Current structure in the Rhine region of freshwater influence

Observation with HF radar, ADCPs and conventional current meters in the Rhine ROFI system in the North Sea indicate that the dominant currents are due to tidal flow, with important contributions from winds and density-driven circulation. The tidal currents are dominated by the semidiurnal components, principally the M2, with tidal ellipses parallel to the coast and a semimajor axis of the order of 1 m s(-1) at the surface. The wind-driven flow varies between 1 and 3% of the wind speed and the rotation varies with depth in qualitative agreement with Ekman theory. After removal of the tidal and wind-driven component the residual flow within this region is generally parallel to the coast (northeastward) with average surface speeds of about 10 cm s(-1). A convergence zone in the surface flow was observed at the outer edge of the coastal zone. The vertical distribution of residual velocity closely follows the Heaps (1972) profile for density-driven flow, with slight deviations probably due to the tidal rectification (similar to 2 cm s(-1)).