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Many worlds have an apocalyptic/cataclysmic event. If mine is a Great Flood, how long would it take for much of the world to be habitable land again? Keep in mind, by the events of the present, the surface of the planet should be ~65% water. The planet is, on average, colder than earth and circa the same size, and civilization doesn't need to recover, camps and villages will suffice. Also, how much would the geography change? And is there a chance of (RELATIVELY) intact artifacts and underwater ruins existing? The civilizations before the floods were quite advanced, so I don't think it's unlikely.

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    $\begingroup$ I presume you have already reviewed all of the literature from Ken Hamm etc. While I personally think he's a fruitcake, he's got an explanation for everything, and there are some decent geological gems buried in his material. $\endgroup$ – pojo-guy Jan 1 '18 at 14:51
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    $\begingroup$ Water never goes away; Earth has just about as much water as it always had. How great is a "Great" Flood? 100 square km, 1000 square km, 10,000 square km, 100,000 square km? Does the (slow in human terms, but lightning fast in geological terms) flooding of low-laying plains at the end of the Last Glacial Maximum count as a "great" flood? (It did make Great Britain into an island, which should count as "geography change".) Does the Black Sea Deluge about 7500 years ago count? (It may have flooded 155,000 square km in about one year.) $\endgroup$ – AlexP Jan 1 '18 at 14:57
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    $\begingroup$ @pojo-guy actually, it's the first time I hear about Ken Hamm. From what I read he may be relevant, so I'll look into his work if I can. $\endgroup$ – Virdex_ Jan 1 '18 at 14:58
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    $\begingroup$ @AlexP I suppose the Black Sea Deluge could be a local variant, but I was thinking about a global event. I don't really need all the previous lands to be dry again, nor do I need the flood to make the planet as a whole an ocean, just something that destroys civilization. An Ice Age thing would do, too, and the flood could just as well be the result of said Ice Age. In fact, yeah, I like it. $\endgroup$ – Virdex_ Jan 1 '18 at 15:06
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    $\begingroup$ Ah, a global flood as in Noah. This question missing a "magic" tag, and seriously misuses the "science-based" tag. No magic means no global flood -- Earth does not have enough water; not even close. $\endgroup$ – AlexP Jan 1 '18 at 17:18
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It can take however long you want, and the faster it goes down, the more destruction it causes.

What Happens

Let's just assume that the planet was dry, and the water magically appeared. Maybe it fell from the sky (an inconceivable number of comets? Shell of ice around the earth?), but whatever happened, the entire planet is now covered with water.

Now we have to get rid of the water again. Barring un-physical things, we have to drain the water somewhere. Now, here is the fastest way of draining the planet that I can think of:

  1. Assume the planet had very few surface features - mountains are only hundreds of meters tall, valleys are only hundreds of meters deep. Maybe the whole world is only 3 kilometers in vertical distance from the tallest mountain to the deepest trench
  2. The entire world floods. You don't need as much water because the world is largely flat
  3. Wait 40 days and 40 nights....
  4. The sudden influx of water causes cools (or heats) the Earth's crust, and this causes massive changes in the tectonics
  5. The planets surface now has mountains 10's of kilometers high, and 10's of kilometers deep
  6. Water races into the trenches that formed, carving huge gullys and washing away everything.

How long it takes

According to the National Park Service, water takes three months to flow down the Mississippi river. I couldn't find stats for the Nile, Congo or Amazon, but at least we have one datapoint. In reality, you'd get dry land forming significantly before then, but you'd end up with lots of swamps that took even longer to dry.

This is only valid if you have a situation like the one presented above where all the water can go somewhere at the same time. The other option is a magic portal: https://what-if.xkcd.com/53/ This assumes a 10m portal draining the water, and it takes hundreds of thousands of years. Thankfully we are not required to use a 10m portal to drain the water from the great flood. So the time really takes however long you want.

The results of the draining

If you drain the water fast, nothing will survive. Water is hugely hugely destructive. Here are some videos for you:

We have a situation thousands of times worse. The ground will also change shape massively, forming new canyons. Big rivers can pulverise rock, so I think it's fair to assume nothing big of your civilisation is left. Maybe small chunks of ground up concrete and shards of metal, but likely nothing easily identifiable.

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