Hydrodynamic controls on alluvial ridge construction and avulsion likelihood in meandering river floodplains

Existing models of alluvial stratigraphy often neglect the hydrodynamic controls on channel belt and floodplain sedimentation, and predict avulsion using topographic metrics, such as channel belt super-elevation. This study provides a first demonstration of the potential for simulating long-term river floodplain evolution (over >500 floods) using a process-based hydrodynamic model. Simulations consider alluvial ridge construction during the period leading up to an avulsion, and assess the controls on avulsion likelihood. Results illustrate that the balance between within-channel and overbank sedimentation exerts a key control on both super-elevation ratios and on the conveyance of water and sediment to the floodplain. Rapid overbank sedimentation creates high alluvial ridges with deep channels, leading to lower apparent super-elevation (the ratio of ridge height to channel depth), and implying reduced avulsion likelihood. However, channel deepening also drives a reduction in channel belt-floodplain connectivity, so that conveyance of water to the distal floodplain is concentrated in a declining number of channel breaches, which may favor avulsion. These results suggest that while super elevation ratios in excess of a threshold value may be a necessary condition for a meandering river avulsion, avulsion likelihood may not be greatest where the super elevation ratio is maximised. Instead, optimal conditions for avulsion may depend on channel-floodplain hydrodynamic connectivity, determined by the balance between coarse (channel bed forming) and fine (floodplain constructing) sediment delivery. These results highlight a need to rethink the representation of avulsion in existing models of alluvial architecture.