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Neck cutoff morphodynamics of compound meanders with sandy banks capped by root-reinforced layers in the Northeastern Tibetan Plateau
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DOI：10.1016/j.geomorph.2026.110239.png)
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Abstract
En 中文
Neck cutoff typically occurs in highly sinuous meanders with fine-grained, cohesive banks; however, the cutoff dynamics of bends characterized by tortuous planforms and predominantly sandy bank bodies remain poorly unravelled. Black River is a major meandering river in the Zoige Basin in the Northeastern Tibetan Plateau, where sandy banks are capped by herbaceous, root-reinforced surface layers. In the lower reach, we identified two compound bends that underwent neck breach during an extreme flood, while several other regular bends remain intact. Particular focus is on bend #7, where two sub-bends at the cutoff neck show acute and broadened morphology with extremely concentrated or distributed curvature. Field surveys, aided with theoretical and numerical analysis, reveal first-order control on bend migration trajectories and cutoff timing by their tortuous geometries. Particularly, the acute bend geometry concentrates flow redistribution near the apex, promoting rapid translational migration. Conversely, broadened bend morphology attenuates local flow asymmetries, allowing the bend to remain comparatively stationary. At the final stage of neck breach, a sufficiently narrowed neck is incised by a flood extreme. We demonstrate that, for bends with sandy banks, the threshold of neck pressure-gradient can effectively distinguish flood-triggered bends from the intact ones. Compound bend morphology increases the flow-path length and thus the hydraulic gradient across the neck, enhancing both surface-flow incision and subsurface hyporheic erosion. This study advances understanding of neck cutoff dynamics in compound meanders and provides a physically based criterion for anticipating flood-induced breaching in high-altitude rivers.
Keywords:
Neck breach
Compound meanders
Sandy banks
Morphodynamic feedbacks
Bar-pool topography
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