Bed roughness experiments in supply limited conditions

Reliable roughness models are of great importance, for example, when predicting water levels in rivers. The currently available roughness models are based on fully mobile bed conditions. However, in rivers where widely graded sediments are present more or less permanent armour layers can develop which limit local entrainment of sediment for bedform formation (supply-limitation). We conducted new experiments to study the effects of supply-limitation on the bed roughness. In these experiments the supply-limitation was systematically varied and the bed roughness was measured. The experiments show that the roughness of a partial mobile sand-gravel bed is strongly influenced by the volume of available mobile sand per unit area, or in other words, the average active layer thickness. For small layer thickness, the sand fills in the pores of the coarse layer and the bed roughness decreases for increasing sand volume. For large thickness, dunes develop and dune dimensions and bed roughness increase with increasing sand volume. An improved understanding of dune dimension under supply limited conditions can possibly help to improve the roughness prediction.