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In "Deep Impact", the president says that "New York City, Boston, Atlanta, Philadelphia, all will be destroyed". New York and Boston are on the water and would be in danger from a tsunami of any size, Philadelphia is 39 feet above sea level and 50 miles inland. Atlanta is over 200 miles inland and between 750 and 1100 feet above sea level. Would an impact event of the size imagined in the movie actually send a wave that far inland and that far up? Would the natural topography of the region (lots of small hills) affect the wave?

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Tsunamis form as a series of moderate amplitude, long interval waves that travel away from the initial impact. The first of these waves has the highest amplitude of the initial waves, and becomes the 'big one' later. Subsequent waves are caused by water rushing into the void created by the initial impact, bouncing off of each other, and starting the next wave out.

As the radius of the impact waves increases, the power in each wave gets spread out over a larger and larger circumference, at a rate of $2πr$. This is a steady geometric decrease in the power any given portion of the wave has.

As these waves start to reach shallow water, the waves in front begin to slow down. This causes the waves behind them to become part of the tsunami. This build up continues until you reach land. Once at land, no more power is being introduced into the wave, no more water is being pulled in. What you see is what you've got at this point. None of the follow up waves will go anywhere near as far inland as this initial wave.

To elaborate, following landfall, the tsunami would encounter:

  1. The barrier islands to Georgia's east protect it from most severe storms. Their arrangement would also introduce an interference pattern into the wave that would incline the big tsunami to spread out more.

  2. Beyond that, there is a large, flat area of low elevation where the wave would travel easily, but it would lose a large percentage of its power crossing this area. Once again, the wave is losing power as it spreads.

  3. Next, you get to the continental shelf that separates North Georgia from South Georgia. In a rather sharp increase, land goes from a few hundred feet above sea level to, as you mentioned, 750+ feet. That is one steep hill to climb for an old, tired tsunami.

Without subjecting this to a full simulation I can't be certain, but I think that any impact capable of creating a tsunami that could reach Atlanta would be completely unsurvivable. Some of the simplest forms of life might make it, but everything else would be gone.

That does not mean that Atlanta wouldn't suffer substantial damage from other factors, such as all of the water launched into the air by the impact of a red hot flaming rock and any chunks of that comet that might break off from the friction of passing through the atmosphere, but right now Atlanta is pretty safe from tsunamis.

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    $\begingroup$ Agreed. It's a good movie, but I think they just randomly picked a major city in that corner of the country. $\endgroup$ – Omegacron Oct 2 '14 at 16:55
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    $\begingroup$ If they wanted a major city in the southeast, they should have picked Miami. Just like NYC and Boston, I have no doubt that a tsunami caused by a rock that big would utterly destroy that city. $\endgroup$ – user940 Oct 3 '14 at 2:43
  • $\begingroup$ Don't tsunamis have periods? So don't Mega-Tsunamis (1500ft) waves have a period of 14 or 15 miles? So if you're 20 miles inland, would you ever see the wave? $\endgroup$ – user4602 Oct 9 '15 at 17:56
  • $\begingroup$ Did you mean to say "Fall Line" instead of "continental shelf "? $\endgroup$ – Spencer May 18 '17 at 0:16
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The bolide depicted in the film seems to have been modeled upon the Chixculub meteor that impacted Yukatan, and created the K-T extinction event. Both the film and Chixculub were about 11 kilometers in diameter. This would result in a roughly $10^5$ Gtonne impact explosion - with some variation depending upon the density of the bolide, and the orientation with respect to the Earth's orbital vector, and hence the velocity of impact. The blast radius would be several thousand kilometers - actually about 7000 kilometers on a theoretical plane surface, but somewhat less due to the curvature of the Earth. So, sorry Atlanta, irrespective of whether the tsunami reaches your fine city, its curtains for you! :-)

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  • $\begingroup$ The comet in the film splits into two fragments, one of which is Chixulub size and will impact Western Canada, one of which is about 1/4 that size and will impact the Atlantic ocean off of Cape Hatteras. $\endgroup$ – philosodad Oct 15 '15 at 15:47
  • $\begingroup$ Some simple napkin math suggests to me that 1/4 the diameter results in something like 1/74 the internal volume, suggesting a blast radius closer to 100km. That may be wrong, but it does suggest that the difference is significant. $\endgroup$ – philosodad Oct 15 '15 at 16:10
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Georgia is a fairly flat state, well known for it's extensive wetlands (swamps, y'all), particularly the Great Dismal Swamp. The Northeastern part of the state is part of the Appalachian and Blue Ridge mountains, but these are very old, low-rolling mountains (unlike the high, sharp peaks of the rockies). So the land slopes down from the North-Northeast to the Atlantic coast in the West-Southwest. Atlanta is pretty far inland, but the land South and East is essentially a ramp leading down to the coast. I imagine it's quite possible for a tidal wave to reach Atlanta.

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