The largest city in the country is mostly a cluster of islands. Its inlets and rivers rise and fall with the tides.

Sandy produced a deadly storm surge, and in 2021, the remnants of Hurricane Ida introduced the damage of extreme rainfall. The next hurricane could bring both.

It would not have to be a major one. A weaker hurricane, dumping sheets of rain and moving in a northwest direction from the ocean, would wreak havoc, experts said.

First Street, a climate risk group in Manhattan, created a model of the damage a storm on such a track could have. In this example, a Category 1 hurricane would make landfall in New Jersey at high tide like Sandy, amid rainfall of four inches per hour — one of the more extreme scenarios.

The results showed a 16-foot storm surge, two feet higher than Sandy’s, which when combined with a torrential downpour, could put 25 percent of the city under water.

Today, such a storm is not impossible. It could happen about once every century, said Jeremy Porter, who leads the group’s climate implications research. “But it will become more normal with the changing climate,” Dr. Porter said.

Highways that hug Manhattan would see up to 10 feet of flooding, while farther north, a part of the Cross Bronx Expressway that dips before an underpass could be submerged up to 47 feet.

But Manhattan and the Bronx would largely fare better than the boroughs that border the ocean. Brooklyn, Staten Island and Queens, with miles of low-lying neighborhoods and dire drainage problems, would bear the brunt – over 80 percent – of the flooding.

Property damage across the city could exceed $20 billion, twice as much as Sandy caused, according to First Street.

Here are some of the neighborhoods, starting inland and moving toward the coast, that would see the worst of the destruction.

A large hilly ridge cuts through the middle of Staten Island, Brooklyn and Queens. Its natural elevation provides some of the city’s most spectacular views.

Bedford-Stuyvesant, in Central Brooklyn and north of the moraine, could see as much as 11 feet of stormwater, including along tree-lined streets with brownstones worth millions. Ground-floor apartments that can rent for as much as $4,000 would fill up like cisterns.

South of the moraine, East Flatbush could see nearly eight feet of water.

Four years ago, rains from Ida flooded the streets here.

“Water was gushing in from everywhere,” said Renée Phillips, 62, a 50-year resident. “That storm was something I’d never seen in my lifetime,” Ms. Phillips said. “And I hope and I pray that I never see it again.”

After Ida, Ms. Phillips escaped by wading through her flooded street while carrying two dogs and a cat. Her waterlogged property grew mold and the first floor had to be gutted.

She did not have flood insurance because she did not live in a designated flood zone. Ms. Phillips took out a loan for $89,000 to replace her boiler and fix the first floor. She was just beginning to consider repairs on the rest of her property when the deluge this fall set her back again. The boiler she had installed after Ida was destroyed, leaving her without heat.

“I’m distraught,” said Ms. Phillips, who was grieving her next door neighbor, and panicked about her finances.

“I feel like I have no control over the situation,” she said.

Ida flooded basement and first-floor homes here, killing three people.

Three years later, in 2024, at a community meeting, Rohit Aggarwala, the city’s climate chief and the commissioner of the Department of Environmental Protection, explained the reasons to residents.

“The area is a bowl,” he said. Kissena Park also was built over waterways and wetlands, he added.

But there was a third factor, Mr. Aggarwala said: A major sewer artery was there, responsible for 20 percent of storm and wastewater in Queens. When the sewer got overwhelmed, it created a bottleneck in Kissena Park.

All of these forces were at work during Ida.

Michael Ferraro, 32, who works in information technology, was returning from moving his car to higher ground, when he discovered that his street had turned into a raging river.

“I tried to swim, but the currents were taking me down,” he said, explaining that grabbing onto a fence saved his life.

Upsizing the sewer for Kissena Park would cost billions and take decades, according to the city’s Department of Environmental Protection.

Southeast Queens was once mostly salt marsh, which provided crucial protection against flooding. But over the years, city leaders filled the marshes in to build neighborhoods, highways and Kennedy Airport.

The water frequently returns.

In Hamilton Beach, when the tide is higher than usual, water pours into the neighborhood from a nearby basin and up through the sewers.

This August, on a clear evening, it flooded again. Some residents moved their cars to higher ground. Others, walking home from work, borrowed plastic bags from neighbors to wrap around their shoes. Sump pumps wheezed, and garbage bags floated through the streets.

Roger Gendron, 63, a retired truck driver and neighborhood flood-watch leader, took it in from his second-floor porch. “A storm that is hundreds of miles off the coast is doing this,” he said. “Just imagine what a direct hit would do.”

Hamilton Beach and other areas surrounding Jamaica Bay, the largest wetland in New York City, are prone to compound flooding, when heavy rain and coastal flooding combine.

The Department of Environmental Protection, which oversees the city’s water systems, has a 50-year plan to build out such a network. It is 10 years in and has spent over $1.5 billion so far. The work includes a major sewer expansion north of Kennedy Airport.

“If the airport were still a wetland, we wouldn’t have to build a gigantic sewer under the highway,” said Mr. Aggarwala, the head of the department, on a recent tour of the work site.

In 40 years, once the entire system for southeast Queens is complete, the pipes here and in other parts of the network will be able to transport over one billion gallons of storm water to the bay.

And this is just one corner of the city. It will take at least 30 years and about $30 billion to improve the parts of the sewer system that are the most vulnerable to storm water, Mr. Aggarwala said.

Throughout New York, city leaders are reckoning with decisions that were made some 100 years ago to build infrastructure on wetlands.

“The work is endless,” said Jamie Torres-Springer, president of construction and development for the Metropolitan Transportation Authority, during a recent tour of a subway yard.

The 30-acre subway yard in the eastern Bronx — the city’s third largest — was built over a salt marsh, where a tidal creek used to flow. Of the transit system’s 24 subway yards, which maintain and store thousands of train cars, 13 are vulnerable to storm surge.

The city’s two biggest yards now have flood walls, drainage improvements and other protections. Work on the eastern Bronx yard is scheduled for next year.

Sandy devastated the Brooklyn peninsula.

“We’re afraid every day that it’s going to happen again,” said Pamela Pettyjohn. During the superstorm, a sinkhole opened under her home.

She and other residents are concerned that the new developments, some of which include higher sidewalks and elevated bases that encourage water to flow under, around or through them, could worsen flooding in lower-lying areas, while taxing an already-overburdened sewer system.

And, with few ways on and off the peninsula, the addition of thousands of residents here could make a hurricane evacuation even more perilous.

After Sandy, Ms. Pettyjohn, a retiree, spent her savings rebuilding her home. She is living without heat because salt water from the storm slowly rusted out her boiler. The soaring cost of flood insurance keeps her from buying a new one, she said.

During Sandy, many bungalows in Midland Beach flooded, and they have since been repaired and put up for sale. Some are so inexpensive that New Yorkers can own them outright. Two neighboring bungalows, for example, are on sale as a package deal for $325,000, in a city where the median price for one home is about $800,000.

Without a mortgage, though, there is no mandate to buy flood insurance. Some homeowners could lose everything in the next hurricane.

“People are still deniers here,” Mr. Tirone said. They will continue to snatch up real estate deals in flood zones, he continued, until the government dictates to them otherwise.

He added: “The question is, ‘What’s that going to take?’ ”