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What features should be present on a planet to have so that a side of an arcipelago of islands or continents have periodically a huge tide or tsunami periodically ?

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Bay of Fundy! Has the most extreme tides on earth .Tidal resonance Caused by the typology of the bay, the time it takes for the incoming tide to fill the bay and begin evacuating and the timing of the new incoming tide.

The average tide in the bay is around 16 metres (52 ft). World wide average is only about 1 Meter (3 ft.).

[EDIT] PS: You can if needed combine this with other exotic land formations. Or Orbital mechanics of moons possibly even large nearby planets, or the alignment of all the above, to make exceptionally large tides only happen when the "stars align" as it were. Then Spill over "natural dams" to create disastrous floods in isolated areas.

[EDIT2] Missoula floods Are a great example of a naturally recurring catastrophic floods.

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A close or big moon

Technically, a huge tide is nothing but an effect of a moon or another gravitating body close enough to a planet to create a force difference. For example, if our moon would be closer, tides on Earth would be "huge" and quite noticeable everywhere. Thus, give your planet a moon that would be closer and/or heavier than our Moon and calculate distance vs mass vs desired period so that tidal frequency would be acceptable for your storyline (remember that planet also rotates around its own axis) and enjoy.

Or the planet to be a moon of a gas giant

Gas giants are bodies of immense gravity, they are a lot heavier than Earth-like planets, and are also producing higher tidal impact along with tectonic impact, so if your story wants tidal waves AND tsunami, you also want tectonic activity, which will be more suitable for a scenario where your planet is a moon of a gas giant. Be warned however, the size of waves or crust deformation on such a planet can be too big to not account for in your story, for example, continental drift would be a lot larger, if the planet wouldn't end up in tidal lock vs gas giant, earthquakes would be more common and deadly, and life would have to accommodate or perish.

Tides on a coast? They always are there

About the desire of your tides to only affect coast - they already do that, just if the tide would be for example 50 meters high, it will flow into the low coast to obliterate several miles of surface, until the local height above sea level together with any features of coast relief would exhaust the initial impulse. You can solve this with some tectonic-based features, like a small piece of crust turned sidewards (at about 10 degrees angle) forming a steep slope at the sea level, so any giant wave would only smash against a short length of a slope, allowing everyone above to watch the raging nature in awe but also in safety. Effectively this will make your archipelago made of granite or similar volcanic material, with very serrated surface, steep cliffs and deep trenches, that would likely experience increased volcanic activity, and whatever plant life existing there would be short-lived and trying hard to spread far because of environment volatility. Still a world where people would possibly survive.

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    $\begingroup$ I mean only on a coast, perhaps I worded wrongly , so I would like to have effect only on one side of a island or a continent or else but not affect all planet . $\endgroup$
    – Naima
    Aug 11, 2022 at 14:16
  • $\begingroup$ @Naima I changed your title.. it's ONE coast, not A coast to make that clear.. if you don't like the edit, you can undo it. $\endgroup$
    – Goodies
    Aug 11, 2022 at 16:30
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If you have something that makes the bottom of the body of water rise/fall of a substantial amount in a periodic or regular way, you will end up with also the level of the body of water being affected.

Tidal forces induced deformation can be one of these something: on Io the crust swings up and down of about 150 meters due to the tidal forces caused by Jupiter. Imagine having a lake that goes up or down of that much just because its bottom is doing the same.

Another reason can be the presence of a magmatic chamber below the body of water: with the increase/decrease of the pressure in the chamber, the chamber itself will inflate/deflate like a balloon and will move what is above of a significant amount.

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  • $\begingroup$ yes but can it be periodical? Like may be once a year or once every X years? $\endgroup$
    – Naima
    Aug 11, 2022 at 14:16
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    $\begingroup$ @Naima tides are periodic, and volcanism can have periodic behavior too $\endgroup$
    – L.Dutch
    Aug 11, 2022 at 14:19
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    $\begingroup$ With magma you'd need some patience.. long periodicity.. on Iceland,there has been a one in 130 years cyclic eruption and it has reoccurred for a long time, six times in recorded history (since 1200 AD) but probably it started much earlier, ref pubs.geoscienceworld.org/gsa/geology/article-abstract/26/10/943/… $\endgroup$
    – Goodies
    Aug 11, 2022 at 16:36
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A place that has periodic tsunami is a place with an opposing coast which has regular landslides. The landslide comes down the mountain, hits the water, and causes a wave that travels over to the other side as a tsunami.

To have the periodic landslides, the place on the opposing coast needs to have very weak rock and underlying geologic activity. For example, Mt. Rainier has produced a number of rockslides as the fumes coming up eat away at the rock structure and it has periodic earthquakes. Hawaii has had numerous tsunamis caused by a landslide on a nearby island and geologists are watching the crack on the big island. When that goes, there will be a large tsunami.

The tsunami will not be on a regular schedule. Any tsunami generated by earthquakes or landslides will be irregular.

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    $\begingroup$ The landslides don't necessarily need to be rocks either. Could be similarly massive amounts of snow, and suddenly you've got seasonal tsunamis every spring $\endgroup$ Aug 11, 2022 at 22:15
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Outburst Floods

Glacial lakes will sometimes fail catastrophically. The result is a massive flood that scours the Earth downstream of the dam / lake for miles.

The Glacial lakes in the Pacific Northwest failed roughly every 50 years for hundreds of years. If you adjusted the flow of water, you could potentially get them to fail more frequently - like annually.

In the case of the Missoula floods, much of the water ended up in the Columbia river. If the glacier was instead adjunct to an arid plain that abutted the ocean, the floodwaters could spread out and become a massive landslide. When this landslide hit the ocean, it would displace a huge volume of water, causing a tsunami.

The landslide could also potentially cause underwater landslides, which could amplify the effect.

You'd need to either vary the location of the outburst, or fill in the land that has been swept away, or else the repetitive floods will create their own river channel. Perhaps the land is arid enough for dust storms the clean the slate in-between floods.

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The first things to come to mind are mangroves and Yellowstone... and one caveat.

I can break your question down into two intrinsic problems; one of them is ensuring that tsunamis occur on a regular basis, and the other is that they only affect one coastline. We'll start with the first problem.

Tsunamis are typically caused by undersea earthquakes on Earth. Let's say we're dealing with regular tsunamis at about 10 m height, which corresponds tightly to a magnitude eight on the Richter scale. That could theoretically do a lot of damage, but a magnitude eight event (6 × 10¹⁶ J) that occurs regularly is not the kind of thing we would typically see from background-noise tectonics.

However, going back to tidal forces, I could see a regularity in moons lining up, or perhaps a sudden alignment in the tidal pull of a supergiant planet, which could cause these to happen. My thought on Yellowstone (specifically the geyser Old Faithful) is how it erupts so regularly; which is an effect of a spring replenishing while trapping a bubble of superheated (heated past boiling, but unable to boil) water; when the spring finally breaches the surface, pressure drops rapidly and the vaporized water shoots to the surface.

This is in itself not enough to cause an earthquake, but the principle could principally be the same in orbital motions.

Now, the second part, only affecting one coast? In Florida, mangroves are trees that grow in spatterings out into the salt water. They're known for rapidly breaking up tides by inducing destructive interference. It doesn't have to be a tree, geological or even man-made constructions are known to do the same thing; but I do know that if a mangrove grove breaks down and dies off, the coast hosting it immediately gets pelted by waves. This can go on over a reach of no more than twenty feet or about seven meters.

So, if you have a synchronized orbit causing periodic extreme geological stress, you can have regular tsunamis. If you pepper one coast with mangrove-like formations to break it up, in proportion to its scale, you can have only one coast be (reliably) affected by the tsunami.

The last thing, the caveat, is that regular tsunamis are geomorpholical actor... they would shape the terrain. Your mangrove substitutes would have to be maintained somehow, or maintain themselves; and the side continually getting hit would need some sort of impetus not to erode away. This should be relatively easy to figure out, though.

Good luck!

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  • $\begingroup$ Thanks , could a Massive undersea geyser work to cause regular tsunamis? $\endgroup$
    – Naima
    Aug 12, 2022 at 8:37
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    $\begingroup$ Certainly nothing on Earth; but the reason I bring them up is that they're evidence of (somewhat) regular tectonic processes that could happen. Really a geyser is just a spring periodically contacting a magma flow, getting superheated, then erupting when the pressure breaks at the surface; so in a sense you would be looking at ocean water contacting magma and performing the same process... actually I think you might be on to something. Just replace the spring with the ocean itself. $\endgroup$ Aug 12, 2022 at 14:58
  • $\begingroup$ so it can be a metric periodic , lets say once a year , magma chamber filling and expelling something under the surface of an ocean to creat the desired effect ? A periodic timed tsunami that comes once a year or once every 50 years for example? $\endgroup$
    – Naima
    Aug 12, 2022 at 15:40
  • $\begingroup$ I think that might work. The thing about superheating is that the pressure on the water has to be large enough to trap the energy without allowing it to vaporize; once it breaches the surface, that pressure drops rapidly, and the bubble just explodes in a jet. The same principle might get you your tsunami, as the bottom of an ocean is a lot of atmospheres of pressure... I was thinking "daily", but fifty years is enough time to develop a massive bubble of superheat. $\endgroup$ Aug 12, 2022 at 18:25
  • $\begingroup$ A slowly orbiting moon or debris in an eccentric orbit could work, such that the object only gets close enough to the planet to drop meteors at perigee. This could be any period you need. If you need daily, then the object orbits daily. If you need yearly, then it orbits more slowly. The impacts cause the tsunamis. $\endgroup$ Aug 13, 2022 at 2:30

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