On the flocculation and settling velocity of estuarine mud

Abstract

This paper presents a three-dimensional model in an Eulerian reference frame for the evolution of the settling velocity of cohesive sediments in estuarine and coastal environments. The variation in settling velocity is the result of turbulence-induced aggregation and floc breakup processes. The mud flocs are treated as self-similar fractal entities because of which Stokes’ formula relating settling velocity and floc size has to be modified. A new hindered settling formula is proposed which accounts for the gelling processes typical of cohesive sediment at high concentrations. When the floc concentration approaches unity, a space-filling network develops and the settling velocity becomes zero. This state of the suspension is often referred to as fluid mud. The flocculation model is implemented in a 1 dv point model and applied to simulate the processes in a reach of the turbidity maximum in the Ems estuary, The Netherlands, where longitudinal gradients are small. The numerical results compare favourably with observed values of the vertical suspended sediment concentration, of the floc size and the fluid mud concentration. The model predicts large temporal variations in settling velocity and flocculation time, which explains why it was not possible to simulate the observed vertical profiles of suspended sediment concentration properly with a constant settling velocity.