A series of idealised numerical simulations has been conducted whose aim was to investigate the interaction of the boundary layer with gravity waves generated by flow over a forested hill. The numerical model used for this study is the Met Office BLASIUS model. A simple forest-canopy drag scheme is described and model results are presented for neutral flow and for stratified flow which supports trapped lee waves. Results for hills with and without a forest canopy are compared. The idealised results are also compared with model simulations of real flows over the Isle of Arran in Scotland and with field measurements made on Arran during 1999/2000.