Surging Water Warnings

Storm surge is not a giant wave like a tsunami, but it is the gradual piling up of water along the coast. It is often the most damaging effect of a hurricane, but is not often clearly understood and it is not easy to predict. New research, and a new computer program developed by the UNC Institute of Marine Science, is helping solve the problem.

CHAPEL HILL - It’s pretty easy to understand most dangers posed by a hurricane, including high winds, crashing waves and tornadoes. However the National Hurricane Center says many people don’t pay much attention to what is often the greatest threat to people and property: the storm surge.

“The storm surge is almost always the most dangerous,” says Rick Luettich, Director, The University of North Carolina Chapel Hill Institute of Marine Science. “If you are in a wind storm, there’s a good chance you cab survive. If you are in a flooding event then you have many fewer options.”

Dr. Luettich has been studying storm surge for almost 25 years and has developed the most advanced storm surge model yet. That’s because it is not only groundbreaking, lifesaving science but it is also personal.

“When people ask why I work on hurricanes, I pull out one of the beautiful satellite images of hurricanes that have come by the coast, I point out the coast, and then I show them the red dot that is my house," adds Luettich. “And then it is easy to have a personal conversation about what a hurricane means.”

Most people believe storm surge is a wall of water, similar to a tsunami. It’s not. Simply put, storm surge is the gradual rise of seawater being pushed by wind. You could also think of it as seawater being piled up along the coast. The storm surge level is higher than normal high tide level. Adding to the danger, the waves of the storm ride above the storm surge, which makes an already high wave even higher and more devastating.

Dr. Luettich’s model incorporates 27 data sources, including information about the size and wind speeds associated with the storm and how fast it is moving. But it also includes the water depth, the shape of the coastline and features on the land surface.

The resulting map readouts are color coded with red being the highest water levels, then orange, then yellow, then green and blue. 

To see the program in action, I asked Dr. Luettich to map the storm surge from Hurricane Irene, starting at 3am, August 27, 2011. The storm was still out at sea but even then, the computer screen showed the effects of the storm’s outer wind bands, already pushing water against the Outer Banks and across Onslow Bay and into the Neuse River.

“Now move ahead three hours and you can see it’s getting a lot worse,” says Luettich as he taps computer keys and motions towards the darkening colors along the shore. “The water is piling up along the Outer Banks and the shallow waters of Pamlico Sound are being blown quickly across the sounds and up the river. The New Bern area will be flooding soon.”

Three hours again pass in the program and Irene is right over Cape Lookout. The winds are at maximum force.

“You now have 10-12 foot storm surges along the barrier islands and you can see from the light blue colors here the eastern most areas of Pamlico Sound are almost dry,” Dr. Luettich comments as he nods towards the screen. People who live in the area still talk about what happened and how you could walk into the sound, if you wanted to walk in hurricane force winds.

“But now as the storm passes, and winds shift to blow towards the east in the opposite direction, the water starts to flow back,” he says.

The storm surge model’s first high profile use was in the forensic study of how badly storm protection systems failed during Hurricane Katrina. Because the storm destroyed most of the monitoring stations along the gulf coast, researchers had no idea just how bad the storm surge was. The system has been used by local and state governments as well as the US Army Corps of engineers, National Weather Service and the Federal Emergency Management Agency in every storm since then.

The storm surge information is influencing a change in how the National Hurricane Center will issue warnings this hurricane season. Because storm surge is such a threat, hurricane and tropical storm warnings will still be posted, even if the storm weakens considerably and loses its tropical characteristics. Hurricane Sandy is a good example of why the change is being made.

Warnings will also be issued using more common language that the agency hopes will easier to understand. For example, storm surge will now be expressed in height instead of depth to explain how water levels might change.

“We’re focusing on two things,” says Rich Bandy, Meteorologist-In-Charge at the National Weather Service office in Morehead City. “We’re trying to improve our modeling capability on storm surge and the kind of information we can put out, but we’re also spending a lot of time, and money and effort figuring out how people process information, and we’re trying to make the information we put out more understandable and help people understand their risk."

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