A case study of wave-current interaction in a strong tidal current

During August 1991, a field program was carried out in the vicinity of Cape St. James, off the British Columbia coast, where a strong tidally driven flow interacts with an active wave climate. Surface current maps were obtained from a CODAR-type HF radar (Seasonde) over an area of about 350 km(2) around the cape. A series of Loran-C drifters were also deployed during the experiment and used as ground truth for the radar. A comparison between the drifter and the radar surface currents indicates reasonable agreement. Wave information was acquired with three Waverider buoys deployed around the cape. A significant modulation of the wave properties at the tidal period was observed for the buoy located in the area where the currents are maximum. The tidally induced changes in the wave field are modeled with a local wave-current interaction model based on wave action conservation and on a high-frequency limiting spectral shape. The model is applied on a period of 11 days for which the wind was relatively steady. The magnitude of the modeled tidal modulation of the wave field is of the same order of magnitude as the measurements but, in general, underestimates the measured tidally induced changes. However, during the first half of the period, the modulation of the total wave energy is significantly out of phase with the buoy data. The effect of refraction by the current on the waves is assessed using a backward ray tracing method and two-dimensional surface current maps. It is proposed that refraction effects are important during the first part of the study period and are a plausible cause for the phase discrepancy between the measurements and the results of the local model.