Vertical Variability of Undertow and Longshore Currents outside the Surf Zone

Abstract

Undertow and longshore current profiles outside of the surf zone were analyzed using a canonical situation involving an alongshore-uniform barred beach. Forcing mechanisms due to the presence of surface gravity waves are of interest. As is customary for surf zone studies, dynamics related to wave breaking and associated wave rollers were included. Also considered were the effects of the wave orbital velocities on the bottom and surface boundary layers. The effect of Earth’s rotation in the dynamics is included through two mechanisms: (1) the Coriolis-Stokes stress that results from the effect of Earth’s rotation on the wave orbital velocities, and (2) the Coriolis terms in the cross-shore and alongshore momentum equations. Model results qualitatively reproduce observed undertow profile shapes inside and just outside of the surf zone, as well as a few surf zone widths offshore and far offshore on the inner shelf. In particular, the results explain the surface-intensified undertow currents often observed outside the surf zone. The Coriolis-Stokes stress is not active inside the surf zone; but it is an important player, along with the other considered dynamics, in shaping the profiles outside of the surf zone. The model also suggests the presence of an alongshore current outside the surf zone that is generated owing to the effects of the Coriolis terms in the momentum equations. An analysis of the temporal spin-up behavior of the predictions suggest that circulation patterns strongly influenced by Earth’s rotation should be observable in the field within a few hours after the onset of wave activity.