Whirlpools explained

THE REMARKABLE television pictures from Japan revealed the awesome power of a tsunami as it reaches land. The coverage also revealed a curiosity – the many whirlpools (above) seen forming along the coastline as the tsunami waves rushed inland.

These were most likely caused by the powerful flow of water as it encountered underwater features, suggested Prof John McCloskey of the University of Ulster. “It is clear that in deep water there is no flow at all [with tsunamis], it is just the water moving up and down.”

The wave as it reaches shallow water generates an extremely strong current, however. The base of the wave is slowed by the seabed, causing the faster-moving top of the wave to rush forward.

“Then you get a significant flow element. If that flow hits [underwater] features that massive flow in shallow water can produce eddies,” Prof McCloskey said. “The surprising thing is you don’t see more eddies.”

In the case of shorelines the eddies are better called whirlpools, but it is just the same as with flowing rivers, he said. The flow constantly forms eddies as it comes up against obstacles and underwater features.

This is also at work as the flows caused by the tsunami encounter features that form whirlpools hundreds of metres across.