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Tidal Wave generated by Nuclear Weapon

  1. Mar 26, 2016 #1
    Hi,

    I was just watching "The Dark Knight Rises" and was reminded of a question I had about the end and Batman's flying the nuke outside Gotham where it exploded over the sea.

    We know tsunamis and even mega tsunamis can be produced by avalances, rock slides, and whatnot. My question is that if those things can potentially cause a wave several hundred feet high wouldn't a nuclear weapon detonating directly above the surface of the water have a similar if not greater effect? It seems like as the energy radiates outward a significant amount of energy would be directed downward causing a tsunami.

    So, would Batman's actions have really saved Gotham or would he have saved it from the blast only to have it smashed by the tsunami?
     
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  3. Mar 26, 2016 #2

    Simon Bridge

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    The bomb was supposed to be a 4MT device ... according to sources[1] it was a neutron bomb. It would have killed a lot of people for a relatively small explosion.
    However, someone better than me[2] has worked out the likely effects for the air detonation of a 4MT fusion bomb.
    You get more of a wave, though, with underwater tests - and you can find footage online showing the wave with ships for scale. I think you can see from [2] that the wave would be the least of their worries.

    [1] http://batman.wikia.com/wiki/Nuclear_Bomb
    [2] https://freakofnature.wordpress.com/2012/07/30/the-dark-knight-rises-nuclear-effects/
     
  4. Mar 26, 2016 #3

    Astronuc

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    In general, large earthquakes are more powerful than the largest thermonuclear explosions. Large thermonuclear explosions are on the order of 10's of megatons (Mt) of TNT. The energy released by Tsar Bomba, the largest thermonuclear weapon ever tested, was about 50 Mt.


    See this table for an equivalence - https://simple.wikipedia.org/wiki/Richter_scale#More_examples
    Mag 7 = 32 megatons
    Mag 7.5 = 178 megatons
    Mag 8 = 1 gigaton (1000 megatons).

    Tsunamis are associated with large ~8 and above magnitudes with a large volume of earth displacement. Tsunamis don't grow to 100's of feet in height, but tens or meters, or less than 100 feet. Runups on land might go more than 100 feet though.

    Much of the energy from a nuclear explosion is thermal and radiative. The bright flash produces substantial radiant thermal energy. The explosion pushes the atmosphere out in a shock wave, but there is not a lot of momentum as compared to that from a mass of the earth being dramatically displaced. An atmosphere detonation weakly couples with the ocean. The effect would be much greater if a thermonuclear device was detonated underwater. The US and French have done underground testing in the Pacific, but I don't believe significant tsunamis were generated.
     
  5. Mar 27, 2016 #4
    I don't doubt that an earthquake releases a tremendous amount of energy. But I've also read that the largest tsunami's aren't caused by earthquakes. There haven't been many but I read that a rock slide entering the water can cause a larger wave than a tectonic plate shifting upward from the sea floor.

    If a rock slide entering the water can cause a large wave, it seems a WMD would likely as well. But maybe the whole idea of a rock slide causing a tsunami is bogus anyway.
     
  6. Mar 27, 2016 #5

    Astronuc

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    Rock slides themselves represent enormous energy and momentum. If the eruption of Mt. St. Helens had happened on the coast, it would have generated a significant tsunami nearby. Krakatoa's eruption in 1883 apparently generated enormous tsunamis. The town of Merak was destroyed by a tsunami 46 m (151 ft) high. Source: https://en.wikipedia.org/wiki/1883_eruption_of_Krakatoa#Tsunamis_and_distant_effects
    https://en.wikipedia.org/wiki/1883_eruption_of_Krakatoa#Climactic_phase

    I've seen an estimate of Krakatoa's yield at around 200 Mt, but I haven't seen a detailed calculation. It would surpass the largest thermonuclear explosion man had accomplished, and it was underwater.

    We discussed megatsunamis here - https://www.physicsforums.com/threads/mega-tsunamis-from-landslides.86519/

    A landslide in Lituya Bay, Alaska did cause a runup in the bay of ~1720 feet directly across from the landslide. As the wave made its way down the bay, the height diminished rapidly.
    http://geology.com/records/biggest-tsunami.shtml
     
  7. Mar 27, 2016 #6

    Simon Bridge

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    The last French nuclear tests at Moruroa were underground iirc. At the time press reported the ground humped about 1m.

    There was a considerable amount of interest here because NZ is quite close, and there was the Rainbow Warrior thing.
    http://www.nzhistory.net.nz/politics/nuclear-free-new-zealand/testing-in-the-pacific

    Nuclear tests at Moruroa:
    (air and ground tests)
    (underground test)
    ... it would be nice if ppl would post stats for the images.

    (underwater tests)
     
  8. Mar 29, 2016 #7
    I watched nuclear test footage from Umbrella and Wigwam. Those were small nukes, 8 and 30kt, and they still generated impressive, kilometer-high splashes. I dread to think what would happen if ~10Mt device would be exploded some 2km under the sea.
     
  9. Mar 29, 2016 #8

    SteamKing

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    A 4 megaton nuclear detonation is not a "relatively small explosion". The A-bomb which leveled Hiroshima had a yield estimated at 16 kilotons of TNT. A 4 megaton nuclear device has a yield 250 times greater than the Little Boy bomb. A 4MT neutron bomb defeats the purpose of having such a weapon, which is to kill the population while leaving the local infrastructure relatively intact. Enhanced radiation weapons have a typically small yield, on the order of 1000 tons of TNT (1 kiloton) to a maximum of 10000 tons of TNT (10 kilotons).

    https://en.wikipedia.org/wiki/Neutron_bomb

    Hollywood and physics do not mix.
     
  10. Mar 29, 2016 #9

    Simon Bridge

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    I could br mistaken about yeild calc for a neutron bomb vs regular nuke yes.
    I think the idea on the wiki is that a neutron bomb is suppoed to kill a lot of people with less blast damage.
     
  11. Apr 4, 2016 #10

    mheslep

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    Apparently there would not be much to dread from a 10 mt bomb generated tsunami, given i) the much larger energy generated by quakes as Astronuc indicated in post 3, and ii) a much larger fraction of an undersea detonation would be realized as an increase in water temperature as opposed to the momentum transfer (100% ?) imparted by a quake. Water vaporized or vertically ejected is energy largely diverted away from tsunami creation, whatever havoc might be created in the vicinity of the blast water plume.
     
  12. Apr 5, 2016 #11
    Burning three kilos of wood releases about the same energy as 1 kg of C4. By your logic which compares only _energy_, 1 kg C4 explosion should be not much more destructive than a small-size campfire...

    My crude calculation which scales Umbrella test to a 10Mt device... it produced 1.5 kilometer high spray dome. With pessimistic assumption of volumetric scaling, 8Mt device (x1000) should produce "only" 10 times higher spray dome. That's 15 kilometer high (!!!). That's *before* depth is optimized for maximum water displacement, and that is 8Mt, not 10. So what, 20 km high spray dome can be achieved? I have a hunch that that might get a wee bit destructive... (and I have an unhealthy urge to see that filmed...)
     
  13. Apr 5, 2016 #12

    mheslep

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    I only commented on tsunami formation, the thread topic.
     
  14. Apr 6, 2016 #13
    That did not escape me either. You are right, the spray dome, however impressively looking, is only locally destructive.

    Earthquake tsunamis are caused by sudden local changes of sea level. For example, if ocean bottom moves up or down by 5 meters over a few square kilometers, water moves up and down too - which creates a high, long-wavelength wave.
    Umbrella test 50 meters underwater: "An underwater crater 3000 feet across and 20 feet deep was produced."
    I crudely estimate that a 10 Mt device .5 km underwater would displace/eject at least some 5-10 cubic kilometers of water, creating a huge "depression" or "cavity" in the ocean surface while ejected water experiences a sudden trip to stratosphere. Then the same mechanism as in earthquake tsunamis converts that depression into a high, long-wavelength wave.
     
  15. Apr 6, 2016 #14

    mheslep

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    Found a source indicating the https://www.researchgate.net/publication/271925061_Displaced_Water_Volume_Potential_Energy_of_Initial_Elevation_and_Tsunami_Intensity_Analysis_of_Recent_Tsunami_Events [Broken]. Thus for a mag 8 quake, E=1000 Mt (4GJ per ton TNT), assuming 0.04% coupling and a 10 meter vertical displacement of water, then the volume of water displaced via 4e-4*E=m*g*h, m=4e-4*E/(g*h) is 16 cubic km (1 gton water per cubic km). The Indian Ocean quake of 2004 was a 9.1 and thus it's tsunami would have displaced 67 times the water volume, or 1072 cubic km under the same assumptions.

    https://goo.gl/zOPpJs
     
    Last edited by a moderator: May 7, 2017
  16. Apr 13, 2016 #15
    1958 Ripple Rock demolition, one of the largest non-nuclear explosions ever. Detonated in a navigation hazard in a contained channel. Tidal effect was "minimal".

    Fourteen hundred tons (1,270 tonnes) of Nitramex 2H explosives (10 times the amount needed for a similar explosion above water) were packed into the drilled rock. Every possible affect of the world's largest non-atomic blast was carefully considered and precautions made.

    When Ripple Rock blew at 9:31:02 am April 5, 1958, the sight was stupendous. Seven hundred thousand tons (635,028 tonnes) of rock and water erupted in a blast that reached a height of 1,000 feet (305 m). The spectacle lasted less than 10 seconds before the debris was engulfed in a cloud of gas.

    Live television coverage, very new at the time, broadcast the event across the country. People in Campbell River saw the blast on the screen, but many commented that they felt and heard nothing of the explosion that was only a few kilometres away. Cushioned by the water, the sound was heard only within a small area, and the tidal effect was slight.
     
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