CGRG Bibliography of Canadian Geomorphology
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Author : Bloomer, D.J.
Date : 2001.
Title : Sediment sorting in the gravel-sand transition along rivers: a field and modelling investigation.
Publication : Unpublished Ph.D. dissertation. Sheffield
Issue :
Page(s) :
Abstract
Gradual downstream fining in gravel bed rivers is often followed by a spatially rapid switch to a sand bed. These gravel-sand transitions (GSTs) can occur in rock types where abrasion rates are low. A common factor is declining shear stress towards base level causing size selective sorting. This study investigates the characteristics of visually abrupt GSTs in two streams of different size and morphology (Allt Dubhaig, Scotland and Vedder River, British Columbia, Canada). A one-dimensional numerical model of width-averaged size selective gravel sorting is enhanced to simulate gravel-sand mixtures. The updated model fails to generate a GST unless an abrupt break of slope is specified at the start of a run, although only one of the fieldsites exhibits this feature today. This finding suggests that additional processes are crucial to initiate a GST. A qualitative method of assessing bed surface facies is developed. Evidence from subsurface probing investigations and bed surface sedimentology indicates a slowly prograding gravel front. The position of the front is dependent on near-bed hydraulics. A fine-gravel tracer experiment shows that the transport of these sizes in the GST reach is size selective, although this is not the case in the distal gravel reach. Field characterisation indicates that the crucial processes missing from the model include: the overwhelming of a gravel framework bed by sand, as the threshold for sand storage is approached, leading to an increased availability of sand on the bed surface; and the lateral sorting of sediment into patches of different ambient grain size, further increasing the availability of the fine fraction.
Bibliography of Canadian Geomorphology