Hydrodynamic modelling with unstructured grid using D-Flow-FM: case study Afferden-Deest

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, two model types are the standard tools in the Netherlands, namely WAQUA/TRIWAQ (Rijkswaterstaat, 2012) and Delft3D (Deltares, 2014). WAQUA and Delft3D are both based on a structured curvilinear grid, which 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 elevation jumps in the river's topography may lead to unrealistic staircase representations in the model, and the inner bends of meandering rivers gridlines may become focussed to unnecessarily small grid cells (Kernkamp et al., 2011). 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, modelling results of Flexible Mesh and WAQUA are presented for a selected river reach near Afferden-Deest and benefits of local grid refinements in Flexible Mesh are demonstrated.