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Suppose you have a modern city in a temperate climate zone that empties. No people, or hardly anyone left, certainly not enough to keep all the infrastructure going.

Our city is not Rotterdam (below sea level), and there is no large natural disaster (Earthquake, tsunami) in the timescale we are looking at. Just time, and weather, plants and animals and maybe a handful of people.

Our city has a mix of industry, high rises, suburbs, etc.

How does our city look like after 1 or 10 or 100 years?

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Ask any resident of Detroit about the empty parts... – Cyrus Feb 17 at 8:29
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Check out The World Without Us by Alan Weisman from your local library. It is a factual book based on the assumption that humans somehow instantly vanish now, and then explore what would happen going forward. It sounds like this should cover pretty much exactly what you are looking for. – Michael Kjörling Feb 17 at 8:42
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If I've learned one thing from Worldbuilding, it's that you'd better hope there isn't a (now-unstaffed) nuclear power station anywhere nearby when this happens... – AakashM Feb 17 at 12:16
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There is an entire show about this. It's called Life After People, and it used to be on the history channel. Maybe it still is. Try looking it up. – XandarTheZenon Feb 17 at 13:23
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@PlasmaHH On Tschernobyl... - or as you should say Pripyat, since that is what the local town called - suffered a waste amount of radiation which "disinfected" the area. For a long time, the radiation levels were too high for molds and pests to survive, so in a certain sense Pripyat was protected from declension. – mg30rg Feb 17 at 15:40

As @TheoclesofSaturn mentioned, Chernobyl (the city of Pripyat) is a very good place to start for this. The slow decay of the city allows you to see the effects over the first 30 years.

"Nature reclaims" is the general theme of this. Chernobyl being continental, it has large mammals including lynx, bison and wolves in equivalent numbers to non-contaminated areas.

Areas with solid ground cover e.g. roads, concrete floors, remain recognisable, though trees grow up in any open areas, it'll take a long time for the hard surfaces to break down in the absence of something like Japanese Knotweed.

If the city was something like Manhattan, all concrete high rise, it's going to be distinctively a city, long after a greener lower rise European city like London has mostly blurred back into the forest. The brick buildings will be weakened and slowly demolished by the trees growing around them. The roads slowly lifted and broken up by the root systems from the street trees. This will take decades, easily up to a couple of centuries. More importantly for the effect, the trees will grow taller than, and eventually over and concealing, the houses.

You could have concrete high rise showing over the forests for centuries. Roman concrete structures still exist 2000 years later, so ours will still be showing for a long time to come.

As has been noted in the comments, reinforced concrete breaks down faster than Roman concrete due to corrosion of the metal bars, however the structures will still remain for some considerable time.

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The steel buildings and rebar in concrete will corrode in @ 200 years, giving a fair limit as to when most modern structures will start to collapse. Roman concrete has no rebar in it, so it lasts much longer. – Thucydides Feb 17 at 10:36
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And people still use Roman buildings today in Italy. – Panzercrisis Feb 17 at 13:30
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2 cents: the city to look at near Chernobyl is called Pripyat. In fact barely typing the name into google images yields an unbound gallery of post-apocalyptic documentary. – Dallaylaen Feb 17 at 14:04
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@XandarTheZenon, I think it has a certain beauty even in the decay – Separatrix Feb 17 at 15:25
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@user16295 I don't normally look at decaying things and think beauty, but to each his own. – XandarTheZenon Feb 17 at 16:34

We have two more real-world examples to draw ideas from, in addition to the already mentioned Chernobyl. Those are Hashima Island, Nagasaki Prefecture, Japan, and certain areas of Detroit, Michigan, USA.

In the case of Hashima, there was a thriving mining town until 1974, then was suddenly abandoned. Today, some buildings stand, some have collapsed. It seems like it has become exactly that thing depicted in apocalypse movies- the very definition of creepy.

In Detroit, entire neighborhoods are mostly vacant. The reasons for this are debated, but the decline of the automotive industry and other economic factors are often blamed[citation needed]. In this case, there are city leaders making an effort to clean up and rebuild. However, they can only do so much so fast, and meanwhile houses and stores sit vacant and decaying.

Both examples show us that anything which is not maintained, is eventually ruined. Rain, wind, fungi, rust, pest animals, plant roots, or something else will take over. The only question is how long will it take.

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It would depend heavily on the type of city and climate. While some buildings may collapse due to lack of maintenance, the general structure of the city should stay intact. If it were a coastal city or one in a very hot/humid climate, the impact would be greater. To keep it interesting, let's assume the city is Bangkok.

Bangkok would be one of the most interesting cities to consider for a scenario like this as it has a horribly warm and humid climate, the wild- and plantlife there is diverse and the infrastructure itself is not as durable as an average modern western city.

Within a 100 years it would likely feature:

  • Heavily decaying buildings: Humidity and temperature have caused wood to rot and buildings to crumble
  • Buildings and streets 'taken back' by nature and covered with trees, vines and plantlife
  • Primate population to increase and use human structures as shelter, especially skyscrapers would be theirs to rule
  • Wildlife in general to move into the city for shelter

All in all the city would still be intact, but decaying at an increasing rate until skyscrapers start to collapse.

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I have just one thought to add to what others have said:

Are you assuming that all human life is gone, or that just this one city has been abandoned? Because if there are people outside the city, you could expect the city to be looted and used as a dumping ground. I live near Detroit. Large sections of Detroit have been abandoned. People regularly break into empty houses and gut the wiring and the plumbing for scrap metal, as well as looking for anything else of value. Also, I recall a news story not long ago about how a certain abandoned neighborhood has become a place where gangsters dump dead bodies. (I guess it's good to have a designated place for things.)

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Not only gangsters have designated places for the dead. – gerrit Feb 17 at 14:53

After one year, the city becomes a literal urban jungle. The high rises collapse after 100 or so years, and the buildings become covered in vines. The pavement would become heavily worn down. Think Chernobyl.

For a better example, check out the TV show Life After People.

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+1 for Life After People. Terrific show that clearly explains what happens to the things humanity leaves behind. – Nzall Feb 17 at 22:53

In addition to the other excellent answers, perhaps you can find some inspiration from the pictures of abandoned buildings on Ross Island, Andaman, India (example below), which shows what happens to abandoned cities in a more tropical environment -- tress quickly take over, turning the city into a Jungle-book fantasy.

Ross Island, Andaman, India

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I think the largest difference between most historical abandonments and a modern city would be that supermarkets might be left largely full, unlike in Detroit and Chernobyl, so a lot of people expect massive population explosion in rodents, since they're already so well adapted to urban life that controlling them is part of city maintenance. They'd die back after all the food was gone but you could probably expect plague sized populations in the mean time. Pretty grim.

Chernobyl is a good model in a lot of ways but there's one interesting caveat; no micro-fauna. In Chernobyl the background radiation is low enough to allow the return of lynx and bears but still high enough to massively inhibit the growth of bacteria and fungi which can't protect their DNA, so dead trees and leaf litter just sit where they fall, for decades. I don't know if there will be any radiation in your abandoned city but if there was I thought these could be useful details.

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Another real world example you could look at for inspiration is the city of Ordos, China.

https://en.wikipedia.org/wiki/Ordos_City

This was a city built to provide a workforce for the Mongolian coal fields. However it was essentially a failure, and the city for the most part is empty.

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Even concrete and steel buildings in abandoned cities do indeed "decay", not in a biological sense of the definition though. I would say the leading cause of structure erosion is water. Water in two forms: frozen and liquid. When water seeps into concrete or asphalt and then freezes (during the winter months) it expands and forces open tiny cracks which grow larger with time due to structural stress pulling them apart. Liquid water also damages steel support beams in structures due to oxidation (rust). In functional (occupied, maintained) cities in places that are exposed to water you'll often see the concrete or exposed steel painted with layers of water resistant paint. Over time however the paint chips off leaving the concrete and/or metal exposed to moisture allowing the water to do its thing. An abandoned city in a desert would perhaps fare much better over time assuming it is not bombarded with too much shearing sand wind. Vandalism and looting are the first things to make an abandoned city appear derelict and cause infrastructure problems. Scrap metal is a valuable industry and abandoned cities are the metaphorical goldmine for scrap.

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