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A seawall is basically a coastal defense to prevent waves and tsunamis. Let's suppose there is a town that is located in a lagoon next to the ocean, a sea version of Esgaroth, the lake town.
In this town, with a 16th century technology, is it possible to build a seawall with length of 5 kilometers in the ocean delta, like this?

This town basically encounters heavy storms more than thrice a year, and big waves (about 5 to 10 meters) hit the town. This causes hundreds to die and causes serious damage to the town. (This town earns money with trading.) But this town harbors more than tens of thousands, so hundreds might not have a serious impact. Also, the town will be located inside the seawall.

I did some research about seawalls and their materials, but it looks like a seawall made out of pebbles and stones might not be a good building material for this. Concrete would be a best solution, and I am not very sure whether 16th century technology can create tons of concrete and cement.

So the conditions are:

  1. Must use solid materials
  2. 5 kilometers of seawall
  3. Has to be stable enough to last a couple hundred years
  4. Maximum construction time: 30 years

Would it be possible to build a seawall with 16th century technology in these conditions? (Assume the town can provide infinite money and workers.)

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    $\begingroup$ If hundreds die thrice a year, I'd get out of this town and build another, rather than a wall. $\endgroup$ – Keelhaul Jun 8 '17 at 12:18
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    $\begingroup$ I'm sorry, this is super pedantic, but always when I see infinity used this way, a bit of me dies and I have to defend myself: If you have infinite money and infinite workers, just place the ocean a bit to the left or the town a bit to the right and be done with it (yes, you have position yourself wisely so that left and right are correct in this context, I hope a bit of another person dies as well). People will still come to your town to sell stuff no matter what. I also don't think it is a wise idea. You always have to assume scarce resources when building worlds. $\endgroup$ – Raditz_35 Jun 8 '17 at 12:42
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    $\begingroup$ You don't need a seawall to prevent coastal damage. You can use a synthetic reef by placing gigantic rocks about 100 yards or more off the beach and in the water. These are called "breakers" and have existed forever. Monks were known to do this technique in the medieval times. There is also levies which have also existed for quite some time. $\endgroup$ – Adam Gent Jun 8 '17 at 12:44
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    $\begingroup$ Infinite workers? You could build a human seawall! $\endgroup$ – Deolater Jun 8 '17 at 12:55
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    $\begingroup$ As some of the answers have touched on, concrete was definitely around long, long before the 16th century. The Romans used it extensively, even in underwater constructions. $\endgroup$ – Adam Miller Jun 8 '17 at 14:12
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It's nice when Wikipedia provides a real-world historical example fitting the question almost exactly...

On December 26, 2004, towering waves of the 2004 Indian Ocean earthquake tsunami crashed against India's south-eastern coastline killing thousands. However, the former French colonial enclave of Pondicherry escaped unscathed. This was primarily due to French engineers who had constructed (and maintained) a massive stone seawall during the time when the city was a French colony. This 300-year-old seawall effectively kept Pondicherry's historic center dry even though tsunami waves drove water 24 ft (7.3 m) above the normal high-tide mark.

The barrier was initially completed in 1735 and over the years, the French continued to fortify the wall, piling huge boulders along its 1.25 mi (2 km) coastline to stop erosion from the waves pounding the harbor. At its highest, the barrier running along the water's edge reaches about 27 ft (8.2 m) above sea level. The boulders, some weighing up to a ton, are weathered black and brown. The sea wall is inspected every year and whenever gaps appear or the stones sink into the sand, the government adds more boulders to keep it strong (Allsop, 2002).

(From the Wikipedia article on Seawalls)

Pondicherry sea wall

The Pondicherry sea wall, by McKay Savage, made available un CC Attribution 2.0 Generic license on Wikimedia.

  • 300 years old.
  • Resisted a tsunami raising 7.3 meters above high-tide mark.
  • Made with splendidly cyclopean technology.
  • Is 2 km long, so we can safely assume that 5 km is possible.
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    $\begingroup$ That's a good find. @Fl.pf. the fact that it is in 1735 doesn't matter, clearly the barrier could have been built using only medieval tech, which is really the requirement. $\endgroup$ – Amadeus-Reinstate-Monica Jun 8 '17 at 12:55
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    $\begingroup$ @Fl.pf. No, you can't, the Empire State Building has a steel core and a great deal of mathematics behind it that did not exist in medieval times. Piling large stones up has been done by humans for 40,000 years, heavy load barges have been in use for at least 7000 years, ropes and levers too. Steel beams and welding have not been. Please think before you write. $\endgroup$ – Amadeus-Reinstate-Monica Jun 8 '17 at 13:01
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    $\begingroup$ @Fl.pf. That is a terrible analogy. piling huge boulders has been done before the egyptians. To go straight up and as high as the empire state building requires steel (hence eiffel tower). $\endgroup$ – Adam Gent Jun 8 '17 at 13:01
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    $\begingroup$ @Fl.pf. There were no spectacular advances in construction tech between the 15th and the 18th century. Steam powered equipment and reinforced concrete came later. The only important advances were in the calculation methods, which were indeed much more advanced both in math and the understanding of the strength of materials. These are not really crucial for this wall. $\endgroup$ – AlexP Jun 8 '17 at 13:02
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    $\begingroup$ Since this answer was accepted, allow me to add one comment which I feel is vital for this seawall, but which seems to me to get glossed over: you need to maintain such a seawall. The quote from the article mentions this but I feel it merits stressing: using medieval tech, you can build this seawall, no problem, but you should really repair it after every storm. $\endgroup$ – Falc Jun 8 '17 at 13:25
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In Europe dikes were built already in medieval times.

A solid wall makes no sense since there was no concrete in 1500 in huge quantities although cement was used as mortar already for building the pyramids. Dikes made out of earth and stabilized with plants are your best choice for that scenario.

You could also use huge stones and cement as mortar, but a natural dike would be better, since it doesn't sink due to its weight.

Edit: To be honest, your conditions make this a very unlikely endeavour...

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    $\begingroup$ en.wikipedia.org/wiki/Roman_concrete $\endgroup$ – Brad Werth Jun 9 '17 at 6:25
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    $\begingroup$ Yes, the Romans had concrete. However during the middle ages the technique was all but forgotten. Their version of underwater concrete also required volcanic ash: en.wikipedia.org/wiki/Concrete#Middle_Ages . From a world-building perspective you could certainly have it, but you would also need to think about all the other ways that concrete might shape the civilization using it. $\endgroup$ – Paul Johnson Jun 10 '17 at 16:00
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Yes. It is easily doable. Alexander the Great created a one-kilometer causeway in order to take Tyre. Depending on the design, they would use a barge to dump the rock or else build a causeway from land to the wall.

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    $\begingroup$ Wow, what an interesting link! I had never heard of that, thanks! $\endgroup$ – Brad Werth Jun 9 '17 at 16:07
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Here's an example that was made in the 20th century, and reclaimed nearly 1000 acres of land from the sea with nothing more advanced that medieval technology - though to be fair, they did used some modern machinery in the later stages, if only because the prison supplying the labour was re-classified and fewer people were available to finish the job.

http://parishes.lincolnshire.gov.uk/Freiston/section.asp?catId=14871

All you need is an "infinite" supply of prisoners, and plenty of time - the project took nearly 45 years to build about 5km of embankment. One part of it was washed away by winter storms in several successive years before it finally survived the whole winter.

They also built this memorial - unfortunately I can't find a picture of the plate (on the right hand side of the picture) with the story of what it is. Reclaimed farm land is on the left of the bank, salt marsh on the right - at high tide the water comes right up to the wire fence. The metal ball on the top is a datum point for the UK Ordnance Survey (the national map database).

Image from http://trigpointinguk-photos.s3.amazonaws.com/031/P31121.jpg enter image description here

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"Wer nicht deichen kann, der muss weichen": This expression is in german and means that communities which could not build their own seawalls have to go, so they can be replaced with people who can. This expression was coined in medieval times since frequent flooding at the north sea required the people living there to build seawalls or drown. Sometimes they drowned anyway, simply because the floods were too great to be handled by the seawalls. See the grote mandrake for example https://en.wikipedia.org/wiki/Saint_Marcellus%27_flood The german article about seawalls has a good overview of the evolution of seawalls and the laws governing it: https://de.wikipedia.org/wiki/Deich#Geschichte_des_Deichbaus

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  • $\begingroup$ this looks more like a comment than an answer. Though the link may provide useful info, links can stop working for any reason. If you could flesh out your answer it would be more useful. $\endgroup$ – L.Dutch - Reinstate Monica Jun 9 '17 at 6:06
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Well, by living in a city with one of the largest tidal variations (you can see in a nice graph here) in the world, I think I can help you with that. In some months, tide variations here are about 8 meters (~26 feet).

I don't know if it can be managed in your time span (probably yes), but by using a mix of seashell dust, clay, and stone you can build a seawall that can last for a good number of years. In my city we still have one like that. It lost its purpose after modern technology kicked in, but it was used since about 1600 as a seawall.

By the way, I live in São Luís do Maranhão, MA, Brasil. A 400-year-old French-founded city, later taken by the Portuguese. The state government building also sits atop this sea wall, because in the 1600s it was a military fort built by the French.

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Yes, obviously, if stone materials are available. The pyramids were built, right? Stonehenge was built. The Roman Coliseum was built, and we know how.

Infinite money and workers means stone walls of great height and width can be built. Unlike the pyramids; it is not necessary to have huge stones that are problematic; just 'large' stones of one ton or so can be shaped (using other rocks) and moved by about 20 men (i.e. they must each carry 100lb some distance). Such rocks can be loaded on a barge and transported to their destination. twenty or forty men on the barge can lower the rock into the water. Divers (natural divers can dive a hundred feet, and hold their breath for minutes at a time) can guide them to put the stones in position. Likewise, using medieval (or even Roman) technology, such a sea wall could easily be built.

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  • $\begingroup$ Not really that good since the heavy weight on the soft ground at the sea would never make a stable wall. $\endgroup$ – Fl.pf. Jun 8 '17 at 12:22
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    $\begingroup$ @Fl.pf. As we have infinite money and workers we can just use more stones (wider base) to make the wall stable. $\endgroup$ – C.Fe. Jun 8 '17 at 12:25
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    $\begingroup$ @Fl.pf. as C.Fe. says. The wall is stable if the base is 100 yards wide, or 500 yards wide, or whatever is needed. And the Romans had better cement than we do now; see io9.gizmodo.com/… $\endgroup$ – Amadeus-Reinstate-Monica Jun 8 '17 at 12:58
  • $\begingroup$ @Amadeus the cement Romans used near water is better than portland cement but is not better than modern performance cement used in critical infrastructures like bridges. Nevertheless Roman concrete is impressive. $\endgroup$ – Adam Gent Jun 8 '17 at 13:27
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A few points to consider:

  • Even in prehistoric times, people could move big stones (pyramids, Stonehenge). So you can build something with enough manpower and a source of stone.
  • If waves are the issue, you don't really need walls, you need very large boulders to dissipate the energy (a rock armour revetment. If you have walls that's in addition; they'll help with tides but need the boulders so they're not undermined.
  • Consider the (defensive) walls of St Malo and many other cities; with a revetment of boulders to stop them being undermined (which in places they have), they'd hold back some oretty severe storms.
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Concrete wasn't invented until the Eerie canal was constructed. They did not have that technology in the 16th century. However, the Romans built aquaducts long before that. Perhaps you should use a more conventional technique but it is just more massive and requires lots of maintenance to endure the pounding. They built seawalls long before the 16th century but they broke frequently. Hope this helps.

I agree with the previous posts, which apparently appeared while I was typing. A natural wall would work best.

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    $\begingroup$ As far as I recall, the Romans used concrete to good effect. And it had a far higher quality than the concrete we use today. $\endgroup$ – Michael Vehrs Jun 8 '17 at 13:11
  • $\begingroup$ It depends on which concrete. Plain portland cement does not beat roman concrete but there are newer formulations that handily beat roman concrete they are just far more expensive to make (time/money). $\endgroup$ – Adam Gent Jun 8 '17 at 13:19
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    $\begingroup$ Either way the comment that it wasn't invented until the Eerie canal was constructed is false by a few millennia... $\endgroup$ – thegreatemu Jun 8 '17 at 15:52
  • $\begingroup$ And the Erie Canal really isn't all that eerie :-) $\endgroup$ – jamesqf Jun 8 '17 at 18:09

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