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In my Pathfinder RPG campaign setting I have two continents separated by an isthmus. An artificial canyon carved through the isthmus from one ocean to another divides the continents. The canyon is five miles wide for the most part, but narrows at The Pinch to a spot one mile wide with a narrow bridge.

The canyon separates the eastern and western continents. It was carved out years ago by wizards, who placed a mountain at the north end of the canyon and a Wall of Force down the south end. These prevent the ocean from filling the canyon.

Between the south end and The Pinch, at the highest point in the canyon but still 2500 feet below sea level, a city has been built against the eastern canyon wall. The city, Blackmount, is the only point on the planet where an elixir that prolongs life may be made. It's also outside the legal jurisdiction of either political state on either side of the canyon.

The part I am stuck on, developing this city, is how they manage water. They're below sea level. How does fresh water get in? What do they do with waste water?

The feel I am going for is Age of Englightment / Pirates of the Carribean (so, roughly 1730 technology level) but in an alternate world where humans compete with elves, dwarves, halfings and gnomes for resources on the planet (and those are just the "good/benign" races). Magic is used by a small percentage of the global population.

EDIT: I agree that I could hand wave away the problems with water and sewage by using the term "magic" and say "The wizards dealt with it." My intent was to find a different way to address it.

With regards to food, trade, and roads: I have not planned this out in enough detail because I got stuck on the water issue. My current idea is that the (small) city is the only place on the planet that makes an elixir that may prolong life. This is sold annually in limited quantities for an exorbitant price. Part of that income is used to bring in food, water and other rare goods on special centaurs that are more mountain-goat than horse, who make their way down incredibly steep channels. Or maybe riders on giant spiders haul cargo up and down. There are no easy roads out, but gravity easily brings all manner of stuff down to the city; this causes its own set of problems.


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    $\begingroup$ How does food get in? Does the city trade with anyone? Are there roads that can be used? $\endgroup$ – nzaman Aug 19 '17 at 12:07
  • $\begingroup$ If you're playing Pathfinder and the city already has the spellcasting infrastructure for supporting a permanent wall of force then they should have the spellcasting infrastructure to cast as many castings of create food and water as they need. $\endgroup$ – sphennings Aug 19 '17 at 13:14
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It heavily depends on local climate, but bottom of a N/S crevice half a mile high by one mile wide would get very little direct little sunshine (around midday); this means having your wastes to evaporate naturally is probably not feasible.

To get water in you can rely on rain, if available, or on some (small) river flowing into the abyss.

In both cases your problem is how to avoid this basin to get filled with fresh water.

You can handwave it away saying the mage who prepared the Wall of Force spell made it unidirectional, so that anything leaning on it from the north side would be instantly pushed unto the south side of the wall (2500 feet under water, you need a good breath to escape that way!... not to speak about your eardrums). [Side Note: in this case you have a very nice way to produce "moto perpetuo" and get free energy just by having a pipe siphoning sea water in, use its 2500 feet drop potential energy with some kind of mill and then shove it through the wall returning it to sea; OTOH such energy might come from the spell itself, which may be weakened and, perhaps, fail after a while (if not "renewed")]

Otherwise you'll need to pump it up, most likely with an Archimedes screw.

Note that this freshwater-filling problem is completely independent from city. Such a closed crevice, if not in fully desert climate, is bound to be filled in reasonably short time, so Magician must have solved it when he built it, somehow.

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Reverse Osmosis

Actually, you have a really easy way to make fresh water in that underwater city: reverse osmosis

Assuming that the air pressure inside this city is actually normal atmospheric pressure, you have it really easy: you simply tap sea water through the wall and let the enormous pressure of being 750 meters / 2 500 feet below the surface do the job for you. The pressure forces the water through the reverse osmosis filters and... well, that is basically it, you get fresh water on the other side. The reason we do not do it more here on Earth is that creating the pressure needed to achieve this requires using a lot of energy. But in your city, you have that pressure for free.

As for waste water... well that too is fairly easy. You have a pipe through the wall leading to a tank. The tank and this outlet pipe need to be very sturdy and not flex when subjected to pressure. You fill the tank to the brim with waste water. Then you close the low pressure side of the tank where you filled it with water, and (very gently) open the outside valve. Then you simply use compressed air to push the waste water out into the sea. Naturally this is something you want to do downstream from inlets to the desalination filters.

And if you cannot get an air pump to produce 75 atmospheres of pressure... well then then you simply need to hoist the waste out of the canyon.

No, you cannot boil away the water, let the steam rise and then just hoist the solid waste that is left behind, because steam cannot rise indefinitely, because stream cools as it rises. The heat of the steam is being converted to potential energy as it rises, and so TAANSTAFL applies, even to steam.

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The water getting in part can be solved by collecting rain water in cisterns assuming the rainfall is suficient.

Similarly if it is hot enough waste water can be disposed of by moving it into shallow pools that evaporate by the heat of the sun, any remaining solid waste can then be shoveled onto carts and carted of.

This assumes a hot an relatively arrid climate similar to the mediteranian region.

If your climate is different other solutions might be needed.

Adding water is probably the simpler part, a system of canals on either side could divert controlable amounts of water over the edge.

Getting rid of the waste, thus avoiding a waterfilled canyon is probably the harder part.

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Create a hoop with Purify Food and Drink on it and use it to recycle the water. Then you just have to figure out how to transport the water uphill to holding tanks. You can do that through siphoning (32 feet at a time) or through some animated water pump.

Another possibility is set up Decanters of Endless Water on the high ground and a Sphere of Annihilation at the low point.

If you combine the two methods, (some Decanters to make up for water loss) you have a pretty good system.

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I'm going to suggest a controlled underground river that is so hot that water turns to steam and burst out of the ground through gysers(or supply a citys' water tank). It would be like a giant sprinkler system and probably look awesome causing lots of rainbows.

Though this seems slightly implausible, especialy as water boils without special geology or magic about 10000 feet below,which might not have the force to reach the surface.Though if this is volcanic terrain or magic is involved it becomes easier. I thought it was a cool idea and couldn't get it out of my head. Also hotsprings are suppost to be great for relaxation and health, maybe a start of a fountain of youth myth.

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