# Thread: Mega tsunamis

1. ## Mega tsunamis

So i was reading this story about a meteor impact that was going to hit the ocean and create a series of mega tsunamis, the worst being 500 meters in height. Can anyone do the math to tell me just how far into say, the eastern seaboard of america a wave that like would flood? I ask because I imagine it wont just go 500 meters in then slowly drain back out, but with the momentum behind it its likely it would travel a significant distance inland. I know it likely depends on a variety of factors such as the terrain involved, so to simplify, lets pick a single state. How much of say, connecticut would be getting its feet wet in this scenario? or pick some other state if you like.

As a followup question, would any island survive a 500 meter tall wave? Or would they be utterly drowned and flattened and barely even break the wave as it travels to a continent?

2. ## Re: Mega tsunamis

I'm useless at math, but it seems there'd be a lot of variables there - not just hight of the wave, but length and width, possibly momentum too? Then there's the landscape. I think I remember something about ... I think when I was a kid (this was 35 years ago) someone told me something along the lines of ... a steep incline will make the wave break hard and fast, while a shallow incline will make it break slow - in other words, flatter terrain means the wave will break with much less of a fuss, but travel further. So a flat county like Holland would suffer much worse than a nice rocky coast like Scotland (I'm guessing, here).

Point is: I think you're asking for rather a lot of calculations =)

3. ## Re: Mega tsunamis

Originally Posted by Kaptin Keen
I'm useless at math, but it seems there'd be a lot of variables there - not just hight of the wave, but length and width, possibly momentum too? Then there's the landscape. I think I remember something about ... I think when I was a kid (this was 35 years ago) someone told me something along the lines of ... a steep incline will make the wave break hard and fast, while a shallow incline will make it break slow - in other words, flatter terrain means the wave will break with much less of a fuss, but travel further. So a flat county like Holland would suffer much worse than a nice rocky coast like Scotland (I'm guessing, here).

Point is: I think you're asking for rather a lot of calculations =)
True, but im willing to accept a less precise answer than 100 east main street, nyc. A reasonable estimate would do if possible.

4. ## Re: Mega tsunamis

The USGS may have a demonstrator for that on their website. I know there was one for JPL showing the effects of various meteor hits (size, composition, target area). I'll take a look.

5. ## Re: Mega tsunamis

While there are impact simulators out there that really let you define things - asteroid size + density, angle of entry, speed of entry, whether impact is on land or on water ....

None that I've found have simulations on water coming ashore following an impact event.

Probably because it's too difficult to model for something that's calculated in moments.

But here's something to keep in mind. Most of the US Eastern seaboard is under 500 meters in altitude.

So, yes, it's going to result in a lot of damage for a city, and flatten a lot of trees.

6. ## Re: Mega tsunamis

Hopefully there's a diminishing return on the energy focusing when you have a thick layer of water compared to a thin one, because the 'run-up factor' of normal tsunamis (in the 10-30m wave height range) ranges from 10 to 40. That's the ratio of the height above sea-level that the water reaches and the half-height (distance from peak wave height to sea level). So if nothing changes nonlinearly with water depth, a 500m wave could reach altitudes of 2500m above sea-level, which basically could reach anywhere in North America. Presumably for these larger waves, the actual volume of water to be distributed is more important than the slope of the land in determining how far they go. If we imagine, say, a 500m x 2000m wave packet depositing water over a 100km range, that would submerge that range to a depth of 10 meters. Over a 1000km range it would be a depth of 1 meter. So even though a 500m wave hitting the East Coast could plausibly cover Denver if you used the normal tsunami math, there probably just wouldn't be enough water volume in the wave to actually do that, though I suppose Chicago could probably get its feet wet.

7. ## Re: Mega tsunamis

Not what I was looking for, but a start: Edumedia.

8. ## Re: Mega tsunamis

It looks like the Lituya Bay tsunami (https://en.wikipedia.org/wiki/1958_L...nd_megatsunami) had a 500m altitude run-up (not the same as a 500m wave height...), but I guess it didn't propagate that far horizontally? From what I can tell, it basically hit things within about a mile of the waterways, but washed out trees 500m up-slope at the inlet.

I'm also looking at the Disenchantment Bay tsunami which I guess was > 35 meters at the initial point, and spread about 5km overland (innundating to a depth of 20 meters) and 24km along waterways (at 5 meter heights). Vajont Dam was a 250m wall of water, but only flooded areas within 1km of riverways, so I guess drainage matters quite a lot for how these things propagate.

9. ## Re: Mega tsunamis

There also may be a real problem with wave spread for conventional mega-tsunami.

I have seen quite a detailed documentary on them and how they found out about them - basically they are caused when a large mass of rock falls into shallow water forcing out of the way. All of the models and examples in the documentary where under conditions where there was no room to spread sideways (narrow valleys and test tanks).
They then jumped from these to Grand Canary, which apparently is in danger of falling apart causing just the sort of rock-meets-water impact that causes them and postulated that this put large amounts of the eastern seaboard of North America (and some South) at rick.

Excuse me? But what happened to the narrow channel that enables them to maintain their force?

Now, asteroid impacts are something different - they could have multiple orders of magnitude more energy, and for these your best bet is probably to look for models of the chicxulub impact which was an asteroid doing exactly that and the dinosaurs not surviving the effects.

10. ## Re: Mega tsunamis

I read in a ... you know, a 'popular science' publication years ago that there is an enormous amount of submarine sediment built up north of Denmark, and that - despite Danmark being seismically stable - a minor earthquake could potentially cause an underwater landslide of epic proportions, resulting in tidal waves of similarly massive scale. Denmark is a nice, flat country, and for a while I imagined sudden doom washing over the entire nation, leaving all in ruin, nothing but rubble left.

Then I forgot all about it. This could have been back in 2001-2002, or so. I think mental health is best served by thinking of water as .. you know, something you bathe in.

11. ## Re: Mega tsunamis

Cumbra Vieja collapsing won't be a rockslide, Khedrac. It'll be 1,500,000,000,000 metric tons of island smashing into the ocean, generating potentially a 600 meter tall mega-tsunami. Now granted, by the time that gets across the Atlantic much of that energy will have been bled of by friction between the water molecules, but will still reach up to 25 km inland.

For how far the effect of a mega-tsunami can be felt, the eruption/explosion of Kraktoa in the 1880s dealt one that affected the tide gauges in the Thames on the other side of the world.

12. ## Re: Mega tsunamis

Originally Posted by Rogar Demonblud
Cumbra Vieja collapsing won't be a rockslide, Khedrac. It'll be 1,500,000,000,000 metric tons of island smashing into the ocean, generating potentially a 600 meter tall mega-tsunami. Now granted, by the time that gets across the Atlantic much of that energy will have been bled of by friction between the water molecules, but will still reach up to 25 km inland.

For how far the effect of a mega-tsunami can be felt, the eruption/explosion of Kraktoa in the 1880s dealt one that affected the tide gauges in the Thames on the other side of the world.
There's considerable disagreement about that. Wiki lists a number of estimates and simulations, all of which are less - though by no means insignificant. The latest of these, from 2008, says:

A 2008 paper looked into this very worst-case scenario, the most massive slide that could happen (though unlikely and probably impossible right now with the present day geology). They find wave heights in the range 10 to 188 metres (33 to 617 ft) in the Canary Isles themselves. But the waves interfere and dissipate as they head out into the Atlantic. They predict a height of 40 metres (131 ft) for some nearby island systems. For continents, the worst effects are in northern Brazil (13.6 metres (45 ft)), French Guiana (12.7 metres (42 ft)), Mid-Atlantic United States (9.6 metres (31 ft)), Western Sahara (largest prediction at 37 metres (121 ft)), and Mauritania (9.7 metres (32 ft)). This is not large enough to count as a megatsunami, with the highest prediction for Western Sahara comparable to the Japanese tsunami, so it would only be a megatsunami locally in the mid-Atlantic Ocean.[26]

Not that wiki is the be-all, end-all of sources, but still.

13. ## Re: Mega tsunamis

Originally Posted by Rogar Demonblud
Cumbra Vieja collapsing won't be a rockslide, Khedrac. It'll be 1,500,000,000,000 metric tons of island smashing into the ocean, generating potentially a 600 meter tall mega-tsunami. Now granted, by the time that gets across the Atlantic much of that energy will have been bled of by friction between the water molecules, but will still reach up to 25 km inland.

For how far the effect of a mega-tsunami can be felt, the eruption/explosion of Kraktoa in the 1880s dealt one that affected the tide gauges in the Thames on the other side of the world.
From what I've read they don't know what it will be so I'm not sure how you're saying what it'll be with any real authority. They're not sure how fast or slow the island will shake down and they're not sure what will come of it when it does happen. Just that it will happen and it's a crap shoot on everything else. It going fast is the worst case scenario but it's not the only model various Volcanologists have put forward. It could be a slow descent and then it really won't be much of an event at all.

14. ## Re: Mega tsunamis

This is a fairly hard to model problem. Massive wave heights generally require specific terrain to force the waves to become shorter and higher. It isn't *just* the terrain, because wave spread is affected by a number of factors. Coastal swamps, for instance, serve to dampen massive waves, where beaches mostly don't. A 500 foot wave would indeed be pretty bad, but it may not be as catastrophic as you would expect. The current world record for a tsunami wave stands at 1720 feet, and while it certainly tore a lot up, it didn't go extremely far inland.

TLDR: It'd suck on the coast, and how far inland it would go would vary strongly by area, but it's not an apocalpyse.

15. ## Re: Mega tsunamis

The current world record for a tsunami wave stands at 1720 feet
Bloody 'Ell, mate, that'd hit Florida and go completely over the state and out the other coast.

16. ## Re: Mega tsunamis

Originally Posted by Tyndmyr
The current world record for a tsunami wave stands at 1720 feet, and while it certainly tore a lot up, it didn't go extremely far inland.
The record breaker didn't go "inland" at all. It occurred from land in a narrow bay and went out towards the sea. While its height was 1720 feet (524m) it dropped to under 400 feet very quickly. This site has a good map showing area of effect and heights of the wave throughout the bay: https://geology.com/records/biggest-tsunami.shtml
It shows terrain has a very big effect, and in reverse, a 500m wave may cover a similar distance if the terrain is right.

17. ## Re: Mega tsunamis

Originally Posted by Tarmor
The record breaker didn't go "inland" at all. It occurred from land in a narrow bay and went out towards the sea. While its height was 1720 feet (524m) it dropped to under 400 feet very quickly. This site has a good map showing area of effect and heights of the wave throughout the bay: https://geology.com/records/biggest-tsunami.shtml
It shows terrain has a very big effect, and in reverse, a 500m wave may cover a similar distance if the terrain is right.
The shoreline along it's path was affected, but not horribly far inland. I didn't mean to imply a specific direction, but I may have spoken poorly.

Something that big is certainly unusual, and dependent on terrain to exist in the first place. While, say, Florida wouldn't want a wave of that magnitude, Florida also doesn't have the terrain to really cause such a wave. Even if it somehow DID get hit by a wave that high at the coast, the wave would lose power as it travelled inland pretty rapidly. It would certainly be pretty awful at the coast, but I don't think it'd make it over the whole state. Energy and water is going to be expended on everything in the way.

18. ## Re: Mega tsunamis

Originally Posted by Traab
Can anyone do the math to tell me just how far into say, the eastern seaboard of america a wave that like would flood?
Short answer: probably not

Longer answer: First, the "eastern seaboard" of the USA runs from Maine to Florida, and experiences significant differences in geology and landscape, which would affect things significantly. Here's a topographic map of the USA, you can see that in Maine the mountains are relatively close to the shoreline, meaning the wave may not travel that far inland. Further south, however, there is the large mid-atlantic plain, which could possibly be entirely inundated; the entire Florida panhandle could also be swamped (fun fact, the highest point in Florida is only 345 ft. high). Also, the east-coast has a much longer continental shelf than the west; I'm not enough of an expert to determine exactly how that might affect things, though.

Second, I think the amount of flooding is mainly determined by the amount of water pushed ashore, which has more to do with the wave's length (distance from peak to peak or trough to trough) than it's height (though the two are frequently correlated). A longer, lower wave may contain more water than a short, high wave, and create greater flooding.

Given a very generous ballpark, I'd wager a wave of that size probably wouldn't go OVER the Appalachian mountains, but would flood much of the coastal plain in front of them. Also, something that large would probably have enough energy to enter the Gulf of Mexico, flooding the southern US and the Mississippi river area. Additionally it would also flood far up any other river channels (of which the east coast has many) such as the Hudson River Valley, which could carry the effects farther inland even into more protected (higher elevation) areas.

Overall, there are just to many variables to answer the question easily. If you're thinking about writing (or telling) a story for a game or something like that, the answer can probably be "as far as you need it to" and very few people will question you on it.

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