What is Tide-Induced Residual Current?

Abstract

We carried out a numerical experiment to study the velocity field of a two-dimensional tidal current in a simple model basin with a narrow strait. It was found that the tide-induced transient eddy (TITE) originated from the low pressure area that is generated downstream behind a headland by the nonlinearity or the centrifugal force of the tidal current flowing with a large curvature through a narrow channel. The transient eddy is maintained during certain phases of the tide, and therefore the Eulerian tide-induced residual current is the result of the averaging process of transient phenomena; the Eulerian residual current is only a mathematical representation of the tide-induced transient eddy and has no physical reality. We should abandon the concept of residual velocity. The lifetime of TITE depends on the magnitude of vorticity and its dissipation. In an inner basin with large bottom friction, the eddy diminishes within a short time (one or two hours) after the generation, and the pressure gradient of the eddy is smaller than the pressure gradient of tidal flow in the strait at the time of high-tide stack water. In this case, TITE is swept away by the ebbing tidal current which flows outward through the strait and soon disappears. On the other hand, in a basin with small bottom friction, because the eddy grows so strong at the time of the start of the ebb tide that the pressure gradient becomes larger than that of the ebbing tide, it is maintained by the ebbing tidal current which runs around the eddy toward the strait. In the latter case, vorticity dissipation caused by horizontal eddy viscosity is larger than that due to bottom friction because of a large horizontal velocity shear near the eddy.