Height and wavelength of alternate bars in rivers: modelling vs. laboratory experiments

Alternate bars are large wave patterns in sandy beds of rivers and channels. The crests and troughs alternate between the banks of the channel. These bars, which move downstream several meters per day, reduce the navigability of the river. Recent modelling of alternate bars has focused on stability analysis techniques. We think, that the resulting models can predict large rhythmic patterns in sandy beds, especially if the models can be combined with data-assimilation techniques. The results presented in this paper confirm this thought. We compared the wavelength and height of alternate bars as predicted by the model of Schielen et al. [14], with the values measured in several flume experiments. Given realistic hydraulic conditions R root Re > 2*10(3), (R the width-to-depth ratio and R, the Reynolds number), the predictions are in good agreement with the measurements. In addition, the model predicts the bars measured in experiments with graded sediment. If R root Re < 2*103, the agreement between model results and measurements is lost. The wave height is clearly underestimated, and the standard deviation of the differences between predictions and measurements increases. This questions the usefulness of small flume experiments for morphodynamic problems.