How to build a tsunami proof island city?

Question

Igbi Island, a remote island located in the middle of the South Pacific Ocean, is notorious for its extremely powerful megatsunamis, huge, deadly waves averaging 500 feet (≈ 150m), but sometimes reaching astonishing of 2000 feet (≈ 600m) that occur every week. Scientists have yet to explain this phenomena.

Recently, a sensational discovery was made. Scientists found vast deposits of diamond and gold and other precious metals and stones at Igbi Island using science-y stuff. This has caused tycoons to go crazy trying to find a way to safely live on Igbi Island, so they can mine all of the precious metals and stones.

Assuming that you have unlimited money (the precious stones and metals provide an endless amount of profit), modern day technology, and three tsunami-free months to build, how could you tsunami proof Igbi Island, so that people could live there?

Igbi Island is circular and doesn't have any mountain ranges. There are a few small lakes and ponds on the island and one large river that splits the entire island in half. It has a diameter of 6 mile diameter, and a land area of roughly 28 miles². There will be skyscrapers, but none will be taller than 500 feet.

I was thinking that it would be near here. The closest (owned) islands are the Pitcairn Islands, but those are still hundreds of miles away.

Answer 1

Going with waves like @pipperchip: boats don't notice the tsunami, and even probes deployed on buoys for the purpose have a very hard time detecting it. Why would an island be bothered? Because of the rise of the seafloor leading up to the visible part. Islands that are too steep will suffer landslides until the slope is sufficient to hold.

Make yours different. Diamond and island in the middle of nowhere points to volcanic origin. Diamond is found in "kimberlite pipes". Now think of another volcanic formation known on land: the butte. If the island is a subsurface butte, there is no rise just a sudden post. The tsunami will not form a wave since there is no shore slope.

Suppose that it is a butte a bit of the way down, and your preparation is to blast off surrounding material to make it post-like sufficiently deep to be invisible to the tsunami passing. That can involve shoring up the rock face to make it stable, and adding a little technobabble: the mound that remains (say, 2 miles down) is reverse-contoured to make a moat-like ring. Waves feeling the bottom will also interact with the shape and cancel out the dangerous rise in amplitude at the spot you are protecting. Cool if it caused sudden waves in complex shapes to appear just past the island after it passes.

As for the setting: how can regular tsunamis be a mystery? A subsea earthquake is quite conspicuous, and whatever it is will have a big bullseye showing you where to look. They are global phenomena, not just something that pops up like a spring shower. They will he hitting costland, not just vanishing after leaving the area. Your back story needs work.

Also, if they are currently a problem for island settlement, isn't it swept bare and eroded off the face of the map? Why do the lakes and rivers stay stable features? How can a river cut a 6 mile island in half, without a mountain in the middle? You do know how rivers work, right?

I think the idea of making the island tsunami-proof is interesting. But the rest of the setup does not hold water.

Answer 2

Dig into the island like underground bunkers.

The island somehow survived these extinction-style events once a week long enough to become an island, it's probably already in the correct shape to handle the water.

The hardest part: you'll probably need some very impressive doors. While a megatsunami is going over your head, there's 2000ft of water overhead. That's a lot of pressure!

I hear they make some really dynamite doors out of unobtanium. Real sturdy. Can we acquire some of that with our untold millions?

Answer 3

3 Months? Tsunami Proof city? One idea is a bit like answering "mu" to a question, but here are some solutions:

1. Make your city a series of floating boats. The Tsunami waves actually do little damage to boats at sea, they are just large waves which boats can ride. If you have a tsunami warning system, you get everyone to the boats, push off, and you have no effective damage when your city re-connects with itself. You have enough money to make everyone live on a boat, so why not? (It should be noted that there would be a recommended distance from the island for people to go to avoid the waves. This depends on local geography, specifically local ocean floor topography.)
2. Tsunami barriers can be made. Once again, 1-3 months is a tight deadline, but you could make walls to help deter the force of the tsunami. It may take longer than 3 months to tsunami proof the entire island, but it can be done. You can even take advantage of natural coral reefs to make this process easier, assuming that those corals exist.

Answer 4

Simply build large submarine(s) that rest on the island like houses. When it's safe they can act just like a house, when the water comes you have an early warning system so everyone can get inside and ride out the waves.

You can either tether them to the ground or if you are adventurous let them float for a bit of excitement.

The subs would need very thick metal (Titanium?) hulls to cope with the battering of debris in the waves and you'd need a lot of workers to buff out the dents afterwards.

The subs could be built somewhere safer and then sailed/towed to the island.

Answer 5

I think you need to consider that this island is not isolated on an uninhabited planet. It's in the Pacific. Seismic waves don't evaporate after passing your island. That means these mega-tsunamis are sweeping all the Pacific coasts, because their energies, while reducing over time, are so stupendous to begin with that they will not disappear before hitting New Zealand, Australia, all of Polynesia, the Philippines, Antarctica, Peru, &c. with destructive force.

They are likely to still be capable of damage when they reach California, Hawaii, and Japan. Hell, the big ones will wipe out harbors in Alaska.

Weekly.

They are going to have to be investigated and explained, because they are too constantly too dangerous to major developed countries. They can't possibly have gone unnoticed this long. Tsunami aren't localized phenomena: when an earthquake happens in Peru or Japan, Honolulu goes on tsunami watch (the Hilo tsunami from the Alaska quake left local civil defence permanently tsunamiphobic). We get them from all angles. Regular tsunami are often discernible here. Mega tsunami? Our local marine research centers would have noticed them first. I used to work at one, and the constant job of the research vessels is placing, replacing, and re-battery-packing tsunami monster buoys.

For all that, the floating city that backs away when it's time for the tsunami sounds good. When the tsunami warnings go up, people here run down to the harbors to take their boats out. But check how far out they have to go.

Skyscrapers of any sort are a pretty crazy idea. They're energy intensive and everything has to be imported to this uninhabitable rock.

It can't provide fresh water, even. Your river and lakes just aren't possible. An island, to have fresh water, needs mountains to intercept rain and enough aquifer above the reach of salt water to hold ground water. Everything else is rain runoff from the last storm, on rock that has been swept clear of life (and soil) on a weekly basis.

If the three-month clear season is when the endangered sea-mammals come to the island to breed, Greenpeace is going to be in the way. ;-)

Answer 6

I think you need to reconsider the tsunami size that you've stated here. Remember the 2004 tsunami that crippled south east Asia? It was really only 10 meters high...it killed over 220 thousand people at this size and effected much of indonesia and south east asia (not isolated to a single island and this wasn't even a megatsunami, megatsunami's are ocean wide events). The height of a tsunami isn't the frightening portion, it's the volume of water. To get the size of wave you are suggesting here, you need a pretty large event...30 million tons of land dumping into the ocean in one cataclysmic event should get you a wave about half the size of the small end you are talking about here...the 'astonishing 600m' wave you suggest could really only be created via an asteroid impact, and much more than your island will feel the effects of that. Subterranean earthquakes will not cause what you are asking for here.

Second note is your island is far too tiny to support lakes or river ways. Rivers have to come from somewhere. The large island of Hawaii (called the island of Hawaii) is around 10,000 km². At this size (with a volcano in the center that supports a snow cap) it is barely able to support a river and most waterways are what we'd consider to be streams. Your island would likely have no fresh water and any standing body of water is likely salt water not fresh (mostly because your tsunamis here have flooded it with salt water).

And third...the size and rate of these tsunamis would literally wash your island away. The size of this wave would completely submerge your island here and repeated hits would literally rub the island off the map and scatter it across the ocean floor. Actually, it'd probably do the same to 95% of indonesian islands, archipelago islands wouldn't last long like this.

All that aside...lets try an answer.

There is no way of resisting this wave. A wall would need to be several KM high and capable of resisting millions of millions of tons of pressure with each wave. This means any form of 'tsunami' proofing this island is going to be from the standpoint of riding out the wave and not resisting it. Submarine and boat answers have been given (I doubt the boat would quite work...a 600 meter wave would begin shaping as a wave when the sea floor is 600m deep, so unless you have a massive incline of this island to the floor, most likely it'd present as a wave long before any boat could get by it). As a last resort, I think you would be better off to maintain a large fleet of helicopters capable of lifting structures off the floor and simply fly over the wave, watch it go by, and reland the buildings (hoping the debris left is flat enough to land the buildings on). Also of note, mining operations would be completely flooded if they weren't properly sealed.

If you drastically reduce the size and scale of your extinction level event tsunami described here, then there are a few other potential techniques that might work. If these tsunami's are coming from a consistent direction, it is feasible to build a sea wall in a triangle shape that diverts the flow of the tsunami around the island.

Answer 7

Gallons per cubic meter: 264
Wave height: 600M
264 * 600 = 158400 gallons per square meter of water column

Inches per square meter: 1550
158400 / 1550 = 102 gallons per square inch of water column
Water weight: 8lbs per gallon

102 * 8 = 816 psi static load.

Typical Concrete is 2000 to 4000 psi.

A tsunami applies load in multiple directions during impact, so the total live load on the structure would probably exceed the static load of 816 psi by a significant margin, but it might be doable.

The surface of the island would be completely flat, whether it started out that way or not.

The underground structure would look a lot like roman catacombs, or subway tunnels or a beehive perhaps. The rooms would be small, and the arches utterly massive.

The impact would probably be violent enough to feel like an earthquake, with massive air pressure fluctuations during the event. Even inside of concrete you're pretty much inside a drum that is getting pounded from above.

Some kind of air pressure damping system would be required, which being subject to seawater corrosion, would be very important to keep working. The floors might be suspended on shock absorbers like they do in nuclear fallout bunkers. Of course concrete has a tendency to crack, so trickles of water would be flowing in. It would be an exceedingly unpleasant experience. A lot like the movie "Das Boot" I imagine.

My best guess.

Answer 8

Honestly the only way you are going to avoid getting destroyed by a tsunami that is 150-600m tall, is to get out of the way of the tsunami. I suggest some ways at the end.

You are going to be having millions of tons of water passing over your island routinely. No matter how you shape your island or try to redirect the water you cannot actually stop the tsunami from passing over your island, if the island has a fixed location like all modern day islands.

The key point to remember is that a tsunami isn't just a wave, it's a rise in water level. Behind the 150m high tsunami, is just more water that is that high. So it may take minutes or hours for the entire tsunami to pass. No matter how you try to redirect water, that tsunami is going exert extreme blunt force against whatever you have constructed, and destroy it.

If you make a diamond shaped 600m tall wall, you don't know what angle the next tsunami will come from. Maybe the next one hits flat on the square side of the diamond and boom it is now gone. Even a cylinder protective wall will likely get toppled, or possibly crushed from the high pressure of being 600m under water. Plus if you have a wall that high you don't have much of an island anymore.

So you really only have 3 options.

1. There is a closable dome over your island/city and the populations aims to let the tsunami pass over. Basically you have an underwater city one time a week.

2. Your island is (literally) a floating island. Its not really an island so much as a large boat. The boat could be anchored to an actual island, or it could just be a giant boat with enough dirt on it to sustain farming, etc.

3. This is a flying island. Stay away from the tsunami.

Answer 9

Place several rows of very large underwater airbags in the path of the MEGA-TSUNAMI. The amount or water that those airbags displace must be on the order of magnitude but larger than the volume of water in the MEGA-TSUNAMI. This should be done in deep water before the slope which causes the formation of the MEGA-TSUNAMI. Release the air at the right time to create a void which can be filled by the MEGA-TSUNAMI. This would work much like noise canceling headphones by causing destructive interference between the anti-(mega) tsunami made by rapidly evacuating the airbags and the MEGA-TSUNAMI. The subsequent rows of airbags would be to dampen the smaller tsunamis made by imperfect cancellation. As soon as the MEGA-TSUNAMI is eliminated, the airbags could be refilled with air and used again. I primarily answered this so I could say MEGA-TSUNAMI over and over again...

Answer 10

Don't go on the island, use submarines and sieve through all the subsurface sediment surrounding the island (bonus since it is already broken up for you). Stay far below wave base and let the tsunami do the work of stripmining the island for you. If mega-tsunami are that regular of an event the island won't be around for long anyway. The island should be surrounded by masses of submarine sediment hundreds of times is size.