“All of this was hit pretty hard,” said Kai-Uwe Bergmann, sweeping his arm from the East River toward the looming sprawl of the Baruch Houses, a public housing complex that sits along the Franklin D. Roosevelt Drive on the Lower East Side. “If another storm hits here in the future, it will be just as bad, probably worse.”

Mr. Bergmann’s job is to ensure that it doesn’t happen. As a partner at the Bjarke Ingels Group, a Danish architecture firm, he is one in a cast of hundreds trying to fortify New York against another storm like Hurricane Sandy, which ripped through the region two years ago this week. In the storm’s aftermath, there were calls for a single big fix, like sea gates that would close off New York Harbor to swells of rising water. But the solutions being tried out now are more widespread, and humbler, including stone revetments on Coney Island Creek to prevent “backdoor” flooding, and solar-powered streetlights on the East 12th Road boardwalk in Broad Channel, Queens, which is often flooded, even by lesser storms.

While only a few of the smallest projects have been finished, the vast constellation of proposals — backed by what one official called a “strange polyamorous relationship” of the city, state and federal governments — will most likely take years and billions of dollars to complete, if indeed that is ever achieved. If there is one guiding principle at work, it is the notion that the city, which has thumbed its nose at the water for 300 years, can no longer keep the sea at bay, but must by necessity invite it in.

“We didn’t want to just build barriers; we wanted to build an ecosystem,” said Henk Ovink, a Dutch water-management expert who now serves as a senior adviser to the Presidential Hurricane Sandy Rebuilding Task Force, a group within the Department of Housing and Urban Development, which has earmarked billions for the program. “For that to happen, we have to live with the water, to understand it, while still understanding our vulnerabilities.”