The Barrier Stops the Wrong Disaster
A Harvey stalled over New York drops ~82 inches in ~60 hours. The surge does ~$1B of damage. A $52.6B harbor barrier would stop the surge — and none of the rain.
Hurricane Harvey didn't sink Houston with a wave. It parked overhead and rained. Drop that same storm on Central Park and the model produces ~82 inches of rain over roughly 60 hours.
For scale: Ida dropped 7 inches and drowned basement apartments. Sandy dropped about 1. This scenario is ~82 — more than eleven Idas, back to back, without a break.
The city's sewers are built to handle 1.75 inches per hour. The storm peaks at 3.9 in/hr. That isn't a flood that drains; it's a system handed more than twice what it can swallow, for hours.
The coastal side, by comparison, is modest. Still-water flood height reaches 4.3 ft NAVD88 — a tidal datum — only +2.0 ft above today's high tide. South Street Seaport floods 1.3 ft deep. Estimated surge damage: about $1B, against Sandy's $19B.
So the threat splits cleanly. The salt water does a billion. The fresh water — 82 inches with nowhere to go — does the rest.
Which is what makes the proposed harbor surge barrier awkward. It would cost $52.6B and it works: it stops surge. It does not stop a single inch of rain falling behind it.
One number for the long view. There are no chronic 'doomed' zones today — nowhere that floods at ordinary high tide with no storm at all. But the NPCC, the city's climate panel, models up to ~5.4 ft of sea-level rise by 2100. That's higher than this storm's entire still-water flood height — before the rain starts.