How to have high tides in the tropics?
How to have high tides in the tropics?
The Bay of Fundy sees the world's highest tides, with ocean water advancing and withdrawing as much as 20 meters or more within a single day.
Here is a list of the 50 tidal stations around the world with the highest tides, all with average tidal ranges of over 7 meters. Note the location of these tidal stations: they are in Canada, United Kingdom, Alaska, Argentina, France, Chile and Russia. None of these locations are anywhere near the tropics.
I want a great tropical river delta, like the Ganges delta, to empty into a sea with massive 10m or more tides. The river will collect waters from a wide swathe of savanna, and be highly seasonal in flow; a great monsoon season will increase flow rate by a factor of 10.
With the least amount of physical changes from Earth, how can I get 10 meter tides or more at a tropical river delta?
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@AlexP The Gulf of Boni has roughly the same dimensions a the Bay of Fundy, and is on the equator. The tides look to be about 1.5 meters on average. I don't think it is just geography.
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– kingledion
Sep 6 '18 at 12:57
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It is just geography. Examples: Alcantara in Brazil: tidal range 5 meters; Curuca: 5.4 m; Antsohimbondrona, Madagascar: 4.5 m; Beira, Mozambique: 6.8 m.
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– AlexP
Sep 6 '18 at 13:24
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Top tidal stations tend to be away from tropics b/c developed countries have more tidal stations, and there are no developed countries in tropics (there is a heated debate about why)
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– Bald Bear
Sep 6 '18 at 15:38
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Do you really need vertical height? A large plain at just the right altitude will cause impressive tides (I'm thinking of St Malo)
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– Law29
Sep 6 '18 at 16:35
4 Answers
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Of course that's a simplification, but it's a useful one for the moment because we're going to look at wave interference patterns.
If we consider the Bristol Channel, which is fairly well represented on your list, it's on the back of a large island relative to the wave of the tide. That means it gets two tidal waves, one comes round the island from the North, the other comes round from the South. The Bristol Channel just happens to be in a location where the two tides meet at a point of constructive interference.
As a counter example, at Southampton on the South coast of England, the tides around the Isle of Wight miss the point of constructive interference at the port but don't quite hit destructive interference. This causes a complex tidal system with a double peak and some very strong currents round the island as the tide first flows in first from one side of the island and then from the other.
Tides around the UK are particularly complex because of this effect and the currents can be very dangerous. Add a high wind, which can hold a tide in or out, and things get considerably more difficult.
The reason for the high tide in Alaska is different.
The tidal range of a particular location is dependent less on it position north/south of the equator than on other physical factors in the area; topography, water depth, shoreline configuration, size of the ocean basin, and others. For example, let's consider the southern coast of Alaska and British Columbia. The configuration of this coastline is very similar to a funnel, with the narrow end at Cook Inlet. The tides travel as a "wave" across the oceans, and in many other respects act as a "wave"; this type of configuration tends to accentuate the "wave" at the narrow end of the funnel. This is part of the reason for the large tidal ranges, 30+ feet, in the area of Cook Inlet. If you look at the tidal ranges for stations on the Bering Sea, outside this funnel but at the same latitude, you will find a tidal range of 5-7 feet.
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Your use of "tidal wave" for the movement of the tidal bulge of water could be misleading, since "tidal wave" is a common term for a seismic sea wave or tsunami.
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– M. A. Golding
Sep 6 '18 at 14:03
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So to sum it up, the only way you will get a decent tide in a wide flat delta is by placing it behind a mid-sized island like England or Madagascar. But for really high tides, you need a narrow canyon.
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– Bald Bear
Sep 6 '18 at 15:40
I guess the reason why high tides are seen in those places is strongly related to their geological history.
To have high tides you need to have a sort of funnel shaped coast line, that compresses the sides of the tidal wave as it propagates, increasing therefore its height.
In regions affected by the ice age, it is possible that the rivers have carved deep valleys when the sea was much lower, or alternatively the glacier itself carved those valleys and left them behind upon melting.
Those valleys now, with an higher sea level, behave as funnels and give high tides.
I guess that in the tropical regions the glaciation didn't have the same effect, either because there were no glaciers or the reduced availability of water didn't allow the rivers to deeply carve the coast line (those closer to the glacier could still count on the melting water coming from the glaciers).
I would say that having a formerly glacial region being migrated around the Equator by plate tectonics would be a suitable way to achieve high tides.
The Bay of Fundy gets especially high tides because the shape of the bay (with the level of water in it) acts as a resonator almost precisely tuned to the frequency of the Atlantic tides outside the mouth of the bay.
It's not just the general appearance of the bay on a map, it's also a matter of the three-dimensional shape of the bay, including the depths at various points.
But if you could somehow have a basin the same shape as the Bay of Fundy much closer to the equator, and exposed to the same frequency of tides, you would likely have a very large tidal range there too.
From the very next question on the web site you linked:
Q: It appears that the range of the tides gets larger the further the location from the equator. What causes this?
If you only consider a few locations, this may seem to be true; but it is not the case. Literally hundreds of locations in the extreme north and south latitudes have small tidal ranges, and a number of stations closer to the equator have large tidal ranges. ...
So latitude is not a problem, per se.
To have a large river delta you need a river that meanders through silty soil, picks up a lot of silt, and slows down as it reaches the sea so that it deposits the silt around the river mouth, forming the delta.
It might strain credulity of some readers to have such a silty river slowly emptying into a basin of the shape of the Bay of Fundy. How is it that we have Bay of Fundy topography just beyond the mouth of the river but Mississippi topography everywhere upstream?
Another thing that might strain credulity: you have a 10-meter tide washing over the delta once or twice a day, necessarily causing fast currents to carry all that water in and out, yet the rushing tidal currents do not wash away the delta.
The last part is what I find most difficult to believe.
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Indeed, high tides are conducive to estuaries, not deltas. The only possible build-up of a delta would be a vegetation that stops the silt being washed away (e,g mangroves), but how such a vegetation can grow/advance in the first place, especially with such high tides?
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– Adrian Colomitchi
Sep 7 '18 at 4:04
There are two solutions: lunar orbits, or geography
Lunar Orbit:
The simplest way to allow greater tide changes in the tropics without altering the Earth, is to simply alter the moons orbit instead. Change the orbit so that the moon orbits directly over the equator. Currently the moon's orbits is 23 degrees displaced off of the equator, which leads to the highest tides also being 23 degrees displaced since that is where the tidal bulge is the greatest.
Altering the moon's orbit so that the highest point of the tidal bulge instead occurs over the equator is the conceptually easiest (and most absurd) solution.
Geography:
The second contender is altering the geography of the tropics. One of the biggest factors contributing to tidal variation is geography.
Specifically we see that the highest tides tend to occur in areas where the geography of the land constricts the ocean; bays, narrow channels, etc. What is happening is the land is essentially funneling the rising water and these areas have exaggerated tidal variations. Where there is less constriction and more open ocean, the tides are less exaggerated.
So have more land in the areas where you want more tide, or change the orbit of the moon.
http://www.astronomycafe.net/FAQs/q2792x.html
https://www.quora.com/Why-are-tides-lower-at-the-equator
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As the moon orbits around the Earth it will move from above to below the equator passing through the equator twice each cycle, moving the tidal bulge with it. ALL orbits will cross the equator. Changing the moons orbit would potentially decrease tides in upper latitudes but wouldn't decrease the effect on equatorial latitudes.
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– Josh King
Sep 6 '18 at 18:04
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The "tidal bulge" is a fiction propagated by oversimplified "science" books. It is simply not possible for a "bulge" like that to travel around the Earth daily, both because the oceans are too shallow and because there are continents in the way. What we actually have is water sloshing around within the oceans. In the North Atlantic, the water sloshes around in a clockwise circular pattern.
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– David K
Sep 6 '18 at 20:42
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If you actually follow your first link and read it all the way to the bottom of the page, it explains that the actual tides have "virtually nothing to do with" the figure you copied.
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– David K
Sep 6 '18 at 20:51
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The tidal range in the open ocean is less than 1 meter (3 feet). Larger tidal ranges are created by the configuration of the coasts. In the tropics there are many points with tidal ranges of 3 meters (10 feet), for example Punta Arenas in Costa Rica. Explore www.tide-forecast.com to find what you seek. Long story short: if the Bay of Fundy would be in the tropics it will still have it's ample tides; it all depends on the configuration of the coast lines.
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– AlexP
Sep 6 '18 at 12:34