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A planet with two orbiting moons (not-tidally locked) and large oceans will have extreme tides. Both moons are about the size of our Moon. I do not know an exact number, but I'm estimating somewhere around 100-meter tides (please correct me if I'm wrong!). Obviously, this would make living on the coast extremely dangerous. So,

What would the continents of a planet with extreme tides look like?

Clarifications:

  • The planet has a slow rotational period of 200 Earth days, and a year of 400 Earth days.
  • Extreme winds circle the planet but are blocked from entering the mainland by thick edge specialist forests.

Map: enter image description here

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    $\begingroup$ An Earth-like planet with two moons about the size of our Moon orbiting roughly at the distance as our Moon will have tides between 0.3× what we have on Earth (when the moons are in quadrature) and about 1.6× what we have on Earth (when the Moons are in conjunction or in opposition). Plus, tidal aplitude depends very much on geography; for example, if the Mediterranean or the Black Sea got tides 100× higher than today, tidal aplitude would be about 2 meters. (Ah, the wind protection offered by forests does not extend far behind the forest. To block strong winds you need tall mountains.) $\endgroup$ – AlexP Dec 4 '20 at 19:59
  • $\begingroup$ @AlexP wouldn't the mountains eventually erode away from the high winds? Or would the tectonic movements create more rock than is removed? $\endgroup$ – Mandelbrot Dec 4 '20 at 20:31
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    $\begingroup$ Judging from Earth, tectonics has so far won out over every kind of erosion. $\endgroup$ – JBH Dec 5 '20 at 5:54
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    $\begingroup$ I'm a little unclear as to what you're actually after: tides certainly affect coastlines, but there's a lot of geography going on in your continents! Can you edit to make clear what specific issue you're dealing with? $\endgroup$ – elemtilas Dec 29 '20 at 1:53
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    $\begingroup$ So, short answer: In about 13 million years, your planet's geography will resemble an exploding DeathStar. Or possibly a heavily-bombarded ring system, if lucky. $\endgroup$ – PcMan Dec 29 '20 at 17:07
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The continents largely could look however you like; they're down to the tectonic plates rather than the tides themselves. With the twin moons as you mentioned, the tides wouldn't be much more than Earth's: even if they were twice or thrice Earth's then there wouldn't be much of a difference from an orbit-level perspective unless the coasts were all virtually flat or there were enormous sandbars (at that point the tides might be able to come all the way in and go all the way out, but it'd be showing/hiding long shelves of land rather than affecting the continent itself).

Especially as you mention that there are severe winds elsewhere, I think all of this could be solved relatively elegantly by making the continents each the result of massive tectonic upheaval: they're essentially a tectonic plate that was forced directly upwards, and so the edges of the continents, rather than descending smoothly into the ocean, are a sheer-ish cliff face. You could have the edges of the continents even higher than the rest of the land mass, resulting in them being ringed by massive mountains, perhaps as a result of the nearby tectonic plates encircling the continent!plate and forcing it upwards.

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Starting with the Earth as we know as a template, you would notice two things:

  1. The ebb and flow of the sea and rivers (specially regarding tidal bores such as Pororoca) has an eroding effect on the landscape. 100m is an expletive lot. It will go over island and flatten them out in geological time. Beaches will also be much wider.

  2. The same forces that cause tides also stretch the planet. Expect much more tectonism.

Earth tide (also known as solid Earth tide, crustal tide, body tide, bodily tide or land tide) is the displacement of the solid earth's surface caused by the gravity of the Moon and Sun. Its main component has meter-level amplitude at periods of about 12 hours and longer. The largest body tide constituents are semi-diurnal, but there are also significant diurnal, semi-annual, and fortnightly contributions. Though the gravitational forcing causing earth tides and ocean tides is the same, the responses are quite different.

For comparison, the highest tide on Earth is 12 meters high. I don't know how the Earth tide would scale, but if we assume it does so linearly, it would mean a 9m Earth tide - you might see a noticeable bulge going up and down everyday around the equator!

Other than that, 200 Earth days rotation makes for a very hot place. Venus is kinda like that. If your planet is in the goldilocks zone, it will need a permanent cloud cover on the sunny side in order to get a chance to have Earth like temperatures and maintain water. It might be possible:

It has until recently been assumed that the rotation rate or day-night cycle of Venus would have to be increased for successful terraformation to be achieved. More recent research has, however, shown that the current slow rotation rate of Venus is not at all detrimental to the planet's capability to support an Earth-like climate. Rather, the slow rotation rate would, given an Earth-like atmosphere, enable the formation of thick cloud layers on the side of the planet facing the sun. This in turn would raise planetary albedo and act to cool the global temperature to Earth-like levels, despite the greater proximity to the Sun. According to calculations, maximum temperatures would be just around 35° C (95° F), given an Earth-like atmosphere.

Otherwise the only way to keep water is through a very unreal level of atmospheric pressure.

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I'm no expert on the relationship between tides and moon and planet, but these are my thoughts.

If the planet has higher tides, the waters will likely be more rough. Depending on how high and rough the tides are, the landscape will be more or less affected. Standard stuff. Here's what comes to mind:

  • finer sand
  • shelves that line the sea will become steep cliffs
  • beaches that line the sea will become wider and further inland
  • things will be tossed around, meaning the sand/sediment deposits might be from a type of rock several miles away, or vegetation that fell into the ocean may end up on a different continent
  • because of the strong winds and tides, birds may not be able to migrate during winter so they will adapt to survive in their ecosystem year-round
  • the fish will either swim deeper to avoid the rough waters (meaning fishing is a RARE source of food for locals) or the fish near the surface would have some sort of way to survive and navigate the rough waters..? (maybe they would evolve to be really big or maybe really small? maybe they're shaped in a way to help them navigate currents? maybe its just not possible at all?)
  • another factor to consider is debris, any trees or rocks or anything that falls into the ocean will likely stay there, trapped in swirling waters, traveling quickly long distances until eventually decomposed or deposited somewhere
  • less vegetation in the sea, closer to the surface means rougher waters, but further means less sunlight. the plants that would exist will have sturdy roots to not be carried away by the current (someone fact-check me on that)
  • ocean currents affect the climate and weather, but I'm not sure how the rougher tides will affect the currents. The currents might be more rapid, meaning that storms and weather in general will come and go quickly on the fast moving winds
  • on the topic of currents, they will affect the shape of your landmass. Real world example (source):

enter image description here

As the tides would be rougher and the currents more strong, your land would be greatly affected by the direction the currents are running.

  • on the civilization side of things depending on how advanced the technology is they may find a way to harness the extreme energy in the tides, similarly to water mills but of course, adapted to the tides and their needs. just food for thought.

I hope this helps, let me know if there's anything else I forgot!

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