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It'd seem, looking at this map, that all the streets of the Lower 48 States are already interconnected, so the alternate scenario is already off to a good start.

But let's look deeper. In this alternate scenario, each red line in this map is a boulevard, spanning a width of 360 feet. Each red line is made of "glassphalt", a kind of asphalt in which 10-20% of its weight is shattered glass. Four meters beneath them, each sewage tunnel is 32 feet tall and 160 wide, and the amount of methane contained from raw sewage depends on where we are, varying wildly from one part per dozen in the downtown area to one part per hundred on the open road.

In this alternate scenario, an ecoterrorist organization has planted a series of Livens Large Gallery Flame Projectors in San Francisco, Los Angeles, Seattle, New York, Chicago, Boston, Philadelphia, New Orleans, Phoenix, Dallas, Austin and Washington DC. On cue, the flamethrowers fire in the direction of the sewers. Within 77 hours, all of the Lower 48's sewers running parallel to all of the Lower 48's streets are lit in fire. Only the densest spots burst out, exposing the smelly holes to sunlight.

Once the fires have eventually cleared, these newly established manmade canyons become inundated with water from nearby rivers, diverting the old routes to the new ones.

Is this worst-case scenario in any way accurate, or do I need to rethink the effects?

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  • $\begingroup$ Where do you come up with this stuff? $\endgroup$
    – kingledion
    Dec 18, 2016 at 6:32
  • $\begingroup$ @kingledion Is there a problem? $\endgroup$ Dec 18, 2016 at 6:39
  • $\begingroup$ Pardon me but a recent conversation with a water utilities worker explained sewer pipes are quite narrow, because that's all they need to be. OK. It's alternative world scenario, but giant sewer tunnels are improbable. $\endgroup$
    – a4android
    Dec 18, 2016 at 12:00
  • $\begingroup$ Just make the methane leak in from mineral sources. Broken fracking plant? $\endgroup$ Dec 18, 2016 at 19:54

2 Answers 2

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Unfortunately, you'd get nothing!

Your sewage pipes have an enormous amount of capacity. Let's do a quick bit of maths to find out what that capacity is:

  • The total length of the road network in the US is about 6.5 million km.
  • The cross section of the sewer is 32 * 160 or 5120 sq feet.
  • That means the total volume of the system - the amount of gas it can hold - is an enormous 109,158,400,000,000 cubic feet.

Alrightey so we know how much gas the system can hold. Next up, let's look at the properties of that gas.

Methane and hydrogen in those sewers would spread out fairly evenly countrywide - this is just the natural diffusion process that gas does - except for the occasional 'hotspot' where the methane hasn't had the chance to diffuse much yet (Like that time you had a bit too much of the Chilli sauce).

Modern sewers - which are far smaller capacity wise - generally remain well below the flammable mixture threshold anyway. Add a lot more capacity and you have something that just won't ignite, except for, potentially, in those hotspots.

So, lets say the flamethrower was in the perfect position at the perfect time - the middle of a busy city on Taco Tuesday. Well, to get an explosion, you need the right mixture and it must be capable of building a pressure wave. After all, that's what an explosion is. In order to build a pressure wave, it has to be contained. The igniting expanding gas would simply spread down the pipes - giving you nothing more than a bit of an underwhelming 'whoosh' for a hundred meters or so down the pipe. No street bursting ball of fire unfortunately!

Side note: Explosives like C4 are 'high speed' because they ignite faster than the speed of sound, essentially making them get contained by the surrounding air to build an unusually large pressure wave. Hydrogen and methane aren't high speed explosives, which is why they whoosh in air when C4 does not.

Could it be made possible?

This seems like an interesting addition - so let's see what we can do!

  • Seal off all the pipes - make it so the pressure builds and has nowhere to go. Get it to the point where the structure is on the brink of failing by the pressure of the gas inside it alone. The expansion caused by ignition then pushes it over breaking point.
  • Add lots and lots of time! The best part of a thousand years of human waste gas building up in such a huge system would maybe get you to the hydrogen/ methane flash point for a truly monstrous explosion.
  • Add lots of water to the system. Unlike air, water isn't compressible, so it acts like a pressure container for virtually any ignition. The US isn't flat so the system won't be either, meaning large portions of it could be completely flooded with undesirables.
  • Poor management would result in a lot of logjams plugging up the system. Those would help pressure build where you need it; the biggest cause of blockages in city sewers is giant lumps of grease which people physically have to break apart. Maybe nobody has done this job for some time.
  • Humans don't naturally produce sewer gas at the perfect flammable ratio; the flash range for methane is rather small (about 5-15%). This means you'd have to actively regulate the mixture too.
  • Pesky safety experts would've shut the system down long before it became dangerous of course, so you'd also have to assume a kind of system that nobody knows how it actually works - a broken system that nobody, for centuries, can fix.
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  • $\begingroup$ Luke, would the fuel-air bomb idea below work, or would the sewers need to be sealed off once filled with fuel vapor? $\endgroup$ Dec 18, 2016 at 5:49
  • $\begingroup$ I don't understand where you came up with 3840 square feet. $\endgroup$ Dec 18, 2016 at 5:50
  • $\begingroup$ @JohnWDailey ah whoops I read 120; in which case it's even bigger of course; I'll edit those numbers. Thom: They'd have to be sealed, otherwise you'd still just get the 'whoosh'. An explosion occurs when gas is under pressure with nowhere else to go. $\endgroup$ Dec 18, 2016 at 5:56
  • $\begingroup$ How did you mean, "undesirables"? $\endgroup$ Dec 18, 2016 at 6:32
  • $\begingroup$ It was the politest word I could think of to describe urine and other, uh, non-solid waste! $\endgroup$ Dec 18, 2016 at 6:36
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Fuel-Air Bomb

You might have a better chance of success if you do not ignite the flamethrower's fuel, but rather make it spray out a fine mist of the fuel until the entire local sewers are full of fuel vapor. Then the sewers need (I believe) to be sealed shut quite thoroughly, since an air-bomb requires significant pressure to build up in order to create the primary shockwave. Once sealed, the sewers can be ignited, effectively making a thermobaric fuel-air bomb.

However, the range of the explosions in each city would likely not extend much beyond the city and also would likely result in a great deal larger canyon than just a trough. I'm no expert in physics, but depending on the amount of exact amounts of fueld, the strength of the enclosures on the sewers, etc., the explosion could likely wipe out much of the city block if you're not careful, leaving you with more of a cratered network, rather than a orderly linear system of mini canyons.

Some other mechanism of filling all the remaining sewers throughout the country with fuel vapor would be needed. I'm not sure the Livens is really the tool for that job.

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  • $\begingroup$ I don't think I follow you. $\endgroup$ Dec 18, 2016 at 5:43
  • $\begingroup$ Which part is unclear? $\endgroup$ Dec 18, 2016 at 5:44
  • $\begingroup$ Pretty much everything. I don't know how to make this clearer, but in this alternate scenario, the nation's population is greater, which means bigger cities, which means a national sewage tunnel system drowning in methane gas. Adding fuel vapor doesn't sound necessary. Also, "a great deal larger canyon than just a trough", which means what? $\endgroup$ Dec 18, 2016 at 5:46
  • $\begingroup$ I've edited this...does it help? $\endgroup$ Dec 18, 2016 at 5:59
  • $\begingroup$ These might help. $\endgroup$ Dec 18, 2016 at 6:01

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