Jm. Phillips et De. Walling, The particle size characteristics of fine-grained channel deposits in the River Exe Basin, Devon, UK, HYDROL PROC, 13(1), 1999, pp. 1-19
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.