[mikekchar]

The height of the tsunami depends on the area. In the area in question, for a very long time people thought that it was impossible for the a tsunami to go above 5 meters. That there were tsunami of that size in other areas is irrelevant. The earthquake in question was the most powerful in 1000 years. Looking back 100 years would be insufficient.

What is interesting is that there was actually a city that raised the sea walls to 16 meters in the area. The mayor of that city was actually sent to jail because it was a scam to siphon money to his brother in law who was in construction. I don't remember the details because it was on a TV program that I watched about a year ago, but unfortunately the mayor in question died a couple of years before the tsunami and never lived to see that his actions actually saved the city.

Speaking of the seawall height, there were, indeed, studies done that showed that under certain circumstances 15-18 meter high tsunamis could be generated in that area. This was disregarded. "Why", you ask.

Let me explain. I live by the sea in Japan. I can go to the sea side and climb up on the sea wall. It is 5 meters above sea level. I can walk along it all practically all the way to Tokyo (200 km away). The only places where it doesn't exist is where there are natural cliffs (which are not as prevalent as you might imagine). Every little stream that empties into the ocean has a 5 meter high metal sea gate that can be closed in the event of a tsunami. Where I live there is one at least every km or so.

The sea walls in Japan are already a miracle of engineering. You want to make them 20m high? Around the entire country? This is an insane notion.

To be honest, I will take the risk. I don't want to live in a jail where the only place I can see the ocean is from on top of a mountain.

I live in Sagara, Shizuoka (now merged with Makinohara). It is the most dangerous place in Japan to live for earthquakes because of the imminent Tokai earthquake (50 years overdue, unless it actually happened in WWII when the place was already destroyed and nobody had the ability to measure the big earthquake that happened then). 5% of the population is likely to die by all predictions.

Which means 95% will live. Terrible tragedy, but not worse than surrounding yourself in a 20m high jail IMHO.

Now, there were many human failures regarding the Fukushima disaster (like not having the correct connectors to power the pumps externally). Sea walls is not one of them.

[Kankuro]

I guess he was talking about higher walls around the (few) nuclear plants, not the whole country. That would not make the country "a jail where the only place I can see the ocean is from on top of a mountain".

[mikekchar]

The water will just go around the wall. I think people don't quite understand the logistics involved. You have this massive wave travelling at 300 km/h. The seawall will break up the wave, but you have to have it long enough to prevent the water from reaching the bits that you want to protect before it retreats (often 20-30 minutes later).

Do a google search for "tetrapod japan images" to see what the beaches around here already look like. The Japanese government is not trying to avoid money on reducing the damage of tsunami. It seems like there should be a simple solution, but there isn't.

[gknoy]

Thank you for suggesting the image search -- I had been imagining a castle-like wall, and seeing the stacked tetrapods [0] was really eye-opening. (Apparently it helps dissipate the wave energy.)

0: https://en.wikipedia.org/wiki/Tetrapod_(structure)

[zurn]

But you could easily build the plant on naturally or artificially higher ground.

[mikekchar]

However, you have to ask yourself, "Why are virtually all the nuclear power plants in Japan next to the sea?" You could answer that with "Because engineers are stupid" or you could look for a more likely answer ;-)

I am not a nuclear engineer and I don't have enough background to really say what the answer is. My guess is that they are set up right next to the sea so that they can do exactly what they did in Fukushima -- pump sea water into the reactor.

Naturally higher ground is actually hard to find near the sea (because the mountains are usually set back by about a kilometre from the sea). Also, if you perch yourself on a cliff next to the sea, then you are at a more serious risk of landslides during the many 6-7 magnitude earthquakes we have in Japan each year. With artificially higher ground, I suspect you would be at even more risk.

The other main reason for not putting a nuclear reactor on high ground is that in the event of a containment breach, the contaminated water will run downhill (in indescriminate directions). The placement in low ground next to the ocean may be the best place, environmentally, in the case of a disaster.

Again, I'm only guessing, but I'm sure if you ask someone who is trained in the field they can give you better answers.

I'm hoping that Japan will transition away from nuclear reactors in the middle term. There is enough geothermal potential to provide base load (though protecting the environment with all the earthquakes we have here is not trivial in that case either). Since Fukushima, the amount of solar panels being installed in my area is insane, so I'm hopeful that things will improve over the next 50 years or so.

(Having said all that, Hamaoka power plant, which is just down the road from me, is perched up on top of a cliff overlooking the sea ;-) ).

[losvedir]

Why can't you just build the wall to encircle the power plant? Why does it have to span the whole island?

[mikekchar]

Not an expert in the matter, but I'm guessing the pressure on such a wall would be too great. One of the things that caused the death of many people in the big tsunami after the Tokoku earthquake was the assumption that if you just went up to the roof of your apartment building you would be safe. Unfortunately, the wave just wiped out everything. Huge concrete buildings were just flattened.

To be honest, I have often wondered how the powerplant survived at all. I guess it was high enough up and that only the generators got clobbered, but I haven't looked into it in detail.

[mackwic]

Thanks for your reply. I now think I don't understand the logistic involved and will look further.