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1 School of Geography & Earth Resources, The University of Hull, Hull HU6 7RX, UK
2 Department of Geography, University College London, 26 Bedford Way, London WC1H 0AP, UK
Downstream changes in bed material characteristics represent an element of the sediment transport process which is of immediate geomorphological and economic significance. Particle size trends represent the joint effect of abrasion and sorting, often complicated by in situ weathering and variable sediment supply. In principle, the combined effects of abrasion and shorting can be explained if attention is paid to the role and characteristics of near-bed turbulence, via the mechanisms of ‘abrasion in place’ and the size-dependent particle acceptance or rejection involved in Bluck’s ‘turbulence template’. Field data describing spatially- and stage-dependent differences in turbulence characteristics by which the significance of such processes can be quantified are, however, lacking.
In this paper, high-frequency three-dimensional velocity data obtained with an ultrasonic current meter in a braided section of Langden Brook, Lancashire, are used to demonstrate significant variations in turbulence parameters at the bar-pool scale. These are related to grain abrasion, sorting and the arrangement of grains in surface microtopographic bedforms using a force-balance model which incorporates turbulent velocity components in two dimensions. Following Bluck, it is suggested that bar forms act as, and result from, sediment trapping phenomena, whose fundamental controls lie in the characteristics of turbulence at the bar surface. Variations in turbulence provide a physical rationale for differing values of sorting/abrasion coefficients used in descriptive models of longitudinal size contrasts. In contrast to single-thread channels, however, spatially- and stage-dependent variation in turbulence characteristics is more marked in braided environments. Consequently, greater research effort is required to fully incorporate this into a spatially-realized form-process feedback mechanism applicable at the reach-scale and beyond.
