University of Twente Student Theses


Hydrodynamic river modelling with D-Flow Flexible Mesh : case study of the side channel at Afferden and Deest

Hagen, Erik ten (2014) Hydrodynamic river modelling with D-Flow Flexible Mesh : case study of the side channel at Afferden and Deest.

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Abstract:Accurate predictions of water levels play an important role in the management of flood safety. Nowadays, it has become common practice to use multi-dimensional numerical hydrodynamic models for such purposes. Currently, WAQUA and Delft3D are standard tools in the Netherlands, which are based on a structured curvilinear grid. The curvilinear grid can follow large-scale topographical changes and uses similar grid resolution throughout the entire computational domain. Drawbacks of the structured curvilinear grid approach are that staircase representation of closed boundaries is sometimes unavoidable, because grid cells are not aligned with the flow direction and in the inner bends of meandering rivers, gridlines may become focussed to unnecessarily small grid cells. To improve on these issues, Deltares is developing the unstructured-grid-based hydrodynamic model Flexible Mesh (also referred to as “D-Flow-FM”). The unstructured grid approach enables the user to use a spatially variable grid resolution. By combining curvilinear grid cells with triangular grid cells, the modeller can increase grid resolution on the locations where, because of local topographical variations, it is most desired. In this study Flexible Mesh is tested and compared with the structured grid based WAQUA and the possibilities of the unstructured mesh of Flexible Mesh are applied on a side channel at Afferden at Deest, where the WAQUA grid is considered to be inaccurate. The main objective of this research is: Evaluate the performance (water levels, flow velocities and discharges) of Flexible Mesh by comparing with WAQUA and assess the sensitivity of the modelling results for the grid resolution in Flexible Mesh. In the first step of the study the Flexible Mesh model is compared to the calibrated WAQUA model with focus on the water levels, discharges and flow velocities. The water levels in the Flexible Mesh model are comparable to the results of the water levels in the WAQUA model. For low discharges there is almost no difference in the water level and for high discharges the water levels are about 12 centimeters higher in the Flexible Mesh model. The discharges over the floodplains and in the side channel are much smaller in the Flexible Mesh model. There are two important sources for the differences between WAQUA and Flexible Mesh. First, Flexible Mesh default uses a different, corrected formula for the Colebrook-White roughness which results in a larger friction in Flexible Mesh and higher water levels. Second, the energy losses due to flow over weirs is modelled different in Flexible Mesh, which results in higher water levels in Flexible Mesh and lower discharges over the floodplain and in the side channel at Afferden and Deest. In the second step local grid refinement was applied at Afferden and Deest to the main channel of the Waal and to the side channel. The grid refinement of the main channel of the Waal showed no clear effects between consecutive grid refinements. The local grid refinement was also applied for the side channel, where the original grid is assumed to schematize the side channel inaccurate. The difference with the reference grid is maximal for the schematization with the largest refined side channel. However, the effect of grid refinement decreased at higher grid resolutions which indicates convergence of the model results. After the grid was refined four times, the results were hardly affected by a grid refinement anymore. Therefore, convergence seems to be reached around the four times refined side channel. The computational time increases because of grid refinement. For high grid resolutions, the time step has to be decreased in order to meet the model condition stability (default Courant number < 0,7). Grid refinement is efficient when model results are not yet converged, so further refinement has still effect on the model results, and computational time is still acceptable. The results of this study show potential for application of local grid refinements with the unstructured grid of D-Flow Flexible Mesh for complex geometries. The accuracy of the computation of the flow in the side channel seems to be improved by the local grid refinement. However, further research is required to assess the accuracy of Flexible Mesh.
Item Type:Essay (Master)
HKV – Lijn in Water, the Netherlands
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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