Effective particle size characteristics of fluvial suspended sediment transported by lowland British rivers

The alluvial flood plains of lowland Britain contain thick sequences of fine-grained sediment that result from the deposition of suspended sediment during flood events. These environments constitute important storage zones for a wide range of sediment-associated nutrients and contaminants and many of the geochemical properties of the deposited sediment are strongly related to particle size. Recognition of the importance of composite suspended sediment particles in fluvial environments has generated a need for new field equipment and monitoring strategies capable of measuring the effective particle size characteristics of fluvial suspended sediment. This paper presents effective and absolute particle size data derived from a range of sampling and measurement techniques for five rivers in central and southern Britain. The study catchments cover a range of geological and hydrological conditions that are representative of lowland Britain more generally. All the rivers display marked contrasts between the effective and absolute particle size distributions of suspended sediment samples. Data from field-based water elutriation systems and scanning electron microscopy (SEM) show that composite particles dominate the suspended sediment load. Composite particles also contain significant clay and fine silt components which are an important control on the nutrient and contaminant loading of the sediment load. Understanding the nature and role of composite particles is essential to the study of the transfer and fate of nutrients and contaminants in the freshwater fluvial environment. Research in the Exe basin suggests that the time integrated data produced by the water elutriation apparatus are to be preferred for estimating the role of composite particles in suspended sediment flux and for calibrating models of sediment transport and deposition.