84-4 Sedimentation of a neck cutoff abandoned bend modulated by unsteady flow dynamics

Abstract:

Abstract: Neck cutoffs on large meandering rivers usually disconnect from the active channel within 2 to 10 years as plug bars build up at the abandoned bend entrance and exit junctions. At Pumps Bend on the lower White River, Arkansas, a neck cutoff formed in 2013 has remained hydraulically connected for more than a decade, with traditional plug bar conceptual models not accurately predict patterns and rates of sedimentation. The elongate subparallel planform geometry of the site redirects flow nearly 180 degrees through the new cutoff channel; yet the role of unsteady discharge on sediment redistribution inside the abandoned bend is not well understood. This study uses Delft3D FM to simulate sediment flux and bed change under three 31-day discharge scenarios: steady flow, a single flood pulse, and two successive flood pulses. All three scenarios have the same total water and sediment input, such that differences in deposition can be attributed primarily to hydrograph dynamics. In the steady case, the upstream limb accumulates roughly twice the sediment of the downstream limb. A flood pulse inverts this pattern with the downstream limb receiving about 30% more sediment than the upstream limb. One of the mechanisms for this shift is a brief reversal in the water-level gradient at the exit junction near peak discharge, which redirects sediment toward the loop entrance. During the rising and falling limbs, the entrance-side gradient dominates and pushes sediment downstream. Additionally, prominent zones of flow recirculation within the entrance and exit of the bend also substantially limit the amount of sediment transported into the abandoned bend and thereby limit the growth of plug bars and the full disconnection to the active channel. Movable-bed runs also show a 20 to 40% reduction in cumulative sediment flux compared to fixed-bed runs, as bed aggradation limits how much sediment can enter later in the simulation. These findings have important implications for predicting planform dynamics of meandering rivers and the alluvial architecture of floodplains.

Keywords: Neck cutoff, Flood pulse structure, Meander loop connectivity, Sediment partitioning, Morphodynamic modeling

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