The Storm That Came From the Sky, Not the Sea
Ida pushed still water to just 0.8 ft NAVD88 — a foot and a half below today's high tide. The flood arrived anyway, at 3.5 inches of rain per hour against sewers built for 1.75.
The harbor stayed put. Ida's still-water flood height reached 0.8 ft on the NAVD88 vertical datum — the national elevation benchmark — which is 1.5 ft below today's ordinary high tide. Coastal flooding: none.
By the storm-surge ledger, this was a non-event. Estimated surge damage came to about $0B. For scale, Hurricane Sandy did $19B.
And yet New York flooded. The water came down, not in — peak rainfall hit 3.5 inches per hour, while the city's sewers are sized to handle 1.75 inches per hour. The storm delivered exactly double what the drains could swallow.
It kept that up. Roughly five hours of rain dropped about 7 inches total — seven times the inch or so Sandy poured down in 2012. The surge that defined Sandy was absent; the rain that defined Ida was the whole story.
This matters because the city's marquee defense is built for the wrong threat. A proposed harbor surge barrier would cost $52.6B and stop surge — but not rain. Against Ida, a $52.6B wall would have done what the absent surge did: nothing.
The arithmetic only worsens from here. The NPCC — New York City's climate science panel — projects sea level could rise up to about 5.4 ft by 2100. At that level the surge math changes; the rainfall math, 3.5 inches against a 1.75-inch drain, was already losing.