Sediment supply controls equilibrium channel geometry in gravel rivers

Abstract

In many gravel-bedded rivers, floods that fill the channel banks create just enough shear stress to move the median-sized gravel particles on the bed surface (D 50 ). Because this observation is common and is supported by theory, the coincidence of bankfull flow and the incipient motion of D 50 has become a commonly used assumption. However, not all natural gravel channels actually conform to this simple relationship; some channels maintain bankfull stresses far in excess of the critical stress required to initiate sediment transport. We use a database of >300 gravel-bedded rivers and >600 10Be-derived erosion rates from across North America to explore the hypothesis that sediment supply drives the magnitude of bankfull shear stress relative to the critical stress required to mobilize the median bed surface grain size (τ bf * =τ c * ). We find that τ bf * =τ c * is significantly higher in West Coast river reaches (2.35, n = 96) than in river reaches elsewhere on the continent (1.03, n = 245). This pattern parallels patterns in erosion rates (and hence sediment supplies). Supporting our hypothesis, we find a significant correlation between upstream erosion rate and local τ bf * =τ c * at sites where this comparison is possible. Our analysis reveals a decrease in bed surface armoring with increasing τ bf * =τ c * , suggesting channels accommodate changes in sediment supply through adjustments in bed surface grain size, as also shown through numerical modeling. Our findings demonstrate that sediment supply is encoded in the bankfull hydraulic geometry of gravel bedded channels through its control on bed surface grain size.river channel geometry | sediment supply | sediment transport W hat determines the shape of alluvial rivers? These selfformed channels emerge through the interaction of flowing water and transported sediment. Explaining widely observed trends in river channel hydraulic geometry remains an ongoing challenge in the field of geomorphology. Gravel-bedded alluvial rivers (whose bed and banks comprise sediment transported by the river) approach equilibrium geometry through feedbacks between deposition, erosion, and bed surface armoring as well as through channel slope change (1-3). These responses in channel geometry and surface grain size accommodate perturbations in the water and sediment supply regimes. Thus, sediment supply is among the key controls on the morphology of all river channels, and understanding linkages between sediment supply and channel morphology is a central question in much of fluvial geomorphology, civil engineering, and river restoration.Decades of observations in gravel-bedded alluvial channels support the pervasiveness of threshold channels (4-6) in which the channel dimensions adjust such that the threshold for motion of the median bed surface grain size (D 50 ) occurs at, or just below, bankfull flow. These observations are reinforced by theoretical work (7) showing that, at bankfull flow, a straight channel with noncohesive banks will maintain a stable channel width with a ...

The equilibrium alluvial river under variable flow and its channel‐forming discharge

“…In addition, for the threshold‐based load relation in Figure c, the gravel‐related channel‐forming discharge is associated with a Shields stress ( ) that is only slightly larger than its critical value ( ). This is consistent with findings with respect to the bankfull discharge in gravel‐bed rivers [ Parker , , ; Parker et al , ; Phillips and Jerolmack , ; Pfeiffer et al , ]. The sand‐related channel‐forming discharge in Figure c is associated with a Shields stress ( ) that is significantly larger than its critical value ( ), which is consistent with the findings by Parker [, ].…”

Roles of Bank Material in Setting Bankfull Hydraulic Geometry as Informed by the Selenga River Delta, Russia

“…It also has been suggested that rivers maintain a roughly constant bankfull Shields number for gravel bed ( = 0.049) and for sand‐bed ( = 1.86) rivers (Parker, ; Parker et al, ). However, recent empirical evidence has shown that the bankfull Shields number is a variable depending on a series of parameters, such as the dimensionless bed material grain size ( (Van Rijn, ), where ν is the kinematic viscosity of water), channel slope, and sediment supply (Li et al, ; Pfeiffer et al, ; Trampush et al, ).…”

Evidence Of, and a Proposed Explanation For, Bimodal Transport States in Alluvial Rivers

“…This model is supported by both global compilations of data and case studies of individual rivers that demonstrate that bedload-dominated gravel-bedded rivers are slightly offset from a threshold channel (Phillips and Jerolmack, 2016;Gaurav et al, 2015;Métivier et al, 2016). Several studies have presented evidence that sediment supply and bank vegetation may drive gravel channels further above threshold (Pfeiffer et al, 2017;Millar and Quick, 1998). Values for Shields stress in gravel-bed rivers reported for a wide range of environments, however, rarely exceed 2-3 times critical.…”