Numerical simulation of turbulent flow in a forested park damaged by a windstorm

The December 1999 Lothar windstorm that affected Western Europe caused extensive damages to French forests. The large spatial variability in the observed damages may be explained in a number of cases by the fragility of tree species or by local soil conditions, but it also seems that the high degree of fragmentation of the forest has induced very heterogeneous flow patterns. In order to understand and explain the origin of such heterogeneity in the damages induced by the windstorm on tree stands, three-dimensional numerical simulations are performed in the particular case of a heterogeneous forested park in the city of Strasbourg (France), in which a few sections were totally devastated by the storm whereas other parts remained unaffected. For this purpose, the atmospheric flow equations are solved within and above the tree canopy over the whole park with a CFD type model adapted to canopy flow. Turbulence is modelled statistically with an energy-dissipation closure scheme. The flow equations in the canopy are modified to account for the drag forces and the production of turbulent kinetic energy by the vegetation. This model has been previously validated in two-dimensional cases (continuous and discontinuous vegetation canopies) against wind-tunnel and in-situ measurements. For the application presented here the model equations are solved over a 1800 x 1500 x 350 m domain with a 20 m horizontal resolution and a 1.2 m vertical resolution within the canopy, described as a spatially distributed set of parameter values (height, drag coefficient, leaf area distribution). Three-dimensional maps of wind velocity, turbulent kinetic energy and shear stress are produced in various cases differing by the mean wind direction, canopy characteristics and hypothesis regarding the state of the park before the storm. It is shown that the spatial variation in the observed damages reflects to a significant extent the spatial structure of the turbulent flow, as calculated in the park.