Surviving in a city under constant threat 1: minimum requirements for survival and maintenance

Question

The fluff:

A new energy source is invented using the energy of the void and it takes the world by storm. By the time they realise it creates connections to other dimensions it is too late, and nightmarish+fantasy creatures will sporadically spawn in and wreak havoc.

A spawning can range from a few individuals to hundreds of thousands. These spawnings can happen anywhere, and while you have seconds to a minute warning by some sound and ripple effects in the air, it is rather problematic if one creature spawns in your bedroom while you try to sleep for example. The creatures are unpredictable, and can be passive and suddenly attack anything in sight. While moving things are preferred, they have no problem attacking lamp posts, doors, walls, roads, trees etc.

As another side-effect of a spawning is that for hours after a spawning, radio waves stop being able to produce understandable messages and any sub 0.2 micron computer component stops functioning properly, stranding the technology at around 2000 level of technology.

Due to the danger posed by these creatures both the infrastructure and humans have been decimated over time. To survive people are banding together and restructuring their society, losing access to much of their ability to transport people and goods or their agricultural food production.

The question:

To ask my original question I need to know the minimum requirements for a large group of people to survive, create weapons (preferably firearms) to fight off a spawning and maintain a city environment. Lets express it in minimum requirements per X population, say 100.000 people. You are allowed to express it differently if you want. For this question it does not matter how those minimum requirements are met, only what they are.

For clarity the question is: what are the minimum requirements for people to survive and maintain themselves in a city environment?

I'm looking for for the most essential things like clean water.

Answer 1

Let's look at clean water, since it illustrates the problem as well as anything else does. For example, according to https://www.cosatx.us/home/showdocument?id=1010, the processing steps for a water treatment plant are:

1. Collection
2. Screening and Straining
3. Chemical Addition
4. Coagulation and Flocculation
5. Sedimentation and Clarification
6. Filtration
7. Disinfection
8. Storage
9. Distribution

That requires, just from a casual glance, a steady supply of chemical coagulants/flocculants, chlorine additives, filters, maintenance and repair materiel for pipes and tanks, replacement parts for motors, pumps, valves, monitoring equipment, and myriad other things. All of that requires factories which depend on other factories that create plastic and metal parts, various chemicals from chemical plants, etc. and those factories and plants require trained personnel to operate them.

All of those things, in turn, requires raw materials like oil, iron, copper, and other ores, rare earths, etc. And all of those require the specialized facilities to extract them, other facilities to smelt/refine/process them, and the additional trained personnel to operate them too.

Now that just provides the supplies and replacement parts. Additional trained personnel are required on top of that to maintain and perform repairs on the water system and it's not a inconsiderable number.

On top of that, to design new equipment to cope with the situation, you also required a variety of specialized engineers: mechanical, electrical, chemical, civil, etc. to create new designs. And all of those engineering fields are big; like physicians, there are many subspecialties for each of those types of engineers. It takes years of education and on-the-job experience to create an effective engineer.

To finally get to the point: in the industrial age, there an enormous web of dependencies to build everything and that means to maintain an industrial age civilization, they need to maintain and staff a full industrial age supply chain. That means a functioning global transportation network, resource extraction and processing facilities, and a very long chain of manufacturing facilities and if a lot of humanity has been lost, pretty much everyone that's alive is either part of a supply chain or the defense forces protecting them. If they can't do that, then it's game over and they slide back to pre-Industrial Revolution tech rather quickly.

(What about scavenging and building your own local replacements, you say? Jury rigging solutions still requires machine shops, electronics labs, etc. that require their own supplies (cutters/bits, cutting oil, welding rods, etc.) and replacements for equipment that wears out or breaks. And even when it works, the output is still more resource and time intensive that a mass produced product. It's a strategy that can buy some time but it's not a long term solution.)