How would you build a harbor in a world with *intense* tides?

Question

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?

Answer 1

LOCKS

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.

Answer 2

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)

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.

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.

Answer 3

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:

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.

Answer 4

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.

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.

Answer 5

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.

Answer 6

Pseudo-drydock

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.

Answer 7

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:

• Peterborough Lift Lock

• Falkirk Wheel

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.

Answer 8

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.

Answer 9

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::

• The tidal range of the Great Lakes is around 5 centimeters - small enough to be functionally called "non-tidal" The Great Lakes have a combined volume of around 22,500 cubic kilometers.
• The tidal range of Long Island, New York has a range of nearly 9 feet. Being adjacent to the Atlantic Ocean, its tides have access to the entire oceanic body of water, which is around 1.3 billion cubic kilometers, however the actual tidal range can vary all along the East Coast.
• The Mediterranean Sea - which is connected to the Atlantic Ocean via a comparatively narrow inlet - only experiences tides in the centimeter range as well. From the same source as the Atlantic Ocean, the Mediterranean Sea has a volume of around 4.4 million cubic kilometers.

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.

Answer 10

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.