Tidal currents in shelf seas—their nature and impacts

Abstract

While accurate maps of tidal elevations have existed for many shelf seas for over 70 years, the complexities of the associated tidal currents are still unfolding. Tidal currents exhibit much greater small-scale variability, both spatially and temporally, and hence accurate descriptions require more sophisticated measurement techniques and finer resolution 3D numerical models. Some simple dynamical theories that explain these intricacies of tidal currents are summarised. Likewise, examples are shown of recent technology developments in both H.F. Radar and ADCP (Acoustic Doppler Current Profiler) that illustrate these intricacies. Practical questions of how to synthesise data from such instruments are also considered. Tidal currents impact on many related phenomena, directly on vertical mixing rates and hence biology, sedimentation and thermal balance and via the associated horizontal residual currents on large-scale distributions of tracers. Examples are shown for the tidal impacts on the distributions of 137Cs, salt, metals and nutrients in the North Sea. Regarding future topics for research, nonlinear interactions of tidal currents with both surge and wave dynamics will need to be resolved in addressing the “Grand Challenge” of predicting coastal bathymetric evolution.