Author(s): A. P. Tuijnder; J. S. Ribberink
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Keywords: No Keywords
Abstract: Under supply-limited conditions a part of the present river bed sediment is immobile. This immobile sediment often forms layers that inhibit further entrainment of the underlying sediment. The volume of mobile sediment on top of the immobile layer limits the development of bedforms. The bedforms play a crucial role in determining the bed roughness and the sediment transport. In morphological models the bed roughness is an important parameter because it determines the water level, flow velocity and the bed shear stress for sediment transport. Therefore the limited development of bedforms due to a limitation of the volume of mobile sediment is important to include in the roughness prediction. In an extensive set of laboratory experiments (>30 experiments in 5 series) the bed form dimensions, bed roughness and sediment transport have been studied. Based on these and other lab data a relation between the mobile layer thickness and the supply-limited bedform dimensions has been developed (recently accepted in‘Sedimentology’, January 2009). Based on these supply-limited bedform dimensions a new method to predict the bed roughness has now also been developed. This method takes decreasing bedform dimensions and the progressive exposure of the-usually coarser-sub-layer into account. After implementation of the new method in a morphological model (Delft3D) it is studied whether large-scale bed level changes (aggradation/degradation) can now be predicted in an improved way when conditions are supply-limited. For this purpose a set of non-equilibrium laboratory experiments has been carried out in which a‘wave’of mobile sediment propagates over a coarse immobile layer. It is concluded that the morphological simulation of this wave can be improved compared to conventional modeling practice.
Year: 2010