Extending some of my earlier questions
My preindustrial* society needs to control water flows on a massive scale. They need vast amounts of water to feed into the head waters of their draught stricken rivers south of the mountains. The only way of accessing it is to tunnel through into a vast lake to the north of a mountain pass.
Once south of the mountain pass, this water then needs to be split into 3 separate channels each 50%, 30% and 20%. They will have previously built canals east and west to distribute this water into the other river systems. They also need to arrange an annual flood of the rivers once a year.
Using this calculator https://www.copely.com/tools/flow-rate-calculator/ It would appear that a 10m wide tunnel dug 400m through the rock 30 metres below the lake surface might release as much as 2100 cubic metres/sec. This scale of tunnel should be possible for them to build.
For comparison (discharge rates in cubic metres per second) Nile 2,830 Mississippi 7,957 Amazon 209,000
Can this order of water flow be managed and controlled by my society? Or are they going to end up with an uncontrolled flooding on a biblical scale down one of their river valleys if they build the tunnel?
Things to consider: can the society carry out the necessary calculations? Can they build sufficient infrastructure that is sufficiently robust for all of that water? What happens when the tunnel through to the lake is first opened? Anything else you think is relevant.
Bonus: roughly how long do they need for construction?
Assumptions *no steam power, no internal combustion engines, no gunpowder. Cement is only available in limited quantities so cement mortar, but no Hover dams. This world is similar to Earth except it has much less water.