Think Sandy was just a 100-year storm that devastated New York City? Imagine one just as bad, or worse, every three years.

Prominent planners and builders say now is the time to think big to shield the city’s core: a 5-mile barrier blocking the entryway to New York Harbor, an archipelago of man-made islets guarding the tip of Manhattan, or something like CDM Smith engineer Larry Murphy’s 1,700-foot barrier — complete with locks for passing boats and a walkway for pedestrians — at the mouth of the Arthur Kill waterway between the borough of Staten Island and New Jersey.

Act now, before the next deluge, and they say it could even save money in the long run.

These strategies aren’t just pipe dreams. Not only do these technologies already exist, some of the concepts have been around for decades and have been deployed successfully in other countries and U.S. cities.

So if the science and engineering are sound, the long-term cost would actually be a savings, and the frequency and severity of more killer floods is inevitable, what’s the holdup?

Political will.

Like the argument in towns across America when citizens want a traffic signal installed at a dangerous intersection, Sandy’s 43 deaths and estimated $26 billion in damages citywide might not be enough to galvanize the public and the politicians into action.

“Unfortunately, they probably won’t do anything until something bad happens,” said CDM Smith’s Murphy. “And I don’t know if this will be considered bad enough.”

Sandy and her 14-foot surge not bad enough? By century’s end, researchers forecast up to four feet higher seas, producing storm flooding akin to Sandy’s as often as several times each decade. Even at current sea levels, Sandy’s floodwaters filled subways, other tunnels and streets in parts of Manhattan.

Without other measures, rebuilding will simply augment the future destruction. Yet that’s what political leaders are emphasizing. So it might take a worse superstorm or two to really get the problem fixed.

The focus on rebuilding irks people like Robert Trentlyon, a retired weekly newspaper publisher in lower Manhattan who is campaigning for sea barriers to protect the city: “The public is at the woe-is-me stage, rather than how-do-we-prevent-this-in-the-future stage.”

He belongs to a coterie of professionals and ordinary New Yorkers who want to take stronger action. Though pushing for a regional plan, they are especially intent on keeping Manhattan dry.

The 13-mile-long island serves as the country’s financial and entertainment nerve center. Within a 3-mile-long horseshoe-shaped flood zone around its southernmost quadrant are almost 500,000 residents and 300,000 jobs.

Proven technology already exists to blunt or virtually block wind-whipped seas from overtaking lower Manhattan and much of the rest of New York City.

This ranges from hard structures like mammoth barriers equipped with ship gates and embedded at entrances to the harbor, to softer and greener shoreline restraints like man-made marshes and barrier islands.

Additional landfill, the old standby once used to extend Manhattan into the harbor, could further lift vulnerable highways and other sites beyond the reach of the seas.

Even more simply, the rock and concrete seawalls and bulkheads that already ring lower Manhattan could be built up, but now perhaps with high-tech wave-absorbing or wave-reflecting materials.