Dynamics of large-scale vortices in the near shore

In spite of the ubiquity of long shore flows, rip currents, and tidal jets in the near shore, large-scale vortices associated with these phenomena remain poorly understood. The objective of this study is to understand the effect of vertical confinement on vortex dynamics through a new laboratory experiment on pulsed jets, which simulate rip currents, in a shallow water layer. The methodology includes analyzing the evolution of three-dimensional turbulence generated by a pulsed jet under varying conditions characterized by a dimensionless parameter related to injected momentum flux, water depth, and injection duration. The results reveal that when vertical confinement is weak, the jet spreads freely, maintaining fully three-dimensional turbulence; conversely, strong confinement leads to a damping of vertical motions and the formation of large horizontal dipolar structures, with localized three-dimensional vortices identified within these structures. A theoretical model developed from these observations shows good agreement with the empirical measurements, suggesting that these insights could be foundational for physical parametrizations in coastal hydrodynamic models.