Surface currents in the Alderney Race from high-frequency radar measurements and three-dimensional modelling
Résumé
Two weeks of HF radar measurements collected at the Alderney Race are compared to the results of a 3D fully coupled wave-current model. Spatial current measurements are rare in this site, otherwise well investigated through modelling. Thus, the radars offer a unique opportunity to examine the spatial reliability of numerical results, and can help improving our understanding of the complex current of the area. Comparison of observed and modelled surface current velocities showed a good agreement between methods, represented by root-mean-squared-errors ranging from 14 to 40 cm/s, and from 18 to 60 cm/s during neap and spring tides, respectively.Maximum errors were found in shallow regions with consistently high current velocities, represented by mean neap and spring magnitudes of 1.25 m/s and 2.7 m/s, respectively. Part of the differences between modelled and observed surface currents in these areas are thought to derive from limitations inthe k-epsilon turbulence model implemented in the hydrodynamic model, when the horizontal turbulent transport is high. In addition, radar radial currents showed increased variance over the same regions, and might also be contributing to the discrepancies found.Correlation analyses yielded magnitudes above 0.95over the entire study area, with better agreement during spring than neap tides, likely due to an increase on the phase lag between radar and model velocities during the latter.
