Three-Dimensional Flow, Morphologic Change, and Sediment Deposition and Distribution of Actively Evolving Neck Cutoffs Located on the White River, Arkansas

Abstract

Neck cutoffs are important and prominent features of alluvial rivers yet detailed field-based research of neck cutoffs has been insufficient to fully characterize three-dimensional flow, morphologic change, and sediment deposition and distribution. The main objectives of this research are to examine the formation and evolution of neck cutoffs by characterizing the flow field, morphology, and sediment distribution through neck cutoffs with complex planform configurations located on the White River, Arkansas. Results led to the production of two conceptual models. The flow model has main hydrodynamic characteristics of (1) tight bend flow resulting from flow redirection of nearly 180° through the point of cutoff, (2) a zone of flow separation and recirculation adjacent to the cutoff junction corner within the downstream limb, (3) zones of recirculation at the entrance and exit of the abandoned loop, (4) highly asymmetric flow through the cutoff channel, (5) a zone of recirculation along the outer bank within the apex region of the downstream loop, and (6) reversal of helical flow. The morphologic model shows (1) the formation of a longitudinal bar in the upstream meander limb, (2) the development of a deep scour hole in the downstream meander limb, 3) erosion of the bank opposite the cutoff in the downstream meander limb, 4) a cutoff bar in the downstream meander limb at the junction corner of the cutoff channel and the downstream meander limb, and 5) perching of the exit of the abandoned bend above the cutoff channel due to channel bed incision. A combination of sediment cores, surface and grab samples, and dune tracking were used to estimate bedload sediment transport and sediment distribution through two of the neck cutoffs. Results indicate similar bedload sediment transport during bankfull and flood stages and revealed mixed load deposition associated with cutoffs that plug slowly is occurring within the abandoned bends. The research should result in invaluable information about hydrodynamic and morphologic processes of neck cutoffs, refine current conceptual models of neck cutoffs, and contribute to our understanding of meandering rivers with complex planform configurations.