Long story short, my world is set on a habitable moon of a gas giant, and as a result has some very intense tides (about 5x those of Earth, averaging a few dozen foot difference between high and low tide.) As a result, coastal areas tend to flood quite easily a few times a day, and it would be a lot more work to set up a coastal city/harbor than in our world. I was wondering how you all would go about setting up docks or a harbor in a world like this?

I think there'd probably be a few ways to go about it, from putting the city itself on an elevated area and having docks in a lower section, to maybe putting the main body of the city further inland and just setting the docks where high tide would meet them? One of my main worries is that if you put the docks in a place where high tide comes right up to them, during low tide they would be essentially unusable. But obviously, if you put the docks any lower than that, they'd flood at high tide—making that not an option. Am I correct to assume that it's impossible to have docks that work 100% of the time? It seems like ships are just going to be stuck in the harbor during low tide (since low tide is presumably low enough that the water is too shallow for big ships to sail.)

Any ideas on solutions, workarounds, etc. to any of these issues?

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    $\begingroup$ Worth noting that very substantial tides are not unknown in some areas; Liverpool, once one of the world's busiest ports, has a tidal range of about 8m on today's tide tables and Newport in Wales has 11m. This is already pushing into the lower end of your extreme tidal range, so might be worth looking at those as examples. $\endgroup$ Commented Nov 8, 2021 at 11:12
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    $\begingroup$ Have you considered the probability of your habitable moon being tidally locked with the giant? $\endgroup$
    – Alexander
    Commented Nov 8, 2021 at 17:09
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    $\begingroup$ How about building the dock floating on top of the water and have it rise up and down with the tides? $\endgroup$ Commented Nov 8, 2021 at 17:19
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    $\begingroup$ Look up the Bay of Fundy. $\endgroup$
    – Daniel B
    Commented Nov 8, 2021 at 18:34
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    $\begingroup$ @supernikio2 it is widely used in river ports. Big rivers can be both navigable and vary ~10 meters or more. $\endgroup$
    – fraxinus
    Commented Nov 9, 2021 at 8:11

10 Answers 10



The answer is one that is used extensively today in canals and rivers: Locks.

Locks are basically double water-tight gates. If you want to go from high water to low, you sail in, the gates are closed behind you, and water is led or pumped out until it is level with the lower water within/outside the harbor. Vice versa for going from low water to high.

This works best if the harbor is situated at the end of a river or large stream. That way, you can maintain the water level in the harbor at high tide levels and use the locks to lower ships going out and raise ships going in.

A lock is used in Chicago Harbor (see picture below) and can accomodate up to 100 vessels at once, handling water level differences up to 1½ m. Increasing this difference is simply a matter of engineering.

enter image description here

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    $\begingroup$ It's probably also worth noting, that if tides are multiple-stories tall as in OP's description, then the harbor wall can/will probably serve as a giant hydroelectric dam too. Let water flow in at high tide, then do controlled releases through hydroelectric generators and the lock system as needed when in low tide. Rinse and repeat. Then, since your harbor is built on the dam's reservoir, you'll have much more stable water-levels at the docks/etc. at any time of day. $\endgroup$
    – DotCounter
    Commented Nov 8, 2021 at 20:17
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    $\begingroup$ The Netherlands is a great case study resource here - 26% of its land area is below sea level and 17% of its total land area has actually been reclaimed from the sea. Coastal engineering projects there are probably some of the most advanced in the world. $\endgroup$
    – J...
    Commented Nov 9, 2021 at 17:57
  • $\begingroup$ I was going to try to answer his question based on the phoenician port of carthage, and then it wasnt very relevant, nice piccies still google.com/… $\endgroup$ Commented Nov 9, 2021 at 18:13
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    $\begingroup$ @LetEpsilonBeLessThanZero and they can harvest energy as the water flows into the lock too. Really, it'd be harvestable energy as long as the water is changing level regardless of which way it's going. $\endgroup$
    – Drake P
    Commented Nov 10, 2021 at 2:36
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    $\begingroup$ Example, too closely related for its own answer: Bristol's floating harbour can only be sailed into at high tide at least by large ships, and is kept topped up by a river/canal so the water level in the harbour remains constant $\endgroup$
    – Chris H
    Commented Nov 10, 2021 at 10:27

Some years ago I went to Scarborough (Yorkshire) and visited the local harbor during low tide: all the ships were sitting on wet sand, as shown in the painting below (for some reason I can only find stock photos)

enter image description here

If you are able to predict the tides, it's something you can live with.

If instead you want to ensure water access at every time, you can resort to using floating piers, which can follow the level of the water, like you can see in harbors in Zeeland, for example.

enter image description here

As long as they have their end in the water, they work. They are also widely used in rivers, which change their level quite a lot during the year.

  • $\begingroup$ Posted a floating dock answer before I noticed you had included it in yours. +1 $\endgroup$
    – Qami
    Commented Nov 8, 2021 at 16:17
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    $\begingroup$ The Liverpool-Birkenhead-Wallasey ferry uses these and the range there is in the 10s of feet. So this would work. $\endgroup$ Commented Nov 8, 2021 at 18:24
  • $\begingroup$ Scarborough, North Yorkshire? There are other Scarboroughs in the world, which are newer but also larger (but I don't think they have big tides). $\endgroup$
    – gerrit
    Commented Nov 9, 2021 at 8:24

Just take a look how communities with much higher than average tides handle the issue IRL.

e.g. these boats are in the Bay of Fundy, which due to quirks of geography has the worlds highest tides at up to 53ft:

extreme low tide

As you can see, people just accept that their boats will be grounded for a significant chunk of time.

Large ships can be flat bottomed and designed to handle being beached without stress.

Also, river ports that can handle ocean traffic, e.g. Portland Oregon, that are far enough inland not to be affected by the tides much will have a much greater importance in this world.

Ships will be smaller for a given tech level, due to the constraints of either being flat-bottomed or riverine.


Given the extreme tide experienced globally, smaller boats as we know them would likely fall out of favor, and the daily driver would be overtaken by amphibious vehicles, which can negate the need for traditional docks entirely.

enter image description here

Several answers here include floating docks, which I think will just have to be the reality of larger cargo and cruise vessels, where they're always anchored fairly far off land during high tide, and accessed via smaller boats. This isn't uncommon now, but the distances will be more extreme in your setting.

  • $\begingroup$ while the most voted ones implies high investment in infrastructure (allowing ports only on big cities), this one can fit in random seashores where there's no such investment for ports. Also, in the world described by OP, surely some towns get to fall apart by periodic tidal seas. $\endgroup$ Commented Nov 10, 2021 at 10:46

I work with some ports that have high tidal ranges, it's a big issue, but it's a problem for which many solutions have been developed.

If the vessels are small, they can simply rest on the seabed when the tide goes out. If the vessels are large then you can schedule the ship movements so they won't ground. If the port would completely dry out during low tide you may need to dredge an area in which water will remain when the tide goes out - keep in mind that water won't flow out of a pool when the tide goes out! A tidal barrier can also be built to trap the water in the harbor like a giant lock.

You're right in thinking it's impossible to have docks that work 100% of the time, but I think you are missing that is true for most big ports on Earth too as there can be a 400,000 ton difference in weight between a ship that is ballast, and a ship that is laden. So if a ship is importing to your harbor it could come in on the high tide, offload its cargo, and leave later on a low tide. This is not unusual at all.

One factor that I think you are neglecting is that having such a big tidal range means water is going to be moving fast. That's a significant problem because ship engines can only exert a limited amount of force. It may be extremely difficult or impossible for you to bring a ship into your port during the ebb or have a ship depart during a flood.

You could also build your port on a river or lake connected to the sea, which does not drain when the tide goes out.

Port evolution

So, you have a port with a 25m tidal range that is drying out your port entirely. If you don't have much money, the first step is just to use small boats - they are quick to load/unload so they can arrive and leave quickly, they can sit on the seabed if need be.

Transshipment If you need to move more tonnage then you might opt to use transshipment - your little boats don't sail all the way to the destination, instead a larger boat waits in deeper water as the boats ferry goods between the port and the boat.

Stronger ships or locks If that is still unsatisfactory, you are going to have to either develop large vessels which can survive sitting on the seabed during low tide, or create a lock which will hold water in your harbor while the vessel can be loaded/unloaded.

Dredging If you absolutely need to be able to move more tonnage, then you have the currently most effective and most expensive option: dredging. Dig a big hole at your harbor so that even at low tide ships can maneuver. Now dig a big trench from your harbor out to the ocean. Both of these need to be deep enough that they always contain enough water. Once you have finished this project ships can sail in and out at any time of day and your import/export ability is maximized. However, that's assuming your ships can actually sail against the tide. In all likelihood the speed of the water is going to be too fast to sail against, so you will still be tidally limited.

Deepwater ports and berths There's actually some other options that can help your ships deal with tides. The first is building a deep water port. Go out far enough from the high tide line that there is always adequate water, and build your port there. Build a bridge or causeway to connect your deep water port to the city. A cheaper option is to build single berths rather than a whole port. There are a lot of options here; build a jetty with a normal berth at the end, drive some piles and have the ship moore against them, add some heavy anchors with lines connected to buoys on the surface that the ship can connect to. To load the ship either use transshipment as above, or construct conveyer belts or pipelines out to the berths.



The low tech solution is to have drydock-like supports at every pier. Sail the ship into the drydock at high tide and, as the water recedes during low tide, the ship is cradled by the supports instead of the water and harbor operations can continue unhindered. The disadvantage is that ships can only arrive or depart at high tide.

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    $\begingroup$ A drydock is major overkill, since setting up a ship to sit in the cradle safely is a major operation(and they are un-loaded as much as possible first to reduce the structural stress). Just a lock that a ship can merrily float in while being (un)loaded until the gate opens and it leaves at next high tide is sufficient. $\endgroup$
    – Eugene
    Commented Nov 8, 2021 at 19:23

Boat lifts

Your harbor could use boat lifts, such as the Peterborough Lift Lock or the Falkirk Wheel. Your harbour would be at the top of the lift, and a channel to the ocean would be at the bottom. Most likely you would have to block off the harbor itself with a dam & spillway system to maintain a constant depth in the basin there.

You can see these lifts in operation (time-lapsed) in the following videos:

Most likely a vertical lift system would be more practical if the lower level varies with time (as it would in your situation.) But a wheel lift is pretty damn nifty and might be allowable under the Rule of Cool.

  • $\begingroup$ The Falkirk Wheel is so cool that I made my wife divert to go see it on one of our three days in Edinburgh! $\endgroup$
    – Andy Dent
    Commented Nov 9, 2021 at 2:01

Build two docks

It's simple. People could talk about the "low-tide docks" and the "high-tide docks". If your story involves violence and whatnot, you could even have this world-building detail, or even plot detail, of crooks tying people to the low-tide docks and waiting for the tide to drown them (or for them to give in and give them the info, for example). It'd be reminiscent of tying someone to train tracks.


As an extension to Klaus's answer about using Locks, I think it's worth pointing out what would make a tide "intense" beyond just extreme gravity - the clarification of which should open up significantly more options.

A tide generally refers to the local change in water level caused by gravity exerted by nearby planetary objects. On Earth, this would be the Moon. The scale of the tide is controlled by two factors: the force of gravity these planetary objects apply, and the amount of water available to move around. In the case of a planet whose surface is primarily water (such as Earth, where the connected oceans account for around 70% of the planet's surface area), this amount of water is staggering.

However, even lakes, ponds, rivers, and any smaller body of water technically experiences a tide. The difference is that these bodies have such a comparably smaller amount of water that the scale of their tide is correspondingly less. As a few examples::

So while Locks are a good answer to this question, so too are any sufficiently isolated or flow-restricted bodies of water such that it is not easily influenced by oceanic tides. As a result, your concerns about tidal levels only need apply to harbors or docks directly on the coastline of the major oceans, and not as much to inland lakes or seas.

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    $\begingroup$ Whilst this is an excellently presented and very informative reply, it doesn't constitute an answer to the question as written and might have been better suited to being a series of comments. $\endgroup$ Commented Nov 9, 2021 at 17:22

Build your cities near amphidromic points where the tide is naturally low because the oscillations interfere.

At the amphidromic points of the dominant tidal constituent, there is almost no vertical movement from tidal action.


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