Lateral 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.
Bibliographical noteFunding Information:
The authors acknowledge the help of R. Christopher, J. Mullin, B. Erickson, S. Mielke, E. Steen, P. Pham, L. Horsager, K. Flemming, A. Poovey, E. Zanella and M. Barros with the set-up of the experiments. Careful reviews from Luca Malatesta, Rebecca Hodge and two anonymous reviewers greatly improved an earlier version of the manuscript. Support from National Science Foundation grant 1050070 to D.W.B. and a UCSB Graduate Student Opportunity award to A.B. is gratefully acknowledged. The project was also supported by the National Science Foundation via the National Center for Earth-surface Dynamics (NCED) under grant EAR-1246761.
- braided alluvial rivers
- channel migration
- physical experiments