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.

Introduction

A longstanding idea in fluvial geomorphology is that balances between sediment supply and transport capacities influence channel geometry (Parker et al., 2007; Parker, 2008), bed slope (e.g. Lane, 1955a; Borland, 1960; Wilcock et al., 2009), streambed texture (Dietrich et al., 1989; Nelson et al., 2009; Venditti et al., 2017) and planform morphology (e.g. Montgomery and Buffington, 1997; Church, 2006; Buffington, 2012; Hassan and Zimmermann, 2012; Métivier and Barrier, 2012). Thus, an abiding goal for a great deal of research in river morphodynamics has searched for a quantitative comprehension of channel adjustments to the balance between water and sediment supply (e.g. Lane, 1955a, Lane, 1955b; Parker, 2004; Blom et al., 2017), yet a full understanding still needs to be elucidated (Eaton et al., 2004; Lawson, 2020). In particular, the prediction of how riverbed texture responds to changes in governing conditions (discharge, sediment supply, valley slope) remains elusive. Very often, the evolution of bed texture is neglected from morphodynamic models or treated with simplistic approaches (Lawson, 2020), even though bed state is a main regulator of river response to sediment supply fluctuations (e.g. Church et al., 1998; Clayton and Pitlick, 2008; Turowski et al., 2011).

In this regard, one major sedimentary feature in gravel-bed rivers is streambed coarsening (often called ‘armouring’). Early field observations (e.g. Harrison, 1950; Gessler, 1967; Willets et al., 1988; Richards and Clifford, 1991) realized that surface coarsening was a pervasive textural feature of gravel-bed rivers. Since then, armouring has been typically reported in degrading beds and river reaches with low or no sediment supply (e.g. gravel-bed reaches downstream from dams or lakes) (Gessler, 1967; Willets et al., 1988; Jain, 1990; Chin et al., 1994; Gomez, 1994; Vericat et al., 2006). In such cases, surface armours were called ‘static’ or ‘pavement’ (Jain, 1990; Yager et al., 2015; Bertin and Friedrich, 2018) and their development is likely driven by sediment winnowing during low flood flows (Gomez, 1983, Gomez, 1993, Gomez, 1994). Static armours can ‘breakup’ during high flow peaks and/or transport episodes with large sand sediment supplies (Laronne and Carson, 1976; Gomez, 1983; Klaassen, 1988; Vericat et al., 2006) and re-form during the falling limb of the flood hydrograph. Non-degrading gravel-bed streams with considerable sediment inputs also exhibit some armouring due to a combination of winnowing during low discharges and kinematic sorting (Parker and Klingeman, 1982; Wilcock, 2001). In truth, bedload transport models (Wilcock and DeTemple, 2005), tracer studies (Haschenburger and Wilcock, 2003), flume experiments (Hassan et al., 2006), and field observations (Andrews and Erman, 1986; Clayton and Pitlick, 2008; Haschenburger, 2017) support the occurrence of such ‘mobile’ or ‘dynamic’ armours and their persistence even during large floods. Apart from armouring, a large diversity of particle arrangements has been reported in the field for gravel-bed rivers (Cin, 1968; Church et al., 1998; Hassan and Church, 2000; Wittenberg et al., 2007; Hassan et al., 2008), which seem to be more prevalent when sediment supply is low and beds are well armoured.

In addition to field studies, flume experiments also contribute to increase knowledge of how streambed texture responds to the balance between water discharge and sediment supply. In this regard, seminal research by Dietrich et al. (1989) reported how reductions in sediment supply tend to promote active channel narrowing, surface coarsening, bedload fining, and transport rate decrease in gravel-bed rivers. Substantial subsequent work documented the influence of sediment inputs on the spatial and vertical patterns of grain size sorting (Nelson et al., 2009, Nelson et al., 2010) and how surface grain-size responds to a decrease in bedload through the expansion of coarse fixed patches (Nelson et al., 2009; Yager et al., 2015), resulting in a general coarsening of the streambed. Flume experiments also tried to elucidate the mechanisms beyond armour development at the grain-scale, such as fine-sediment winnowing during low discharges (Chin et al., 1994; Gomez, 1994), infiltration of fine sediment (Marion and Fraccarollo, 1997; Curran and Waters, 2014; Berni et al., 2018), and kinematic sorting during bed load transport (Wilcock, 2001; Bacchi et al., 2014; Ferdowsi et al., 2017) and on the conditions beyond armour breakup under no sediment supply (Wang and Liu, 2009) or triggered by large fine-sediment inputs (Venditti et al., 2005; Venditti et al., 2010a, Venditti et al., 2010b). More recently, Orrú et al. (2016) studied the breakup and reformation of static armours, documenting armour breakup with release of subarmour fines and quick armour reformation during high flows. Hassan et al. (2006) and Plumb et al. (2019) delved into the effects of hydrograph characteristics on surface coarsening, documenting how experiments with flat and sustained hydrographs developed a well-armoured structured surface, while sharply peaked hydrographs did not result in substantial armouring. Follow-up research by Berni et al. (2018) and Hassan et al. (2020) continued exploring the timing of armour formation. In addition, another major line of experimental research investigated the coevolution between armouring and bed structures (e.g., Venditti et al., 2017; Bertin and Friedrich, 2018; Hassan et al., 2020), highlighting the influence of bed structures on bed topography, particle entrainment, and bedload transport (Hassan and Reid, 1990; Gomez, 1993, Gomez, 1994; Nikora et al., 1998; Butler et al., 2001; Church and Hassan, 2002; Marion et al., 2003; Smart et al., 2004; Cooper and Tait, 2009; Hodge et al., 2009; Mao et al., 2011; Heays et al., 2014; Perret et al., 2020).

In summary, both field and flume research point at the linkages between streambed texture and sediment supply regime as a key question in river morphodynamics and fluvial geomorphology. As we have outlined above, flume research has significantly augmented the theoretical and quantitative understanding of streambed adjustments to sediment supply. Field research, however, has often been case-study focused and the intrinsic complexity of bedload measurement complicates the field assessment of streambed response to sediment supply fluctuations (Pitlick et al., 2012). For these reasons, the question of how well the results from laboratory studies could be extrapolated to interpret field observations is still open. Hence, in the first part of this paper we propose an extensive review of previous flume and field research on gravel-bed rivers, first introducing compiled data from the scientific literature for 49 natural gravel-bed rivers and then addressing surface coarsening and its different controls. We illustrate this literature review by a systematic re-examination of compiled grain-size measurements and bedload discharge. In the second part of the paper, we performed a meta-analysis of the compiled data in order to quantify the covariation of surface coarsening with channel hydraulics, hydrology, and bedload fluxes in natural rivers. The main outcomes of this meta-analysis are twofold: (i) documenting an asymptotic rise in armouring with sediment supply decline; and (ii) reporting some correlation between bedload fluxes, surface grain-size, and channel morphology.