GENERATION OF A RESIDUAL CURRENT BY INTERACTION BETWEEN THE COASTAL BOUNDARY-LAYER AND THE EKMAN LAYER IN A TIDAL MOTION

Some experiments have been made with the large rotating tank at the In stitut de Mecanique de Grenoble to reproduce a sinusoidal flow along a vertical wall, over a flat bottom, with or without slope, in a homoge neous fluid. The objective of these experiments was to simulate a tida l motion, restricted to its main harmonic component M2, parallel to a vertical coast; and to demonstrate a particular mode of generation of residual circulation near the coast as a result of the interaction bet ween friction, background rotation of the system (Ekman pumping) and o scillatory motion (Stokes effects). Moreover, the stabilizing (or othe rwise) effects of the Earth's rotation on this shear flow may modify t he rectification process. The configuration is in a good agreement wit h the general topographic and hydrographic situation along the French coast of the Pas-de-Calais near Boulogne-sur-Mer. In the Northern hemi sphere, the bottom current is diverted to the left of the main current due to. the balance between the Coriolis force and the pressure gradi ent. This imposes a dissymetric behaviour of the flow between flux and ebb, as well as the generation of an alternative component perpendicu lar to the coastline, together with lateral transport (vertical close to the coasts) of horizontal momentum which modifies the coastal bound ary layer. A rectified (residual) current is observed with the coast t o its right. Experiments were systematically made on a flat bottom mov ing in the rotating tank by varying the main parameters of the flow; a nd we measured the velocity field with ultrasonic velocimetric probes and photographs of surface floats. The results (amplitude and width of the rectified current) show a good correlation with the few observati ons at sea and with the scaling analysis we provide.