Application of a Three-Dimensional Unstructured-Mesh Finite-Element Flooding Model and Comparison with Two-Dimensional Approaches

Urban flood modelling plays a key role in assessment of flood risk in urban areas by providing detailed information of the flooding process (e.g. location, depth and velocity of flooding). Accurate modelling results are the basis of reliable flood risk evaluation. In this paper, modelling of a flood event in a densely urbanized area within the city of Glasgow is presented. Modelling is performed using a new three-dimensional (3D) flooding model, which is an unstructured mesh, finite element model that solves the Navier-Stokes equations, and developed based on Fluidity. The terrain data considered comes from a 2 m Light Detection and Ranging (LiDAR) Digital Terrain Model (DTM) and aerial imagery. The model is validated with flood inundation area and flow features, and sensitivity analyses are conducted to identify the mesh resolution required for accuracy purposes and the effect of the uncertainty in the inflow discharge. Good agreement has been achieved when comparing the results with those published in other 2D shallow water models in ponded areas. However, larger vertical velocity (>0.2 m/s) and larger differences between the 3D and 2D models can be observed in areas with greater topographic gradients (>3 %). Finally, performance of the proposed 3D flooding model has been analysed. Through the modelling of a real flooding event this paper helps illustrate the case that 3D modelling techniques are promising to improve accuracy and obtain more detailed information related to urban flooding dynamics, which is useful in urban flood control planning and risk management. To the best of our knowledge, this is the first paper to apply a 3D unstructured mesh finite-element model (FEM model) to a real urban flooding event. It highlights some of the differences between the 3D and 2D urban flood modelling results.