The settling behaviour of fine sediment particles: some preliminary results from LISST instruments

The settling velocity of suspended particles is a dominant factor in controlling the transfer and fate of sediment and sediment-associated substances. The properties of fine particles can vary significantly throughout a catchment, especially in terms of the degree of aggregation/flocculation, but relatively little is known about the consequences this has on settling velocity. This study attempts to explore the significance of the particle size distribution in influencing the settling behaviour of natural particles. Particles were collected from a range of sources across two contrasting catchments, giving natural variability in the grain size composition and degree of aggregation/flocculation of the samples. Particle size and settling velocity were measured using novel LISST-100 and LISST-ST laser diffraction devices. Significant differences in settling velocity were found between samples, notably between aggregated/flocculated and dispersed samples, and between individual size classes. The results emphasize the importance of aggregation/flocculation in the hydraulic behaviour of sediment.