Tidal Flow through the Straits of Dover

Results are presented from the first long-term deployment of the Mark II OSCR high-frequency radar system. This new system measures surface currents at 700 preselected locations every 20 minutes at a range up to 25 km offshore. Tidal analysis, in 11 monthly blocks, of measurements made in the Straits of Dover indicate consistent results for seven major constituents, M2, S2, N2, O1, K1, M4, and MS4. Synthesized current ellipse distributions for these constituents are compared with corresponding values from a 2.5-km grid numerical model. (The synthesis involved both spatial and temporal averaging with quality checks on the associated standard deviations.) A simple point model of vertical current structure is used to extrapolate the surface observations to provide depth-integrated flows. For horizontal integration to compute net fluxes, spatial harmonic functions are used to fit streamline distributions to the array of point measurements. By these means, the net residual transport arising from nonlinear coupling of M2 elevations and currents (Stoke's drift) is calculated. A similar approach using the depth-averaged M2 tidal amplitude and direction (semimajor axis) is used to map both the M2 tidal flux through the straits and the M2 tidal energy flux. The respective values obtained are (i) tidal flux (amplitude) 1.06 × 106 m3 s−1, (ii) Stokes drift 40 000 m3 s−1, and (iii) net energy 1.1 × 1010 W. While the widely used POL 35-km grid Northwest European Shelf Model computes a similar value for (i), this model overestimates both (ii) and (iii) by 35%. By reducing the resolution of this model to 9 km, close agreement is found for all three parameters. The point model for current structure also derives surface gradients. Thence by again using spatial harmonic functions for horizontal integration, both an M2 elevation cotidal chart and a corresponding mean sea level distribution were produced from the radar data. This use of high-frequency radar in providing continuous observations of strategic flows demonstrates its capabilities in relation to future oceanographic monitoring systems.