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My planet has fairly large tides, with the difference between high and low tide being roughly thirty meters, and the spring high/low tide different being roughly sixty meters.

There is a civilization living on the coast of a large bay that only connects to the sea via a small strait, reminiscent of the Mediterranean and Gibraltar.

In order to gain more land and be able to build safer ports, I thought that this civilization could attempt to put locks into the strait, similar to those found in the Suez and Panama canals, and thus not only control the water level in the bay, but also allow ships to pass through it for a limited time each day, between high and low tide.

The catch is that this civilization has late 19-century technology. But given that the Suez Canal was built in 1859, I don't think that should be much of a problem.

Here are some figures to work with: The strait is roughly 150 meters deep and 2 kilometres wide. The bay itself has an area of about 17 500 km2 (a bit smaller than Lake Ontario), and averages around 200 meters deep.

Is putting locks on this strait (not damming it) possible with 19th-century technology?

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    $\begingroup$ Suez has no locks, it's a false comparison. $\endgroup$
    – Separatrix
    Commented Nov 20, 2020 at 14:59
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    $\begingroup$ I'd also suggest putting a 30m sea level rise into something like floodmap.net to see what it looks like in the real world. Entire countries disappear from the map. There are very few places in the world where you'd be able to build near enough to tidal water to be able to use boats. At low tide, Doggerland returns, at high tide the Netherlands is lost, and that's for the average tide. $\endgroup$
    – Separatrix
    Commented Nov 20, 2020 at 15:17
  • $\begingroup$ My concern is that this large body of water inside the lock will still be tidal as tides are the result of the gravitational pull of the moon causing the water to "rise" towards the moon. Tides occure on large bodies of water that are seperated from the oceans as well. Lakes Superior and Eerie do experince tides (though they are smaller), so something as large as a Great Lake would see some tidal swell given the magnitude of oceanic tidal force. $\endgroup$
    – hszmv
    Commented Nov 20, 2020 at 15:25
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    $\begingroup$ This is an excellent world to build some really serious tidal power plants... $\endgroup$
    – cmaster - reinstate monica
    Commented Nov 20, 2020 at 15:31
  • $\begingroup$ (1) The Suez canal has no locks -- it is a sea-level canal. (2) Only the eastern half of the Suez canal is tidal, and those tides are only have only about 1 meter amplitude. (2) Unless the strait is made of adamantium, those 60 meters tides will greatly enlarge and deepen the strait in no time; as in, the first 60 meters tide will double the width and depth, and it will only continue from there. $\endgroup$
    – AlexP
    Commented Nov 20, 2020 at 17:20

3 Answers 3

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No

This is very doable. For tides of 1.5m (3m up & down) Using late-20th century technology.

The city of Venice has such a system, which was used for the first time on 3 Oct 2020.

To do the same for a 60m tide is.... wow. No chance. It is well out of feasibility even using present day technology and unlimited budget. It would be like building and removing the Three Gorges dam, twice each day!

The other option (but declined by the OP) of permanently DAMMING the strait would be possible, but ultraMegaSuperGargantuan in scope.

P.s. A 150m deep strait, impeding a 60m tidal ocean? It will very rapidly become a 1km deep strait! Mere granite rock cannot stop that sort of scouring ability.

P.P.s. My flood calculator says the water will flow through that strait at a rate of some 395m/s (1422km/h), and a volume of about 1/4 cubic kilometer (55 billion gallons) per second.

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    $\begingroup$ Given that the speed of sound (in air) at sea level is only 344m/s, that flow rate is going to be fun to deal with. $\endgroup$
    – Separatrix
    Commented Nov 20, 2020 at 15:08
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    $\begingroup$ The venice floodgates took about 30 years to be built up fully! The 60 meter tide would widen the channel from 2 to 8 kilometers rapidly, deepen it to the mentioned 1 kilometer and utterly rip away the land on both sides within a matter of weeks $\endgroup$
    – Trish
    Commented Nov 20, 2020 at 15:29
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    $\begingroup$ "Like building and removing the 3 Gorges Dam twice each day" - isn't it simply filling the reservoir of the dam twice a day? Once it's built, it's built. Two large dams in series seem like they'd serve perfectly well as a lock for even very large tides. The issue doesn't seem to be holding back 60m of water (which can certainly be done), it's handling the fast-moving flow, but that's relegated to a postscript here. $\endgroup$
    – Nuclear Hoagie
    Commented Nov 20, 2020 at 17:28
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No it's not possible. But you can still have a safe port with 60m tides with 19th century tech.

This answer gives an overview of why it isn't possible.

60m tides with a narrow straight will flow very fast (~ half speed of sound in water) at these speeds water tries to ignore the turns suggested by the rock - eroding the narrow straight very quickly giving you a very smooth coastline, so you'll end up with a wider opening than Gibraltar pretty quickly. I'd be surprised if the entire continent didn't end up convex after a few millennia.

Your tides may be strong enough that you can use them for thrust in many cases - that's an interesting story idea. No need for sails or oars, astronomy and decent mathematics may be discovered before the sail, as it's superior for navigation.

The trick is to use pontoons as your dock. Since you're tides are massive. You need to a build a structure underwater to rest the pontoons on at low tide. I've ms-painted it as a trestle but you could rock fill it using 19th century steam shovels. (Or find a perfect coastline).

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Pontoons are anchored laterally to the coast so they stay in the correct position and don't apply excessive torque to the attachment point, and the ship ties to the pontoon(s) itself and importantly doesn't drop anchor - it needs to drift forward and backwards in space over time to remain stationary relative to the pontoon.

In the peak spring tide, you'll be looking at the pontoon moving at an average of about 2mm/second vertically. (4mm/s peak. Stationary 4 times per day). This is about 1/4000th the speed of a typical 20th century building lift / elevator. You'll feel it, but your dock workers can transfer heavy cargo efficiently under this speed.

Your society will also probably discover standardised shipping containers faster than we did. You can't take a freight train to the water with this setup, so you're probably going to have to unload sacks / cartons onto small carts pulled by horse or a steam-engine mounted at the top of the hill. Then unload the cart onto a train. The triple handling will make the standardised 20ft container look really attractive in the late 19th / early 20th century.

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  • $\begingroup$ What an amazing piece of worldbuilding advice! Thank you for the ideas! $\endgroup$
    – D. Daniels
    Commented Nov 20, 2020 at 16:13
  • $\begingroup$ The Landungsbrücken in Hamburg (Gernany) use such a system. A long row of pontons are fixed in place by vertical anchor beams and connected to one another (level!) with small pieces of metal so the entire ponton row acts like one long harbor wall without any tide. The pontons are connected to the land by several longer bridges that turn however necessary. At low water you walk down to the ships at a steep angle, and at high water you walk slightly upwards along the same bridge. $\endgroup$
    – cmaster - reinstate monica
    Commented Nov 20, 2020 at 17:38
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Maybe. But it seems you have two separate problems. You could certainly put locks around the strait, if the terrain is favorable. For instance, the Welland Canal https://en.wikipedia.org/wiki/Welland_Canal bypasses the "strait" (AKA Niagara Falls) between Lakes Ontario and Eire, and has more than your 30 m elevation change. The first version of the canal was built in the 1820s, so easily doable with late 19th century tech. That would allow ships to pass safely, avoiding the currents in the strait.

However, you also ask about controlling the water flow, and that is going to take a dam. While 30 m high dams are certainly possible, the problem is going to be building the thing, since your ongoing construction has to withstand the currents. If you could build it, though, it should work, and would make a great tidal power plant.

The height itself is not a problem, as the Romans were able to build taller oness: https://en.wikipedia.org/wiki/Subiaco_Dams Quite a few Roman dams still exist today: https://en.wikipedia.org/wiki/List_of_Roman_dams_and_reservoirs

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