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Comment by lukan | Hacker News Reader

3 m sound not too bad?

It can be a very long wave. If you have a coastal area that is mostly at or very close to sea level then that entire area could now be 3m underwater which is very bad.

Also, even under normal conditions the type of wave you would see at the beach with a height of 3m is insanely powerful. I think the highest I have body surfed in was maybe 2-2.5m and if you get caught in the break of the wave you are literally powerless and get dragged under water and spun around for quite some distance. It is a bizarre experience and doesn't really match the basic intuition of how powerful it should be.

My crude understanding and associated mental imagery is that tsunami waves are more like tide than "usual" waves: your typical 3m wave is mostly "surface-ish" in comparison, while 3m tsunami has both greater wavelength and "depth", which is what makes the "usual" wave break quickly when it reaches shore, whereas tsunami waves continue pushing real hard for real long because of all the length * undersurface water you don't see.

Gently shaking a glass of water sideways or blowing upon it won't make much water spill out, but jolt it vertically and most of it goes out real quick.

This is all probably very incorrect physics but it helps drive my intuitive understanding that 3m wave and 3m tsunami are quite different, the former being scary but the latter being muuuuch scarier.

Yep, where I live (rent) a 3m wave would fill my house with 1m of water. Even if it was one quick wave, that washed over without staying long enough to completely flood the inside of my house, I'd probably at least have to deal with a bunch of water damage.

I would be more worried about the force of the water hitting things and about objects it drags with it than about water damage.

One meter of water weighs 1000kg/m². If a wave a few meters long slams into a wall of your house, that can easily be more force than that of a decent sized car doing the same.

Our local offshore break that we grew up with as kids was, and still is, a total monster in storm (a long way back and out to sea) conditions:

https://www.youtube.com/watch?v=xjHaFOGBPzk&t=138s

https://www.surfertoday.com/surfing/the-right-the-monster-we...

It's a little bigger than 3m at the face and is a "slab" .. not a thin wave but one backed by a mass of water.

Somewhere in that video you might see a triple lip .. three curling breaks, one within another and another again.

I know 3 m waves are powerful, I was within them.

But they are normal on most coast and that they are bad on a flat beach I can imagine. But Taiwan is quite steep, so aside from some damaged boathouses, I cannot see the big danger (compared to the consequences of the earth quake). But apparently no Tsunami came.

Think of it as a 3m tall bulldozer the width of the country with effectively unlimited pushing power. Watch some videos of the Japan tsunami, it's basically an unstoppable force that gobbles up everything in it's path. Very uncool.

It's generally very bad, but it also depends on the shoreline. If your city is on the coast at sea level with no real barrier, it's devastating. If there's a big cliff and everything is at higher elevation, you can ignore it.

Unfortunately, people tend to build settlements at sea level.

We should set up some stone monuments to delineate the high water mark, for future generations.

Same but different, there's an old town near a lake that suffered a devastating flood way back in its history. The downtown area has markers on the buildings that show the high water mark for the flood.

The Japanese have those. A lot of the monuments are hidden in the forest in the hills above the coastal villages...

That's what parent poster was referring to. They have high tide markers showing where tsunamis have reached, and then in the past 50-100 years people started saying "Waves Of Unusual Size? I don't think they exist" and building closer to the shoreline.

Consider for instance this place

https://commons.wikimedia.org/wiki/Category:Diablo_Canyon_Po...

which is on top of a high bluff thus much more secure than the Fukushima NPP.

Unfortunately, that power plant is very close to two fault lines, and they didn't even know when they built it, so it's questionable how well it'll withstand an earthquake.

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

They increased the anticipated seismic loads and also reinforced the structure accordingly. I don’t think any NPP has been scrutinized to the extent that Diablo has been.

i'm always amazed how little appreciation people have for massive amount of water. a 3m wave will more than ruin your day. trust me, it's plenty

I lived in a coastal region where, every storm season, people would get swept out to sea and drown because they were watching storm waves from the beach. Sometimes they'd even go in the water, out on jetties or out on piers, without any appreciation for the sheer forces at work that push the water around. With enough force, even ankle to knee deep water can sweep you off your feet and suck you out to sea.

Sadly parents let their kids do this and some have lost their lives.

We had two tourists just get killed last week during storms. Both swept off harbor walls wanting to look at the waves. There are literally warning signs everywhere.

Yeah, it's usually tourists or people vacationing from their beach houses. It's sad when more than one person drowns this way, because it usually means the additional people that lost their lives were trying to rescue the others. Recently happened to a parent and their kid here.

Well, I was in the ocean saving a person getting caught by such a big wave on the beach, so I do appreciate the power of the water here. But I think 3 m waves are quite normal on most coast lines in rough sea.

So my question was, why would a Tsunami of the same height be that much worse. And in comparison to the destruction of the earthquake in general here.

A 3 m tsunami isn't like a 3 m wave from wind action. It is like a 3 m tide or storm surge added on top of whatever normal tide level you are at. So if it hits at low tide it might be somewhat negated, but if it hits at high tide it may overwhelm sea walls and other coastal infrastructure.

When people talked above about the long wavelength of a tsunami, they mean it doesn't crest and possibly break in seconds like a surface wave. It can keep building for many minutes and continue to flow inland after the crest.

Look for some tourist video of the 2004 tsunami that flowed over islands in Thailand. It looks more like a river or levee breach flooding over the whole field of view. In steeper coastal terrain, the wave can sometimes be funneled or focused into a narrower port or valley and then reach much higher elevations than you would expect.

Imagine a 10 foot wall of water flooding the coast that just doesn't stop. Smaller tsunamis have devastated coastal regions before. People, beach houses, etc are swept away under that force even though the height is rather low.

How tall are you?? Imagine standing at the shoreline and the water level suddenly being 3m higher...

It’s not really a wave, it is more of a tide that comes in quickly. So everything gets flooded to 3m above the normal water level, and stays that way for a while.

3 meters is significant if you live in an inundation zone. You can see the effect on a seaside community here: https://www.floodmap.net

3m is about 10 ft which is huge, though there are huger waves.

a 3-meter high wall of water is not really good either, due to kinetic energies, if no significant tsunami coastal protections, that water flow can reach far inland

This comment author was not correct, but this question prompted a lot of great answers and improved the quality of the discussion. Per HN rules, this comment should be upvoted, not downvoted.

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