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Andrew Alden

Tidal Wave or Not?

By May 9, 2008

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The flood disaster in Myanmar was caused by a severe storm, but it might as well have been a tsunami — indeed, the Indian media used the traditional name "tidal wave" for the storm surge. I've argued that geologists should stop objecting to the press using that term because it isn't really confusing, but now I wonder if I'm wrong.

On the one hand, the results of tsunamis and storm surges are the same for human purposes and probably the same in the sedimentary record. When we look for ancient tsunamis, the historical record is ambiguous. Some of history's most deadly tsunamis might have been storm surges. History mentions very deadly "tidal waves" along the South Asian coast, comparable to what we're seeing in Myanmar, and for planning purposes we need to beware both earthquakes and storms.

On the other hand, for geology the two causes must be distinguished, if possible. In some places, such as the American Gulf coast, there are no earthquakes, and the prehistoric hurricane record can be deciphered by experts in paleotempestology. But if the Myanmar cyclone had happened a thousand years ago, could we tell from the sediments what had happened? Hard to say. Without sure signs of earthquake such as sand blows or suggestive signs like mud volcanoes, we're stuck.

But probably arguing about "tidal waves" versus "tsunamis" is immaterial, just as it is when the press talks about the "Richter scale" for earthquakes. The important thing is that geologists get to explain Earth hazards to the public.


May 9, 2008 at 9:31 pm
(1) Lab Lemming says:

I think that the press means “wave generated by tides” when talking about last week’s Burmese catastrophe: The storm surge was boosted by the fact that it came ashore at high tide during a new moon.

At least here in Australia, they have been good about referring to the damage 4 years ago as being from “the tsunami”, and they’ve used the term “tidal surge” to describe the unusually strong flooding associated with the cyclone.

May 9, 2008 at 11:25 pm
(2) Kim says:

A friend of mine from college works on the transport and deposition of sand in very high-energy environments – mostly he works on old tsunami deposits, but he has also looked at the sand moved by Hurricane Katrina. A few GSA’s ago, he had these sticky cheesecloth things with sand stuck to them, which showed the sedimentary structures in sand moved by the hurricane versus the tsunami. He could tell them apart – the energy (and the flow regime, and the sequence of structure) were different.

(I don’t know where the work is published. And I’m not a sedimentologist.)

May 10, 2008 at 10:58 am
(3) BrianR says:

I’ve never studied storm deposits … at least not specifically like that. I’ve never done any research.

I think it could be very tough to differentiate in many cases. It’s always a matter of preservation. A lot of the structures preserved in high-magnitude events can be redistribution and winnowing of sediment as the waters fall. It also depends on the source of sediment – how voluminous is it?, how erodable?, etc.

My own bias would be to look in offshore settings – a lot of times these events can trigger failure and generation of various types of gravity flows. My intuition says that onshore deposits would likely be destroyed/modified over long time scales (>10^4 yrs?).

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