Tidally induced residual circulation in estuaries with cross-channel bathymetry

Date of Completion

January 1996


Physical Oceanography|Biology, Oceanography|Physics, Fluid and Plasma




Tidally driven residual circulation in estuaries with lateral depth variation has been studied using-several 2-D and 3-D analytic models as well as a numerical model. Exchange flow is found to be correlated with the topography. The magnitude of this exchange flow mainly depends on four parameters: the ratio between the length of the estuary and the tidal wave-length, the ratio between the minimum depth on the shoal and the maximum depth in the channel, the ratio between the tidal amplitude at the mouth and the mean depth, and the ratio between the tidal time scale and the decay time scale due to friction. Generally, a net landward flow occurs over the shoals and is balanced by a return flow in the channel. The mean elevation shows a setup at the head. This produces a pressure gradient that drives the outward residual flow which is larger in the channel than on the shoals. The residual velocity is highly dependent on the length of the estuary. When the length of the estuary is about a quarter of the wave length, the magnitude of the residual velocity reaches its maximum. When the length of the estuary is much larger than a quarter of the wave length, the residual circulation approaches to a pattern in an infinite length estuary. For an estuary that is much shorter than a quarter of the wave length, the motion in the estuary is close to an inviscid standing wave in which the phase difference between the elevation and the longitudinal velocity is about 90$\sp\circ$. As a result, the surface Stokes transport is close to zero. The advective transport of vorticity is then dominant and the argument of Zimmerman (1981) applies. The resulting residual exchange flow is then landward in the channel and seaward on the shoal.^ Results indicate that the magnitude of the tidally induced residual velocity in typical coastal plain estuaries with shallow shoals and deep channels can be comparable to the density driven flows. The present solutions can be used for development of numerical models and guidance for future field and laboratory studies. ^