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

Large-amplitude river meanders develop multiple scour holes within a s ingle bend that are observed to be part of overlapping shingle bars (W hiting 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 mul tiple pools and lobes within a bend has been shown to cause localized bank erosion and channel trace distortion expressed as planform asymme try and compound heads. In this second paper we describe experiments d esigned specifically to test hypotheses that propose multiple pools to be the result of an overshoot phenomenon, to be a consequence of an a lternate bar-like instability, or to develop from a flow/planform inst ability. Although our experiments favor the hypothesis that multiple b ars in bends of large amplitude arise from an alternate bar-like insta bility, experiments conducted over an immobile flat bed also reveal lo ngitudinal oscillations in flow that may arise in these long bends. Th is secondary effect interacts with shingle bars to amplify pool expres sion at particular points along the channel.