Development of supply-limited transport due to vertical sorting of a sand-gravel mixture

If a partially mobile sediment is transported an immobile sediment layer can form below the bedforms. This immobile layer can cause a supply-limitation, i.e. the volume of mobile sediment on top of the layer available for transport becomes limited. This causes the bedforms, roughness and sediment transport to be reduced compared to alluvial conditions, i.e. the situation where all bed surface material consists of mobile sediment. We studied the development of the bed stratification in a series of flume experiments with different initial sand-gravel mixtures. In all experiments a thin immobile gravel layer developed with supply-limited bedforms on top; showing a strong similarity with the supply-limitation as observed in situations of sand transport over pre-installed flat immobile beds. Two phases were observed in the temporal development of the stratified bed: I) first a relatively quick development of dunes with gravel accumulating in the dune troughs, followed by II) a slower development of the level of the immobile gravel layer in the bed. In the final equilibrium situation the thickness and composition of the immobile layer appeared to be more or less independent of the initial sand-gravel mixture composition. However, the thickness of the mobile sediment layer (active layer) and the average dune height strongly reduced with increasing gravel concentration in the initial mixture.