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I have a city, and let's say this city gets cut off, Berlin '45 style. Nothing in, nothing out. Classic siege. I want to be more specific than that though: Suburbs won't be in it, it will be all the most densely populated parts of a modern city, like Munich or London. No prep time, just suddenly as if a dome were put over it. No Rosinenbomber flying in food or any other input. No one can also leave or enter. While I want to know both with and without electricity/running water, if you can only answer one I'd like to know without either more.

Of course a city can't sustain itself for long, but how would the population curve look like? How quickly until the population has halved? How many people could, in a city like Munich (~1M people), actually live/where would the population level out? The people could fish in rivers that flow through the city, but if it has a coast they could not swim out much. (I know this still leaves a lot of variables, but I am still not sure how the city will actually end up. You can really take any western modern city as example for this for I can translate it to what I will need)

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    $\begingroup$ @JohnO That should be an answer. $\endgroup$ – Zeiss Ikon Mar 3 at 14:24
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    $\begingroup$ What Kind of research have you made? Stalingrad, Danzing, Warsaw. Water source is more important than food. Orlando or Las vegas would have much much much less time than New York or Washington DC. $\endgroup$ – SZCZERZO KŁY Mar 3 at 15:41
  • $\begingroup$ @SZCZERZOK┼üY - Orlando has a lot of fresh water laying around. Also tasty alligators. $\endgroup$ – Willk Mar 3 at 17:04
  • $\begingroup$ I think you need to specify a real world city to use as a model. Different cities would be VERY differently affected based on availability of various resources, geography, and local culture. $\endgroup$ – Nosajimiki Mar 3 at 19:57
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Since the question is not very specific, let's see what some of the issues might be:

  1. Breathable Air. This is only an issue in case of a "large dome covering everything" scenario. If the dome was air tight, air might run out sooner or later. The variables are too many, but the dome dimension is the most important one. If it is large enough and it includes a body of water and vegetation, you might have a mini-biome forming int oit, with it's own weather and air cycle. If it's not, or it has too little vegetation, air won't be recyled and it will run out eventually. The amount of people and animals, and every pollutant (eg. cars, engines, and so on) used inside it would make a big difference in the amount of time left before everyone suffucates.
  2. Water. Other than air, water is the first thing people need. A person can survive only a few days without water. If there is a source of clean water, then it's a no brainer. If there is a source of tainted water, then depending on the city the authorities (or someone else inside) might be able to set up a purification system; worst case scenario, boiling it makes it safe enough to drink. If there isn't a water source, there are two more things to consider: can one be found? what's the current amount available per person? Digging wells is probably the first thing people would do in order to find a water source. If none can be found, and there's no tools avaiable for digging deeper, then the town supply is all that's left. Depending on the amount of it, and the number of people inside, it could probably last a few weeks or a few days, not much longer.
  3. Food. Things get more complicated. In a perfect situation, any given town has a lot more food than it needs. Rationing that food could allow you to last for months or years. But that's a perfect world. In the first few days, most people woudn't ration their food, hoping everything to be over soon. A few more days go by, and perishables begin to go bad. If people are smart and capable, they could save a lot of it with several preservation techniques, but chances are it would just rot. I wouldn't give any town more than a month or two before food becomes a life or death issue. Consider people can survive about 3 weeks without eating, but that's not a pretty picture. Any animal, including pets, would soon become food. And assuming any is left, some animal food would become human food (not all, most is so disgusting you wouldn't be able to keep it down). Then there is the possibility of cannibalism: we've seen it in some extreme survival sitation in real life, and this would be one of them. Cannibalism is not a sustainable long-term diet, and it causes some severe damages to your brain if done regularly, but in the short timeit would work. So then again, lots of variables. And this is assuming there isn't the possibility of growing crops and farming animals inside. Hunting and fishing might help for a bit, but wouldn't do much to help a large city. It's mostly something the last few survivors might find useful.
  4. Medicine. This would be a real killer. Modern society relies heavily on medicine: from insuline to heart medicine, there's lot of people who rely on medical supplies to survive daily. These supplies would run out fast, and most would be hard if not impossible to replace or create. In a perfect scenario, all medicine would be gathered and given to medical staff, that would then give them only to those who can't survive without it. But we know better, and people would immediately raid pharmacies and hospitals, making it run out much faster than it should. And this brings me to the real threat:
  5. People. This is the main thing you should be concerned about. People don't react well in this kind of situation. Looting would begin almost immediately. Food and medicine would be the two main things people would hoard and steal, but especially in larger cities (meaning more people) you would probably see riots in the streets, stuff broken and set ablaze for no good reason, and destruction everywhere. Crime would raise, authorities would have a hard time controlling the situation, and things would get very ugly. This is especially true if people have no physical way to get out and have no common enemy to fight against (something that a real war siege would give you, for example). People are the main threat, since in a survival situation they wouldn't think straight and would do anything to last one more day.

Those are the 5 main things I would consider priorities and dangers. Electricity is important, but most western cities have some renewable energy sources now which would allow for at least some basic needs (eg. hospitals) to be covered. For a more specific timetable you would have to get a very specific example.

I can suggest you a couple of series that somehow touch this topic. One is called Jericho (2006), and one is Under The Dome (2013). I would suggest watching the first few episodes, as they do a pretty good job to show how people would behave in similar situations (later episodes are not really relevant to this)

Hope it helps

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I don't know about population curves and so on, but most people wouldn't last beyond a week (~3 days without water, ~7 days without food).

However, it's unlikely that the city would ever reach population 0. Beyond the odd I Am Legend-style scavenger, even in the concrete-junglest of cities you'd have groups of survivors who would eventually tear up a road here and there to create patches of tillable land. Assuming a steady supply of fresh air and sunlight (and fish in the rivers you mentioned), the city's decimated population could live on for generations.

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    $\begingroup$ They'd probably start farming the parks before tearing up the roads. And most cities have houses with back yards/gardens that would be easier to convert to farmland than ripping up blacktop or concrete. (Wait a few decades & Mother Nature would do most of the work for you.) $\endgroup$ – llywrch Mar 3 at 18:54
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It is perfectly possible for a future city to be self sufficient, growing its own food indoors in downtown farm buildings using hydroponics or aeroponics.

Or the city might have a totally closed cycle enclosed ecosystem like a moon base and use its waste matter to synthesize food, powered by fusion generators. If people can theoretically build enclosed ecosystems in moon bases they can theoretically build enclosed ecosystems on Earth.

So depending on the details of the fictional city's systems, it is possible that half the population will die every week or every day until only a few hundred are left alive out of millions. Or if the city has advanced enough future technology, nobody might die when the city is totally surrounded and besieged.

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