University of Hawaii researchers have updated scientific models that confirm that low-lying areas of the islands face a lesser known but more insidious threat from sea-level rise generated by global warming: groundwater flooding.

A new study from UH scientists shows a large part of urban Honolulu and Waikiki increasingly swamped with groundwater as the rising ocean pushes up the water table, threatening some $5 billion of taxable real estate, nearly 30 miles of roadway, and a host of other facilities and assets.

This will happen regardless of any seawalls built, according to the study, thus requiring special engineering, costly public works and other measures to deal with the standing water.

“We’re talking about new wetlands forming in our urban core, and it’s all driven by sea-level rise,” said Charles “Chip” Fletcher, UH professor of geology and associate dean of the School of Ocean and Earth Science and Technology.

The study not only found the water table already close to the ground surface — within 2 feet at high tide in many places — but 86 percent of the 256 active cesspools in the study area are already likely flooded by groundwater.

That’s troubling news, researchers said, because it suggests that cesspool sewage is already entering the coastal groundwater and potentially finding its way to the ocean.

What’s more, this could grow into an even greater health concern as contaminated waters begin to appear in streets, sidewalks and gardens.

Fletcher and other UH colleagues joined doctoral student and lead author Shellie Habel in producing the research paper, published last month in the science journal Water Research.

Fletcher, Hawaii’s top expert on things related to the intersection of ocean and land, first identified the problem in a research paper published five years ago. He and colleague Kolja Rotzoll, who also added to the current paper, along with Aly I. El-Kadi, named it “groundwater inundation.”

Now, Habel has taken the research a step further by developing a computer model that considers a variety of factors — ground elevation, groundwater location, monitoring data, estimates of tidal influence and three-dimensional groundwater-flow modeling — to simulate flooding in the urban core as sea level rises.

The study concluded that a quarter of the study area will be flooded from 3 feet of sea-level rise, a figure that many researchers suggest may happen before the end of this century.

But it could be even worse. New analysis from the National Oceanic and Atmospheric Administration that considers recent studies of melting ice worldwide suggest twice that amount of sea-level rise is possible by the end of the century.

Making things worse, Hawaii and other Pacific tropical locations are expected to bear an extra 25 percent of sea level flooding compared to the global average.

“We’re talking about sea-level rise that is accelerating,” Fletcher said. “We have the West Antarctic Ice Sheet melting irreversibly. We have Greenland melting. We have the ocean expanding as it gets warmer and absorbs heat from the atmosphere. Sea-level rise is a huge problem that models indicate is going to continue for centuries.”

4-pronged threat

Groundwater inundation is one prong of a four-headed monster expected to batter our coastal regions on an accelerated basis over the next several decades and beyond. The other three will see waves sweeping over the shoreline, the land eroding away, and storms and tsunamis penetrating increasingly inland over time.

As for groundwater inundation, the rising ocean will literally lift Oahu’s “caprock aquifer,” an underground lens of brackish and polluted water that floats on a base of higher-density saltwater connected to the ocean.

The water table will eventually break the land surface to create new wetlands and expand others with flooding that becomes especially bad when high tide and rainfall coincide.

In actuality, the water table onslaught already is happening in some low-lying places. Ocean water is overwhelming some storm drains, flooding street gutters at high tide and leaving a smattering of sand on the street.

Habel said much of Waikiki sits on so little unsaturated soil that many construction projects working below the ground surface have to pump out water from their excavations before work can begin.

Need to ‘yield to water’

Fletcher called groundwater inundation the most insidious of the consequences of sea-level rise.

“Because how do you adapt to all these new wetlands?” he said.

One way, he said, is to pump down the water table. That’s what they’ve been doing in the Netherlands for centuries.

“You know all those windmills that the Netherlands is famous for? You know what they’re doing? They are actually pumps that are driven by the wind that lower the water table so they can have enough dry land to grow food on. They pump the water through a pipe to the ocean,” he said.

Hawaii could do that. But it may not be cheap. Holland sits on the Rhine Delta, which is slowly sinking. But that region hasn’t endured sea-level rise at the rates Hawaii is expected to face, the professor said.

Fletcher said Hawaii’s coastal communities have been developed with the typical mid-20th-century approach that assumes the land is permanent.

“That’s no longer valid in the coastal zone,” he said.

“The Dutch have a rule of thumb: If you wage war with water, you will lose. You need to yield to the water and you need to elevate, which means our communities will have to become completely different types of communities,” Fletcher said.

He said going forward Hawaii must build with a light footprint, with modular, flexible construction that can be relocated, elevated and ready for retreat.

“It’s a whole new concept, a whole new mindset for coastal community resilience,” he said.

For example, while you can’t stop a hurricane from hitting Ewa Beach, the low-lying bedroom community can be redesigned so that it comes back safer and faster after the hurricane, Fletcher said. Electricity and power lines can go in waterproof vaults at ground level instead of on telephone polls that can be knocked down. People with chainsaws can be designated for clearing roads in their respective areas. Others can be responsible for certain emergency medical supplies, such as bringing insulin to those who need it.

“Communities need to start communicating and planning and talking to each other,” he said.

A resilient community should be up and running within 24 hours of the storm, Fletcher said, “so that you’re not writing ‘Send ice’ in the sand like they did after Hurricane Iniki.”

“It’s got to be a whole new mindset,” he said, “but we still have a lot of work to do to get there.”

Habel, a former Pacific Beach Hotel aquarium diver who has studied under Fletcher since 2011, said adapting communities to the growing impacts of sea-level rise doesn’t have to be horrible.

“Waikiki could be just as cool, if not cooler,” Habel said. “Everybody loves water. If you look at the top 25 most romantic cities, most of them are right on the water.”

In this case, however, you might have to abandon first floors of buildings, put the lobbies on the second floor and build on stilts above a flood zone “like an elevated city,” she said.

Mass transit would have to serve the region from up above, and planners could create a walkable, bikeable community, with boardwalks and gardens below with places the water can’t reach. The beach, she said, will find its natural place below the buildings.

“You can have your beach and your city at the same time,” Habel said.

Fletcher said world-famous Waikiki Beach will certainly be forced to change under the growing pressure of sea level rise.

For now, nourishing the beach with imported sand (like it was done in 2012) can continue, he said. But as the ocean rise advances, sand replenishment will be needed more frequently and will become more expensive, with the sand receding faster and faster each time.

“If we want a Waikiki Beach, at some point you will have to yield to the ocean and allow the beach to find a new equilibrium, a new location for itself,” he said.

Working together

Climate change is happening faster than anybody expected, Fletcher said, but America, with the new administration in Washington, has “come to a dead stop in terms of dealing with it.”

Fortunately, Hawaii officials aren’t burying their heads in the sand, he said. In fact, the results of this report and its new model have been forwarded to the state’s Interagency Climate Adaption Committee, which was created by the 2014 state Legislature to produce a statewide sea-level rise vulnerability assessment and adaptation report by the end of this year.

The model is also ready for use in any other community around the world facing the same problem, including Florida and Miami, in particular, as well as other coastal areas in the Southeast.

Fletcher said the researchers five years ago didn’t have the faith in their flood maps to confidently suggest to the government any bold moves to address the problem.

“These maps we have the confidence to say, ‘This is a scenario for the future. Let’s work together. It’s worth investing in,’” he said.