Sinking coasts are increasing the threat from rising seas. What can NC do about it?

Land sinking on the North Carolina coast could lead to thousands more people facing threats from high-tide flooding, according to a new study.

Researchers used satellite measurements to understand how 32 coastal cities, including Wilmington, are rising or sinking and then modeled flooding under different emissions scenarios. The study, “Disappearing cities on US coasts,” was published in the peer-reviewed journal Nature.

About 2,250 additional people in Wilmington will be exposed to high-tide flooding if subsidence continues at the same pace, the researchers found, with the value of homes exposed to flooding in the Southeastern North Carolina city rising from $570 million in 2020 dollars to $950 million. The study was conducted by researchers at Virginia Tech University, Brown University, the University of East Anglia and IISER Mohali in Punjab, India.

If coastal planners fail to account for how subsidence worsens the impact of rising seas, the authors warned, they will not accurately understand how many of their residents will be harmed by future floods.

“People only think of 100-year floods and sea-level rise and planning for those, but they do not think about how what happens on the land could significantly increase the trends that they already have in most communities,” Leonard Ohenhen, a Virginia Tech Ph.D. candidate studying coastal vulnerability, said in an interview.

How quickly are NC’s coasts sinking?

North Carolina’s average rate of subsidence is about 1.4 millimeters per year, with a maximum rate of 4 mm per year, Ohenhen said. If those stayed consistent for a century, that would mean that state’s coast sinks between 5.5 and 15.75 inches.

“That would definitely have an effect on flooding extent for the coast,” Ohenhen said.

The Outer Banks are subsiding more quickly than the southern part of the North Carolina coast, with a 2016 research paper finding that Duck is sinking at a rate of 2 mm per year. That’s about 7.9 inches over 100 years.

At the same time, sea levels globally are rising about 4 mm per year.

Land subsidence is not accounted for in some sea level rise projections, which focus primarily on the expansion of water as it warms and on the addition of water to oceans as ice sheets near the poles melt.

As land sinks, though, the effects of rising waters are exacerbated. That impact is best accounted for in relative sea level rise measurement.

What causes subsidence?

Subsidence can be caused by geologic forces or by human activity.

Sinking land caused by geologic forces happens slowly, often over the course of thousands of years, said Andrea Hawkes, a professor at the University of North Carolina Wilmington’s Center for Marine Sciences.

Known as glacial isostatic adjustment, the change is occurring because the sheer weight of glaciers pushed gooey magma under the earth’s crust to the edge of the continent during the last ice age, about 18,000 years ago. When the ice melted, Hawkes said, pressure on the inner part of the continent eased up, allowing the molasses-like magma to creep back toward the inner part of the continent.

That process is causing Wilmington to sink about 1.2 to 1.5 mm per year, Hawkes said, or about 4.7 to 5.9 inches a century. That’s less than coastal cities to the north like Virginia Beach and Atlantic City and more than the Georgia or Florida coasts.

Another geologic factor that can cause subsidence is the compaction of sediment, particularly in coastal areas where the underlying rocks tend to be newer.

That’s less of a factor around Wilmington than in other coastal areas, Roger Shew, a University of North Carolina Wilmington geology professor, said in an email.

“We are underlain by harder rocks over much of the area that are nearer the surface because of a special geologic positive feature that is less compressible/less compaction,” Shew wrote, adding that some areas like Eagles Island on the western side of the Cape Fear River and marshes are subsiding more quickly.

What role can humans play?

Pumping water from aquifers for drinking water or industrial uses can also lead to subsidence, often much more quickly than the geologic factors.

The same process plays out on the Gulf Coast with oil and gas drilling.

“Within a very short timeframe you can see the effects of such extraction on the rate of sinking in a community,” Ohenhen said.

Sinking land in Wilmington

The study’s subsidence estimate for Wilmington is likely conservative, Hawkes said.

Part of that is because the researchers used current rates of subsidence and projected them forward to 2050. Subsidence often doesn’t take place in a linear fashion, though, particularly if it’s caused by pumping groundwater.

Another factor is Wilmington’s steady population growth and the development patterns that is causing.

If more intense development happens in places that used to hold marshes, Hawkes said, that could mean increased weight on rock that is still settling. Adding pressure there could mean increased localized subsidence, further increasing the threat posed by flooding.

Some of those places are obvious, like low-lying Eagles Island, which is currently being eyed for development. But elsewhere in the city, places where people could be most vulnerable to flooding exacerbated by subsidence are along creeks that rise and fall with the tides.

“They aren’t the ones on the oceanfront, they aren’t the ones necessarily on the river. They’re the ones up these creeks that see this flooding in a devastating way,” Hawkes said.

When friends or colleagues are thinking about buying homes in Southeastern North Carolina, Hawkes urges them to consider the property’s elevation. A home should sit at least 15 feet above sea level to be safe, Hawkes said, and ideally 20 feet or higher.

“People have to be smarter about where they choose to develop,” Hawkes said.

Are there solutions?

It’s impossible to counter the effects of longer-term subsidence caused by compressed sediments or by the Earth’s crust gradually shifting. But there are efforts to repair some effects of human-caused subsidence.

Amanda Martin, North Carolina’s chief resilience officer, pointed to wetland restoration as one way that subsidence could be mitigated. Taking land that was ditched and drained for agricultural use and returning it to a peatland or a pocosin would allow it to hold more water and prevent sinking, Martin said in an interview.

“If we can basically re-wet the soil, it will hold more water and it will be better for the natural habitat that we’re trying to restore,” Martin said.

In Virginia, the Hampton Roads Sanitation District is working on a $1 billion project that aims to treat 10 to 20 million gallons of wastewater a day and then pump it back into the over-drawn aquifer underneath the region. The project could scale up to 100 million gallons a day pumped back into the aquifer.

By refilling the aquifer, the Sustainable Water Initiative for Tomorrow project aims to reverse decades of withdrawals and effectively prevent the low-lying region from sinking further.

“The lesson here is that we do not want everything to go on as usual and secondly we do not want it to be worse than what the current rate is because that would significantly increase the number of current people that are affected,” Ohenhen said.

This story was produced with financial support from the Hartfield Foundation and Green South Foundation, in partnership with Journalism Funding Partners, as part of an independent journalism fellowship program. The N&O maintains full editorial control of the work. If you would like to help support local journalism, please consider signing up for a digital subscription, which you can do here.