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I've done a bit of research and small bodies of water don't have significant tides, even the Mediterranean doesn't have particularly large tides.

So how can I get bigger tides on a small body of water?

The sea in question is, for simplification sake, on what amounts to a floating continent. I'm not entirely sure what size the body of water will be yet (really there will be quite a few of varying sizes, I'm happy with only the largest having more severe tidal effects), but I would guess no bigger than the Black Sea or Caspian Sea.
There is magic in the world and I'm happy with a magical solution as long as it makes sense, I don't want to just handwave it and say it just happens with no reason.

What else could cause a tidal effect, or the appearance of one?

Edit: It appears I was not entirely clear, when I say a floating continent I mean floating in the air, not in another body of water.

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  • $\begingroup$ It is often stated that the Great Lakes are not tidal, but this is not entirely correct. What is the case is that the tides account for only a couple centimeters variation, whereas a good steady rainfall after a week of dry conditions can change Lake Michigan by dozens of centimeters. I know you are looking for more significant tides, but this might give you a place to start from. $\endgroup$ – cobaltduck May 8 '17 at 15:28

11 Answers 11

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Rocking

Your continent is floating. Maybe it is rocking back and forth as it floats. That can happen with floating things. Any number of forces can put a body into an oscillating motion.

As your continent rocks back and forth the water contained in it will flow back and forth, moving toward the low side. I could imagine the rocking being subtle enough that persons living on the continent do not notice it. But the water will notice it and flow accordingly.

The period of the rocking will determine the period of the "tides" produced. The amplitude of the rocking will determine the amplitude of the tides.

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    $\begingroup$ Simple and elegant solution. $\endgroup$ – Kilisi May 8 '17 at 0:22
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    $\begingroup$ Yes, all water will experience these mock tides, but a tenth of a degree shift in tilt across a glass of water, or swimming pool, or even a medium sized pond will not be easily detected. Remembering our trigonometry, adjacent*tanθ=opposite, with θ being our tilt and adjacent being the width of our body of water. plugging in the numbers, we see that a 100m wide body of water will have a 17cm "tide" if the tilt is 0.1°. $\endgroup$ – Geezle May 8 '17 at 12:13
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    $\begingroup$ Of course, your actual tilt will be a lot less than 0.1° to keep small seas mostly in their basins. A tilt of .001° should be enough to give 100km wide seas a 2m tide. $\endgroup$ – Geezle May 8 '17 at 12:20
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    $\begingroup$ @adaliabooks The "slope" is constant across the surface of the water. So lets say you have a slope of 1 in 100. That means a 1 meter wide bathtub would over the course of the day vary from one side to the other by around 1cm. Not very noticeable. On the other hand though a 100m wide lake would see a "tidal" range of 1m. The tides would always pivot around the horizontal center of the water and would be far more severe in the direction of the rocking than in the other. $\endgroup$ – Tim B May 8 '17 at 15:26
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    $\begingroup$ The rocking direction could rotate slowly to give you a variation in tidal strength or could be constant and predictable. The tides in this would would generally be more regular than our tides since you have no sun+moon interaction to give "spring" and "neap" tides. $\endgroup$ – Tim B May 8 '17 at 15:28
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Quasi-magical mega-geyser

There are plenty of geysers around, the most famous probably being Old Faithful which has been shooting about 8000 gallons of water into the air every 40-120 minutes since at least 1870. A little magic and imagination can extend a geyser principle to something large enough to affect a lake or inland sea.

Your geyser is the lava tube of a massive underwater volcano. Every 12 hours or so, the geyser erupts sending cubic kilometers of water into the sky, and causing 'rain' in whatever the downwind part of the lake is (sounds like the world's wettest rainforest). With that amount of water blasted out of the lake, the water level lowers appreciably all along the lakeshore. Then, the huge lava tube, recently emptied of water, fills with lakewater further lowering the water level. Over the next 12 hours, water comes back down into the lake, both directly raining from the sky, and flowing back from rivers in the downwind rainforest, and the lake level rises again.

You can adjust the size of the blasts to the desired tidal variance, and the timing to your desired tidal period. This is obviously not believable for any substantial lake on Earth, but whatever force is powerful enough to make a continent float can surely make a nice mega-geyser as well.

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  • $\begingroup$ Hmmmm. That's certainly an interesting idea, I don't suppose it would even necessarily have to be a single geyser causing the effect. It may not work for all bodies of water but certainly could for some of them. $\endgroup$ – adaliabooks May 7 '17 at 22:32
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The tides are part of a balancing system

If your flying island is actually continent-sized, the moon's gravity would affect parts of it at different times, causing it to rock or sway just a little, but enough to put serious stresses on the internal structure, reducing its lifetime.

Whether by design or natural selection (all the other continents crashed long ago), your continent has long underground water channels running in the direction of the moon's orbit. The water moves back and forth with the moons, balancing the moon's upward pull with extra mass and keeping the continent still. Some or all seas are in contact with the channels, giving them tides much stronger than their own size would suggest.

Alternatively, the water is moved around by magic. Nobody knows how, but smart people theorize it keeps the continent from tipping over somehow. This scenario would not even need a moon or predictable tides. Seas could experience ebb and flood at seemingly random times and ranges.

(old answer below, invalidated since the continent floats in the air, not on the water).

The inland sea is connected to the ocean (underground)

You say that your continent is floating and for this answer I'll assume it's on the ocean, not on the rocky mantle deep under any ocean.

To have high tides, you need a large mass of water (larger than just a lake or sea) and a favorable coastline that compresses the water flow in the direction of the tidal flow. On Earth, those happen mostly around Alaska and Canada as explained here, while in the southern hemisphere, there is no similar effect.

Your continent is probably looks something like a giant island (say Australia) on a map and is surrounded by open ocean, so it might not experience very large tides on its outside coast, but it still gets them. While there is no narrowing coastline, there is still a compressing of the flow happening as it goes under your continent, increasing the pressure.

If your inland seas are connected to the ocean through holes in the bottom of the continent, there might be water flowing in and out with the tides, causing the sea to have ebb and flood, although it will happen nearly simultaneously around its coast.

Of course, this would probably be accompanied by extreme currents and a maelstrom here and there, depending on how deep they are. If that is not the environment you want you seas to have, you could instead make the thin bottom of the sea somewhat flexible, to the point that the entire sea bottom rises and sinks a meter or two from the pressure changes in the ocean under it. Whatever material or magic (or turtle) is holding your continent together (and floating) can include the flexible aspect.

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  • $\begingroup$ Ah, I should clarify in the question, I actually meant floating in the sky. Otherwise this is probably what I would go with, but there is no mechanism for them to be attached to a larger body of water. $\endgroup$ – adaliabooks May 7 '17 at 22:07
  • $\begingroup$ Yes, that's an important distinction that completely invalidates my answer :-\. I guess I'll have to go with "Oscillations in the floating magic" instead. $\endgroup$ – Cyrus May 7 '17 at 22:16
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    $\begingroup$ Sorry! It seemed obvious to me what I meant and didn't even consider floating could mean a variety of things... $\endgroup$ – adaliabooks May 7 '17 at 22:19
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Seiches

Seiches are standing waves in a bounded water body, which can give a resemblance of tides. They occur in several lakes on Earth, and are usually caused by the wind (or occasionally seismological activity). In extreme cases, they can be of equal or greater magnitude than tides - from the Wikipedia page,

On Lake Michigan, eight fishermen were swept away from piers at Montrose and North Avenue Beaches and drowned when a 10-foot (3.0 m) seiche hit the Chicago waterfront on June 26, 1954.

On a floating island, this could amplify the effect of a periodic rocking, causing significantly larger water level changes than would otherwise occur.

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The island is floating, so there are anti-gravity forces at work. Rather than having the island rock (as in the solution provided by will), you could have the island remain perfectly perpendicular to the planet it orbits, however the magic/science that holds it up is constantly readjusting to do so.

Imagine the island is on pillars of anti-gravity. To maintain balance the pillars move. The pillars are restricted to a certain depth, they can't come all the way to the surface or people would fly off the continent!

If the depth of the water is deep enough to intersect the anti gravity pillar. Then the sea would contract. Bedrock provides a good support but water does not. It would be lifted, forced up wards. The ocean would bulge, perhaps only slightly, over these spots. Large helical vortexes would be created.

This would create the following effects:

  • The tide-like-force would contract the body of water on all sides, expansion would likewise be uniform; as if breathing slowly over time.
  • Tides typically move slowly, these tides could be abrupt, depending on the how the force is diminished/reallocated.
  • Things dropped into the ocean such as a body, could remain submerged for a long period of time due to the undertow that would be created.
  • Travel could benefit, if a vessel could make half the trip during high tide (ocean is lying flat) then take advantage of driving force of the low tide (the ocean is pulled inwards by the upward bulge) then the ship would have an inclination advantage, in addition to favourable outward currents, it could make strong progress.

Of note: This mechanic could foreshadow the direct manipulation of the powerful anti-gravity magic supporting the continent, which could produce all manner of natural disasters earth quakes or tsunami, or simply throw people off the world.

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Greater outside forces
The tides on Earth are caused by the pull of the Sun and the Moon and their relative orbits. A closer moon and/or one with a stronger gravitational pull would increase the tidal effect on smaller bodies of water. A more elliptical orbit around your setting's star would cause a tide that shifts through the year, between very strong and much weaker.

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    $\begingroup$ Would these have any other effects though? Presumably a more elliptical orbit with also mean more severe seasons? $\endgroup$ – adaliabooks May 8 '17 at 12:41
  • $\begingroup$ Yes, they probably would have more extreme summers/winters (how much for each depending on the orbit in relation to their star). $\endgroup$ – sirjonsnow May 8 '17 at 17:10
  • $\begingroup$ I was thinking along these lines, but could you go one step further and have your world set on a moon like titan (but slightly more inhabitable)? That way, any large soft bodied liquids would be pulled at pretty fiercely by the planetoid below. This could even contribute to the floating continent's floatyness. $\endgroup$ – Brent Hackers May 9 '17 at 7:39
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Closed-cycle weather system?

The center region of air above your body of water could be affected by a periodic, regular change in pressure - leading to periodic and regular changes in sea level.

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  • $\begingroup$ Wikipedia's article on tides states that Storm Surges are a great example of costal tides based on barometric pressure or wind. +1 $\endgroup$ – Jammin4CO May 8 '17 at 18:09
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Sloshing

Any body of water will slosh back and forth with a period related to its overall mass. Water in a drinking glass will slosh very quickly, while water in a bathtub will slosh more slowly. The point, however, is that the period (the time between cycles) is constant as long as the mass of water is constant. In your lake/sea, if you "push" the water in one direction with series of pushes that match the correct frequency, you can get it to slosh back and forth. Think of it like pumping your legs when using a playground swing. Even a small push is enough as long as you have the timing right.

I remember reading in National Geographic a long time ago about the Great Lakes, and how natural sloshes sometimes occur in Lake Superior (due mostly to wind). Alas, I don't remember any other details, but I found this technical article which may be enlightening. You could use either magic or some advanced engineering to make a big paddle that pumps the water at the required frequency. Depending on the mass of your lake, it would probably have a slosh period of several hours to a few days.

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Resonance

Tides are periodical. If you can make a water mass that oscillates with the same frequency of the tide, tides build up to larger tides even in an small water body, just as we make swings oscillate with repeated small pushes. In fact, real tides larger than average are caused by resonance phenomena.

You can tune natural frequency of a large lake or small sea by adjusting its geometry. With a rectangular long sea with reflective ends a few hundreds of kilometres length can be enough to allow a wave to go end to end one time per high tide.

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I mean floating in the air, not in another body of water.

Looks simple to me. Take a filled bowl with water. Try place it up your head using only your arms. Ask some one to look on it. It is close to the impossible to not get waves! Magic (or science) is not perfect. Floating continents dangle, believe me :)!

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Carbonize the water

I'm no physicist so this answer assumes (probably in error) that carbon-saturated water takes up more volume than just plain water, and enough to be noticeable in a lake/small sea.

Have a naturally occurring buildup of CO2 in your lake that occasionally gets released into the air. So as more and more CO2 builds up, the tides will rise and people that have seen it before will start to worry. Then one rainy day the water level will suddenly drop because a Limnic eruption was triggered. This releases a cloud of foul smelling CO2 into the air. Which is deadly by the way, because CO2 is heavier than oxygen and will displace it. So if you want the small sea and areas around it to be livable, make sure the CO2 can be drained somehow (maybe off the edge of your floating continent) fast enough to keep the death toll to a minimum.

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