Controls on the lateral channel migration rate of braided alluvial channel systems

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. Here we develop, for the first time, an expression that describes the lateral migration rate of braided alluvial channels in 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, lateral channel-migration rates scale inversely with the channel-bank height. Furthermore, when changes in channel-bank heights are accounted for, lateral migration rates appear independent of channel slope, channel-system width, and water depth. These constraints allow us to derive two dimensionally consistent expressions for lateral channel-migration rates under different boundary conditions. We find that migration rates are strongly sensitive to channel-bank heights and water discharges and more weakly sensitive to sediment discharges in braided equilibrium channel systems. In addition, the strong dependence of lateral migration rates on channel-bank heights implies that external perturbations (for example, perturbations of sediment and water discharges) that modulate the depth of channel incision and can indirectly affect lateral channel-migration rates.