Bathymetric and shear stress effects on an island's wake: A computational model study

A numerical model has been refined and used to predict the depth-averaged tide-induced flow patterns around Rattray Island, northeast Australia. Rattray Island is approximately 1.5 km long, 300 m wide and is inclined at about 60° into the direction of the dominant semi-diurnal tidal currents in a well-mixed sea. The local depth around Rattray Island is approximately 25 m and a large horizontal eddy has been observed and measured in the island's wake during the flooding tide. This eddy has been modelled using a semi-empirical turbulence model for the Reynolds stresses, with reasonable agreement being obtained between the numerically predicted and the field-measured velocities recorded at 26 sites in the southern lee of the island. An analysis of the vorticity equation has been undertaken to establish the relative importance of the various terms in generating and maintaining this eddy, with numerical simulations being reproduced to confirm the significance of the advective accelerations, bed- and free-shear-generated turbulence, bottom friction and geostrophic effects. The effects of the bathymetry on the characteristics of the leeward eddy have been studied, with numerical simulations for the actual bathymetry, a horizontal bed and sloping beds indicating a marked difference in the vorticity structure and the shear stress distributions.