Long-Wave Modulation of Stokes Drift and Mass Transport in Short Waves

Abstract The Stokes drift generated by surface gravity waves has been widely investigated in both laboratory and field studies. However, key uncertainties remain, particularly regarding the modulating role possibly played by long-term oscillations induced by infragravity-like waves locally varying the mean depth. This study addresses these gaps by evaluating Eulerian and Lagrangian mean motions under weakly nonlinear monochromatic and bichromatic surface waves in controlled laboratory experiments. Particle tracking velocimetry ( ptv ) is used to infer fluid and particle flows underneath the waves. Short-wave-related Stokes drifts are then extrapolated and compared with theory. Results show that the inferred Stokes drift dynamics for the periodic monochromatic wave align well with the theoretical profile. Moreover, albeit long wave flow components are dominant on both short-wave Eulerian and Lagrangian dynamics as they are capable of shifting mean flow profiles onwards and backwards according to long-wave phase, the structure of the ensuing Stokes drift is relatively unaltered and still follows the theoretical law. The experimental findings suggest that the theoretical Stokes profile for a monochromatic wave may still be valid when short-wave motions are modulated by long period fluctuations in water surface and velocity, as long as a weakly nonlinear regime is attained.