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

Abstract

Abstract:Deposition and storage of ®ne-grained (562 . 5 mm) sediment in the hyporheic zone of gravel bed rivers frequently represents an important cause of aquatic habitat degradation. The particle size characteristics of such ®ne-grained bed sediment (FGBS) exert an important control on its hydrodynamic properties and environmental impact. Traditionally, particle size analysis of FGBS in gravel bed rivers has focused on the absolute size distribution of the chemically dispersed mineral fraction. However, recent work has indicated that in common with¯uvial suspended sediment, signi®cant dierences may exist between the absolute and the in situ, or eective, particle size composition of FGBS, as a result of the existence of aggregates, or composite particles. In the investigation reported in this paper, sealable bed traps that could be remotely opened to sample sediment deposited during speci®c storm runo events and a laser back-scatter probe were used to quantify the temporal and spatial variability of both the absolute and eective particle size composition of FGBS, and the associated suspended sediment from four gravel bed rivers in the Exe Basin, Devon, UK. The absolute particle size distributions of both the FGBS and suspended sediment evidenced c. 495% 5 62 . 5 mm sized primary particles and displayed a seasonal winter±summer ®ning, while the opposite trend was displayed by the eective particle size distribution of the FGBS and suspended sediment. The eective particle size distributions of both were typically highly aggregated, comprising up to 68% 4 62 . 5 mm sized particles. Spatial variation in the eective particle size and aggregation parameters was of secondary importance relative to temporal variation. The eective particle size distribution of the FGBS was consistently coarser and more aggregated than the associated suspended sediment and there was evidence of aggregate break-up in samples of resuspended bed sediment. The implications of these ®ndings for sediment transport modelling are considered.

The Fine‐Sediment Cascade

“…It clearly shows the shift in size with the median size of the flocculated sediment lying between 20 and 80 mm compared with an absolute value of about 5 mm. Phillips and Walling (1999) also show a seasonal variation in particle size, with the size of flocculated particles being higher in the summer when biological activity is greatest but with the absolute particle size being higher in winter when erosion is more active.…”

“…Grain sizes between 1 and 2.6 mm are entirely incorporated into flocs (Droppo and Ongley, 1992). Figure 10.4b shows example distributions of both effective particle size and absolute (disaggregated) particle size for both suspended sediment and bed sediments taken from Phillips and Walling (1999). It clearly shows the shift in size with the median size of the flocculated sediment lying between 20 and 80 mm compared with an absolute value of about 5 mm.…”

Identification of Fine-Scale Marine Benthic Ecoclines by Multiple Parallel Ordination

“…Sediment particle-size properties, recorded to describe conditions experienced by the macrofauna, should therefore preferably be recorded in situ [45,46]. Depth, which was not strongly correlated with any other variable in Subset-, was the only variable strongly correlated with the main coenocline in that subset.…”

Size characteristics of sediments eroded from three soils in China under natural rainfall

“…These data of particle size are mostly obtained after fully dispersing the sediment into primary mineral particles (Slattery and Burt 1997), which, as a traditional sizing technique, provides a consistent baseline for evaluating the spatial and temporal variability of sediment size (Droppo and Ongley 1994;Walling et al 2000). However, recent work has indicated that much of sediment is eroded in aggregated form and the dispersed sediment-size distribution is of limited value in studying sediment movement (Walling 1990;Phillips and Walling 1999;Martínez-Mena et al 1999). In contrast, the particle-size distribution analyzed without chemical dispersion reflects the size composition of the sediment when eroded and serves as a better indicator for sediment transport behavior (Slattery and Burt 1997;Walling et al 2000).…”