AbstractA numerical model of the local flow above an infinite series of two dimensional ridges is used to investigate the extent to which form drag on the ridges is balanced by reductions in surface shear stress compared to that for a plane surface. With a constant eddy viscosity Mason and Sykes (1979) results an confirmed and there is only a relatively small net increase in the horizontal force on the terrain due to the presence of topography. A larger increase is found when height dependent eddy viscosities are used. the nature of velocity defects produced by individual ridges are examined for the two cases.