EXPERIMENTAL STUDIES OF BED TOPOGRAPHY AND FLOW PATTERNS IN LARGE-AMPLITUDE MEANDERS .2. MECHANISMS

Large-amplitude river meanders develop multiple scour holes within a single bend that are observed to be part of overlapping shingle bars (Whiting and Dietrich, this issue). These bars, having wavelengths near 4 channel widths, consist of a pool along the outer concave bank and a depositional lobe along the inner convex bank. The development of multiple pools and lobes within a bend has been shown to cause localized bank erosion and channel trace distortion expressed as planform asymmetry and compound heads. In this second paper we describe experiments designed specifically to test hypotheses that propose multiple pools to be the result of an overshoot phenomenon, to be a consequence of an alternate bar-like instability, or to develop from a flow/planform instability. Although our experiments favor the hypothesis that multiple bars in bends of large amplitude arise from an alternate bar-like instability, experiments conducted over an immobile flat bed also reveal longitudinal oscillations in flow that may arise in these long bends. This secondary effect interacts with shingle bars to amplify pool expression at particular points along the channel.