INFLUENCE OF BOTTOM STRESS ON THE 2-LAYER FLOW-INDUCED BY GRAVITY CURRENTS IN ESTUARIES

Baroclinic circulation in highly stratified and partially stratified e stuaries is characterised by a two-layer flow: a bottom saltwater infl ow and a surface brackish-water outflow. Tidal period variation of the thicknesses of a two-layer how is observed to be associated with mixi ng, bottom stress and hydraulic characteristics of superposed tidal an d gravity currents. Here, both analytical two-layer hydraulic equation s with weak friction and a numerical model including a turbulence clos ure were utilised to understand the mechanism of the layer tendency wi thin a two-layer flow under different barotropic flow conditions. It h as been found that in the weak bottom friction case, a gravity current has two critical solutions at the layer thickness equal to 0.5H and 0 .292H. The layer thickness towards a particular critical solution is d ependent on the sign of the bottom stress, i.e. when the bottom stress is opposite (favor) to the bottom gravity current, its layer thicknes s converges to 0.5H (0.292H). In the case of strong bottom stress and mixing opposing the gravity current, the solutions of the gravity curr ent layer thickness at 0.5H and 0.292H will not be valid. Both mixing and vorticity produced by bottom stress erode the halocline, and produ ce a high velocity core in the mid-depth, which leads to the thickness of a bottom gravity current greater than 0.5H. These internal hydraul ic tendency and mixing processes, varying with time-dependent barotrop ic tidal current forcing, determine the tidal period variation of the gravity current structure. (C) 1998 Academic Press Limited.