SHALLOW-WATER FRICTIONAL EFFECTS IN ISLAND WAKES

Only one eddy is present in the island wake in the lee of Rattray Isla nd, a 1600 m wide island in shallow coastal waters off Australia. This observation is contrary to the predictions of a number of two- and th ree-dimensional, fixed grid, numerical models which all predict that t wo eddies are present, one eddy being shed from each of the two tips o f the island. The discrete vortex model suggests this observation resu lts from the frictional effects in sub-grid scale details of the topog raphy close to the tips of the island. This model also successfully re produces the observations of a Karman vortex street in the lee of isla nds in Rupert Bay, Canada. By contrast, fixed-grid models predict no s uch eddy shedding. Island wakes are thus sensitive to details of the b athymetry near the separation points and these must be either paramete rized in fixed grid models, whether regular or boundary fitted, or exp licitly calculated in non-fixed grid models such as the discrete vorte x model. (C) 1998 Academic Press Limited.