Suspended sediment structure: implications for sediment and contaminant transport modelling

This paper examines the influence of sediment (floc) structure (size, shape, porosity and density) on the physical behaviour (e.g. settling and transport) of suspended sediment (flocs) and its implications for sediment and contaminant transport models. Results demonstrate that as floc size increases, the settling velocity increases in a linear fashion. The rate of settling is, however, substantially below that predicted by Stokes' law for solid spherical particles of the same size. Floc density and porosity demonstrate strong negative and positive relationships respectively with floc size. As floc size increases, the density of the flocs approaches that of water. It is found that whilst a change in density can affect floc settling, the size of the floc is a much more important influence on settling. Floc shape also influences flox settling, with elongated flocs settling with their long axis parallel to the direction of settling. This paper concludes that suspended sediment can no longer be viewed and modelled in the traditional manner as solid spherical units. Suspended sediment must be observed and analysed in its natural flocculated form due to its significantly different behaviour (transport/settling) from primary and theoretical solid spherical particles.