Exploring meandering river cutoffs

Abstract

Cutoffs, which include neck and chute cutoffs, are the results of the fluvial processes that fundamentally influence evolution of meandering rivers. Neck cutoff happens when the two limbs of a highly sinuous bend touch, whereas chute cutoff refers to the formation of a shortcut channel passing through a meander bend. In this review, we begin by distinguishing the morphological and hydrological conditions of the two cutoff types. Mechanisms driving the development of neck cutoff are embodied in a variety of kinematical and hydrodynamic models simulating processes governing the long-term evolution of meandering rivers. These models adopt a morphological threshold for judging the occurrence of neck cutoff, b = αw where α is a constant ranging between 0 and 1, b is the bend neck width, and w is the mean channel width. The potential underestimation of the evolutionary period during the late-stage of bend evolution toward neck cutoff by using this morphological threshold and uncertainties in quantifying the migration-curvature relationship limit the abilities of the existing models in predicting the occurrence of neck cutoffs. We then suggest three possible directions for future research on meander neck cutoffs. Mechanisms controlling chute cutoffs are relevant to six key factors representing meander hydrological regime, planform morphology, bed topography, and floodplain characteristics. The combination of these factors gives rise to four distinguishable triggering mechanisms: headward-erosion, embayment, mid-channel bar, and scroll-slough chute cutoffs, though initiation of chute cutoff may be caused by some of their combinations. However, the hydraulic and morphological characteristics of meander bends under these triggering mechanisms are so complex that they are often site-specific, making it extremely challenging to generalize the known morphodynamic and hydrodynamic processes driving the formation of chute cutoffs in individual meander bends. We close the review by recommending three possible research directions on chute cutoffs for tackling the existing challenges in the future.

Hydrodynamics of Meander Chute Cutoffs in Microtidal Mudflats

“…Image source: Google Earth. (i) A conceptual sketch depicting the different geomorphological evolution processes between the neck and chute pattern for fluvial cutoffs (adapted from Dieras, 2013 and P. Gao & Li, 2024).…”

“…River cutoffs are triggered by two complex hydrological processes, such that the neck cutoff results generally from the self‐intersection between downstream and upstream bends of a meander via overbank flow erosion or progressive bank failure (Figures 1a, 1b, 1g, and 1h), whilst the chute cutoff is primarily caused by continuous enlargement of swales, propagation of headcut on the concave bank, or relatively large flows in flood seasons eroding through the inner bank or point bars of the bend (Figures 1c–1g; Allen, 1965; Camporeale et al., 2008; Gagliano & Howard, 1984; P. Gao & Li, 2024; Z. Li & Gao, 2019; Z. Li et al., 2023). Regardless of this morphodynamic difference in meander cutting processes, flow diversion always occurs once cutoff forms (Constantine et al., 2010; Dieras et al., 2013).…”

“…Thus, comprehensive studies on the controlling factors and driving forces of tidal cutoff evolution are required, not only to understand their roles in coastal environments, but also for coastal management purposes. (i) A conceptual sketch depicting the different geomorphological evolution processes between the neck and chute pattern for fluvial cutoffs (adapted from Dieras, 2013 andLi, 2024).…”