Effect of variable fractal dimension on the floc size distribution of suspended cohesive sediment

Flocculation of suspended cohesive sediment, well-known to impact the floc size distribution and vertical fluxes, and cause morphodynamic changes of marine and riverine environments, is modelled by means of a population balance equation that implements a novel description of floc geometry: the capacity dimension of fractal flocs, normally assumed constant over the population, has recently been argued to change during flocculation. Our experiments have shown that a power-taw function of the dimensionless floc size can conveniently be used to describe these changes. This description of floc capacity dimension is used to explore in detail the extent to which the geometrical properties of flocs affect aggregation and breakup processes, and contribute to shaping their size distribution. A comparison of experimental floc size distributions from settling column test with computed distributions for two hypotheses of floc capacity dimension (i.e., constant and variable) and two hypotheses of flocculation reactions (i.e., semi-stochastic and deterministic) are shown. This suggests that the use of variable rather than constant floc capacity dimension, and the use of semi-stochastic and asymmetric reactions rather than deterministic and symmetric, result in better predictions of the floc size distribution in the environmental conditions herein analysed. (C) 2007 Elsevier B.V. All rights reserved.