The particle size characteristics of fluvial suspended sediment in the Humber and Tweed catchments, UK

This paper presents information on the absolute (chemically-dispersed) particle size characteristics of the suspended sediment transported by rivers in the Humber and Tweed basins during the period 1994-1998. For most of the rivers, > 95% of the suspended sediment load at the time of sampling was < 63 microm (i.e. silt- and clay-sized material) and < 5% was > 63 microm (i.e. sand-sized material). The < 2 microm fraction (i.e. clay-sized material) typically ranged between 15 and 25%. The average median (d50) particle size for the individual rivers ranged between 4.1 and 13.5 microm. Generally, the particle size characteristics of suspended sediment transported in the two basins were similar. There were, however, noticeable spatial variations in the particle size composition of suspended sediment within the study basins, which reflected the particle size of the sediment sources and their spatial variation, and the selectivity of the sediment mobilization and delivery processes. When particle size parameters were plotted against discharge, there were no significant relationships, although there was some evidence of trends varying between sites. The lack of significant relationships with discharge reflects the fact that sediment particle size is largely supply-controlled, rather than a function of flow and hydraulics. When particle size variations were examined during individual storm events, there was evidence of a pulse of coarse sediment on the rising limb of the hydrograph. This may reflect the remobilization of coarse channel bed sediment as flow velocity and shear stress increase. Finer sediment was transported subsequently during the hydrograph peak and on the falling limb. The findings reported have important implications for understanding and modelling suspended sediment, and associated contaminant, dynamics in river basins.