When the coastal town of Wrightsville Beach, which buffers nearby Wilmington from the open ocean, began to come back to life on Wednesday after Hurricane Florence hit North Carolina, many people returning home were surprisingly relieved.

Despite Florence roaring ashore in this exact spot last Friday and now causing catastrophic flooding inland, the hurricane had not destroyed the beach, as many had expected.

That could be because the US army corps of engineers and the state and local government this winter spent more than $9m dumping sand onto the beach to build it up, in a precarious and controversial effort to hold back nature. The corps has done this every four years for more than 30 years – and is now approaching a spending cap for the project.

Maintaining beaches this way, through what is called “renourishment”, will soon be more complicated than just finding funding, however.

Sea levels will rise up to several feet by 2100, and tougher hurricanes are likely to form in a warmer ocean, whirling in and repeatedly beating against the coast. With Wrightsville Beach, the city of Wilmington and the wider Carolinas suffering one of the region’s worst storms of all time, expensive, human-bolstered beaches like this one won’t work, ultimately.

That will be a problem, not just on the eastern seaboard, but everywhere with a beach. Rising oceans could also destroy two-thirds of southern California’s beaches by the end of the century, experts predict.

Some scientists have begun to argue for a retreat from the shore, after relentless development, but are emphatically shot down, particularly after a disaster. On Wednesday, visiting the storm zone, Donald Trump was upbeat about recovery from the hurricane.

But scientists are alarmed.

“Within a decade or two or three, it depends on where you are, we’re looking for a time when beach nourishment doesn’t work, so what do we do next?” asked Orrin Pilkey, an emeritus geology professor and coastal expert at Duke University. “Nobody, I don’t think, is thinking about how, when sea level rises, beach nourishment will be out of the question, will be economically impossible.”

The corps remakes beaches that are eroding from waves in an attempt to brace against major storms. It is hard to immediately determine how much sand survived after a storm. A beach can flatten out and look normal but have lost significant mass.

The corps is assessing the damages, but the Wrightsville Beach town manager, Tim Owens, said the area was lucky the storm landed where it did and wasn’t more intense and that renourishment “did what it was supposed to do, kept the water from overwashing and taking over property and infrastructure”.

While the replenished beach seemed to help, Owens said “all bets are off when you have a category 4 storm or 5 storm.” Florence landed as a category 1 but easily could have slammed into North Carolina with higher winds.

Asked about Pilkey’s retreat proposal, however, Owens quickly interjected.

“It won’t happen. It just won’t happen until it’s absolutely necessary … I won’t have to worry about it. It will be after my lifetime,” he said.

Typically, the US government pays for 65% of storm protection projects on public beaches. In Wrightsville Beach, the state and the town together pay the other 35%, with the town using revenues from a tax on room rentals. Sometimes after a disaster, the federal government pays to make a beach whole again. Owens said Wrightsville Beach may inquire about a touch-up following the hurricane.

Natalie English, the president of the Wilmington chamber of commerce, said the projects are a way of life on the coast, where tourism is a significant part of the economy.

“I think there’s been a lot of claims that beaches have been obliterated in North Carolina, English said. “I think it’s best that people understand that’s not the case. We will have a strong robust tourism economy again. We will be right back where we were.”

Erin Carey, the coastal coordinator for the North Carolina chapter of the environmental group the Sierra Club, lives about a mile and a half from Wrightsville Beach and was at headquarters in Raleigh earlier this week after evacuating.

Carey said: “When people fall in love with a place like they do with Wrightsville Beach, it will take an act of God or nature – maybe not even that – to pull them away.”

Because North Carolina has a law prohibiting more invasive seawalls, renourishment is seen as the main option for protecting development along the ocean. State lawmakers in 2012 also barred state agencies from considering long-term predictions of sea level rise.

North Carolina has about 320 miles of shoreline, with around half owned publicly and half privately, said Andrew Coburn, the associate director of the Program for the Study of Developed Shorelines at Western Carolina. Most developed coastline has been supplemented with extra sand in many states, he said.

Coburn said the effectiveness is unproven.

“The bottom line is we just do not have any evidence that spending that much money nourishing shorelines is even close to worth it,” Coburn said.

If the government is always willing to step in and recreate a beach, people will keep buying and building in vulnerable places, he warned.

“We are artificially changing shorelines and trying to make shorelines do what we want them to do,” Coburn said. “The problem is they’re going to do what they’re going to do.”

David Cignotti, who was mayor of Wrightsville Beach until the end of 2013, said beach towns are stashing away money in case the federal government ever stops paying for restoration.

“These beaches are used by people from all over the country … To me it’s worth it, ” he said.

Cignotti, who acknowledges climate change, favors stronger federal restrictions on development.

“It would be good to see our government not be one of only a couple in world that’s in denial this is happening,” Cignotti said.