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Scenario
The straits of Gibraltar never quite opened and the entire Mediterranean basin is dry land apart from a few large salt water lakes at the lowest points ~5% by area. The Europe – Africa land bridge has been slowly eroded and is now just 2 kilometres wide and no more than 10 meters high at any point.

It is April 100 AD and a civilization similar to that of Rome holds sway over the lands of the Roman Empire and the whole of the Mediterranean basin. A particularly high and fierce storm tide temporally breaches a one kilometre stretch of the centre of the bridge allowing a little salt water in and reducing the barrier height to just 1 meter above normal high tide level before retreating again. The recent breach is the first sign of the seriousness of the situation. The composition of the land bridge is a simple extension of the coastal rock on either side of the straights as we know them.

Question
Would a civilization similar to that of Rome have been able to understand the danger they faced and be willing and capable of taking effective action to prevent catastrophe over the short and long term?

Out of scope
Any arguments about the Roman Empire not being able to evolve without the Mediterranean sea. History would have been very different but it’s there.

No magical involvement.

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    $\begingroup$ Would the Romans be capable of understanding the threat? Yes, they were not idiots. I believe that your question should assume that the authorities notice and decide to take action, as the answer to what action they might take is problematic enough. Realistically, no man made structure (even today) can keep the ocean out of that basin if the land-bridge is swept away. $\endgroup$ – AndreiROM Oct 18 '17 at 19:35
  • $\begingroup$ You like Glen Cook's books? $\endgroup$ – Alexander Oct 18 '17 at 19:59
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    $\begingroup$ I would also like to point out that this question can't really be science based unless you provide us with incredibly detailed engineering specs as to what the issue that must be overcome is. Do you intend for this civilization to simply build a wall all along the land-bridge? To somehow stop the erosion? To build a gigantic dam? We can't answer "scientifically" if you don't define the problem in the same manner. $\endgroup$ – AndreiROM Oct 18 '17 at 20:00
  • $\begingroup$ @Alexander No I am not familiar with Glen Cook’s work, I take it that he has written a fictional work with a similar scenario? If so I would be interested in any reference you have. $\endgroup$ – Slarty Oct 18 '17 at 21:07
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    $\begingroup$ Willing? On the evidence of at least one far more advanced civilization facing a situation much more severe and better documented than the one you describe, I'd have to say, surprisingly (and sadly), no; unless you can somehow explain how inertia, denial, deceit, and short-sighted self interest can be overcome or is not present to the same degree in your world as it is in ours. $\endgroup$ – orome Oct 18 '17 at 23:18
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The Romans made spectacular dams which by itself is enough to read about for an hour.

Long seawalls (much longer than the Strait) were under construction in the Netherlands even in Roman times and the principles involved there would be obvious to roman engineers familiar with what was going on.

The thing about this is that none of the lands we are familiar with would be threatened by the Mediterranean filling up, because none of them are under the Mediterranean. Italy is a long way from the strait - so far they would not immediately think that something at such a distance could affect them.

The deciding factor here are the people occupying the basin east of the land bridge. This world is not the ancient world plus a huge empty seabed. That seabed would be loaded with people, cities, farms, etc. If the Iberian Romans / North Africans who lived next to this breach realized their farms and cities could be flooded, they would be extremely motivated to fortify that wall, with help from the capital or not. Even the Dutch barbarians figured that out.

But if there was no-one east of the land bridge because it was a worthless salt marsh, people might not worry too much about it flooding. It might not be until it started filling in earnest and the gap was huge that the civilizations might do the math and figure out what would be next. It might then be too late.

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  • $\begingroup$ Land levels can be roughly projected by assuming the straights are entirely filled and the shore profile and slope of the land to either side are what exists from the land bridge facing east. The realisation or lack of it is part of the question, would the Romans have understood the scale of the calamity that would overtake their world within months or a few years at most if they did nothing? I would assume that all arable land in the basin was at least sparsely populated. $\endgroup$ – Slarty Oct 18 '17 at 21:45
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Would the Romans be capable of understanding the threat? Of course they would. This civilization spawned generals, statesmen, poets, philosophers, etc. and left a legacy behind still remembered today (2000+ years later). Those people did not think small. In fact, they conquered a good chunk of the world. They were innovators, and incredibly ambitious.

Now, that being said, could they take on the ocean and win? Could we?

If the land bridge was being eroded at a particularly narrow point, I believe that the Roman civilization would rally, and implement a pretty inventively engineered solution to the problem. They knew all about cement, and were experts at building fortifications, roads, and other infrastructure (such as aqueducts, and sewage systems).

As far as taking on a grand engineering project is concerned, that's not really an issue. If the Emperor decides that something should be done, then the entire might of the empire will be thrown against this challenge.

The Roman empire did not only have access to vast resources, but they also made extensive use of slave labor (plenty of those around at the time), as well as having access to many trained/skilled engineers (trained in the legions).

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  • $\begingroup$ Did Romans know about elevation over long distances? ie anyone knowingly living below sea level might grasp the problem, but would they know? $\endgroup$ – user25818 Oct 18 '17 at 20:09
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    $\begingroup$ @notstoreboughtdirt - not a lot of civilizations had to deal with that on a large scale. In fact, the first ones to really have to tackle that problem extensively were the Dutch, and they did so significantly later. However, this version of the Roman civilization has had extensive territories which exist under the sea level for a while now. The problem they are dealing with, is not reclaiming flooded territories, it's preventing the flood in the first place. And they certainly built canals/dams, so they can comprehend what would happen if the sea level rises over the land-bridge. $\endgroup$ – AndreiROM Oct 18 '17 at 20:16
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    $\begingroup$ Certainly the locals know which parts are above or below the sea, but will the next precinct over? And after some hills? Long distance (upto 100's of km) surveying was done when they needed an aqueduct, but they don't need salt water 1000km away. I don't think they had a concept of sea level that they marked everything against (and even more doubtful without the sea). Did they know if say Milan was higher or lower than Rome? $\endgroup$ – user25818 Oct 18 '17 at 20:45
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    $\begingroup$ @notstoreboughtdirt the Romans were also masters of communications, they had to be as far as their reach extended. Remember, these are a people who expected the empire to last forever, they only thought big. $\endgroup$ – Richard U Oct 18 '17 at 21:01
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    $\begingroup$ @notstoreboughtdirt They built one aqueduct system that was 170km long, including a 90km long tunnel. I don't think they did that without understanding "elevation over long distances!" And an anecdote on a slightly smaller scale: in the UK I used to live close to the site of a Roman villa that was known to have been built by a retired military commander. On a clear day from the site, you could see Lincoln cathedral to the south and York Minster to the north, 55 miles apart as the crow flies, major Roman military settlements, and linked by a Roman road (Ermine Street). Just happenstance??? $\endgroup$ – alephzero Oct 19 '17 at 4:11

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