On the exchange flow in tidal rivers and shallow estuaries

An analytic model is developed to investigate the barotropic tidally driven residual exchange flow in shallow estuaries. Ebb dominated flow in deep channel and flood dominated flow on the shoals produced by the model are consistent with some observations in tidal rivers and shallow estuaries. Analysis shows that this type of exchange flow is caused by the combined effect of nonlinearity and the lateral variation of the depth. The inward flux is mainly due to the surface elevation of the wave. A seaward residual pressure gradient has to be maintained to drive the water outward for mass balance. Since the surface elevation in an estuary has only small lateral variation, the depth integrated pressure force is mainly dependent on the depth whose value in the channel is larger than that on the shoals. As a result, the return flow in the channel is larger than that on the shoals. An ebb-flood flow system is thus generated.