While it's good they are starting to think of technology solutions they might consider cheaper alternatives. Man-made fog can be generated for about $1 per square mile covered. Fog can be used to reflect sunlight out to space then burn off and let the ocean radiate its heat out at night. It can also be used as a blanket at night to retain heat. If you run the numbers it is much cheaper to make fog to regulate the climate than it would cost to build and maintain a multi-billion dollar dike around one American city. We only need to cool the temperature at the latitude where it is 32 degrees to keep the water from melting.
Water vapor is a greenhouse gas. The net effect is warming.
What NYC is planning for is tidal surges like the one that leveled the Mississippi coast.