|Whirlpools near Iwaki City|
A fascinating poster presentation of tsunamis and their whirlpools in art and mythology by R.S. Ludwin and A. Colorado can be found here. Whirlpools occurred in the 2004 Indian Ocean tsunami, in the Lisbon earthquake of 1755, Papua New Guinea, 1957 and 1998, Manzanillo Mexico, 1995, and Puerto Rico 1867. They are prominently mentioned in mythology, including the Haida myths of the Pacific Northwest. The poster and other information from a workshop in 2005 can be found here.
In addition to observations and reports of vortices in myth and history, their existence has been inferred from whirlpools and potholes formed during paleo-tsunamis. Whirlpools can reach up to 70 m in diameter and often have a central plug protruding up 2-3 m from the floor of the pit at the center of the vortex. These features are prominent in New South Wales, Australia. There are often potholes within the vortices, indicating nested vortices. Other features indicate different types of vortices: comma marks, "muschelbruche", V-shaped grooves and flutes.
Features are sculpted into bedrock during tsunamis by a number of different flow phenomena. Three large-scale processes are Mach-Stem formation, jetting, and flow reattachment. Mach-Stem formation is a phenomenon that develops when waves travel at an angle to a cliff--at the (traveling) point where the wave contacts the shore, the amplitude is amplified 2-4 times because of oblique reflections. This phenomenon is one reason that on-lookers are sometimes swept away by an unexpectedly high pulse of water. Jetting is a phenomenon that occurs when a tsunami approaches a rocky promontory--the geometry forces vortices and the velocity increases due to momentum conservation. Velocities in excess of 10 m/s are easily generated by this process, and plucking of rocks then occurs. Flow separation, the third phenomenon, occurs if the tsunami runs up a slope and encounters a relatively flat top. It can have so much momentum that it detaches from the slope (goes airborne). The forces on the rock where it lands (reattaches) can be enormous, hydraulic jumps can be formed, and erosion is dramatic.
An excellent book on tsunamis, and the source of this discussion is "Tsunami: The Underrated Hazard" by Edward Bryant, Cambridge University Press, 2001.