Particle Size Selectivity Considerations in Suspended Sediment Budget Investigations

The delivery of suspended sediment front drainage basins has frequently been quantified in mass terms by use of the sediment budget approach, which identifies sources. Storage and output of mobilised sediment. An attempt is presented here to define the main components of a generalised suspended sediment budget for a drainage basin in Devon, U.K. in teens of particle size characteristics and grain size selectivity, rather than total amounts of sediment. Samples of sediment mobilised from the hillslopes, fluvial suspended sediment and suspended sediment deposited on the river bed were all collected for particle size characterisation. These samples were then treated to remove organic matter and their chemically dispersed (absolute) particle size composition was measured using a Coulter LS130 laser granulometer. Where possible, measurements of the naturalin-situ particle size distribution (effective particle size) were also undertaken. Samples were collected at different times of the year so that temporal variation of hydromcteorolgical and ground conditions was represented. Comparison of the results for the different components of the delivery process shows that significant particle size selectivity occurs in the mobilisation and transfer of sediment from the hillslopes to the basin outlet. This reflects the particle size selectivity of detachment, transport and deposition processes, which is in turn influenced by the aggregation or flocculation (effective particle size) of the sediment.