Mr Musk,

while the water may inevitably cause damage to the tunnel,

it doesn't really matter, as whether the water is

entering the crevices of the damage or not.

it's just a container that will be pressured by the volume

of the water and the gravity, or forced gravity by pumps.

so, the material in which the tunnel is made,

earth + construction aggregate + some coating,

will last quite a long while before it needs repairs.

it's the length that will make it costly so better off

to use most cost-effective materials....

if to create a network like this and re-route the water

during times of need...