Since water seeks it's own level, blocking it at one point will just cause it to rise up somewhere else. When those so affected find out, they will demand an artificial barrier too. Therefore you will never come to any final conclusion.
This is similar to building new roads, tunnels and bridges-----the more that are built, the more you need. As Robert Moses found out.
No, the low lying areas that are vulnerable, will just be abandoned---when replacement costs get too high and new building codes too onerous.