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Long story short the earth-like planet is being bombarded by ice comets crashing into the oceans and the sea levels are rising. I want there to be a huge wall(probably around 20 meters or so) that keeps the water at bay, having started being built the moment they noticed the waterline of the docks going up, but I don't know if that sort of feat is possible for technology of the medieval ages, 14th century. If not I'll adjust the tech level. Other continents have also built their walls, leading to bowls of dry land across the world with the docks on top of the walls which the walls need to be able to deal with as well.

Sure there's the great wall of china and sure people were capable of building water-proof walls but I want to know if the people of the times would be able to build walls strong enough to not only constantly be beaten by waves but also handle the equivalent of... oceanic pressure, if the term is correct? Basically the ocean would be pressing against the wall from all sides at some point, at least that's how I'd imagine it would be.

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    $\begingroup$ Frame Challenge: if people see that the ocean rise isn't stopping, why would they build a wall to stay? Would they not become environmental refugees and seek higher ground? I've seen a prediction that Bangladesh will generate about 6 million environmental refugees due to sea level rise. That will cause significant conflict with neighboring countries. $\endgroup$
    – David R
    Nov 1 '21 at 14:30
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    $\begingroup$ Stephen Baxter has a trilogy of novels with this exact question. In it, the people who build the sea wall are neolithic... and they keep Doggerland from flooding. You know, the now-flooded area that connects Great Britain to the mainland (all the way to Denmark?). I read it and found it plausible. I recommend them for people who like alt history. $\endgroup$
    – John O
    Nov 1 '21 at 14:47
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    $\begingroup$ "being bombarded by ice comets" - I daresay your people have much bigger problem than rising sea levels. For example, Chicxulub impactor was probably a comet, but even if it was an all ice comet, we wouldn't be able to detect the tiny amount of sea level change brought by the amount of ice in this impactor. $\endgroup$
    – Alexander
    Nov 1 '21 at 18:48
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    $\begingroup$ engulf -- 'cause I can't edit for one letter $\endgroup$
    – Dúthomhas
    Nov 1 '21 at 19:50
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    $\begingroup$ Agree with @Alexander - you'd need tens of thousands of Chicxulub-sized impacts to raise sea levels by this much on an Earth-like planet. No one's going to be left to worry about the sea level. $\endgroup$ Nov 1 '21 at 19:51
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Other answers have stated that:
a) the people at the time knew how to keep water at bay
b) what pitfalls there are.

What was done for many centuries in frisia (~ the coast of the north sea) was that villages were surrounded with dikes.
I doubt that in medieval times you could actually coordinate all of the population of any given continent to build that wall as a single, connected structure. Also, there's the risk of one large dam failing in one place and causing large areas of your continent to be flooded with salt water.
So i guess in a situation as you describe it people would build local dams, much like they built local city walls to protect against other threats. Against the sea, however, they will also need to protect the fields, which were notoriously left unprotected by city walls. But gradually, they would build those walls and dams enclosing ever more space, eventually leading to every "useful" piece of land that needs it surrounded by dams and dikes.
The speed of progress will depend on workforce and building materials available, but this decentralized approach will be manageable enough, and it will not waste time and resources protecting regions that are considered of low value.
As an interesting bonus, you get pretty good infrastructure (i.e. roads) on top of your walls or dikes, which will help society along a great deal.

To sum it up, and to actually answer your question: Yes, i think they could, although in a slightly different matter than you had in mind.

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    $\begingroup$ Very interesting. Now I'm imagining a honey-comb effect of walls being built. I imagine this sort of cordoning off of things would allow better handling of floods and accidental wall breaks too? $\endgroup$
    – Lemming
    Nov 3 '21 at 7:23
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    $\begingroup$ Yes, that's both the intention and an emerging effect, that a single failure would only lead to a controlled and controllable flooding. $\endgroup$
    – Burki
    Nov 3 '21 at 8:32
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Dutch started building dikes from the Iron age.

The earliest indications of dike building date from the late Iron Age. During excavations of terps in the Frisian villages of Peins and Dongjum, among others, dike bodies were found – small dikes predating the building of the terp. These little dikes, no more than 70 cm high, were composed of neatly-stacked peat sods against a core of loose bulk material. Later on the structure was reinforced by adding an outer wall with a gentler gradient.

In medieval time the construction slowed down

The Netherlands witnessed little dike-building activity in the early Middle Ages. With the departure of the Romans began a period of political instability and population decline. From the eighth century we see renewed, if slow, population growth, after which the population of the Netherlands increased tenfold between 800 and 1250. Once again settlements were formed in the salt marshes, which abounded in fish and in grazing pastures for livestock. On a small scale, streams were dammed and low dikes built, following the contours of the existing differences in elevation.

Toward the end of middle age, dike construction stepped up

In the fourteenth century, the combined effects of soil subsidence and rising sea levels meant, in many parts of the Low Countries, that sea level and ground level converged to the same height. This was the period that saw the first large-scale building of dikes. The population was falling in some parts of Europe, as a result of economic recession and a succession of epidemics, but the Netherlands, especially Holland, was doing relatively well.

So, technically building dikes was done even earlier than medieval time, but not in the scale you are asking. The scale of the work you are putting in place is probably beyond the economic capabilities of many medieval cities. At most it can happen on few spots, not on a whole continent, the same way large cathedrals were not built everywhere, but only in those cities which could afford the large and prolonged expenditure.

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    $\begingroup$ "God created the Earth, but the Dutch created the Netherlands". $\endgroup$
    – Demigan
    Nov 1 '21 at 11:41
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    $\begingroup$ True @Demigan.. when the rivers got under control. But L.Dutch is right, large sea dikes were always too expensive. Coastal protection was of little concern for Dutch dike builders until 1953. The old Dutch dikes and waterworks (the gemalen) controlled rivers and helped to win land, where there used to be swamp, like in the Northern part of the province of Holland. Some sea dikes existed (e.g. protecting Zeeland, Dordrecht), but the 1404 st Elisabeth flood easily broke them. The effort to build really large sea dikes actually started in the 20th century (Afsluitdijk, Delta Plan) $\endgroup$
    – Goodies
    Nov 1 '21 at 19:36
  • $\begingroup$ Is L.Dutch Dutch? $\endgroup$
    – OmarL
    Nov 2 '21 at 10:32
  • $\begingroup$ @Goodies the Dutch have a long history of maintaining and strengthening the dune system along the coast. This helps against the occasional flood event, though it's of course not perfect. Fortifying hundreds of kilometers of coastline to the level that's in place for the Delta works is simply too expensive, let alone the tens of thousands of kilometers needed for an entire continent. $\endgroup$
    – jwenting
    Nov 2 '21 at 11:27
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The Jawa Dam in Jordan dates back to about 3000 BC, is 80 metres long, 9 metres high, and consists of a 1 metre thick stone wall supported by a 50 metre wide earth rampart.

the Great Dam of Marib in Yemen was 580 metres long and originally 4 metres high, built about 1700 BC.

The Kallanai dam in Tamil Nadu, India, is constructed of unhewn stone, over 300 metres long, 4.5 metres high and 20 metres wide. That's from the 2nd century AD.

Hence, the basic technology is ancient.

Although not a dam, as a demonstration of the capability to build large structures we can look to the Walls of Benin in Nigeria, of which the banks and ditches in rural parts are an estimated 16,000 km long! They're thought to have been built around 1500 AD by the Edo people.

It's worth noting, though, that there are other ways of dealing with rising sea level besides man-made dams and walls. Coral islands are formed by nature when coral reefs grow up to the sea surface. As corals have survived historic floods at the ends of the ice ages, like Meltwater Pulse 1A, where the sea rose more than 16 metres in a period of 500 years (4 cm per year), they're pretty effective. In tropical parts of your continent, plant coral reefs around flooded land to provide a platform on which to build.

And river deltas naturally rise to meet sea level. Rivers carry a load of eroded sediment depending on the speed of water flow. When the water reaches sea level it stops, and deposits its load. If the channel is a fixed width and the deposition rate is faster than sea level rise, the delta extends out into the sea, spreading out in a triangular fan. As the area of the delta increases, the sediment is spread over a wider area until the rate of deposition (minus subsidence, erosion, etc.) equals the rate of sea level rise, and an equilibrium is reached. If the rate of sea level rise increases, the delta shrinks until a new equilibrium is reached (same amount of sediment dropped on a smaller area) and then again remains constant. So the land rises to track sea level, and the rate of rise automatically adjusts to match the rising of the seas.

Since the last ice age ended around 14,000 years ago the sea has risen about 100 metres. So the tops of pretty much all the river deltas in existence today are less than 14,000 years old, and consist of a 'wall' of sediment up to 100 metres thick.

Rivers automatically meander across flat 'flood plains' to evenly deposit sediment across them. A lot of the modern problems with the low lying land are due to humans engineering to stop the recurrent flooding that keeps the land rising. The land continues to sink by compression, and falls ever further below the water level, so when it does finally flood the result is catastrophic. Nevertheless, with a bit of intelligence, a medieval technology capable of building canals, dams, and irrigation ditches should be perfectly capable of engineering the process to increase sediment deposits where it is needed across wide areas. Build a pair of widely separated walls, maybe a few miles apart, allow the river to meander between them and deposit its load, and keep raising the walls. This raises the level of a much broader area which can then loop round a much bigger chunk of land that you want to protect. The end result is that you can build a dam a few miles wide, using the river to do most of the heavy lifting work for you.

I'm not aware of any case of ancient people actually using this trick - but if they had enough understanding of how rivers worked to think of it, the engineering itself should be well within their capabilities.

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Submerge-proof an entire low-pan continent is not possible, unless:

  • it's almost a desert-like one; or...
  • ...your medieval population have a reliable source of power to pump all the flowing rivers on that continent against the height the raising ocean
  • ...or both.

Because, letting aside the sea seepage, I expect it will rain quite enough on a continent that is supposedly habitable - if you don't drain that water, it will fill the pan until it overflows.

It is possible to keep limited land areas of the continent dry - many answers here mentioned Netherlands already.

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  • $\begingroup$ ... and the water cant drain if the external water level is higher than the coastal land level... $\endgroup$
    – Stilez
    Nov 2 '21 at 7:08
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    $\begingroup$ And even if you had the technology, where'd you get the materials? You'd need to demolish some pretty serious mountains, which presents its own challenge :) $\endgroup$
    – jwenting
    Nov 2 '21 at 11:28
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    $\begingroup$ You don't need to pump up the rivers. You just need to make canals to drive them to the see at high levels. You just need pumps to get rid of the rain and groundwater arriving to the areas under sea level. $\endgroup$
    – Pere
    Nov 2 '21 at 22:49
  • $\begingroup$ @Pere those rivers that you speak of, are they something else than the excess of rain water that collects and flow downhill? I don't know, maybe some magic that makes the "rain and groundwater" assembling itself as a flow on higher ground, so that you're safe if you just build enough flood walls? $\endgroup$ Nov 3 '21 at 4:48
  • $\begingroup$ @AdrianColomitchi - Most of the waters the rivers carry in the low lands has arrived there by the same river flowing from higher ground. If sea level rose a few meters, most rivers could still be driven down to the ocean just by raising their banks in the last kilometers. That's how the Rhine, the Meuse and other large rivers of the Netherlands arrive to the sea without flooding the country. Of course that could be more challenging in very flat countries with very large areas under sea level. $\endgroup$
    – Pere
    Nov 3 '21 at 12:03
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The real problem is not building walls or dikes but to keep the land behind the dikes dry.

A simple method is to use the tidal forces: Open up some gates when the tide is low and closing them at high tide. This can even be automated by the construction of the gates. This method can keep land dry that is approx. 1 m below average sea level. It works for small creeks and brooks, but not for larger rivers.

For more, you need an elaborate system of pumps (the famous dutch windmills were just that) but that wasn't available at medieval technology level.

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    $\begingroup$ Two plausible pumps existed over 2000 years ago: reciprocating pumps and Archimedes screws. The technology existed; the hard part is powering it and scaling it up. Small areas could be kept clear even in the absence of suitable tidal ranges. Windmills go back far enough - 9th century Persia at least, though we could conceivably claim Heron of Alexandria invented one and thus have all the technology in a classical setting. $\endgroup$
    – Chris H
    Nov 2 '21 at 10:29
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    $\begingroup$ @ChrisH "the technology existed; the hard part is powering it and scaling it up" - or, to paraphrase, the technology didn't exist. $\endgroup$
    – N. Virgo
    Nov 3 '21 at 1:24
  • $\begingroup$ @Nathaniel "hard", not "impossible". Compelled labour could help with construction, but the skilled craftspeople needed to make the moving parts would be a bottleneck. $\endgroup$
    – Chris H
    Nov 3 '21 at 6:48
  • $\begingroup$ Further early developments could be based on irrigation methods - the saqiyah was an ancient device using oxen to lift water (that's the important bit, gravity does the rest). If there's a lot of river flow to deal with, dams upstream will reduce the peaks. Some flooding of farmland after heavy rains in the off season may be acceptable. $\endgroup$
    – Chris H
    Nov 3 '21 at 6:55
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Don't build a seawall; raise the land.

You mentioned "bowls of dry land" across the world, so I am envisioning your world consists of many archipelagos occupied by mostly seafaring civilizations. The problem with a seawall is that even absurdly high ones would occasionally get breached by a natural disaster (e.g. a hurricane surge, a tsunami, cracks from an earthquake, etc.) and would flood the "bowl" with nowhere for the water to drain. That would be an utter, civilization-ending disaster depending on how bad it was.

Another problem with seawalls is that it isn't sufficient to just add to their height when the sea level rises. You'd also have to reinforce the base of the wall or drive it deeper into the seabed to withstand the increased pressure. That's probably more of an engineering problem than the wall itself.

A better solution would be for your various civilizations to be continually raising their cities above the water line over time. The good news is that there is already precedent from real-world history of ancient cultures doing exactly that.

There are many cities in the world today that are built directly on top of the ruins of ancient cities -- sometimes in multiple layers over thousands of years.

Venice, Italy, is another example. The city was built 1500 years ago on platforms that rest atop wooden stilts. I remember a seeing a documentary once upon a time (can't remember when exactly) that mentioned archaeological evidence that the ancient Venetians may have jacked up portions of the city as it expanded.

The Uru people of modern-day Bolivia have lived on artificially-built floating islands in the middle of Lake Titicaca for the past 500 years. The islands are made of reeds that naturally grow in the lake, woven together to support the people and structures.

In more modern times, Chicago, Illinois, USA was raised 14 feet (4.2 meters) in the 1850's. Obviously not medieval times, but Chicago is (and was) a very large city. Large medieval cities were rare; most cities had only a few hundred to a few thousand people, so this seems doable with medieval technology.

The city of Seattle, Washington, USA was also raised after a fire destroyed most of the waterfront in 1889. They just built a new waterfront on top of the old.

I'm sure if you did more research you could find many more examples of entire cities being raised up even or moved elsewhere due to environmental issues. These are just the ones I could think of off the top of my head.

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    $\begingroup$ Raising the cities won't save the arable land. Apart from fishing people in the new cities would have little to support themselves. Raising all the land would be a huge task even with modern technologies, unlikely. $\endgroup$
    – FluidCode
    Nov 3 '21 at 18:00
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A wall would not stop water infiltration through the ground

There are many coastal areas below sea level because the sea currents deposited enough sand to create a barrier of dunes keeping out the sea. However those areas are usually marshland because the pressure of the sea eventually causes some water to seep through the ground. The Dutch solved the problem because the constant wind provided them a lot of free energy. But constant winds cannot be found everywhere, you will have to solve the problem in another way.

You will need expendable manpower

The Dutch managed take from the sea so much land because they had a lot of coastal dunes, their dams just closed the gaps between the dunes. To build a big wall along the entire coast people would not have time to tend their fields and a lot of them would die of hunger. The productivity of medieval agriculture is not enough to support such huge works.

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