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In my world there is a neolithic civilization of people who inhabit earthen trenches. The people first soften the earth with ... fluids ... and dig out 6 - 12 foot deep trench systems that extend out for many square miles with bamboo and ceramic trowels & spades.

Though digging out massive trench systems with what is essentially a grass stick is extremely labour intensive, they can provide all that they need in and around the trenches; planted oak trees to make acorn bread, planted potatoes, yams, snails and mushrooms for extra food sources, bamboo, hemp and stone for building material, rope, cloth, you get the point.

Also, since the trenches are easily defendable , they need not worry about invasions. However, there is one issue which they cannot overcome:

The Rain

When it rains any significant amount the entire trench system is partially flooded. Crops get drowned and buried, food is spoiled, paper and clay tablets get ruined and lost in the sludge, fungus grows on stored seeds and earth is turned into mud, which makes the trench practically unlivable until the trenches completely dry, which may take days.

That, however, isn't even the worse part. Any sort of flooding necessitates a complete evacuation. Everything a trench dweller may own is completely lost under the murky water, sometimes including their life (since they can't swim) and it basically turns their homes into a permanent canal, forcing them to start all over. On Top of all of that the problem follows them wherever they go, since they have to live near some sort of water source.

My Question is: how might such a civilization keep water out of their trenches?

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    $\begingroup$ Why are they using the trenches in the first place? What advantage do the trenches give them over living on the surface? How deep are the trenches? Why can't they just put up a roof to keep the rain out? $\endgroup$ – kingledion Sep 14 '16 at 4:03
  • $\begingroup$ @kingledion As said , trenches are much more defendable that houses , and you can't burn it down , lay siege to it , etc. And even if you manage to avoid spears , atal atals , sling stones , and arrow fire , it is really hard to defend small sections of a hostile trench , while really easy to stop the advance of invaders already in the trench. Also , trenches are cool in the summer , and heat easily spreads in the winter. As for how deep they are , I'll edit my post to include the info $\endgroup$ – user15036 Sep 14 '16 at 4:13
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    $\begingroup$ First, if things are the same as on earth, no one would use trenches. No earth civilization wasted time digging trenches when there were irrigation canals or pyramids to build. You need a logical reason for trenches to be useful, like a bunch of Tyrannosaurs running around eating people who aren't in trenches. Second, I have lit a fire in my house, which has a roof that keeps out rain, and I haven't been suffocated. Furthermore, I have been able to see during the day in my house with no lights on. The same windows and chimneys that I use would work just as well in a trench house. $\endgroup$ – kingledion Sep 14 '16 at 4:23
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    $\begingroup$ I suppose if I was going to attack these people, I'd probably just dig another trench connecting their trench to a lake. $\endgroup$ – sh1 Sep 14 '16 at 4:41
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    $\begingroup$ I would think twice about the defensibly. While yes, the trench cannot be burned down, the people in the trench can. You've ceded the high ground to the attacker before they even arrived. Any reasonable attacker would stay out of the trenches and leverage this advantage. There may be other reasons to live in a trench, but defensibly is not one of them. Trenches were good defenses in WWI because if you popped your head up, you got shot. Without guns, they become weak. $\endgroup$ – Cort Ammon Sep 14 '16 at 5:02
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With the massive amount of manpower they put into digging these trenches, they would certainly learn how to dig them correctly. Basic civil engineering would suggest that the entire network be dug in areas where water can flow out of the trenches. It'd be the inverse of what we do for irrigation channels. Instead of one source of water flowing into many channels, the trenches would need to be designed with a slope such that the water all flows into trenches dedicated to moving the water elsewhere.

Such a civilization would never build at the lowest point of a basin, so that they could always drain their trenches.

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    $\begingroup$ And trenches on hill tops or sides are even better defendable. $\endgroup$ – Jan Doggen Sep 14 '16 at 9:54
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    $\begingroup$ I'm imagining homes and gathering places would be Earth lodges built on or into hills, while the lower trenches could divert water to the fields. $\endgroup$ – Kys Sep 15 '16 at 18:10
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They would slope the trenches so that they would naturally drain to a sump. They would also build French Drains as part of their trench systems - layers of gravel or other quick-draining porous materials at the bottom of the trenches, and near the surface on the upslope side of trenches that run parallel to the landscape's contour. Even something as simple as a layer of wood planking 20-40 cm above the bottom of the trench would work.

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Ancient civilizations that did massive earthworks were often quite good at large scale earth shaping. In some cases, they flattened the tops of hills and made very level pads similar to what we can do with a bulldozer today. If a civilization lives in trenches, it is very likely that they would be not only expert diggers, but very good at laying out long straight runs and figuring out slopes.

I would think that each trench would probably have a sub-trench/gully either down the center or along one wall, and that the ENTIRE trench complex would be cut at a slight angle, creating a slope. Long trenches would follow the slope, shorter trenches would never quite be perpendicular, allowing good flow of water down to the lowest corner. Optimally, the entire thing would be built where there was lower ground nearby (or perhaps take up the top of a large, flattish hill). The lowest corner of the trench complex would have a channel cut to the lower terrain.

If you look at ancient sites like Petra, you see that ancient peoples had some pretty impressive hydro-engineering skills. Obviously in the case of Petra, they were collecting water in a dry environment, but they still cut a network of trenches and cisterns that collected rainwater from a wide area and stored it for them.

Which just gave me a second idea: instead of dumping excess water, they might make a BIG cistern lined with clay or whatever and store the water for later use. There are ways this could be done that would not lead to a contaminated pool of stinky badness but rather a cool "tank" full of drinkable water year round.

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Use bamboo piping into a lower reservoir (hydrodynamics) will keep water flowing out and where ever you want them to go, you could use it to produce a clean water source internally even by them throwing, I think Coal, and some sort of fiber to filter the water out which makes yourself even more defensible... You can also use this as a built in simple plumbing, which can then be used for fertilizer too...

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Building a rampart on each side of the trench would help further, and be the natural place to deposit the cutting material. Also, the trenches would be deeper as a consequence.

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The Trenches will have small trenches next to the "downhill" wall. Slabs of stone or wood can be used to mostly cover it, leaving openings for drainage. Small trenches are created out of and on the uphill side of Trenches to divert as much run off from entering the Trenches in the first place.

Wide canopy trees are planted on the rims of the Trenches to create a natural roof to cut down on the direct precipitation.

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