Intrinsic steady alternate bars in alluvial channels. Part 1: Experimental observations and numerical tests.

Alternate bars in straight alluvial channels are migrating or steady. The currently accepted view is that they are steady only if the width-to-depth ratio is at the value of resonance or if the bars are forced by a steady local perturbation. Experimental observations, however, seem to indicate that steady bars are also present in cases of migrating bars in the absence of a persistent perturbation. The companion paper by Mosselman (2009) provides a theoretical explanation. We review some experimental observations as well as long-term numerical tests using a 2D depth-averaged morphological model of a straight channel with non-erodible banks. Small random variations in total discharge are imposed at the upstream boundary. Rapidly growing migrating bars are found to develop first, but slowly growing steady bars are found to evolve subsequently, starting either from upstream or from downstream. Since steady bars are seen as a prerequisite to explain meandering of alluvial rivers, our findings imply that neither resonant width-to-depth ratios nor steady local perturbations are necessary conditions for the onset of river meandering.Hydraulic EngineeringCivil Engineering and Geoscience

Experimental observations on long-term behavior of migrating alternate bars

Migrating alternate bars form in alluvial channels as a result of morphodynamic instability. Extensive literature can be found on their origin and short-term development, but their long-term evolution has been poorly studied so far. In particular, it is not clear whether they eventually reach an equilibrium shape, since short-term experiments show that they may tend to elongate with time. We studied the long-term evolution of alternate bars by performing two independent long-duration laboratory experiments. In a straight flume, we carried out two tests, characterized by the same hydrodynamic conditions. In the first test (duration 3 weeks), a transverse plate created a permanent disturbance at the upstream boundary, forcing the formation of steady bars. In this case, both migrating and steady bars formed, but steady bars rapidly dominated the scene. In the second test (duration 10 weeks), the incoming flow was uniform, without any external disturbance. Migrating bars initially dominated the bed topography, but steady bars slowly developed from upstream, locally suppressing migrating bars. In both tests, migrating bars showed a periodic behavior, characterized by vanishing and reappearing at intervals of one or more weeks. Recurrent bar vanishing occurred also in another long-term experiment carried out in an annular flume, this time at intervals of 6-8 days. Migrating bar vanishing appears related to progressive steepening of the bar fronts. Without any external forcing, steady bar development seems to be caused by the presence of migrating bars, which may operate as forcing factors. Steady alternate bars are found to act similarly to point bars in sinuous channels, in their ability to locally suppress migrating bars. In conclusion, in the performed experiments migrating bars appear as a transitional phenomenon of alluvial channels having a cyclic character, whereas the bed topography is eventually dominated by steady bars.Hydraulic EngineeringCivil Engineering and Geoscience

Long-duration laboratory experiment of slow development of steady alternate bars

The current view is that migrating bars are the result of morphodynamic instability in straight or mildly-sinuous alluvial channels and are therefore an inevitable feature of alluvial river beds. Steady bars, instead, require some external forcing or specific morphodynamic conditions to develop. Yet, recent numerical tests showed that steady bars may develop as a result of spontaneous morphodynamic instability, just like migrating bars, without meeting the specific conditions. We investigated this possibility in the laboratory, following the temporal evolution of alternate bars in a straight flume with mobile bed. The experiment was run with a constant discharge for about 10 weeks. Initially, the bed topography was dominated by the presence of fast growing migrating bars. After three weeks, however, slowly growing, larger, steady bars emerged. These bars had the same wavelength as the ones that formed in another experimental test in which the flow was perturbed by the presence of a transverse plate. The experiment confirms the recent numerical results. Considering that the presence of steady alternate bars is a prerequisite for initiation of meandering, this is now shown to be an inherent feature of alluvial rivers.Hydraulic EngineeringCivil Engineering and Geoscience