Measurement in situ of the effective particle-size characteristics of fluvial suspended sediment by means of a field-portable laser backscatter probe: Some preliminary results

Abstract

Sediment-associated transport frequently represents an important component of the flux of nutrients and contaminants through fluvial systems, and particle size exerts a fundamental influence upon the hydrodynamic behaviour and geochemical properties of the suspended sediment particles involved. Recent work has highlighted the need to distinguish between the ultimate and effective particle-size characteristics of fluvial suspended sediment. The latter may differ appreciably from the former owing to the existence of composite particles (aggregates or flocs). Obtaining representative data of the effective particle-size distribution ideally requires measurements to be made in situ. This paper describes the use of an immersible, portable laser backscatter probe (Par-Tec 200/300) to make measurements in situ of the effective particle-size characteristics of suspended sediment transported by the River Exe and its tributaries in Devon, UK. Within the study basin the effective and absolute particle-size characteristics of suspended sediment were documented at four sites. Significant spatial variation and inter- and intra-event temporal variation in the mean effective size are identified. The degree of aggregation is assessed by comparing the effective and absolute particle-size data. Such comparisons show aggregation to be an important process in the study basin, although its precise magnitude varies in both space and time.