The significance of particle aggregation in the overbank deposition of suspended sediment on river floodplains

The effects of suspended sediment aggregation on rates of overbank deposition and on the grain size composition of deposited sediment were investigated using a combined field measurement and numerical modelling approach. Measurements of the in situ grain size characteristics of the suspended sediment load (termed the effective size distribution) of the River Culm, Devon, were obtained using a custom-built water elutriation system. Samples of suspended sediment and of sediment deposited on the floodplain were analysed in the laboratory to determine the size distributions of their constituent discrete mineral particles (termed the ultimate size distribution). Interpretation of these field data was aided by the development of a two-dimensional finite difference model of flood hydraulics and suspended sediment dispersion and deposition. Field measurements and model predictions show that particle aggregation results in higher relative contributions of fine size fractions to overbank deposits. Aggregation may also provide a mechanism for explaining the poor agreement between theoretical and observed trends in relationships between mean deposit grain size and distance from the main channel. Suspended sediment aggregation is shown to cause significant increases in rates of floodplain deposition. Increases in deposition rates resulting from aggregation may, however, be less than expected because of the effects of aggregate shape and density. The latter may offset increases in particle size so that the settling velocities of the largest aggregates do not increase appreciably with particle diameter.