The Water Wasn't the Rain's Fault
A simplified Sandy redux floods South Street Seaport 8.3 feet deep and does ~$18B in damage — yet only 3 inches of rain fall. The surge does the work; the sky barely shows up.
The storm arrives and the harbor climbs to 11.3 feet on the NAVD88 scale — a fixed national elevation benchmark, sea level's official zero. That's 9.0 feet above where today's ordinary high tide stops.
That extra 9.0 feet has to go somewhere, and it goes inland. South Street Seaport sits under 8.3 feet of water, Red Hook under 6.3, The Battery under 5.3. LaGuardia Airport takes 4.3 feet, Rockaway Beach 3.3, Coney Island 1.3.
Here is the quiet part: the rain is almost an afterthought. Peak rainfall hits 1.0 inch per hour, comfortably under the 1.75 inches per hour the sewers can drain. Over roughly 8 hours that totals about 3 inches.
For scale, Ida dropped 7 inches and the real Sandy dropped about 1. This scenario's sky is unremarkable. The damage — roughly $18 billion — comes almost entirely from the harbor, not the clouds. Hurricane Sandy itself did $19 billion.
Which is why the proposed fix is awkward. A harbor surge barrier would cost $52.6 billion and stop the surge — the part doing the $18B in damage — but it does nothing for rain. In a wetter storm, the barrier holds the front door while water comes through the roof.
The encouraging number: at this sea level, the count of chronic "doomed" zones — places that flood at ordinary high tide with no storm at all — is still none. That zero is the budget being spent. The NPCC projects sea level could rise up to 5.4 feet by 2100, and a 9.0-foot surge built on a 5.4-foot floor reaches a lot more than the Seaport.