Deadly deception
From Brian Todd
CNN
WASHINGTON (CNN) -- On a Sri Lankan beach, an amateur cameraman captures part of a suddenly receding tide. It's a common occurrence during tsunamis, but a deadly deception.
Oceanographers call this "wave withdrawal." It's a phenomenon where the tide sweeps unusually far out, sometimes farther than the eye can see.
Entire bays can be drained. Witnesses say it looks like the ocean has turned into a desert.
"All the water was sucked out to sea," said one tsunami survivor. "Two hundred- to 300-yards and that gave everyone the sense that something very, very weird was happening."
Weird, but only in its scope.
In fact, experts say this is the same effect you see every few seconds on your average beach: an undercurrent washing down, away from shore, followed by a tiny wave, sweeping up.
In tsunamis, the waves' energy is magnified. The entire sequence takes longer, and is thousands of times larger.
"The more that the water withdraws, generally speaking, the bigger the wave that's coming at you -- and it could come at you very fast," Eddie Bernard, an oceanographer at the National Oceanographic and Atmospheric Administration said. "You cannot outrun the wave once it is upon you."
Bernard said that people were lured to the shoreline to get a better view. Even fishermen get taken in when they see an easy catch flopping on the sand.
Then the waves come back with a fury.
"It is a warning sign for you," Bernard said. "If you're on the coastline and you see this happening, that's a natural sign that something is wrong, and you should go away from the coastline."
In southern Asia, many didn't. There are various accounts of people putting themselves in harm's way just to get a better look, and then being swept away.
The size and speed of the wash-back depends on the beach itself. In shallow areas, with reefs of gradual slopes, experts say the ways drains away more quickly
Regardless of topography or depth, it always returns with force.
