MY AMERICAN SCIENTIST
LOG IN! REGISTER!
SEARCH
 
RSS
Logo IMG

ESSAY

Natural and Unnatural Disasters

Reflections on a city made possible and made vulnerable by reliance on technology

Brian Hayes

Backdoor Flooding

The topography of southern Louisiana is just the opposite of what you would find in an upland river valley. Instead of having a concave profile, where elevations increase as you move away from the river, the delta lands are highest right beside the river bank, where centuries of floods have piled up alluvial deposits. The oldest neighborhoods in New Orleans, including the French Quarter, were built on a particularly high bank on the outside of a sharp bend in the river. As the city grew, however, the only land available was in the "back swamp," which gets lower as it extends north toward Lake Pontchartrain. Much of this area was never more than inches above sea level, and as the land was developed and drained, it sank lower still. Despite the levees and drainage canals and pumps, the "bottom of the bowl" is inevitably subject to flooding.

Craig E. Colten of Louisiana State University gives a chronicle of the city's floods in his recent book An Unnatural Metropolis: Wresting New Orleans from Nature. It's a long list, with two dozen major events in the 19th century alone. In the 20th century, as levees and other protective measures became more robust, urban floods were less frequent; when the barriers did fail, however, the consequences were often more severe. In 1937 the gates of the Bonnet Carré Spillway were opened for the first time; the strategy succeeded in lowering the Mississippi, but Lake Pontchartrain rose so fast that a lakefront levee slumped and allowed water to pour into Jefferson Parish, west of the city. A 1947 hurricane overtopped several levees along the lake shore, flooding 9 square miles in New Orleans and 30 more in Jefferson Parish. In hurricanes Flossy (1956) and Hilda (1964), lake water spilled over the walls of the Industrial Canal, which connects the lake and the river along the eastern edge of the city. Hurricane Betsy in 1965 caused a breach in the west wall of the Industrial Canal, submerging the city's Ninth Ward to a depth of eight feet. Four years later, Hurricane Camille devastated the Mississippi Gulf coast, just as Katrina did this year, but damage was lighter in New Orleans; again, however, a section of the Industrial Canal failed. The canal was also the source of flood water during a 1983 winter storm; in that case, officials neglected to close flood gates.

Below ground zeroClick to Enlarge Image

The exact sequence of events that led to the post-Katrina inundation has not yet been made clear as I write this in mid-September, but preliminary reports suggest a familiar pattern. Storm winds and tides pushed water up against the south shore of Lake Pont-chartrain and into the canals that penetrate from the lake into the city. Again there were breaks in the Industrial Canal, caused, according to some reports, by a barge that broke loose and battered through a concrete wall. On the 17th Street drainage canal—the one that carries the outflow of Pumping Station No. 6—a huge gash opened up sometime on the night of the storm or the following morning. Two more major leaks appeared in the London Avenue drainage canal, a few miles east of 17th Street. According to the Army Corps of Engineers, the two drainage canals failed when water overflowed their concrete walls; the resulting cascade eroded the earthen foundations, eventually allowing the wall panels to topple over. Ivor van Heerden, deputy director of the Louisiana State University Hurricane Center, offers a different view. His analysis indicates that the storm surge was not high enough to overtop the walls, and so they must have collapsed merely from the pressure of the water they were holding back.

It may seem surprising that drainage and navigation canals would so often be the soft spot in the city's flood-control ramparts. The canals are narrow waterways, protected from the wind-driven waves that pound levees facing Lake Pontchartrain. But there is another risk factor that may help to explain the vulnerability of the canals. Whereas lake levees are erected atop embankments at sea level or above, the canals extend into areas of the city where the ground lies 5 or 10 feet below sea level. Hence the walls of a canal must be taller and built to withstand greater pressure at the base. The canals also pass through urban neighborhoods where land is scarce, and so the canal walls tend to be more-vertical structures.

Mississippi River drains most...Click to Enlarge Image

Once the floodwalls failed, the network of pumping stations could not possibly save the city. The aggregate capacity of all the pumps in New Orleans is a little less than 50,000 cubic feet per second. This is a mammoth flow—roughly equal to the mean discharge of the Missouri River—but the influx of lake water through the burst flood walls was an order of magnitude greater. The pumping stations themselves were soon flooded. Even if they could have kept running, their effort would have been pointless. The discharge from Pumping Station No. 6, for example, would have immediately poured back into the streets through the hole in the 17th Street canal.

Flooding in areas east of New Orleans, such as the hard-hit neighborhoods of Chalmette and Arabi, apparently followed a different time course. Whereas most of the city remained relatively dry until the morning after the hurricane passed, the eastern suburbs were submerged during the night of the storm. The water that washed over those places probably came not from Lake Pontchartrain to the north but more directly from the Gulf, across Lake Borgne to the east. A particular point of vulnerability is the V-shaped area where two major canals converge. The wind-driven water may have reached a depth of 20 feet there, far above the levees. This surge may have also entered and overflowed the Industrial Canal, causing at least some of the flooding in eastern wards of the city.

Experts on hurricanes and on New Orleans say that no one should have been surprised by the impact of Katrina on the city. As mentioned above, many earlier storms had similar effects, although at lesser magnitude. Computer models constructed by workers at the Louisiana State University Hurricane Center predicted that a storm of Katrina's strength would produce "back door" flooding from Lake Pontchartrain and the canals. In 2002 a prescient series of articles by John McQuaid and Mark Schleifstein, published in The New Orleans Times-Picayune, brought the results of those simulations to public attention. Just a year ago, emergency planners from the area took part in an exercise focused on a fictitious Hurricane Pam whose effects on the city were quite similar to those of Katrina.



 

EMAIL TO A FRIEND :

Subscribe to American Scientist