Sea Change: Researchers Use Computer Modeling to Understand Rising Seas and Coastal Risks

In reality change is the norm along the coast, and although nothing is certain, a warming climate could mean quicker or more significant change over the next few centuries.

Scientists at RENCI and the University of North Carolina at Chapel Hill use the latest modeling techniques and high performance computing power to understand how expected increases in sea level over the next 100 years could affect coastal communities, wildlife and the coastline itself.

Most scientists believe that melt water from glaciers, the Greenland ice sheet and possibly the West Antarctic ice sheet, along with thermal expansion from warming oceans, will raise sea levels by one-half to 1 meter (1.6 to 3.2 feet) over the next century and by 1 meter to 2 meters (6.5 feet) over the next 200 years.

If that happens, North Carolina’s coast will change dramatically by 2100 or 2200, according to Tom Shay, a UNC-Chapel Hill marine scientist who conducts research with the UNC Institute for the Environment, the UNC department of marine sciences and UNC’s Center for the Study of Natural Hazards and Disasters.

“Some areas of the Outer Banks are only a few meters above sea level now, and there will be an increased tidal range and larger areas inundated by tides,” said Shay. With deeper water in the sounds, storm wave heights would increase as well, he said.

Some land areas, such as parts of the Albemarle and Pamlico peninsulas, will become submerged regardless of storms or high tides, said Shay. Coastal estuaries and the marine and plant life they support also might change due to the influx of higher salinity ocean water.

If sea level rises by a meter, “we will see higher tides, higher tidal velocities and tidal inundation every day,” said Shay. “And we’ll have a different shoreline.”

Shay uses the ADCIRC coastal storm surge modeling software and the SWAN (for Simulating WAves Nearshore) wave modeling software to create new “What If” models, showing how a major storm could affect the coast if sea levels were higher. The work complements work done at RENCI, which uses ADCIRC and SWAN to compute thousands of different possible storm scenarios. The Federal Emergency Management Agency (FEMA) uses the RENCI model output to develop new Flood Insurance Rate Maps (FIRMs) for coastal North Carolina as part of the North Carolina Floodplain Mapping Program.

Shay’s work zeroes in on one storm: Isabel, which made landfall in North Carolina in 2003 as a Category 2 hurricane. Using data on coastal topography and bathymetry compiled for the floodplain mapping modeling effort, Shay models the storm at current sea levels and at sea levels ranging from .5 to 2 meters higher.

Although his simulations show more pronounced storm surge and flooding during the hypothetical Hurricane Isabels, Shay stressed that the results depict a possible future, not a certainty.

“There are a lot of unknowns and uncertainties,” he said. “A warming ocean will cause some sea level rise just because of thermal expansion, but we don’t know how fast the ocean will warm or how fast the Greenland ice will melt or whether the West Antarctic ice sheet will melt. All those things have a range of possible outcomes and we want to understand as many of the possible outcomes as we can.”

Rising seas and changing risks
RENCI Senior Scientist Brian Blanton develops simulations using the same ADCIRC and SWAN models, but focuses on understanding the risks coastal communities will face from hurricanes and extratropical storms if sea levels rise.

“We are looking at the changes in risk associated with these storms—particularly risks from waves and surge—while making various assumptions about the future climate, such as increases in sea level, increasing rates of sea level rise and increased storm intensity,” said Blanton.

The work, funded by the North Carolina’s Floodplain Mapping Program, uses the same datasets as the floodplain modeling project and data from Applied Research Associates on probable hurricanes that could head toward North Carolina over the next 100 years. Blanton then uses that data to generate probable hurricane and severe storm events under different conditions that could be affected by climate change, such as higher sea levels, increased storm intensities and changes in the frequencies of storms.

The work will harness the power of a RENCI supercomputer to run about 2,000 individual storm simulations in six different climate scenarios, according to Blanton. Each simulation set will be crunched into statistical analyses that will help to clarify how coastal risks change under each climate scenario.

The end product, he said, will be a scientific evaluation of the risk of living on the coast—whether economic risk, risk to infrastructure or risk to lives—under changed conditions, which planners, emergency managers, scientists and policymakers will then be able to compare to their current understanding of coastal risks.

“With an understanding of not only the flood hazard data and the risk data but also of the uncertainty that goes along with that data—because the uncertainty can be quite high when your talking about a 100-year future climate—we hope to be able to inform the discussion on policy at the state, regional and maybe down at the county level as to what possible future risks may be,” said Blanton.