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Spatial variation in armouring in a channel with high sediment supply

Recent advances in our understanding of the origin and function of armouring in gravel-bed rivers have not addressed the role of non-uniformity and unsteadiness of flow. These flow attributes have important influences on both the surface and subsurface bed material size distributions which are observed at low flow, from which we commonly make inferences concerning bedload transport at high flow. Bed armouring, measured as the ratio of surface to subsurface bed material grain size, is highly variable spatially in a channel in which past and present bed aggradation indicates that the sediment supply has exceeded the transport capacity. Alternate bar topography induces strong variations in boundary shear stress, especially at low flow. The resulting winnowing of fine sediment from zones of high stress, such as riffles, and its deposition as thin sheets in zones of low stress, such as pools, produce wide variations in armouring. Linked winnowing and deposition of fine sediment in each bar pool sequence creates pronounced, local, streamwise sorting by grain size. Streamwise sorting can lead to pronounced apparent size-selectivity in bedload transport during subsequent rising stages. The relatively fine bedload transported during initial rising stages can originate from unarmoured deposits of fines, where all particle sizes present on the bed surface may be nearly equally mobile. These surfaces are devoid of larger particles, however, that appear in the bedload only when coarser armoured surfaces become more widely entrained.