Numerical Modeling Of Two-Dimensional Hydrodynamics In A Highly Curving And Actively Evolving Neck Cutoff Under Different Hydrologic Conditions

Neck cutoffs are a common morphologic feature of meandering rivers though their occurrence on natural rivers is intermittent. Morphologic adjustments following cutoff initiation are typically rapid making detailed field investigations of the interactions between flow and form challenging. However, numerical models can offer insights into the hydrodynamic characteristics within the early stages of cutoff evolution. Field measurements of channel bathymetry and three-dimensional velocities were collected during the first two years of morphologic change for a neck cutoff on the White River in Arkansas, USA. These data were used to calibrate a two-dimensional numerical model to investigate the influence of hydrologic variability on the hydrodynamic characteristics of neck cutoffs during the early stages morphologic adjustment. The modeling results reveal complex patterns of the two-dimensional flow field, including multiple regions of flow separation and recirculation related to strong redirection of the flow nearly 180 degrees through the cutoff. These regions of recirculation are also shown to vary depending on discharge conditions and affect hydrologic connectivity between the main channel and abandoned bend. Model results for the initial cutoff morphology show a substantial portion of discharge still conveyed by the original bend for all three discharge scenarios, resulting in a shear layer within the downstream limb of the site that limits advection of high-momentum fluid against the bank opposite of the cutoff. The findings from this investigation are among the first detailed flow structure studies during the initiation of a neck cutoff and are used to improve conceptual models for cutoff dynamics.