Experimental studies of bed topography and flow patterns in large‐amplitude meanders: 1. Observations

Large amplitude river meanders have been observed to contain multiple bars within a single loop and to exhibit planforms that are asymmetric or have subsidiary bends. Here we report experiments conducted in symmetric sine‐generated meanders of large amplitude which document the bed topography, flow patterns, and sediment transport fields in such bends. Multiple pools spaced at a distance of 3–4 widths develop successively along the outer concave bank. Downstream of each pool a distinct bar lies along the inside convex bank. The series of bars overlap and appear shingled downstream; the leading edge of each bar is oblique to the channel trace and curves toward the inside bank. The first and most pronounced pool, opposite the first inner bank bar, is consistently positioned where the projection of the inner bank tangent at the upstream crossing intersects the outer bank. Successive shingled bars in the bend migrate downstream if bars arriving at the persistent first pool deliver large enough pulses of sediment to the pool to form new bars. In experiments with an erodible bank, the banks near pools are sites of accelerated retreat. This suggests that the shear stress fields engendered by these multiple features may cause the planimetric distortion seen in larger rivers.