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.
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.
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.