Interaction among mean flow, turbulence, bed morphology, bank failures and channel planform in an evolving compound meander loop

The dynamic evolution of the planform of meandering rivers often leads to the development of compound loops with multiple lobes of maximum curvature. At present, the interaction among spatial patterns of mean flow, turbulence, bed morphology, bank failures and channel migration in compound loops is poorly understood. In particular, field studies of this interaction over the timescale of planform evolution are lacking. This study examines the co-evolution of flow, bed morphology, and channel planform in a compound meander loop and relates patterns of near-bank velocity and turbulence to planform change within the loop. Data consist of repeat surveys of channel change in a compound loop over an 11-year period, coupled with Acoustic Doppler Velocimeter (ADV) measurements of 3-D instantaneous velocities for similar magnitude flows at the beginning and end of this period. Results confirm that this compound loop is highly dynamic, with major changes in planform occurring over the 11-year period. Spatial patterns of near-bank velocity and turbulence correspond to patterns of bank erosion and channel migration within the loop; however, these patterns are not strictly a function of planform curvature. Instead, local factors, including deflection of the flow by point bars and failed bank blocks, enhance or inhibit the development of high near-bank velocities and turbulence kinetic energy. The loop has elongated and become more asymmetric over time-a pattern of development consistent with patterns of near-bank velocities and turbulence at the beginning of the study period. The pattern of near-bank velocities and turbulence for measurements at the end of the period indicate that the loop will continue to elongate in the near future, supporting the hypothesis that compound loops evolve continuously over time rather than developing into a stable configuration.