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In what ways could an island be protected from tsunamis, given the following constraints?:

  • It lies in the middle of the ocean, at least several hundred kilometers from the nearest continent.
  • It does not rise up from a continental shelf - the surrounding ocean is at least a couple of kilometers deep.
  • It is at most ~25km wide in the longest dimension.
  • It has "line of sight" (along the surface of the planet) to a fault line, subduction zone, anything that might result in a tsunami.

(See Easter Island for a real-life example that satisfies these rules.)

Possible parameters to consider include the shape of the island, the slope of the insular shelf, the island's topography, etc.

So far, I've been able to come up with the "obvious" solution of having the island have high cliffs all around the coast, but I'm curious what other solutions there are that I haven't considered.

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  • $\begingroup$ I don’t have time to do much research on it, but my understanding is that Hawai'i is pretty tsunami-resistant, thanks to the massive buffers provided by Kilauea and Mauna Loa and the angles that tsunamis come from. The island of Hawai'i is far larger than the island you’re suggesting, but the idea still might be workable. $\endgroup$
    – KRyan
    Mar 22 '17 at 20:35
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You have the right idea, but misplaced. All that is required is that the island be the peak of a seamount which rises steeply from the ocean bottom. Tsunamis in open ocean have little amplitude (ships hardly notice them), but extremely long wavelengths. When these approach a shallow, sloping coast the water in the fromt of the wave starts to slow down, and the rear part catches up, the velocity of the wave increases and water starts piling up into a much higher, shorter wave. Coasts with a steep underwater approach simply don't get the "tidal wave" effect.

Another way to look at it is that, since a shallow approach produces a short, high, fast wave, the wave has no chance to move laterally around an obstacle, and tries to go over it. A longer, slower wave has the time to be deflected around an obstacle without much amplitude increase.

EDIT - In comment, the question was asked, "what do you mean by "the velocity of the wave increases", and a response will be too long for a comment, so I add it here. The short version is that I misspoke - the velocity does not increase.

As the wave propagates into shallower water, it loses velocity (proportional to the square root of water depth), and gets higher.

As its height increases, though, at some point the wave shape changes from being part of a wave system to that of a solitary wave - basically, once the wave trough touches the sea bed the behavior of the wave changes radically. I misspoke in saying that the wave speeds up. It does not. What I meant was that the higher the wave, the faster it propagates once it becomes a solitary wave. It may help to consider that a wave is in constant collapse, as the weight of the total wave causes the lower sections of the wave to squirt forward. The higher the wave the greater the pressure and the faster the base of the wave moves - and the dynamics of the wave as a whole keep the upper section in place, although the actual water molecules are constantly recycling their positions within the wave. And, of course, at this point the wave will begin dissipating as it travels inland.

If you ever watch one of those documentaries about the crazies who surf the Condition Black (40 and 50 foot) waves in Hawaii, you'll notice that all those surfers get carried out on jet skis and start their runs behind the jet skis. That is, they don't get dropped off, stationary, at the base of the wave. There's a good reason for this. If they are stationary in the water, the wave moves so fast they they can't accelerate fast enough to keep up with the surfable portion of the wave face, and will either fall down the vertical portion or get left behind by the wave, depending on the exact shape of the wave. Mostly, they fall down the face and get driven under.

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    $\begingroup$ I had thought about seamounts, but I came across reports of tsunamis affecting both Hawaii and Easter Island. However, Hawaii is much bigger than what I'm thinking of, and now that I look at it, the slope around Easter Island is not as sharp as I had assumed. Though as I understand it, the wave slows down as the water gets shallower, what do you mean by "the velocity of the wave increases"? $\endgroup$
    – andrej88
    Mar 22 '17 at 1:09
  • $\begingroup$ @AndrejPetrovic - See edit. My bad. $\endgroup$
    – WhatRoughBeast
    Mar 22 '17 at 3:15
  • $\begingroup$ "[...] into a much higher, shorter wave." "[...] produces a short, high, fast wave [...]" Are we to understand these statements as a measurement of length vs height (such as shorter/longer vs lower/higher)? The consecutive use of higher to shorter and short to high has me slightly confused. $\endgroup$
    – OhBeWise
    Mar 22 '17 at 18:26
  • $\begingroup$ It seems line a long-thin island, like a needle pointing into the wave, would be ideal. Maybe the slope of the needle extends all the way down to the faultline to start the wave diverting as it is formed? $\endgroup$
    – Daron
    Mar 23 '17 at 13:59
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I live on an island where we have had a couple of tsunami. The parts of the island least affected (on the side it was hit) were the ones with steep dropoff into the sea. A gradual rise like a tropical beach is what makes tsunamis so dangerous. A boat in mid ocean might not even notice a tsunami.

So natural defence is steep dropoff without sloping beaches. Don't rely on reefs, those get destroyed and they don't recover within a human lifetime. But tsunamis main danger is an uneducated populace living in the prime habitat of the lowlands and beaches.

If your island has a history of tsunami caused by a fault line, it's only going to get hit hard from one side. The reef if any on that side would be long gone, the land itself would be shaped by successive tsunamis, and you may have a situation like some of the Marquesas where all the beaches are on one side and the other side is mostly sheerish cliffs with no reefs.

So you only need cliffs on the affected side, fault lines don't move around arbitrarily, all the tsunamis will be coming from the same direction.

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Mangrove forests.

Video of a model in action: https://www.youtube.com/watch?v=ljLAfU132sw

https://www.newscientist.com/article/mg22430005-200-mangrove-forest-planted-as-tsunami-shield/ They break up the waves, the deeper the row of trees the better. It's quite simple really. It will make that part of the coast hard to use for fishing and genera; boat usage.

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  • $\begingroup$ That article is fairly convincing. Sounds like they don't "stop" the tsunami, so much as slow and depower it. Some quotes would help. $\endgroup$
    – DCShannon
    Mar 23 '17 at 0:02
  • $\begingroup$ Last time I put down quotes I got scolded because they amounted to 50% of my post. Don't think I can add any menaingful ones without doing that. $\endgroup$
    – Mormacil
    Mar 23 '17 at 9:12
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    $\begingroup$ Also very related, wetlands in general can rob a tsunami of momentum leading to less destructive outcomes. Additionally, they would recover much quicker than reefs. I would think slowing and decreasing the destructive force of a tsunami would be stopping it. ncbi.nlm.nih.gov/pmc/articles/PMC3594144 $\endgroup$
    – RomaH
    Mar 23 '17 at 17:48
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What about a barrier reef?

enter image description here

For example, the island in the center of this atoll (Bora Bora) is about 1/5 the size of Easter, and it is going to be much less susceptible to waves of any sort. I'm sure an island the size of Easter could have such a barrier reef as well.

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    $\begingroup$ Do you have any evidence that the wave won't just destroy the reef? $\endgroup$
    – Pureferret
    Mar 22 '17 at 8:33
  • $\begingroup$ @Pureferret it might, depending on the size of the wave. Or the surge may pump enough water into the lagoon to cause flooding on the island that way. $\endgroup$
    – jwenting
    Mar 22 '17 at 8:54
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    $\begingroup$ @Kilisi Is that a fact? Didn't happen in the 2004 tsunami. $\endgroup$
    – kingledion
    Mar 22 '17 at 11:55
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    $\begingroup$ @kingledion that was lucky for them, we lost a lot of reef here. If their tsunami was a bit smaller it would have been worse. And if they have another tsunami it will get worse. $\endgroup$
    – Kilisi
    Mar 22 '17 at 12:01
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    $\begingroup$ I'm not sure the reef, of itself, will do anything at all. If the island has a shallow sloping seabed leading up to it then the tsunami will still pile up dangerously high, and just wash right over and a small wall near the coast. $\endgroup$
    – Grimm The Opiner
    Mar 22 '17 at 12:55
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Bio-barrier

Stack a series of large rocks inland from the beach, and add plantings like mangroves, palmettos, and other beach plantings to fill in amongst the rocks. This does not prevent flooding, but it does calm the wave system when it comes rushing in. You can see these rocks in action about a little after a minute into this film.

The surge of a major tsunami can get up to 30m height, but the level raise is usually about only 3m. Your bio-barrier can prevent the majority of the damage caused by surge, and your protected buildings can be naturally on a rise 3m or more uphill from the beach, or on solid stilts.

Education

The death toll difference between the 2004 and 2011 tsunamis was likely to do with missing infrastructure in the former, but also due to citizens of Japan being more aware both culturally and formally by education: there was an earthquake, something's wrong, get to high ground.

In your question, we can't provide artificial infrastructure, so I would say that in addition to your bio-wall, you can provide education.

Geography

If you know in general which direction your tsunami is approaching from, make sure you do not develop on the beaches of the opposite side of the island. On smaller islands, the wrap-around effect (refraction) makes the back side of the island much more susceptible (but of course the front gets slammed, too).

Prioritize residential units on higher ground - if you have to prioritize. If the tsunami occurs during the night, people will likely be sleeping, so they may have no clue that it's coming. So, if you have to have buildings near the water, use the types of buildings that will be occupied during the day (commercial units) where people are awake and can be better informed to get to high ground.

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A breakwater at sea

A 'barrier reef' grows out of the continental shelf and would be destroyed by successive waves. Let's go one step bigger: Half way between the faultline and the island there is a chain of underwater mountains (pretty much other islands that don't quite reach the surface) that rises from the seabed for several kilometes.

This causes any incoming tidal waves to break over the range rather than the island itself. For story purposes the breaks should be visible from the inhabited island -- though the islanders might not know exactly what they are seeing. The energy from the broken wave then has time to be reabsorbed into the deep water before it gets to the island.

Problem: I don't quite understand where the energy from the tsunami 'goes' after it breaks over the mountain range. Why would it hit the island with any less energy than it left the range? Would it really be reabsorbed into deeper water?

Another problem: Tidal waves have MASSIVE wavelengths when traveling over deep water.. They can stretch into the hundreds of kilometres. Boats might not notice them. 'line of sight' is about 5km at ground level. I can't imagine 5km is enough distance for a tsunami to properly form in the first place.

A Solution: Okay the faultline is not visible from groundlevel, but it might be visible from the island's highest mountain. Still at 500m the horizon is still less than 80km away. . .

Another Solution: Since they cannot form properly in the space provided, these are 'not normal tsunamis'. They do not have to behave like normal tsunamis and 'realistic' countermeasures do not apply.

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  • $\begingroup$ I was going to suggest something like this. The energy doesn't disappear, it just loses it focus. Instead of a unified wave, parts of it are now out of sync with others and they drag against each other. Given a proper "pattern" of subsurface seamounts, the island itself may be in a calm of overlapping waves. $\endgroup$
    – Michael Richardson
    Mar 23 '17 at 17:09
  • $\begingroup$ The wave still has forward momentum though. I don't see that going anywhere after the breakwater. Maybe if it somehow 'stretched' itself wider the island wouldn't experience such a rise in water level when the wave passes. . . . $\endgroup$
    – Daron
    Mar 23 '17 at 17:56
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    $\begingroup$ By "line of sight", I meant that there are no significant landmasses between the island and the source of the tsunami. I don't know if there's a word for "line of sight" where "line" refers to the a great circle, as opposed to a nice straight Euclidean line. $\endgroup$
    – andrej88
    Mar 25 '17 at 1:48
  • $\begingroup$ The barrier doesn't need to breach the surface. It only needs to be as tall as a continental shelf -- you could sail right over it without noticing. So this should still work depending on what you count as significant landmasses. $\endgroup$
    – Daron
    Mar 25 '17 at 10:44
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Lowering the possibilities of a Tsunami : would be pretty hard, if not outright impossible, but if you could destroy a part of the ocean around you, make the depth there lower and so on ( it is impossible )

Another impossible way is to : stop earthquakes, hence stopping tsunamis. Problems ? Countless... Earthquakes are created by the formation of new land, explosions in the mantle ( if I remember well ), the tectonic plates moving and crashing against each other, huge explosions on a scale of an atomic bomb...

Hmmm what other ways have other people not mentioned... Oh yeah, if you have let's say limitless resources, a more available way would be to build a dam or something like that, as defensive wall against tsunamis ( also impossible, since tsunamis carry the force of an atomic bomb and I suppose we don't have much to go through after that. )

And lastly making an underground city, not affected by tsunamis, something like a secret base and sooo on... Problems ? Yes, the earthquakes would be a lot more severe in this case.

Welp... that's it from me.. not anything useful, but nice ideas... I think.

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