A model to determine the advective circulation in a three layer, salt wedge estuary: Application to the Ebre River estuary

A model to determine advective fluxes of water and salt in three-layer salt wedge estuaries under steady river discharge is presented. This model is a generalization of the Knudsen's two-layer model when applied to highly str atified estuaries. The reason for this is that Knudsen's model neglects the interface and produces biased estimates of the salt fluxes at this layer. ii basic assumption of the model is that velocity and salinity have linear profiles across the interface, which is appropriate for such types of estua ries. A physical description of the Ebre estuary is given in terms of river discharge, mean salinity in upper and lower layers, and cross-sectional ar ea of all three layers. These data are introduced into the model in order t o obtain estimates of horizontal and vertical fluxes. The equations for hor izontal fluxes are: the steady salt and water transport conditions at any c ross-section and a flow relation between the three layers. Solving the thre e mentioned equations, explicit expressions for horizontal fluxes are obtai ned. introducing the latter in the salt and water continuity equations of u pper and lower layers, vertical fluxes of water and salt at any given compa rtment along the estuary are obtained. These vertical fluxes among the thre e layers allow more precise information to be obtained about mixing process es within the estuary. A comparison of the results with those corresponding to the Knudsen's two-layer model shows that the model gives a more accurat e estimate of advective fluxes in highly stratified estuaries. (C) 1999 Aca demic Press.