Three-dimensional flow structure and channel change in an asymmetrical compound meander loop, Embarras River, Illinois

The planform dynamics of meandering rivers produce a complex array of meander forms, including elongated meander loops. Thus far, few studies have examined in detail the flow structure within meander loops and the relation of flow structure to patterns of planform change. This field-based investigation examines relations between three-dimensional fluid motion and channel change within an elongated, asymmetrical meander loop containing multiple pool-riffle structures. The downstream velocity field is characterized by a high-velocity core that shifts slightly outward as flow moves through individual lobes of the loop. For some of the measured flows this core becomes submerged below the water surface downstream of the lobe apexes. Vectors of cross-stream/vertical velocities indicate that skew-induced helical motion develops within the pools near lobe apexes and decays over riffles where channel curvature is less pronounced. Maximum rates of bank retreat generally occur near lobe apexes where impingement of the flow on the outer channel bank is greatest. However, maximum rates and loci of bank retreat differ for upstream and downstream lobes of the loop, leading to increasing asymmetry of loop geometry over time - a finding consistent with experimental investigations of loop evolution.