One thing to consider, though, is that this flooding would have taken place slowly over decades; it would have started with flooding during storms, which became

One thing to consider, though, is that this flooding would have taken place slowly over decades; it would have started with flooding during storms, which became more frequent over time until the sea slowly moved in. So they had plenty of time to move or waterproof that infrastructure. Though that calls into question the premise of the book, because they would have likely either filled in the land or built levees sometime in the half-century or so that the sea was rising.