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 gravelbed 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 strong and 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, but they are small compared to the signal related to sediment supply. 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 supply regime.

Coarse sediment supply sets the slope of bedrock channels in rapidly uplifting terrain: Field and topographic evidence from eastern Taiwan

“…The abundant presence of rounded and imbricated (transported) volcanic rock large boulders (e.g., diameter > 1 m) in channels several kilometers away from its source supports the partial role of debris‐flow processes in shaping the fluvial channel profiles (Figure 4a,c), which require unrealistically large high‐flow hydraulic radius ( R h ) to exceed the threshold of sediment entrainment of these boulders by fluvial bedload processes (Equation ). This scenario is plausible because the efficiency of channel bed erosion scales with coarse‐sediment flux per unit channel width (Sklar & Dietrich, 2001; Vázquez‐Tarrío et al, 2020) and the frequency of episodic debris flows (Stock & Dietrich, 2003; Stock et al, 2005), which increase with uplift rate and the occurrence of landslides on steep hillslopes (Attal et al, 2015; Larsen & Montgomery, 2012).…”

The active layer in gravel‐bed rivers: An empirical appraisal

“…Additionally, bedload transport in gravel‐bed rivers is largely controlled by the relative mobility of the streambed surface, which is often armoured (Vázquez Tarrío et al ., 2020). Thus, we could expect some co‐variation between the relative mobility of the streambed surface and the depth of the active layer.…”

“…DeVries, 2002). However, gravel‐bed rivers are often armoured, which means that the surface layer is coarser than the underlying substrate (Gomez, 1983; Parker and Sutherland, 1990; Richards and Clifford, 1991; Hassan et al ., 2006; Vericat et al ., 2006; Venditti et al ., 2017; Vázquez Tarrío et al ., 2020). At this stage, larger grains prevent entrainment, increasing the chances of sediment by‐pass and decreasing the amount of vertical mixing during bedload (e.g.…”

Experimental Insights Into the Threshold of Motion in Alluvial Channels: Sediment Supply and Streambed State

“…They tend to develop rapidly in gravel‐bed streams and may remain largely intact even during relatively large flow events (Lamarre & Roy, 2008). The impact of sediment supply on bed surface structuring has been explored experimentally (e.g., Hassan & Church, 2000; Johnson, 2017; Qin et al., 2017; Vazquez‐Tarrio et al., 2020). Like armur layers, the development of gravel‐bed structures is affected by sediment feed rates relative to transport capacity, and influences the amount of shear stress available to transport sediment.…”