My immediate thought is at what point does desalination tech + clean energy reach the crossover where building a 60 mile tunnel over 60 years not make sense?
It feels like very soon, and coastal cities can stop relying on hinterland reservoirs for water.
Probably never. The tunnels cost a lot to build but, once built run almost for free - they're powered by gravity and will keep running for close to a century before major maintained is needed.
Yeah that makes sense but if growth dictates another tunnel... And it takes another 60 years, your capital expense starts to look a lot like an operating expense. Not to mention one of the big stated purposes of this tunnel is actually to facilitate maintenance of the other tunnels. There is probably more operation cost hidden here than seems obvious.
The big reason for tunnel 3 isn't new population growth, it's so that the other tunnels can be shut down for maintenance and inspection. NYC's population is more or less stable over the last 90 years.
Today as in Ancient Rome the aqueduct terminates at fountains. It does not connect to the standard modern municipal plumbing. Whether that counts as being operated for "tourist purposes" is a matter of definition.
Capital vs operating is a big factor here. The tunnels operations & maintenance cost is probably far lower than a desalinization plant that could produce an equivalent volume of potable water.
> Desalination will be a West Coast thing. The East Coast has abundant fresh water.
It's not entirely accurate to say that the West Coast doesn't have enough fresh water. Oregon and Washington have a lot of rain, and many groundwater resources.
California kneecaps itself with perpetual deeded water rights and mismanagement/closure/lack of improvement to reservoirs and related infrastructure. There's a long history of this kind of stuff in the state (see the watering LA desert, the Salton Sea experiment, and many others).
It feels like very soon, and coastal cities can stop relying on hinterland reservoirs for water.