Formulas to estimate storm surge inadequate, scientists say

ShareThis

Tens of thousands of Texas and Louisiana Gulf Coast residents who had to make life-or-death decisions last month about leaving ahead of Hurricane Ike faced one big uncertainty -- just how high would water driven by the storm rise.On the one hand, Ike was "only" a Category 2 hurricane, though a strong one, with top winds around 110 miles an hour -- just below the threshold for a Cat 3. On the other hand, the storm was spinning tropical storm force winds out over a rough circle nearly 600 miles across, one of the biggest hurricanes in memory.The National Hurricane Center warned Ike could produce a storm surge of up to 20 feet above normal coastal water levels, plus waves. But plotting storm surge, even with sophisticated computer models, is still as much art as science, because weather forecasters can't perfectly predict where a hurricane will go, how strong it will be when it gets there or where the strongest winds will be generated, even how fast it will be moving forward at landfall.Many researchers are concluding, though, that the rough formulas long used to match the winds of the Saffir-Simpson scale of hurricane strength to potential storm surge levels are inadequate, particularly along most parts of the Gulf Coast, as well as other spots where a wide, shallow continental shelf tends to push surge higher.What may matter more to surge size, a recently published analysis of nearly two dozen Gulf storms going back to the 1940s found, is the size of a storm, and particularly how far out a storm's most extreme winds extend from the center."The size of the storm as measured by how far out its peak winds extend seems to be the big determining factor for surge in the hurricanes we analyzed,'' said Jennifer Irish, an assistant professor of coastal engineering at Texas A&M University and lead author of the study, published in the Journal of Physical Oceanography.So, while 1969's Hurricane Camille, at Category 5, was the benchmark for storm surge for generations of Mississippi and Louisiana residents with a storm surge as tall as 22 feet in places, Camille's worst winds were only about 26 miles wide.Hurricane Katrina, at Category 3, hit much of the same coast in 2005 at a bit different angle, but with a top wind field twice that of Camille. That produced a storm surge of 25 to 28 feet along the beachfront and around some shallow bays many miles inland the water was even higher. Surge estimated even 10 to 15 feet lower was enough to overtop some levees around New Orleans and helped many others to fail.Irish said data on very large hurricanes is hard to come by, since there have been relatively few of them since the late 1960s, when satellite imaging and other surveillance systems became able to give a full ocean view of a storm.Likewise, instruments to measure storm surge have improved, although even during Ike, some were overwhelmed by the water. "Until the 1970s, all we have to go by were the high water marks on buildings that survived, and in some spots, none of them did,'' Irish said.Human nature being what it is, many people, particularly on Galveston Island, heard Ike was a Cat 2 and based on a little history and some internal scale of danger, opted to stay put.Ike's scope caught some residents by surprise, flooding low spots on barrier islands more than 12 hours before actual landfall and cutting off last-minute escape routes. There were hundreds of harrowing escapes and dozens of rescues.As it turned out, Ike's worst was more like 15 feet, maybe a few feet higher on the Bolivar Peninsula east to the Louisiana border -- the bad side of a storm that came ashore a little east of Galveston Island. But the big, battering waves that accompanied the storm eroded the Gulf shoreline from Florida to Texas for days."Ike gives us another example to model,'' Irish said. 'The preliminary data we're seeing shows that it was much like Katrina, although not quite as strong at landfall. Katrina's surge was also made worse because it developed over the shallow bottom of the Mississippi River Delta.Irish said she and study co-author Donald Resio of the U.S. Army Corps of Engineers Coastal and Hydraulics Lab are working on a new system for categorizing hurricanes that would take their findings into account. "We want to take 3 or 4 numbers for intensity, size and particular the slope of the ocean floor the storm's likely to cross, to give people a better sense of the potential threat."Understanding this threat in the long run should help people build and protect structures along a coast more wisely, and also help people living in a threatened area make better decisions,'' Irish added.'"If we can tell them that this is going to be bad, and why it is going to be bad for these particular sections of coast, we might be able to get them to take warnings seriously in a way that the Saffir-Simpson scale isn't doing for us now."On the Net: ams.org www.noaa.nhc.gov(E-mail Lee Bowman at bowmanl(at)shns.com. Distributed by Scripps Howard News Service, http://www.scrippsnews.com)