Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732A theoretical analysis is conducted to study qualitatively the development of alternate bars as the flow transitions from low- to high-Froude numbers. The model formulation adopted is that of Schielen et al. (1993), excluding their rigid-lid approximation for the flow free-surface and re-expressed in terms of local water depth, leading to the appearance of the Froude number in the dimensionless flow and sediment transport equations. For width-to-depth ratios above a minimum value, the alternate bars are influenced by the width of the channel; below this value, solutions to the model do not exist as such values of width-to-depth ratios correspond to the small-scale bed–wave regime. However, once the width-to-depth ratio is in the large-scale bed-wave regime, the Froude number plays an important role in separating stable and unstable regions. The quasi-steady nature of the flow model does not support the formation of neither stationary nor migrating alternate antibars (alternate bars that migrate upstream). It was found that the model can yield alternate bars under trans-critical and supercritical flow conditions, such as those in the laboratory observations of Ikeda (1984). When the width-to-depth ratio is greater, but still close to the minimum value required for the alternate bars to exist, low-Froude number instability does not occur as the bars only appear at sufficiently high Froude numbers. Low-Froude number instability takes place when the width-to-depth ratio increases past a certain amount above the minimum value required for the existence of the bars. In addition, the region of instability grows in size with higher width-to-depth ratios
