Downstream grain-size distribution of surficial bed material and its hydro-geomorphological significance in a large and regulated river: the Rhône River in its delta area (France) / Distribution granulométrique longitudinale des dépôts de fond superficiels et sa signification hydro-géomorphologique dans un grand fleuve chenalisé : le Rhône dans son aire deltaïque (France)

Abstract

Abstract This paper characterises the bed material in the two regulated distributaries of the Rhône Delta and examines the relationships among grain size, channel geometry, hydraulic conditions and distance from the coast. The Rhône River in its delta area is characterised by four bed- material types (cobble-pebble, sand, compact silt, mud). Sand beds are the most frequent. During low water flow, a small proportion of sand grains is mobilised by rolling. Sand transport capacity strongly increases at bankfull discharge, and bedload transport of sand is replaced by graded suspension. Finest cobble-pebble beds of the Grand Rhône are strictly mobile at bankfull discharge. Bed-material grain size is distinct between the Grand Rhône, which mainly transports cobble-pebble and coarse to medium sand, and the Petit Rhône, which predominantly transports medium to fine sand. Specific stream power differences, combined with local structural and geomorphological factors, explain this distinct bed-material grain size distribution in the two channels. An abrupt transition from cobble-pebble to sand beds occurs in the Grand Rhône 25 km upstream from the river mouth. On the Petit Rhône, a downstream fining occurs in the sand beds located in the lower section. At present, the Grand Rhône has a higher competence and a higher bed-material transport capacity than the Petit Rhône. High specific stream power explains the presence of numerous incised channel sections that create a bed-material discontinuum.