[wolfekeeper (OP)]

Apparently in Russian state media they're threatening to do this. Is this even remotely practical? I'm doubting they've got a big enough bomb to do that, but I'm prepared to accept a worked example proving otherwise.

[SeanB]

Would have to be big, and to make a tsunami in deep water that shelves towards the UK mainland. North sea would be possible, would swamp parts a little of the north, but any one that is detonated would result in harm to many other coasts at the same time, leading to the USSR getting a few replies sent to them space express.

[Bored chemist]

Is this even remotely practical?

It would be mad.
https://en.wikipedia.org/wiki/Mutual_assured_destruction

[alancalverd]

Whilst much of the east of England can be covered by a high tide and a northeasterly storm (February 1953), you'd need a wave up to 3000 ft high and capable of travelling at least 100 miles to ensure that the rest of the country even got wet. Little point in dropping the bomb in the North Sea where most of the energy would flood the Netherlands (who are quite used to it) nor the Irish Sea, where it would do more damage to the flat east coast of neutral Ireland before it made much impact on Wales.

[paul cotter]

Not possible with a single bomb, in my humble opinion. There was discussion in the early days of nuclear weapons of a thousand megaton device but it was realised that the fireball would extend beyond the atmosphere wasting a huge amount destructive energy. To produce that degree of tsunami I would guess many thousands of megatons in multiple devices which not only sterilise the british isles, but the whole planet. Silly infantile scaremongering.

[evan_au]

This was attempted at Bikini Atoll in the Pacific, by the Baker test, with an underwater blast of 23 kilotons.
- At 6km range, it produced a tsunami of 5m in height.
- It was apparently not the blast itself that caused the tsunami, but the rush of water to refill the vaporized/displaced void
- It did not manage to sink many ships, but did cover them with so much radioactive fallout that they could not be used for later target practice
- This test was conducted in fairly shallow water (a tropical atoll), so it might be a bit different than open ocean.

A back-of-the-envelope calculation:
- Presumably, a modern repeat would use a much more powerful bomb, say 23 Megatons (1000x explosive yield).
- The volume of displaced/vaporized water would be about 1000x larger
- The wave energy is proportional to the height squared.
- The tsunami might have a height of 150m at a range of 6km - certainly enough to destroy any coastal city, and severely damage a city on a river.
- The energy in the tsunami wave reduces in inverse proportion to the distance from the point source (ignoring focusing/interference effects), so perhaps 75m height at 100km.

This would certainly destroy nearby shipping, and damage coastal cities, but would not cover Ben Nevis at 1300m.
- The long-term radioactive pollution may be as bad as the short-term tsunami damage
- And the retaliation would only make it worse...

See: https://en.wikipedia.org/wiki/Operation_Crossroads#Sequence_of_blast_events

[alancalverd]

Which does raise the question: What, exactly, has the "nuclear deterrent" deterred? The Soviet Union was an ally of the western powers until they acquired nuclear weapons, since when there have been wars all over the world, mostly with little point and no desirable outcome.

Anyway, developing Evan's theme, there are plenty of 10 MT warheads available. The North Sea is quite shallow, and the advantage of an underwater explosion is that you can arrange sequential detonation to produce whatever focussing effect you want, in exchange for the destruction of all major Russian cities by a more conventional airburst. Hence the invasion of Ukraine - nobody dares to oppose any other nuclear power, but the lesson of the abject failure of anyone to occupy Afghanistan since 1842 is of no consequence to an unpopular dictator.

[Bored chemist]

OK, imagine you wanted to drop a few inches of water onto Manchester. (not an unreasonable goal :-)  )

That's not going to destroy the city but we have numbers for it.
"In a typical thunderstorm, approximately 5×10⁸ kg of water vapor are lifted, and the amount of energy released when this condenses is 10¹⁵ joules."
From
https://en.wikipedia.org/wiki/Thunderstorm

That's of the order of a megaton blast.
And Manchester survives those regularly (Thunderstorms, rather than bombs).
Flooding the whole country using weapons just isn't energetically plausible.
The Russians are lying- but we knew that.

What, exactly, has the "nuclear deterrent" deterred?

The use of nukes- that's the point.

[Petrochemicals]

https://en.m.wikipedia.org/wiki/Wave_power#Wave_power_formula

The 2004 Indian ocean tsunami released 24 megatons worth of energy.
https://en.m.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami#Energy_released

To flood somewhere with a wave you require the wave amplitude and length to be sufficient to achieve it in one fell swoop, otherwise the wave withdraws. Given tsunamis generally flood with sea level rise there wavelength you would need to move enough water to literally flood the land, there is not enough water in the North Sea to achieve this entirely to a height of 100m. From the Atlantic side in a circular blast radius raising roughly 500,000 km2 of water averaging height of 50m 

[wolfekeeper (OP)]

I mean it's basically a tsunami bomb:

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

Which does raise the question: What, exactly, has the "nuclear deterrent" deterred? The Soviet Union was an ally of the western powers until they acquired nuclear weapons, since when there have been wars all over the world, mostly with little point and no desirable outcome.

I know what it didn't deter. It didn't deter Russia from invading Ukraine. There was no deterrent, after Ukraine agreed to give up the nuclear weapons they actually had to Russia, shortly after the USSR fell.

[evan_au]

The 2004 Indian ocean tsunami released 24 megatons worth of energy.

During the 2004 Indian ocean tsunami, the Sunda Trench displaced water by 15m over a length of 1,300 km, displacing 30 cubic kilometers of water.
This produced a line source parallel to the coast, which doesn't attenuate much with distance, and drove a lot of water onto land.

A single explosion is a point source, which produces circular waves spreading out in all directions, and the energy reduces linearly with distance from the source. That is why the New Zealand study talked about a line of charges, to produce something like a linear source.

the total energy of the tsunami waves was equivalent to about 5 megatons of TNT (21 PJ), which is more than twice the total explosive energy used during all of World War II (including the two atomic bombs) but still a couple of orders of magnitude less than the energy released in the earthquake itself.

So the energy coupling from the earthquake into tsunami is something like 1%.
- I imagine that the energy coupling from an atomic bomb into tsunami would also be fairly low
https://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami#Tectonic_plates

[Petrochemicals]

The 2004 Indian ocean tsunami released 24 megatons worth of energy.

During the 2004 Indian ocean tsunami, the Sunda Trench displaced water by 15m over a length of 1,300 km, displacing 30 cubic kilometers of water.
This produced a line source parallel to the coast, which doesn't attenuate much with distance, and drove a lot of water onto land.

A single explosion is a point source, which produces circular waves spreading out in all directions, and the energy reduces linearly with distance from the source. That is why the New Zealand study talked about a line of charges, to produce something like a linear source.

I imagine so, but a straight line would also create waves in the opposite direction and will eventually round out. I imagine it is going to be somewhere in the region of 2diameter rather than PIdiameter ammount of water moved. Do it too close and the wave will run out of water.

the total energy of the tsunami waves was equivalent to about 5 megatons of TNT (21 PJ), which is more than twice the total explosive energy used during all of World War II (including the two atomic bombs) but still a couple of orders of magnitude less than the energy released in the earthquake itself.

So the energy coupling from the earthquake into tsunami is something like 1%.
- I imagine that the energy coupling from an atomic bomb into tsunami would also be fairly low
https://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami#Tectonic_plates

Total energy involved in the quake was 400,000 times the energy released, which in turn as you quote is 5 times the energy in the waves. As a generalisation you would assume a bomb would have a bout the same wave energy conversion.

Interestingly to raise 500,000km2 of  water 50m is about twice the energy in all of the nuclear weapons on earth

[Bored chemist]

So... we have conclusively proved that the Russian State media are still telling lies- just like Pravda and Tass did in the 80s.

[Petrochemicals]

The 2004 Indian ocean tsunami released 24 megatons worth of energy.

During the 2004 Indian ocean tsunami, the Sunda Trench displaced water by 15m over a length of 1,300 km, displacing 30 cubic kilometers of water.
This produced a line source parallel to the coast, which doesn't attenuate much with distance, and drove a lot of water onto land.

A single explosion is a point source, which produces circular waves spreading out in all directions, and the energy reduces linearly with distance from the source. That is why the New Zealand study talked about a line of charges, to produce something like a linear source.

I imagine so, but a straight line would also create waves in the opposite direction and will eventually round out. I imagine it is going to be somewhere in the region of 2diameter rather than PIdiameter ammount of water moved. Do it too close and the wave will run out of water.

the total energy of the tsunami waves was equivalent to about 5 megatons of TNT (21 PJ), which is more than twice the total explosive energy used during all of World War II (including the two atomic bombs) but still a couple of orders of magnitude less than the energy released in the earthquake itself.

So the energy coupling from the earthquake into tsunami is something like 1%.
- I imagine that the energy coupling from an atomic bomb into tsunami would also be fairly low
https://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami#Tectonic_plates

Total energy involved in the quake was 400,000 times the energy released, which in turn as you quote is 5 times the energy in the waves. As a generalisation you would assume a bomb would have a bout the same wave energy conversion.

Interestingly to raise 500,000km2 of  water 50m is about twice the energy in all of the nuclear weapons on earth

I suppose if we have a 20 percent efficiency in water energy transfer, a 1% in the individual waves radiating out from the source and the world's nuclear arsenal only being 50 percent of the energy required in each wave it would take something like 1000 times the world's nuclear arsenal to flood britian