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I'm the last person in a large city. Everyone else has left and I don't know when they will be back, if ever. I'm planning on staying in the city as long as I can.

I want to avoid, or minimize, a situation where tens of thousands of homes and businesses, that would have been otherwise usable, are destroyed when all the pipes burst during the spring thaw. I can take care of my own water needs, so that's not an issue.

This is a northern latitude city that sustains sub freezing temperatures for months at a time. The city has an extensive potable water system but this all feeds from a single reservoir. I believe there's a big valve (or set of valves somewhere that stops water flow but I'm not sure where they are.)

The goal is to remove as much water from the city's water system as possible. Removing all of it is impossible. Passive means of emptying all the pipes is preferable since it's just me and electricity service will stop in a few days, a week at most.

Shutting down natural gas and electrical service are different questions.

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    $\begingroup$ Are you talking about a city that has been built for the temperatures you will be experiencing, or are we talking a city that has not been built for it? $\endgroup$
    – dot_Sp0T
    Commented Aug 12, 2018 at 21:19
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    $\begingroup$ How long must the city be mothballed? If more than a couple years, then rodents/vegetation/frost heave/entropy may leave little worth returning to without extensive rehabilitation anyway. $\endgroup$
    – user535733
    Commented Aug 12, 2018 at 21:56
  • $\begingroup$ You may have a bigger problem. Backflow prevention devices aren't just required for sprinklers and hoses. Some municipalities are required to have them in the mains to prevent source contamination. Consequence: there is no way to drain the system. $\endgroup$
    – JBH
    Commented Aug 12, 2018 at 22:26
  • $\begingroup$ @dot_Sp0T the city is over 300 years old. It was made and grown for harsh winters. $\endgroup$
    – Green
    Commented Aug 13, 2018 at 1:47

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You actually have two separate issues: closing the outlet from the reservoir and then draining all the pipes in the city.

Shutting the municipal water supply might not be as difficult as you think. If you can find the city engineer's operations centre, it may be possible to close the valves from that point and stop water from flowing into the city water system. Based on the OP, there is still electrical power so the computers and vales should still work.

Draining a city sized system is going to be more difficult. You can theoretically drain part of the system by dumping the municipal sewage system and essentially creating an "open" system between the inlet and outflow, but the problem is the water trapped in millions of pipes between the reservoir and the outlet. For that you would literally need to go from house to house and building to building and open the taps so air can get ion the system and the water has a way to flow out (the air can get into the taps, and the water drain back down to the inlet manifolds, I'm not talking about draining out through the sink). Larger building might have valving to allow the water system to be closed off and drained from the utility room, but this largely depends on the building, when it was built and what systems were installed. You might have to turn off and empty the water heaters in order for the water in the pipes to flow out.

This could take quite some time. XKCD once did a comic asking how long would it take to visit every room in NYC, with the answer being @ 10 years. NYC is a very large city, so unless you are thinking about a megacity like New Deli, Tokyo, Shanghai, Mexico City etc. you could conceivably drain the water out of most buildings in a year or so.

Incidentally, this really is only an issue if the temperature is below 0C for several days in a row, when the water sitting in pipes freezes and expands. If you are really all alone in the world, it may be much simpler to head south and find a place with is not prone to sub 0 temperatures, in which case your major issue will be spoilage of foodstuffs and organic materials.

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  • $\begingroup$ Consider adding a third issue: Sources feeding the reservoir may need to be diverted (or un-diverted) to prevent wear or blockages and their associated damage. $\endgroup$
    – user535733
    Commented Aug 12, 2018 at 22:00
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    $\begingroup$ Most of the water in the mains will be safe though, as the water mains are all buried below the frost line underground. You'd simply have to go through and winterize every last house, and the works facilities.... $\endgroup$
    – Shalvenay
    Commented Aug 13, 2018 at 0:35

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