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Worldbuilding

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    $\begingroup$ "All" you need to do, is to control the rate at which water can enter the tunnel. And to make the tunnel wide enough to handle that max flow rate. So, your flood defense will actually be situated outside of the city, at the place where the river enters the tunnel. $\endgroup$
    – cmaster - reinstate monica
    Commented Nov 12, 2018 at 8:08
  • 4
    $\begingroup$ Singapore has very similar problems with a majority of all canals/river beds in the city (due to rain mostly), you can look into the solutions they use $\endgroup$
    – Nicolai
    Commented Nov 12, 2018 at 8:55
  • 15
    $\begingroup$ 4 metres wide is more a large stream than a river, we'd casually put such a thing through a sewer if it was in the way. There are bigger rivers running under London. $\endgroup$
    – Separatrix
    Commented Nov 12, 2018 at 9:40
  • 4
    $\begingroup$ Bypass tunnel set at a slightly higher elevation than the normal run of the river, of 3-4 times the capacity of the main tunnel. If the water overflows, it will enter the bypass first and exit the city. $\endgroup$
    – nzaman
    Commented Nov 12, 2018 at 11:58
  • 3
    $\begingroup$ The Grand Canyon (Arizona, USA) will never overflow. Unless something is extreme, there will be the occasional flood. Because you build for 99 years out of 100. That unusual circumstance is too hard to build for and is the one that will get you in the end. $\endgroup$
    – Cyn
    Commented Nov 12, 2018 at 16:22