The restless and dynamic geological history of the Mississippi River Delta, says Frank Pazzaglia, makes it foolhardy at best to rebuild New Orleans to anything resembling its pre-Hurricane Katrina configuration.
“The rebuilding of New Orleans is not a problem open to the best engineering solution,” says Pazzaglia, an associate professor of earth and environmental sciences who specializes in tectonic processes, rivers and erosion.
“Any artificial solution will work for only a very short period of time, during which the problem will get worse. The city is going to continue to sink farther below sea level, requiring levees to be built higher and higher.”
New Orleans can be restored as a viable port if necessary infrastructure is located on high ground, Pazzaglia says. But it would be irresponsible to rebuild the Big Easy as a major city, he says, with the neighborhoods that were devastated by Katrina reoccupying land that lies below sea level.
President Bush has promised unprecedented federal assistance to rebuild New Orleans and portions of Louisiana, Mississippi, and Alabama that were devastated by Hurricane Katrina. Experts say reconstruction will take years and require an estimated $200 billion or more.
But any attempt to engineer an artificial resurrection of New Orleans, says Pazzaglia, will ultimately be undermined by two factors – the constant compaction of sediment in the Mississippi River Delta and the Gulf of Mexico, and the Mississippi’s desire to relocate westward into the nearby Atchafalaya River.
Against the backdrop of geological time, Pazzaglia says, the Mississippi prowls restlessly along Louisiana’s 400-mile coastline, changing positions almost overnight in a constant search for the lowest-lying land and the shortest route to the Gulf of Mexico.
From 7,500 years ago until about 5,000 years ago, says Pazzaglia, the Mississippi contented itself with a westward course and emptied into the gulf on the Texas side of where New Orleans now sits. Then the river swung eastward and charted a new course mostly on the eastern side of the future city.
The eastward orientation lasted until about 2,000 years ago, when the Mississippi switched tracks and again swung back. This westward sweep would have moved the Mississippi well past New Orleans, says Pazzaglia, but for the levees and dams, built in the past two centuries, that now force the river to flow through New Orleans to the Gulf of Mexico.
Water flowing downhill
Pazzaglia is no stranger to uncommon natural phenomena. He spent last year on sabbatical in Italy, where he is trying to determine why the rocks on one side of the Apennine Mountains are compressed while those on the other side are stretched out, and how this might influence future earthquakes in the region. His research is supported by the National Science Foundation.
New Orleans, says Pazzaglia, is hostage to its own dramatic geological features: The ground on which the city sits is sinking 3 feet every 100 years. Many neighborhoods lie 11 feet below sea level, and sea levels worldwide are rising, exacerbating the city’s battle to keep the water out.
Much of this subsidence, Pazzaglia says, is due to the frequent relocations of the Mississippi River Delta, which changes positions once every 1,000 to 2,000 years, or an eyeblink in geological time.
The delta’s restless shifting is triggered by the tons of heavy, water-saturated sediment that the Mississippi deposits daily into the Gulf of Mexico. The weight compacts the sediment and squeezes out its water, causing the shoreline to sink, or subside. Aiding and abetting this subsidence is a thick layer of salt coating the floor of the Gulf of Mexico several thousand feet below sea level. Oozing like toothpaste, the salt is displaced by the weight of the sinking sediment, causing additional subsidence. The process, says Pazzaglia, is typical of large river deltas throughout the world.
The holes created by this subsidence are gradually filled as the Mississippi River deposits new sediment, enabling the river delta to remain at sea level. Meanwhile, a short distance to the west, the basin of the smaller Atchafalaya River also subsides. But the Atchafalaya deposits little or no sediment, and its basin subsides below sea level – until the Mississippi River, yielding to gravity, shifts course and fills the sinking Atchafalaya basin.
“It’s all a matter of water wanting to flow downhill to the lowest hole,” Pazzaglia says.
Two hundred years ago, when New Orleans was first settled by Europeans, the river emptied into the Gulf of Mexico at New Orleans, but was in the process of beginning to shift position and edge westward towards the Atchafalaya basin, Pazzaglia says.
This time, the river ran into a new obstacle – human activity.
“Human intervention has prevented the natural redirection of the river from happening,” Pazzaglia says.
The Mississippi River has been dammed, leveed and channeled to bypass New Orleans and prevent flooding, Pazzaglia says. The silt and sediment which would naturally be deposited in and near the city, helping maintain its minuscule elevation, are instead diverted to the Gulf of Mexico, where they contribute to the city’s sinking.
“The levees that were built upstream on the Mississippi River to prevent flooding in New Orleans cause the sediment to bypass the city. The sediment is dumped in the river delta and in the gulf, causing all of New Orleans to sink—with no new sediment to fill the hole naturally and to keep the land at least at sea level.”
Left to its own devices, Pazzaglia says, the Mississippi would abandon its artificial channel at New Orleans and take a more direct route to the sea. Upstream flooding on the Mississippi will eventually cause this to happen, he says, resulting in another disaster similar in scope to Katrina.
“This is an artificial situation that is untenable and unsustainable,” Pazzaglia says. “The thought of moving tens of thousands of people, most of them very poor, back into neighborhoods lying below sea level on land that is the most difficult in the city to drain—that, to me, seems almost criminal.”
Pazzaglia says government planners should heed the lessons of the Galveston Hurricane of 1900 and seriously explore opportunities to relocate New Orleans. In the aftermath of the Galveston devastation that took more than 6,000 lives, Galveston’s port activities and much of its commercial and cultural life were permanently moved to Houston.
Posted on Wednesday, October 05, 2005