Self-formed levees and floodplains in an annular flume

Various river channel patterns have been produced in experiments recently, including dynamic meandering. The key to produce more realistical patterns is the formation of levees and sedimentary floodplain. However, experiments to date only produced wide floodplains or bedload-generated levees and overbank splays, but not the classical levee with decaying thickness and particle size away from the channel. The objective of our work is to understand the subtle balance between inundation level, flow velocity and sediment properties, and to design experimental conditions that form levees in channel pattern experiments. We designed and built an annular flume with floodplains, where flow is driven by vanes in the preformed channel. The channel sediment was mobile and developed a transverse bed slope in response to the strong spiral flow. The transverse water surface gradient and the level of inundation controlled the flow on the floodplain. We experimented with sediments varying in diameter and density to obtain levees and floodplain under constant forcing and depth. The flow on the floodplain developed horizontal circulation when shallow relative to the channel, and vertical (spiral) flow when it was deeper. Silt-sized silica flour was either not entrained from the bed onto the floodplain, or suspended so much that the floodplain was covered entirely. A channel-flanking levee only formed in a very narrow range of flow depth and velocity for this sediment. Preliminary tests with low-density sediment did not form levees for channel flow conditions with mobile bed sediment. The difficulty in forming levees, and some numerical modelling with tides, suggest that fluctuating water levels due to floods or tides are conducive to levee formation.