PARTICLE-SIZE CHARACTERISTICS OF SUSPENDED SEDIMENT IN HILLSLOPE RUNOFF AND STREAM-FLOW

This study examines the particle size characteristics of hillslope soi ls and fluvial suspended sediments in an agricultural catchment. Sampl es of surface runoff and stream flow were collected periodically and a nalysed for the size distributions of the effective (undispersed) sedi ment. This sediment was subsequently dispersed and the ultimate size d istributions determined, The median effective particle size of stream suspended sediment was considerably coarser than the median ultimate p article size, indicating that most of the load included a substantial proportion of aggregates. Moreover, the proportion of fine material (i .e. silt and clay) increased, and the proportion of sand-sized materia l decreased, with increasing discharge, This decrease in sediment size with increased how, which is contrary to the traditional assumption o f a positive discharge/particle size relationship, is thought to refle ct: (i) the influx of silt and clay, predominantly the former, origina ting on the catchment slopes and brought to the stream by overland flo w along vehicle wheelings, roads and tracks; and (ii) erosion of fine material from the channel bed and banks. During large storms, however, the proportion of sand-sized sediment increased during the rising lim b of the hydrograph, as a result of the entrainment of coarser source material from the valley floor during overbank flooding. The stream su spended sediment was finer than the catchment soils and considerably f iner than material eroding from the catchment slopes during storms. Th e degree of clay and silt enrichment in the suspended sediments was la rgely the result of preferential deposition of the coarser fraction du ring the transport and delivery of sediment from its source to basin o utlet. The data from this study confirm that a significant mode of sed iment transport in fluvial systems is in the form of aggregates, and t hat the dispersed sediment size distribution is inappropriate for dete rmining the transportability of sediment by flow. (C) 1997 by John Wil ey & Sons, Ltd.