Abstract
AbstractLateral movements of alluvial river channels control the extent and reworking rates of alluvial fans, floodplains, deltas, and alluvial sections of bedrock rivers. These lateral movements can occur by gradual channel migration or by sudden changes in channel position (avulsions). Whereas models exist for rates of river avulsion, we lack a detailed understanding of the rates of lateral channel migration on the scale of a channel belt. In a two‐step process, we develop here an expression for the lateral migration rate of braided channel systems in coarse, non‐cohesive sediment. On the basis of photographic and topographic data from laboratory experiments of braided channels performed under constant external boundary conditions, we first explore the impact of autogenic variations of the channel‐system geometry (i.e. channel‐bank heights, water depths, channel‐system width, and channel slope) on channel‐migration rates. In agreement with theoretical expectations, we find that, under such constant boundary conditions, the laterally reworked volume of sediment is constant and lateral channel‐migration rates scale inversely with the channel‐bank height. Furthermore, when channel‐bank heights are accounted for, lateral migration rates are independent of the remaining channel geometry parameters. These constraints allow us, in a second step, to derive two alternative expressions for lateral channel‐migration rates under different boundary conditions using dimensional analysis. Fits of a compilation of laboratory experiments to these expressions suggest that, for a given channel bank‐height, migration rates are strongly sensitive to water discharges and more weakly sensitive to sediment discharges. In addition, external perturbations, such as changes in sediment and water discharges or base level fall, can indirectly affect lateral channel‐migration rates by modulating channel‐bank heights. © 2019 The Author. Earth Surface Processes and Landforms published by John Wiley & Sons, Ltd. © 2019 The Author. Earth Surface Processes and Landforms published by John Wiley & Sons, Ltd.
Highlights
The lateral movement of channels across fluvial surfaces is a fundamental characteristic of alluvial river systems
All changes in channel-system geometry are caused by internal dynamics, and we can isolate the impact of channel-system geometry on lateral migration rates independently of the boundary conditions
We demonstrated that an inverse scaling between the effective channel-bank height and the lateral channel mobility explains most of the variability of lateral migration rates under constant boundary conditions (Figure 6)
Summary
The lateral movement of channels across fluvial surfaces is a fundamental characteristic of alluvial river systems. The rate of this lateral movement controls the dynamics and scales of diverse landscapes, such as deltas, floodplains, alluvial fans, and alluvial sections of bedrock rivers (Figure 1). Channels move laterally through two main mechanisms: abrupt rerouting of water into new channels by avulsions (Slingerland & Smith, 2004) and gradual sideways migration of individual channels (Einstein, 1926; Hickin & Nanson, 1984). We focus on the gradual lateral migration (as opposed to avulsions) of braided alluvial river systems
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