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 sym metric sine-generated meanders of large amplitude which document the b ed topography, flow patterns, and sediment transport fields in such be nds. Multiple pools spaced at a distance of 3-4 widths develop success ively 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 t o the channel trace and curves toward the inside bank. The first and m ost pronounced pool, opposite the first inner bank bar, is consistentl y positioned where the projection of the inner bank tangent at the ups tream crossing intersects the outer bank. Successive shingled bars in the bend migrate downstream if bars arriving at the persistent first p ool deliver large enough pulses of sediment to the pool to form new ba rs. In experiments with an erodible bank, the banks near pools are sit es of accelerated retreat. This suggests that the shear stress fields engendered by these multiple features may cause the planimetric distor tion seen in larger rivers.