence are further analyzed. The flocs are treated as self-similar fractal entities. As a result, the settling velocity is shown not to scale with the diameter D squared, as predicted by Stokes' law, but with Dnf-1, where nf is the fractal dimension. Based on a collision frequency function and a flee breakup formulation, a simple flocculation model is proposed that can be solved analytically for uniform conditions. The flocculation and flee breakup coefficients were obtained from experiments described in literature. It is shown that the maximal attainable flee size and settling velocity are determined to a large extent, especially at low turbulence levels, by the available residence time, i.e. water depth.