Wave-current interactions in a high-flow tidal channel

Abstract

High-flow tidal channels (Re~O(108)) are characterized by strong bidirectional currents (3 m/s) and high turbulence intensities. As in most coastal environments, these strong flows are affected by wind and waves, especially during severe weather events. The combined occurrence of strong currents and wind-generated surface gravity waves is a nonlinear coupled process in which both the currents and the incident wave field are modified. In this work, the evolution of mean wave parameters in a high-flow tidal channel is evaluated for various wave and tidal flow conditions. Synchronous current, turbulence, and surface altimeter data measured by a bottom-mounted acoustic Doppler current profiler (ADCP) deployed in Grand Passage, a tidal channel within the Bay of Fundy in eastern Canada, are analyzed. Results indicate that wave growth and wave propagation are coupled with the magnitude and relative direction of the current. During each measured high-wind event, the significant wave height consistently increased, and the wavelength decreased, as the current magnitude increased when locally generated waves opposed the currents. To the contrary, the significant wave height was drastically reduced when the waves follow the currents even for a small current magnitude. In addition, elevated turbulence kinetic energy and vertical plumes of elevated acoustic backscatter amplitude were observed in the upper water column during high-wind events, likely bubbles injected into the free surface by breaking waves (whitecaps) and transported through the water column by the turbulence generated by tidal currents. Obtained results allow to identify when and where wave-current interactions are significant, what their implications are for the complex dynamics of tidal channels, and suggest that currents must be incorporated into forecast wave models to improve local sea state predictions and consequently navigation safety.