Storm surges, which can inflict some of the worst damage that comes with hurricanes, are getting extra attention these days from scientists working to better understand – and predict – the phenomena.
Scientists are setting up arrays of sensors and putting forecast models through their paces in hopes of giving emergency personnel better information about surge hazards from a given storm. Their research also holds potential for helping planners evaluate options for improving coastal defenses against storm surges.
Research began to rise in 2004, when four hurricanes crisscrossed Florida, but hurricane Katrina in 2005 provided the biggest jolt, says Rick Luettich, director of the Institute of Marine Sciences at the University of North Carolina at Chapel Hill. "Every year out from a natural disaster, the national attention span wanes a bit," he says. "We just hadn't had a major catastrophe from hurricane storm surge" in many years. "Katrina rang a whole bunch of chimes."
That event gave the endeavor a fresh sense of urgency – and unleashed new funding to pay for the work, he says.
Surges occur as a storm's circular winds race over a swath of ocean. The wind's friction with the sea surface hustles the water along, piling it up until it bulldozes its way onto land. In the Northern Hemisphere, a storm's biggest pileup occurs to the right of the storm's eye along its line of travel, when viewed from the seaward side of the storm.
The surge height at landfall depends on the expanse and intensity of the storm, as well as the slope of the sea bottom offshore. A long, shallow slope – similar to the Gulf Coast's profile – can spell trouble. A surge at high tide compounds the problem. And the wind continues to whip up waves atop the surge. From a surge-saturated building's perspective, each cubic yard of water breaking against it is like getting punched by a wrecking ball weighing nearly 1,700 pounds.
The forecasting devil is in the details. Prior to hurricane Ike's landfall early Sept. 13, the National Hurricane Center issued "certain death" warnings based on an expected 25-foot surge near Galveston, Texas. Early estimates of the actual surge put the highest point at about 15 feet, near the Texas-Louisiana border.
Errors in surge forecasts stem from uncertainties about a storm's track, intensity, and size, says Wilson Shaffer, who heads the evaluation branch of the National Weather Service's Meteorological Development Laboratory. This year, the agency has started to issue forecasts based on the likelihood that a surge exceeding five feet will strike a given location.
Improving the accuracy of forecast models – by seeing how well a model reproduces past surges – can be tough. Surge-height data often are poor. Typically, scientists go into a surge-stricken area after a storm and gauge surge height using everything from water marks on buildings to debris hanging from trees.
Starting with hurricane Rita in 2005, the US Geological Service has, in advance of a tropical cyclone's landfall, strapped Magic Marker-size pressure sensors to building piers, telephone poles, or any sturdy mount they can find. The sensors track a surge's evolution over five to seven days. After a storm passes, researchers use surveying techniques to provide a standard height reference, then see what tales the sensors tell.
Sensors "were tremendously valuable in validating our modeling," says Joannes Westerink, a scientist at Notre Dame University who has helped develop a detailed surge model now used by several federal agencies.