Click on the image to go to the North Carolina Flood Risk Information System map.
Click on the image to go to the North Carolina Flood Risk Information System map.

By Stephanie Soucheray

For Brian Roth, the mayor of Plymouth, any political back and forth about sea level rise holds little interest. Plymouth lies at the mouth of the Roanoke River, and Roth sees the water rise every time the community weathers a storm.

“The road to our sewer treatment plant is a quarter mile long,” said Roth. “And we can’t get out there when it rains. The roads go underwater. Our public infrastructure goes underwater more than our residences.”

Rising water, whether due to sea level rise or a storm surge, threatens the public-health infrastructure of North Carolina’s coastal communities. From standing water to corroded sewer pipes to overflowing waste-treatment sites, sea level rise has direct effects on the underground network of pipes and pumps that keep human waste away from potable water.

And depending on the rate of sea level rise, those impacts could become a problem quickly.

Two years ago, North Carolina state legislators passed House Bill 819, which banned projections of sea level rise based on climate change models and data.

While scientists agree North Carolina is, along with Louisiana and Florida, among the most vulnerable American coastlines to suffer the effects of rising water, HB 819 made sea level rise a political grenade lobbed between pro-development groups who argued sea level rise can’t be predicted and indignant environmentalists who believed that denying sea level rise is akin to calling the earth flat.

Roth was first elected mayor 12 years ago, and like most in a pre-Hurricane Katrina world had never really heard of sea level rise. But now, the former Navy pilot is trying to face the issue head on.

“None of us will be here in 100 years to say we were right or wrong, but if we believe this is true, the federal government needs to put money into solving [this],” said Roth. “There’s no harm in fixing infrastructure now that was built in the 1930s and 1950s. We need to go ahead and get it moved, not start studying it.”

What we talk about when we talk about sea rise

When people talk about sea level rise, they’re really talking about two different things, according to Tom Allen, associate professor of geography at East Carolina University. The first and most contentious factor is the height of the ocean rising globally.

“It could be from climate change, thermo-expansion, melting glaciers, you name it,” said Allen.

The second, and less controversial, factor is this: The eastern coast of North Carolina is subsiding under its own weight.

“There’s a regional trend of land sinking,” said Allen. “It’s sinking most rapidly in the Outer Banks.”

Those two factors converge to create situations like Hurricane Sandy, which, according to Allen, was a once-in-a-century storm. But sea level rise means more and more storms will become Sandys as the base level of the ocean rises and encroaches on the shore.

Click on the image to go to the North Carolina Flood Risk Information System map.
Click on the image to go to the North Carolina Flood Risk Information System map.

Sea level rise is a gradual process, and because its predictions rely on modeling, there are minimal and maximal estimates.

“We’re looking at a rise of three feet by 2100,” said Alex Glass. Glass, who has been outspoken against HB 819, is a lecturer and director of undergraduate studies in earth and ocean sciences at Duke University. Both Glass and Roth said that even a limited rise has the ability to severely damage coastal infrastructure.

“If the new baseline is one to two feet higher than what we currently have, it’s a compounding situation,” said Roth. “If you have commercial districts with water-inundation issues, that creates a whole other set of problems and challenges waterfront districts.”

Roth said that replacing parts of the existing infrastructure bit by bit tends to be the state’s modus operandi. But if sea level rise predictions are accurate, infrastructure will eventually have to be permanently moved.

“Moving to new locations, that cost is astronomical,” said Roth.

Sewers and sea level rise

For Larry Cahoon, a professor of marine biology at UNC-Wilmington, examining the effects of sea level rise on infrastructure boils down to one idea: “I and I.”

photo of flood damage that's destroyed a storm sewer.
Damage from 2011’s Hurricane Irene included damage to storm sewer systems in New Bern (Craven County). Photo courtesy NC DOT Communications

“Inflow and infiltration,” he said.

Inflow is the water entering a sewer collection system from above, like rainwater. Infiltration is water from the ground leaking into collection pipes through faulty joints. In 2011, Cahoon examined sea level rise’s effect on four waste-treatment sites around Wilmington. He matched the site’s flow data with national rainfall records.

“We found a surprise,” said Cahoon. There was a statistically significant effect of sea level on the flow of sewer systems. “It was not trivial. High tide can vary by half a meter or so, a storm surge could be a meter higher than normal, which produced quite significant effects of half a million gallons of water per day.”

In other words, the sewage systems can get easily overwhelmed.

Cahoon also had the wastewater operator at the Carolina Beach plant measure salinity to see if inflow contained sea water during high tide. Salt was there.

“By the time you can see [standing sea water] in the marshes, it’s already in your sewer systems,” said Cahoon. “That’s not a good thing. Sulfur in sea water [and sewage] generate hydrogen sulfide.” He said hydrogen sulfide smells bad and is corrosive, ruining sewer pipes.

“Sewage is nasty stuff,” Cahoon said. “Throw in a little salt, and it’s not good.”

Cahoon is currently collecting data from more than 100 sewer systems along the coast. While he hasn’t yet published his data, he said a number of these systems are showing I and I problems.

Image of Hurricane Sandy on Oct. 28, 2012 courtesy NASA Goddard Space Flight Center
If “100-year storms” like Hurricane Sandy become more frequent, more seawater could encroach on sewage systems in low-lying areas, making them essentially uninhabitable. Image of Hurricane Sandy on Oct. 28, 2012, courtesy NASA Goddard Space Flight Center

What’s next?

For Cahoon, the I and I problem means future development in coastal communities is rife with problems.

“There are serious concerns about extending central sewer systems into low-lying areas,” he said. “It’s going to be more and more problematic with more storms and weather situations.”

Cahoon said central sewer is a “hideously expensive” system, costing taxpayers approximately $1 million per mile of pipe. And, because of sea level rise, it’s a system that’s not working well. He said redoing one county’s sewer system can easily cost taxpayers $1 billion.

“The two allowed waste sites in North Carolina are septic tanks or central sewer,” said Cahoon. “Those two approaches for working and living at the coast aren’t working too well.”

Tom Allen said he’s currently working on the N.C. Coastal Atlas, which tries to share maps and flood information online. The atlas can help smaller communities look at risks flooding poses to infrastructure.

“There are vast differences in the ability of local jurisdictions to do anything,” said Allen. “Wilmington is able to handle things better than scores of small towns and poor counties still trying to discover their vulnerabilities.”

photo of sewage treatment plant ffrom the air
Sewage treatment plants located just above sea level are at risk of contaminating local waterways during flood events. Image courtesy David Weekly, flickr creative commons

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