Detailed 2D flow simulations as an onset for evaluating socio-economic impacts of floods

A 2D hydraulic numerical model is presented. It is based on the fully dynamic shallow-water equations, solved by means of a finite volume scheme on multiblock structured grids. The model handles highly accurate DSM derived from airborne laser altimetry for the floodplains and sonar detection for the bathymetry of the main channel. The flow simulations are performed with a typical grid spacing of 2m, comparable to the resolution of the DSM and fine enough to represent reliably the flows at the scale of each building. Consequently, the model succeeds in predicting accurately the pattern of flood depth even in densely urbanized floodplains, as confirmed by several validation examples, which also demonstrate the ability of the model to reproduce depth measurements for a wide range of flood discharges without the need for recalibration of the roughness coefficient. Finally, the hydraulic model is exploited as a component of a complete flood risk modelling chain, accounting for socio-economic and ecological impacts of floods and dedicated to the assessment of flood protection measures in the context of climate change. 2 HYDRODYNAMIC MODEL As an onset for evaluating socio-economic impacts of floods, simulations of floodplain inundation flows are conducted with the numerical model WOLF 2D, developed at the University of Liege and based on a finite volume scheme. 2.1 Mathematical model The model is based on the two-dimensional depthaveraged equations of volume and momentum conservation, namely the “shallow-water” equations (SWE). In the “shallow-water” approach the basic assumption states that velocities normal to a main flow direction are smaller than those in the main flow direction. As a consequence the pressure field is found to be almost hydrostatic everywhere. The large majority of flows occurring in rivers, even highly transient, can reasonably be seen as shallow everywhere, except in the vicinity of some singularities (e.g. weirs). Indeed, vertical velocity components remain generally low compared to velocity components in the horizontal plane and, consequently, flows may be considered as mainly twodimensional. Therefore, the approach presented here may definitely be regarded as suitable for floodplain inundation modelling. 2.1.1 Conservation laws The conservative form of the depth-averaged equations of volume and momentum conservation can be written as follows, using vector notations