INSTABILITIES OF THE UNDERTOW

The steady undertow created by waves breaking at a beach and slowly fl owing offshore can become unstable and create a train of submerged off shore migrating vortices with shorter length scales and longer time sc ales than the incident waves, as shown by Matsunaga, Takehara & Awaya (1988, 1994). These vortices rotate about horizontal axes parallel to the shoreline. Our larger-scale laboratory experiments show that an ad ditional layer of vortices can exist over the water depth, with vortic es near the water surface rotating in the same direction as the wave-i nduced water particle trajectories, while those located at about mid-d epth rotate in the opposite direction. A theoretical and numerical ana lysis shows that these vortices are due to instabilities of the undert ow. Far offshore of the surf zone, the vortex trains decay because the velocity profile for the undertow becomes linear over depth, hence ne utrally stable to any disturbances.