Sediment supply-limited bedforms in sand-gravel bed rivers

The stability of bedforms in mixtures of gravel and sand is not well understood. Two bedform types are characteristic: flow-parallel sand ribbons and flow-transverse barchans. Flume experiments and field data presented here show that gradual transitions exist from sand ribbons to barchans, and from barchans to fully developed dunes. Barchans and sand ribbons occur when not enough transportable sediment is available for the formation of fully developed ripples or dunes. The reason is that a part of the bed sediment is immobile, e.g., with an armor layer, which limits the sediment supply and thus the volume of sediment available for the formation of bedforms. Bedform stability diagrams are shown to be extendable to sediment supply-limited bedforms in sand-gravel sediment, if the particle parameters of the diagrams are derived from the transported sediment instead of the bed sediment. Barchans and forms transitional to fully developed dunes plot in the dune stability fields. Sand ribbons, on the other hand, plot in the ripple, lower plane bed, and dune fields. In the case of sediment supply limitation, bedforms are partly or completely related to the characteristics of the sediment supply from upstream. The sediment underlying the bedforms may be a stable armor and the exchange of sediment between this armor and the bedforms may be small or non-existent. Consequently, bedform characteristics in sand-gravel mixtures in supply-limited conditions often are not predictable from the local hydraulics and sediment characteristics.