The particle size characteristics of fine-grained channel deposits in the River Exe Basin, Devon, UK

Deposition and storage of fine-grained (<62.5 mu m) sediment in the hyporhe ic zone of gravel bed rivers frequently represents an important cause of aq uatic habitat degradation. The particle size characteristics of such fine-g rained bed sediment (FGBS) exert an important control on its hydrodynamic p roperties and environmental impact. Traditionally, particle size analysis o f FGBS in gravel bed rivers has focused on the absolute size distribution o f the chemically dispersed mineral fraction. However, recent work has indic ated that in common with fluvial suspended sediment, significant difference s may exist between the absolute and the in situ, or effective, particle si ze composition of FGBS, as a result of the existence of aggregates, or comp osite particles. In the investigation reported in this paper, sealable bed traps that could be remotely opened to sample sediment deposited during spe cific storm runoff events and a laser back-scatter probe were used to quant ify the temporal and spatial variability of both the absolute and effective particle size composition of FGBS, and the associated suspended sediment f rom four gravel bed rivers in the Exe Basin, Devon, UK. The absolute partic le size distributions of both the FGBS and suspended sediment evidenced c. >95% < 62.5 mu m sized primary particles and displayed a seasonal winter-su mmer fining, while the opposite trend was displayed by the effective partic le size distribution of the FGBS and suspended sediment. The effective part icle size distributions of both were typically highly aggregated, comprisin g up to 68% > 62.5 mu m sized particles. Spatial variation in the effective particle size and aggregation parameters was of secondary importance relat ive to temporal variation. The effective particle size distribution of the FGBS was consistently coarser and more aggregated than the associated suspe nded sediment and there was evidence of aggregate break-up in samples of re suspended bed sediment. The implications of these findings for sediment tra nsport modelling are considered. Copyright (C) 1999 John Wiley & Sons, Ltd.