Textural signatures of sediment supply in gravel-bed rivers: Revisiting the armour ratio
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.
Section snippets
Compiled field data
The dataset used in the present paper consists of grain-size and bedload measurements collected at 49 river sites (summarized in Table 1). An important amount of the compiled data derives from the extensive campaign of sediment transport measurements carried out on Idaho, Nevada (King et al., 2004), Colorado, and Wyoming rivers (Ryan et al., 2002, Ryan et al., 2005). These data have been presented previously and analysed in several papers (Ryan et al., 2002, Ryan et al., 2005; King et al., 2004
Surface coarsening in gravel-bed rivers: introducing the ‘armour ratio’
According to both field (e.g. Harrison, 1950; Gessler, 1967; Gomez, 1983; Willets et al., 1988; Richards and Clifford, 1991; Bunte and Abt, 2001) and experimental observations (e.g. Parker and Klingeman, 1982; Dietrich et al., 1989; Parker and Sutherland, 1990; Chin et al., 1994; Parker and Toro-Escobar, 2002) surface coarsening is a widespread textural feature in gravel-bed rivers. Indeed, it has been postulated that surface coarsening represents an intrinsic consequence of bedload transport
General aim
The previous review highlights how a long tradition of field and flume research has largely confirmed the existence of close links between sediment supply, channel morphology, bedload transport, and streambed texture in gravel-bed rivers. This idea, which has been present in fluvial geomorphology and river morphodynamics for many decades (e.g. Lane, 1955b), is supported by our systematic review of a wide database of field studies compiled from the scientific literature on fluvial bedload
General discussion
Surface coarsening is a major textural feature in gravel-bed rivers that has long been studied by fluvial scientists. Our systematic review of previous literature conflates the results from cumulated field and flume research into showing how streambed in gravel-bed rivers responds to a decrease in bedload supply through channel narrowing and a general surface coarsening (Dietrich et al., 1989; Nelson et al., 2009). Hence, the degree of the latter may provide, in principle, an indicator of the
Concluding remarks
A large body of research in fluvial geomorphology has contributed to establishing the general idea that sediment supply, bedload fluxes, channel morphology, bankfull shear stresses, and surface grain size are intimately related in gravel-bed rivers. In this paper, we aimed to quantify the existing links between sediment supply and surface coarsening. Based on a meta-analysis of a large database of bedload discharge information for gravel-bed streams, we have proposed semi-empirical relations
Notations
- D
Grain-size (particle diameter)
- Dis
i-th percentile of the surface grain-size distribution
- Diss
i-th percentile of the subsurface grain-size distribution
- Di⁎
Armour ratio, or the ratio between the i-the percentiles of the surface and subsurface grain-size distributions
- Di⁎⁎
Corrected version of the armour ratio, i.e. armour ratio computed accounting for the inherent covariation between surface and subsurface grain sizes
- f
Ratio between the armour ratio estimated based on the 84-th percentiles (D84) and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The present work has been possible thanks to the financial support provided by the grant ACB17-44, co-funded by the post-doctoral ‘Clarín’ program-FICYT (Government of the Principality of Asturias) and the Marie Curie Co-Fund. This work was also performed within the framework of the EUR H2O’Lyon (ANR-17-EURE-0018) of Université de Lyon (UdL), within the program ‘Investissements d'Avenir’ operated by the French National Research Agency (ANR). We would like to thank Pablo Turrero García and
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2021, GeomorphologyCitation Excerpt :In accordance with the results of Preciso et al. (2012) and Vázquez-Tarrío et al. (2019), CBMT estimates obtained at the most impacted reaches were between 10 and 20 times lower than those obtained at reaches with high sediment availability (i.e., reach 1). Armouring increases with decreasing sediment supply and coarse transport (Vázquez-Tarrío et al., 2020). Accordingly, the reaches where an extremely limited morphodynamics was recognized are characterized by high armouring, especially affecting the main channel unit.