A Disaster Long in the Making
How man distorted the delicate dance between the delta and the sea
Wexelblat's thesis was that our decades-long back-and-forth between nature and technology would explode in our faces. We're shortsighted in engineering technological systems and stupid in taking the measure of nature. Now the two are engaged in a destructive, possibly apocalyptic dance. "Humans have a very hard time thinking and operating on geological time scales," Wexelblat said. "... We think only about short-term effects: The Mississippi is flooding-let's build some concrete along it." Wexelblat disasters, as they came to be known, happen in slow motion, without anyone noticing for years. When they do happen, it's too late.
Louisiana State University geologist Kam-biu Liu had an interesting insight when he began studying the geological evidence of ancient hurricanes in the 1980s. Storm surges picked up sand from the Gulf, carried it over dunes and ridges, then dumped it miles inland in the pattern of a giant fan. Hurricanes thus left sand where it otherwise would never have been found. If scientists dug deep enough, they might find sand layers sandwiched between other soils. Since only a powerful hurricane could wash sand past the coastline, those ancient sand layers would be the fingerprints of the biggest hurricanes of the past.
Liu and some colleagues staked out five sites along the Gulf coast, including the Pearl River basin just northeast of New Orleans. Taking soil borings down 50 feet or more, Liu's teams tested the samples to distinguish squishy organic soils from dense, granular sand. When imaged and graphed, the sand layers leaped out in clear light bands.
Those patterns of light and dark helped Liu piece together a history of superstorms over the past 5,000 years. His research showed the Gulf coast to be a violent place. The results also indicated there had been only a handful of big hurricane strikes over the past 1,000 years or so, but during the 3,000 years before that, giant storms barreled ashore three times as often. This history suggests that the settlement of the Gulf coast over the past millennium had come about thanks in part to dumb luck: People arrived during a lull in the action.
It had been clear from the time of the city's founding in 1718 that hurricanes could flood New Orleans. In the 1970s, as the Corps of Engineers' levees rose around the city, they gave the shallow New Orleans "bowl"-and those of its suburbs-a higher protective rim. If a hurricane dumped enough water inside the levees, however, the resulting flood could now get quite deep-as much as 30 feet in some spots in the Lakeview neighborhood, 13 feet below sea level. Hurricane Betsy's flood had been trapped inside the St. Bernard and the Ninth Ward levees, but the central part of the city had never filled up. The very idea seemed almost unthinkable.
When computer modeling of storm surges improved in the 1980s and 1990s, however, it became clear that even a weak hurricane could put virtually any point under water. Thousands, maybe tens of thousands could end up dead. Hundreds of thousands could be trapped on rooftops. New Orleans itself could be destroyed.
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