Aren't tidal waves usually preceded by a pulling back of the water at the natural shoreline?

Question

I'm watching this movie one the Sci-fi Channel, Tidal Wave-No Escape, and each tidal wave comes in with no warning at all. It doesn't seem realistic. Can they do that, come in without warning?

Answer 1

Although many exceedingly high waves are preceded by a withdrawal, not all of them will be. For example, the waves created by a meteor impact would not recede at the coastline before the wave hit. Or the waves created in a bay by the cleaving of an iceberg would not recede first. In other words, impact created waves would not recede first because all the forces point outward from the point of impact.

In fact, the recession from the coastline would occur after, not before, the wave due to impact. This results because the water displaced by the impact would rush back into where it came from before the impact. [NB: You can test this by dropping a large ball into a vat of water and paying attention to the wave action. You will see the crest comes before the trough.]

On the other hand, as most of us know by now, tusnami waves are preceded by a withdrawal at the coastlines. This results because tsunami waves are caused by a sudden collapse of the ocean floor during an earthquake. Thus, as the floor collapses, the water first rushes in to fill that vacancy.

This results in pulling the water away from the coastlines. The tidal wave crest follows that recession because it is a reaction to the water impacting with itself as it rushes in to fill the vacancy left by the ocean floor collapse. Thus, even here, the wave, the crest of it, is created by an impact of sorts.

I didn't see the scifi show, but if the wave initially resulted from an impact, there would be no recession at the coastline...no warning.

PS: The tsunami a couple of years ago around Indonesia, Sri Lanka, etc. was in fact preceded by a withdrawal from the coastline. In fact, some of the people who were washed to sea were out on the newly dry ocean bed around the coast looking for whatever might be left on the seabed after the water receded. Not a good idea....

Answer 2

It depends on the nature of teh sea bed and shape of the bay.

If you watched pictures of the devastating tidal wave in South East Asia a couple of years ago, there was no real drop in water level, but also no wall of water. The surge just rushed in and kept on rushing.

In other cases, the water level does indeed drop a long way first.