Experimental observations on long-term behavior of migrating alternate bars

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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.