Experimental constraints on bar migration through bends: Implications for meander wavelength selection

Abstract

The bed of an initially straight channel often deforms into a series of migrating alternate bars that can cause bank erosion and the development of periodic planform curvature which is often expressed as a meandering channel trace. The planform curvature in turn leads to topography similar to alternate bars, but stationary with respect to the meander. The migration of alternate bars over this steady topography may lead to interactions influencing meander initiation and wavelength selection. We studied these interactions in flume channels with bends of various lengths and angles, and having various width‐to‐depth ratios. In channels whose wavelength was equal to the length of bars in straight channels (nearly eight channel widths), bars migrated through sine‐generated channels with angles up to 10°. The critical bend angle was smaller for channels of longer wavelength and smaller width‐to‐depth ratio. Migration was nonuniform, and bars temporarily stalled when in phase with the curvature‐induced topography, leading to amplification of the topography. Flow against an opposing bank can lead to such a strong boundary shear stress divergence that migration of the bars is prevented because sediment that otherwise would have led to bar migration is scoured from the bar front. We propose that the amplification of bar‐pool topography in bends where free alternate bars are stalled enhances scour of the bank which leads to selection of a meander wavelength equal to that of the alternate bars.