WE all have an intuitive sense of how water works: block it, and it flows elsewhere. When a storm surge hits a flood barrier, for instance, the water does not simply dissipate. It does the hydrological equivalent of a bounce, and it lands somewhere else.

The Dutch, after years of beating back the oceans, have a way of deciding what is worth saving with a dike or sea wall, and what is not. They simply run the numbers, and if something is worth less in terms of pure euros and cents, it is more acceptable to let it be flooded. This seems entirely reasonable. But as New York begins considering coastal defenses, it should also consider the uncomfortable truth that Wall Street is worth vastly more, in dollar terms, than certain low-lying neighborhoods of Brooklyn, Staten Island and Queens — and that to save Manhattan, planners may decide to flood some other part of the city.

I think I was the only journalist who witnessed the March 2009 unveiling of some of the first proposed sea-wall designs. “Against the Deluge: Storm Surge Barriers to Protect New York City” was a conference held at N.Y.U.’s Polytechnic Institute in Brooklyn, and it had the sad air of what was then an entirely lost cause. There was a single paying exhibitor — “Please visit our exhibitor,” implored the organizers — whose invention, FloodBreak, was an ingenious, self-deploying floodgate big enough to protect a garage but not at all big enough to protect Manhattan. When we lined up for the included dinner, which consisted of cold spaghetti, the man waved fliers at the passing engineers. But as I look back over my notes, I can see how prescient the conference was. A phrase I frequently scrawled is “Breezy Point.”

One speaker got a sustained ovation. He was an engineer from the Dutch company Arcadis, whose $6.5 billion design is one with which I suspect we will all soon be familiar. It is a modular wall spanning 6,000 feet across the weakest point in New York’s natural defenses, the Narrows, which separates Staten Island and Brooklyn. Its main feature is a giant swinging gate modeled on the one that protects Rotterdam, Europe’s most important port. Consisting of two steel arms, each more than twice as long as the Statue of Liberty is tall, Rotterdam’s gate is among the largest moving structures on earth. And New York’s barrier would stretch across an even larger reach of water — “an extra landmark” for the city, he said triumphantly. That’s when everyone began clapping.