Modelling of the morphological interaction between a river and its groyne fields

Groynes are dam-like structures in a river, which are constructed at an angle to the direction of the flow. Their purpose is to maintain the navigable depth during low water conditions and to divert the flow from critical zones during high water conditions. The spaces between the groynes, the groyne fields, are areas of shallow flow. Groynes have an impact of the hydrodynamic and morphologic conditions of the river. They cause specific turbulent structures in the groyne field. The morphological impact of groynes are bed level changes, such as scour holes near the groyne tips and deposition ridges in the groyne field. In flume experiments, which have been carried out at the Laboratory of Fluid Mechanics at the TU Delft, these flow conditions and bed level changes were studied for various test cases. In this thesis a numerical simulation of an emerged case of the laboratory experiment is performed. Here, the exchange processes between the main channel and the groyne field are of special interest. The simulation was done using the Delft3D-FLOW system. The add-on Horizontal Large Eddy Simulation (HLES) models the small-scale turbulences on a sub-grid scale. For the morphological computations the module Online-sediment was used, which calculates the sediment concentration, the sediment transport, and the bed level changes simultaneously with the flow. The model was calibrated against data from the experiment. The results of the hydrodynamic computations show the typical turbulent structures in emerged groyne fields, that have also been observed in previous experiments and described by several authors. However, a comparison with the experimental results could only be made at the locations, where the velocity was measured. With a sensitivity analysis, the influence of some input parameters on the computed flow field was studied. For this analysis some physical parameters were modified and the influence of the use of a high-pass filter for the velocity signals was investigated. The results of the morphological computation showed a qualitative accordance with the measured bed levels. Yet, the suspended sand settles too quickly at the entrance of the groyne field instead of entering it. Furthermore, the scour holes are less pronounced than in the experiment, while the erosion of the main channel is computed to be stronger than in the experiment. Another sensitivity analysis was carried out for the morphological computations. In conclusion, it can be stated, that Delft3D-FLOW computes the morphological pattern qualitatively well. When analysing the details, though, some major differences with the observations occur. Especially the sediment transport into the groyne field is not reproduced realistically at the moment.Civil Engineering and Geoscience