A simple tree swaying model for forest motion in windstorm conditions

A simple tree swaying model, valid for windstorm conditions, has been developed for the purpose of simulating the effect of strong wind on the vulnerability of heterogeneous forest canopies. In this model the tree is represented as a flexible cantilever beam whose motion, induced by turbulent winds, is solved through a modal analysis. The geometric nonlinearities related to the tree curvature are accounted for through the formulation of the wind drag force. Furthermore, a breakage condition is considered at very large deflections. A variety of case studies is used to evaluate the present model. As compared to field data collected on three different tree species, and to the outputs of mechanistic models of wind damage, it appears to be able to predict accurately large tree deflections as well as tree breakage, using wind velocity at tree top as a forcing function. The instantaneous response of the modelled tree to a turbulent wind load shows very good agreement with a more complex tree model. The simplicity of the present model and its low computational time make it well adapted to future use in large-eddy simulation airflow models, aimed at simulating the complete interaction between turbulent wind fields and tree motion in fragmented forests.