Science Advances on Calculating Storm-Surge Risk in New Orleans
In southeast Louisiana, crews are laboring hard to build miles of new fortifications to defend the region against another Hurricane Katrina-like disaster. But as local flood-protection officials learned last month, understanding and applying the evolving science of storm-surge and flood risk modeling is an even tougher race.
“What we know about storm surge is changing faster than most science and engineering today,” says Robert Turner Jr., a civil engineer and regional director of the Southeast Louisiana Flood Protection Authority-East, one of two consolidated levee boards in New Orleans formed in 2005 after Katrina devastated the region. “We have to be vigilant and re-evaluate periodically to make sure we have the protection we think we have.”
To gain new perspective on storm-surge science and its impact on federally designed levees as the region increases its level of storm risk reduction to 100-year levels of protection by the start of the hurricane season in June 2011, board officials called in a host of flood-protection and risk modeling experts on Jan. 26.
One of the board’s major concerns is the level of disparity in surge-height and flood-depth predictions among recent major studies, says Turner. These include a report by the Louisiana Coastal Protection Restoration Authority, Katrina risk-analysis maps developed by the federal Interagency Performance Evaluation Task Force (IPET) and local hydraulic studies that formed the basis of the region’s current surge-risk-reduction designs. “We want to know if there are valid reasons why they differ and if the processes leading to their conclusions were valid,” says Turner.
Back to school
“The physics is very complicated,” says Joannes Westerink, an applied mathematics professor at the University of Notre Dame, Indiana, noting the complexities of wave and storm surge modeling. “There is a lot of mathematics involved in this, and that mathematics is continuously evolving. We keep refining models.”
Don Resio, senior research scientist at the U.S. Army Corps of Engineers’ Engineer Research and Development Center in Vicksburg, Miss., says the agency was contracted by the U.S. Weather Bureau in 1959 to develop hypothetical hurricane models along the Gulf and Atlantic coasts. At the time, its model of destruction potential, which was based solely on wind velocity, was “the best of the best,” he says. Successor agency the National Weather Service later redefined the model to include storm-surge calculations.
Even so, Katrina proved that historical data isn’t enough to analyze a storm’s potential effects. “The Corps is trying to shift to more risk-based decision-making because hard-and-fast numbers don’t always give a real depiction of reality, ” says Resio, who introduced the concept of joint probability that factored significantly in the Corps-led IPET analysis.
“There is a problem with an approach that is not risk-based and not probabilistic,” says Ed Link, IPET study leader and a civil engineering professor at the University of Maryland, College Park. “You use fundamentally the same data steps to get at your best estimate, but you don’t know how much uncertainty you get. The traditional way to deal with uncertainty is to apply a factor of safety, but it’s only good for the failure mode you assume. If failure comes from something you haven’t analyzed for, your factor of safety wasn’t relevant.” Risk-based analysis already factors in uncertainty “that can creep into computations,” says Link. “You understand or deliberately try to manage and estimate it.”
The science underlying storm-surge modeling will also continue to change, says Robert Jacobsen, associate vice president of Baton Rouge-based Taylor Engineering Inc., which specializes in coastal environmental hydrodynamics. More research is needed on how waves react to vegetated onshore areas; such a study could provide the same level of empirical data for storm-surge prediction that now exists to calculate river flooding potential, he says.
Turner says the upgraded storm defense system will significantly reduce damage potential and satisfy federal and insurance industry flood-protection requirements, but “that doesn’t mean there isn’t any risk.” While the board should understand the science behind its decision-making, participants agreed its larger view on storm protection should include evacuation planning and personal responsibility.
Officials need to “step back and look at the big picture,” Link says. “That’s how you manage risk for communities.”