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"The applied theory of price", by Donald N McCloskey claims:

If all people flushed their toilets at once, the city water mains would burst.

enter image description here

How much damage (if at all) will a drainage system in a modern city incur if every household flushed their toilets at once?

  • Removed "what if" part as off topic and retagged. Can you edit in the specific textbook in question? Thanks. – Sklivvz Mar 4 '17 at 10:38
  • @Sklivvz Thanks for editing. Yes, I've added that. – ghosts_in_the_code Mar 4 '17 at 11:21
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    Long ago, before cable, you could watch a TV show only when it was broadcast. And of course everyone watched the Ed Sullivan show on Sunday nights. It was said that there was a noticeable decrease in water pressure in certain cities whenever Ed Sullivan had a commercial, presumably from many toilets flushing then. But I heard no stories of water main breaks because of that. – GEdgar Mar 4 '17 at 15:57
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    Recommend this be submitted to what-if.xkcd.com – Shadur Mar 4 '17 at 21:49
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    I know that waterworks and electrical companies / electrical grid operators in germany prepare for big soccer events, because the flushing of toilets during the half-time is difficult to handle, but more difficult is the sudden increase in power consumption when people switch lights on and open the fridge to get fresh beer. For events that have a lot of viewers, like e.g. the 2006 WM this is important. I sadly don#t have sources at hand, otherwise I'd post an answer. – Polygnome Mar 4 '17 at 22:41
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There have been fairly similar things happening before. Consider Edmonton during the 2010 US-Canada gold medal hockey game:

enter image description here

Here the green represents the previous day while the blue is the day of the game. Note the preparation before the game and the intermissions. The difference in the peak and trough at the end of the ceremony is 140 million liters. In a city of less than 900k people, this is about 160 liters per person. That's a couple dozen flushes each.

However, the biggest issue is that people aren't the only things that use water. According to this article, industrial water usage represents 40% of municipal water usage. So you would expect to see only an increase from 100% to 160% if someone did this.

Now, my city, Calgary, used 176 billion liters of water in 2014. This is over 20 liters per minute on average. Surely the peak times are much higher than this mean. The 1.2 million flushes of one gallon of water taking one minute to fill would only increase the mean by about 6%, and surely the peak is significantly less.

I think it's safe to say no, it would not come close to breaking.

  • I think it's safe to say no, it would not come close to breaking. At least, not in Canada... – xDaizu Mar 6 '17 at 12:44
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    I don't see any particular reason why major cities in Albeta (the two I'm most familiar with) would have drastically different water parameters than other cities in the world at least in the context of the claim. – corsiKa Mar 6 '17 at 15:00
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This is a hypothetical question - one that is unlikely to be answered by empirical evidence, so we need to rely on the opinions of appropriate experts.

Huffington Post set out to speak to two experts:

According to Michael Johnson, a civil engineer at the Utah Water Research Laboratory who models fluid flow in sewer systems, the consequences of mass flushing would range from negligible to pipe explosions depending on where you are.

[...]

Most cities could handle the flicks of all those handles without the risk of a water hammer. “I would think that most cities have water supply systems that are robust enough to handle the refilling of all toilets simultaneously,” said Ed Maurer, a civil engineer at Santa Clara University and a hydraulics expert. “For example, if one tank served 25 percent of my city (25,000 people) and even if we assume everyone had their own toilet to flush, the standard 1.6 gal/flush toilet would create a total need for 40,000 gallons, which is equivalent to a small pool.” Not so much in the grand scheme of things.

As for the outgoing sewage, we’d probably be saved from a plumbing disaster by a technicality: Simultaneous flushes don’t seem simultaneous from the perspective of the main sewer line. “A simultaneous flush would result in sewage arriving at a main trunk line from toilets further from the branch connection arriving much later than sewage nearer the branch to trunk connection,” Johnson said. In other words, rather than a peak, the main would receive a smooth curve of commode contributions.

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