The sediments found in tidal rivers, estuaries and coastal zones are usually a mixture of sand and mud. The accumulation and the erosion of these sediments have a large impact on the accessibility of harbours and on the necessary maintenance dredging operations. The ability of the mud fraction to adsorb large amounts of contaminants makes it even more important to fully understand the mixture behaviour. The sediment bed in a combined sewer system can also be described as a mixture of cohesive and cohesionless sediments. The accumulation of sediments reduces the sewer capacity and the erosion of the sediment bed can cause environmental problems when part of the flow is discharged into surface waters. Again, it is important that the behaviour of the sediment mixture is properly understood. This thesis represents the results of over five years of laboratory experiments on the erosion of mud/sand mixtures in uniform flow. The aim was to examine the effect of the mixture composition on the behaviour of the sediment bed in uniform flow conditions. The transition from cohesionless to cohesive behaviour, with respect to erosion resistance and mode of erosion, has been studied by adding more and more cohesive sediment to sand. A critical mud content is found above which the mixture can be described as a cohesive sediment; below this limit the mixture acts like sand only. The existence of a critical mixture composition has been explained in terms of the different structures formed inside the homogeneous mixture. Furthermore, the formation of a layered bed from a mud/sand suspension waS followed. The influence of the mixture composition on the sedimentation and erosion processes has been studied. The sand fraction seemed to enhance consolidation and reduce the resulting bed thickness. The erosion of the stratified deposit was a sequence of suspended load and bed load transport phases. In order to extrapolate the laboratory results to field conditions, the influence of the shape and the scale of the flume cross section on bed shear streSS distributions has been studied in detail. Both the shape and the aspect ratio of the flume were found to have an important effect on secondary currents and hence on bed shear stresses. Finally, based on the results of the experimental study, some guidelines to model the erosion resistance and erosion rates of mud/sand mixtures have been formulated.
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