Rip current instabilities

A laboratory experiment involving rip currents generated on a barred beach with periodic rip channels indicates that rip currents contain energetic low-frequency oscillations in the presence of steady wave forcing. An analytic model for the time-averaged flow in a rip current is presented and its linear stability characteristics are investigated to evaluate whether the rip current oscillations can be explained by a jet instability mechanism. The instability model considers spatially growing disturbances in an offshore directed, shallow water jet. The effects of variable cross-shore bathymetry, non-parallel flow, turbulent mixing, and bottom friction are included in the model. Model results show that rip currents are highly unstable and the linear stability model can predict the scales of the observed unsteady motions.