Formation of standing waves in a mountain river and its consequences on gravel bar morphodynamics
Formation d'ondes stationnaires dans une rivière de montagne et ses conséquences sur la morphodynamique d'un banc de galets
Résumé
In May 2008, a 10-year return period flood occurred in the Arc River, France, with a maximum discharge over 500 m3/s. During this event, large standing waves (approximately 1 to 2 meter high and 5 to 10 meter long) were observed at several locations in the river and for a given discharge range (approximately 250 m3/s, for both rising and falling flood limbs). These standing waves appeared to be unstable (lasted 15 to 30 min), and induced large bedforms located at the edge of a gravel bar. These bedforms form typically an antidune system. Two other assumptions were proposed in order to explain the formation of these standing waves in a compound channel flow. First, the waves may be the consequence of the formation of large eddies in the interfacial zone between the main channel and the floodplain. Secondly, the waves may result from an instability due to the juxtaposition of a supercritical flow in the floodplain and of a subcritical flow in the main channel, which induces a small surge at the interface. Finally, a discussion is provided on the consequences of these standing waves on the bed morphology. First, these standing waves are directly associated with bedforms observed later on in the side of a gravel bar. Also, the location of transverse channels on the gravel bar appeared to be strongly influenced by the location of these bedforms during the ebb flow.