Alternate bar dynamics in response to increases and decreases of sediment supply

作者：Bankert Andrew R^*; Nelson Peter A

来源：Sedimentology, 2018, 65(3): 702-720.

DOI：10.1111/sed.12399

• Alternate bar
• flume
• fluvial geomorphology
• gravel-bed channel
• sediment supply
• stratigraphy

Gravel-bed rivers can accommodate changes in sediment supply by adjusting their bed topography and grain size in both the downstream and cross-stream directions. Under high supply aggradational conditions, this can result in spatially non-uniform stratigraphic patterns, and the morphodynamic influence of heterogeneous stratigraphy during subsequent degradational periods is poorly understood and has not been studied through physical modelling. A flume experiment was conducted to analyse channel response where alternate bars were developed in a gravel-sand mixture under constant discharge and sediment supply before two supply increases led to the development of heterogeneous stratigraphy beneath alternate bars. The supply was then reduced back to the initial supply rate, causing degradation through that self-formed stratigraphy. Stratigraphic samples were collected, and the bed topography and flow depth were measured frequently, which were used with a two-dimensional hydrodynamic model to characterize flow conditions. Migrating alternate bars stabilized during the first equilibrium phase, creating bed surface sorting patterns of coarse bar tops and fine pools. During the first supply increase, the bars remained stable as the pools aggraded. During the second supply increase, the pools aggraded further, causing the boundary shear stress over the bar tops to increase until the bars gained the capacity to migrate and eventually stabilize in new locations. As aggradation occurred, the original sediment sorting patterns were preserved in the subsurface. During the degradational phase, the pools experienced incision and the bars eroded laterally, but this lateral erosion ceased when coarse sediment previously deposited during the bar-building phase became exposed. The results suggest that if a sediment supply increase is capable of filling the pools, it can cause stable bars to migrate and the bed to be reworked. These findings also show that heterogeneous stratigraphy can play an important role in determining whether bars persist or disappear after a sediment supply reduction.

• 出版日期2018-4