Abandoning cities is the least inventive form of engineering.
The Dutch have been spending centuries on their land reclamation systems for most of their country. Half of their area is less than 3 feet above sea level. They have about 13,000 square miles to work with, both above and below.
Louisiana has over 51,000 square miles to work with and most of it is not below sea level.
There is no shortage of elevated land in the US.