The role of bed morphology and channel curvature in the redistribution of momentum in a series of meander bends, Pearl River, USA

<p>The process of meandering in alluvial rivers is the result of spatial variability in erosion and deposition that systematically alternates from one side of the channel to the next along sequential meander bends. The localized erosion of outer banks, typically downstream of the bend apex, occurs due to an outward advection of high-momentum fluid leading to an asymmetric lateral distribution of streamwise velocity. Two primary mechanisms are responsible for the lateral flux of streamwise momentum within meander bends. The first is an increasing bed elevation along the inner bank in the form of a point bar that causes a shoaling effect, steering fluid mass and momentum from the inner bank toward the outer bank. The second mechanism is curvature-driven secondary circulation that leads to helical motion of flow through the bend and an outward advection of high-momentum near-surface flow. While these two mechanisms have been studied previously, their relative contribution to the net redistribution of momentum within a series of consecutive bends have not yet been fully documented. For this study, we obtained three-dimensional velocity measurements using a boat-mounted acoustic Doppler current profiler along regularly spaced cross-sections for six consecutive meander bends on the Pearl River (Louisiana, USA) during two different discharge conditions. Velocity data were processed using the Velocity Mapping Toolbox in Matlab, and calculations were performed to evaluate the lateral flux of streamwise momentum due to topographic steering and secondary circulation using the Rozovskii frame of reference. These momentum flux terms are systematically compared to spatial series of curvature, bed elevation, and channel width to elucidate the interactions between form and flow structure. Results show that lateral flux of streamwise momentum is primarily driven by topographic steering, and that values of momentum flux due to curvature-driven secondary circulation are on average an order of magnitude lower. The spatial pattern of these flux components through the bends show that momentum redistribution due to topographic steering is highest at the entrance to the bend, and momentum redistribution due to secondary circulation is typically highest downstream of the apex.&#160; The results of this study emphasize the important role that interaction between process and form play in dynamics of natural meandering rivers.&#160; Point bars form through spatial variations in bed-material transport capacity within curved channels, but, once in place, these bars reinforce their own persistence through their influence on the lateral redistribution of streamwise momentum.&#160;</p>