The influence of sediment texture on the mobility of mixed beds : Annular flume experiments and numerical modelling

The research objectives in this dissertation were inspired by studies investigating the influence of fine sediment on the mobility of a mixed sediment bed on different grain scales and in different flow environments. While studies investigating fluvial sediment transport have concluded that the addition of fine material mobilizes the riverbed, other studies investigating mainly estuarine and marine sediment transport have found that the addition of fine material leads to bed stabilization. Two series of laboratory flume experiments (with a) spherical glass beads and b) sand and silt) and a numerical model were used to analyse the influences of the sediment texture and the particle shape on the near-bed flow field and the mobility of a mixed bed, and to find a possible transition between the different modes of behaviour. The sediment texture was characterized by the fine-grained fraction and the grain-size ratio RD = Dcoarse/Dfine between the diameters of the coarse and the fine particles. It could be shown that the bed texture influences flow processes at the bed surface that subsequently control particle entrainment.