Textural signatures of sediment supply in gravel-bed rivers: Revisiting the armour ratio

Abstract

The surface of the streambed in gravel-bed rivers is commonly coarser than the underlying bed material. This surface coarsening, or ‘armouring’, is usually described by means of the ratio between surface and subsurface grain-size metrics (the ‘armour ratio’). Such surface coarsening is typical of river reaches that are degrading due to a deficit in sediment supply (e.g. gravel-bed reaches below dams or lakes), but non-degrading gravel-bed streams may also exhibit some degree of armouring in relation to specific hydrological patterns. For instance, selective transport during the recession limbs of long lasting floods may coarsen the bed more significantly than flash floods. Consequently, regional differences in bed coarsening should exist, reflecting in turn the variability in sediment and water regimes. In this paper, we explore the trends linking armour ratios to sediment supply, taking into account the differences in hydrological context. We based our analysis on a large data set of bedload and grain size measurements from 49 natural gravel-bed streams and four flume experiments compiled from the scientific literature. Our main outcome documents how the balances between sediment yields and transport capacities have a quantifiable reflection on the armour ratios measured in the field: we report statistically significant correlations between bedload fluxes and surface grain-size, and an asymptotic rise in armour ratios with the decline of sediment supply. Hydrological controls are also observed. Additionally, the trends observed in the field data are comparable to those previously documented in flume experiments with varying sediment feed. In this regard, different kinds of bedforms and particle arrangements have been commonly described with progressive reductions in sediment inputs and the subsequent coarsening of the streambed. Hence, armour ratios serve as a proxy for the general organization of the streambed of gravel-bed streams, and our results quantify this streambed adjustment to the dominant sediment regime.