Hydrodynamic Processes of Incipient Meander Chute Cutoffs: Laboratory Experiments With Implications for Morphodynamics and Depth-Averaged Modeling

Abstract

Meander chute cutoffs are a common and geomorphically important feature of meandering rivers, exhibiting complex dynamics and distinctive morphologic features. To date, however, the geomorphic processes governing the evolution and formation of these features are poorly understood due to limited knowledge of cutoff hydrodynamics. This paper investigates three-dimensional mean flow structure, turbulent flow structure, and bed shear stress distribution from high-resolution flow velocity data in a fixed-bed, sediment-free physical model. The results show that (a) the chute channel conveys around 1.4 times the unit-width flow discharge as the cutoff bend; (b) mean flow structure is highly three-dimensional, with strong convective acceleration throughout the bends and pronounced flow separation zones in both the chute channel and the cutoff bend; (c) turbulent kinetic energy is intense at shear layers bounding the flow separation zones at several locations in the channel; and (d) bed shear stress is elevated due to strong turbulence in the chute channel and is low in the cutoff bend. The unique hydrodynamics of meander chute cutoffs explains their distinctive morphologic behaviors, including the rapid widening and deepening of chute channels and locations of bars and pools. Moreover, this paper compares quantitatively the depth-averaged flow structure before and after the cutoff, demonstrating that cross-sectional redistribution of streamwise momentum by secondary flow remains largely unchanged in the presence of the chute channel. This implies that 2D depth-averaged hydrodynamic models, parameterized and calibrated for secondary flow in single-channel meanders, are suitable for simulating flow within chute cutoffs.