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My world involves several floating islands extending some way beneath sea level (enough for their to be deep mines on the islands). How would tides work? Could some areas have tides strong and frequent enough to harness for energy generation? Assume the islands aren't ever in danger of being flipped over.

Edit: By "[h]ow would tides work", I'm curious as to how high the waves crashing on the shore would be.

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    $\begingroup$ if the islands float then just like ships they move up and down with the tides, they will not have any normal tidal effects we see on normal shorelines. why not wind power, wind mills and wind turbines work great near the shore. $\endgroup$
    – John
    Commented Jul 13, 2023 at 14:45
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    $\begingroup$ @LordNobody It seems like you are confusing "waves" and "tides" here... Your question is about tides, but your edits and comments are about waves. $\endgroup$
    – DotCounter
    Commented Jul 13, 2023 at 14:50
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    $\begingroup$ @LordNobody - it's a really important distinction in this case between waves and tides - tides are movements of large amounts of water, over a long time, sufficient to move a large body like your islands. Waves are not, and will act as though the island was a regular one $\endgroup$
    – lupe
    Commented Jul 13, 2023 at 14:58
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    $\begingroup$ Your floating island is like a ship without motors. Or an iceberg. It will see sea-waves but it will go up and down with the tides. If you had a large island with a critical size, it might bob up and down pumped by the tides, but I have never heard of icebergs doing this. If you lived on such an island, I would worry where the ocean currents were taking it. $\endgroup$
    – Richard Kirk
    Commented Jul 13, 2023 at 15:23
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    $\begingroup$ @LetEpsilonBeLessThanZero, lupe, Richard Kirk: Thanks for clearing that up. $\endgroup$
    – Lord Nobody
    Commented Jul 13, 2023 at 19:18

2 Answers 2

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The islands would be pushed and lifted by the tides. Tidal generation would be worse on these islands than on a regular island. There might, however, be a quirky way to generate power.

A tide is essentially the moon pulling the water to it, which creates a current. Your free floating island would be affected by the same gravitational pull, and by the current created by the tides, so it would move in the same direction, but maybe less fast than the surrounding water.

Tides also cause changes in water level, as the water moves. Your island would just rise with these, however, as it keeps the same boyancy, so no observable increase in sea level would occur

So, here's why it'll be worse for tidal generation - your island is moving in the same direction as the water, so, relative to it, the water is moving more slowly. Slower water = less energy generated.

However, wave power would still be fine.

Edit:

Waves are too small to move the island, so they'll break on the shore. Beaches will be more difficult - a horseshoe shaped island might accumulate sediment in the middle.

The bit that I can't figure out, at least without someone willing to lend a wavepool and a nice island model, is if the islands tilt with the tides. I'm guessing they would, which would do an excellent job dislodging sand, and make beaches uniquely unlikely to form here. You have another source of sediment removal (west edge of the island goes slightly further underwater than usual, and as it rises, you'd see a whole load of water movement.

Thinking about this, this might be the source of a different generator - an island with a horseshoe bay would tip down as the tide hit the horseshoe edge, filling it more deeply with water than normal. As the tidal pressure equalizes, it'd tip back to upright, causing a large rush out of water out of the bay. It's not a tidal generator, per se, but relies on the rocking of the island, and the movement of water into the bay.

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  • $\begingroup$ So, just to be clear, there wouldn't be any waves at all lapping at the shore? Come to think of it, would (or could) there be beaches on the islands, or would it be completely rocky? $\endgroup$
    – Lord Nobody
    Commented Jul 13, 2023 at 14:47
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    $\begingroup$ @LordNobody There would still be waves and those could be harvested with wave focused devices. Tides and tidal power really relies on a steady current of water going in/out, which is likely to not work well on a floating island. It's easy to confuse these devices. $\endgroup$
    – PipperChip
    Commented Jul 13, 2023 at 14:52
  • $\begingroup$ Beaches would likely not exist, as loose aggregate material regularly washes back and forth on islands. Most islands are held together by mass, so the islands would likely need to be a strong material to avoid being eroded away. Likely stone-like shores. $\endgroup$
    – DWKraus
    Commented Jul 13, 2023 at 18:35
  • $\begingroup$ @DWKraus: Could beaches form if each island had a small undersea shelf? $\endgroup$
    – Lord Nobody
    Commented Jul 13, 2023 at 19:16
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    $\begingroup$ @LordNobody - thinking about the beach problem, and I think there might be a way that a very specifically shaped island would generate more power from tidal forces - it has to be a horseshoe shaped island with a shelf in the middle, under the bay, however. $\endgroup$
    – lupe
    Commented Jul 13, 2023 at 21:48
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It strongly depends on how big your islands are.

Larger islands have a huge mass so they wouldn't just float at the same level of surrounding water.

When tide arrives and thus water (pulled by Moon or pushed by centrifugal force) rises there will be a force (Archimedes's) pushing island "up", force will increase as water rises and island is too slow to follow; in this condition water will rise on shore.

When water retreats island is moving up with a definite speed and will not stop immediately, but will "overshoot"a bit, as buoyancy decreases and an effective "down" force starts to build.

Given the "right size" you could even get a "resonance" frequency where movement increases (up to a point: friction would be very large also!) getting very high "relative" tides.

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