128-8 Evaluating the Impact of Distance-Dependent Overbank Deposition on Avulsion Loci, Pathfinding and Fan Morphology in RiverWalk, a Cellular Model of River Avulsion

Abstract:

River avulsions are the abrupt relocation of a channel onto a new course on its floodplain and are the primary mechanism by which sediment and water are redistributed across aggrading alluvial landscapes; however, the extent to which floodplain deposition rates and styles impact avulsion locations and pathfinding is unclear. In this study, we build on RiverWalk, an existing cellular model of river avulsion and megafan growth, to test how distance-dependent floodplain rules impact avulsion processes and fan shape. In the original model, Martin and Edmonds (2022) incorporate floodplain dynamics in the form of abandoned channel attraction and repulsion rules, and they show that when abandoned channels repulse avulsing flow, avulsion locations shift farther downstream from the mountain front, but when abandoned channels attract flow, avulsion locations occur closer to the apex. Outside of abandoned channels, RiverWalk overbank deposition is uniform across the floodplain and varies only with distance from the mountain front; therefore, floodplain aggradation is decoupled from active channel location. 

However, field and theoretical studies have shown that overbank deposition rates often decline with lateral distance from a channel as flow depth and transport capacity decrease. To address this, we add a new overbank deposition method in RiverWalk that incorporates a lateral distance-decay function based on the exponential formulation in Mackey and Bridge (1995), in which each floodplain cell’s deposition rate depends on its distance from the nearest channel margin. We test exponential decay (lambda) values ranging from 0.12 to 0.45 and distance rules ranging from 2-10 times the channel width to test model sensitivity to floodplain rules. 

 Preliminary results show that in the distance-decay method, avulsions increase in the distal fan, and the avulsed channel shows a clear tendency to return to previously occupied paths compared to the uniform floodplain aggradation models. This change in loci leads to a narrowing of the fan shape. Overall, our preliminary results show that incorporating more complex floodplain-filling rules influences avulsion behavior and fan shape in RiverWalk, suggesting that overbank deposition outside of abandoned channels is also important for avulsion dynamics and fluvial fan architecture.

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