Katrina: Behind the Tragedy
Taming the River to Let In the Sea
Southern Louisiana is sinking into the Gulf of Mexico.
The surprising culprit is overambitious flood control.
By Shea Penland
http://www.naturalhistorymag.com/mas..._feature2.html
( to see the AMAZING graphics, you must go to this site. )
In February 2005, writing in Natural History, Shea Penland, Director of the Pontchartrain Institute for Environmental Sciences at the University of New Orleans, and the University’s Braunstein Professor of Petroleum Geology, outlined the threat of hurricanes and catastrophic flooding to the city of New Orleans and the surrounding Mississippi Delta region. Now, in the wake of Hurricane Katrina, the full horror of that threat has been realized. The toll of the dead mounts daily. The survivors are refugees: when—and whether—they will be able to return to what is left of their homes is unclear. Behind the tragedy lie both natural trends and the impact of human settlement and engineering. Penland’s article, reproduced below, explores these factors and provides a context for understanding the tragedy, as well as a guide to the obstacles that must be addressed as the devastated region is rebuilt.
If you live in Louisiana and don’t know how to swim, now might be a good time to learn. The state is rapidly disappearing into the Gulf of Mexico. As a result of hundreds of years of natural-resource exploitation and modifications to the flow of the Mississippi River, whose silty waters created the delta region of southern Louisiana, the state’s coast lost more than 1,900 square miles of land in the twentieth century alone. At the current rate of loss in Louisiana, an area of wetlands the size of the Baltimore-Washington, D.C., metropolitan area will disappear by 2050. Without putting a massive program of ecological restoration into effect immediately, the fertile crescent of the Mississippi River is doomed to wash away sometime in this century.
Habitable land has always been a critical issue in coastal Louisiana. Much of the state lies only a few feet above sea level; the highest elevation in Louisiana is only 535 feet. Much of the city of New Orleans is actually below the level of the Gulf of Mexico. No wonder, then, that the earliest European explorers, colonists, and entrepreneurs became preoccupied with “taming” the Mississippi. Wherever people settled, they built levees, channels, and canals to control the floods; they reclaimed land from the bottoms of swamps and running rivers; and they did whatever else they could to harvest the bounty of Louisiana’s deltaic Eden.
Actual and projected areas of land loss and gain in coastal Louisiana are detailed on the map above, based on a map by the Louisiana Land Change Study. Land builds up when the Mississippi, its flow slowed as it meets the still waters of the Gulf, deposits its burden of silt. Some land loss results from erosion, but the most important effect is subsidence. Silt left by flooding subsides with time, and without regular flooding to renew the layers of silt, the ground can sink low enough to become immersed in the Gulf. The areas running from just east of Morgan City, eastward to Head of Passes and the Chandeleur Islands are the ones affected by the absence of regular flooding; two centuries’ worth of levee building and flood control on the Mississippi have led to that unintended consequence. The most active areas of land buildup are the Atchafalaya Delta, south of Morgan City, and limited areas around Head of Passes, at the present mouth of the Mississippi.
Map © U.S. Geological Survey, National Wetlands Research Center
The Mississippi and other rivers, of course, are not the only threat to Louisiana’s lowlands. Every year hurricanes pose a threat from the Gulf of Mexico. The accompanying storm surges cause local, short-term flooding, but they also lead to permanent erosion of the coastal marshes and barrier islands in the Gulf, which provide the only protection for the inhabited lowlands farther inland. Louisianans have focused on river flooding for hundreds of years, yet only in the mid-1970s did the state begin to take the coastal erosion problem seriously.
The breakdown of the marshes and beaches coupled with the drainage of reclaimed lowlands remains a disaster in the making. New Orleans’s defenses would simply crumble if a truly enormous storm lingered over the city for long: a storm the size of, say, Hurricane Ivan, which made landfall along the Gulf coast of Alabama last fall, roughly 150 miles to the east. Even many residents do not realize that New Orleans is on the brink of becoming the next Atlantis, the fabled island that, according to Greek legend, sank into the sea.
Beach erosion and flooding are still not the heart of the problem facing Louisiana: subsidence of the delta plain is.
Important as they are, though, beach erosion and flooding are still not the heart of the problem facing Louisiana: subsidence of the delta plain is. Before the levees were built to channel and “control” the Mississippi and other nearby rivers, floodwaters would spread out and slow down as they flowed over the delta. When the flow slowed, the river would deposit its burden of silt, forming a new layer of earth. But the levees, which now constrict floods along a 1,200-mile corridor of the Mississippi, keep the floodwaters from spreading across the delta. Instead, the river-borne silt is lost off the edge of the continental shelf.
The delta, primarily mud that already filled the Mississippi River valley before the levees were built, is continuously being compacted under its own weight. As it compacts, it loses elevation, and without floods, no new sediments can arrive to build the land back up. In the past several hundred years, subsidence rates have ranged from a foot to four and a half feet per century.
Compounding the risk of catastrophic flooding is global climate change. Many climatologists expect such change to cause hurricanes even more frequent and more violent than the ones of the past several years. Sea levels are expected to rise by ten to twenty inches. As Louisiana’s marshes and barrier islands sink farther into the sea, the people of Louisiana could find themselves exposed to the elements. But the present trends and ominous signs of coastal land loss in Louisiana threaten much more than just the environment, economy, and people of the state. The importance of the Mississippi and, in particular, New Orleans to the commerce of the nation makes the crisis a threat to the entire United States.
The importance of the Mississippi and, in particular, New Orleans to the commerce of the nation makes the crisis a threat to the entire United States.
The threatened collapse of coastal Louisiana has been centuries, even millennia, in the making. Eighteen thousand years ago, with the end of the last ice age, sea levels began to rise dramatically. For thousands of years the great glaciers that had formed in the preceding era melted into the ocean, until, four thousand years ago, the sea level stabilized. But the Mississippi now met the sea in what had been its old valley. The river water, halted in its course by the Gulf of Mexico, no longer had the energy to carry its sediment. The sediment, falling out of the flow, began filling in the ancient river valley. The result was a subsidence-prone delta that could maintain its elevation only so long as sediment from upstream reached the delta plain each year.
Meanwhile, for thousands of years, the Mississippi Delta has undergone a process known as “delta-lobe switching.” The path the river takes to the Gulf is constantly changing, because the river is continuously drawn along the most efficient path to the Gulf [see map on opposite page]. Thus time and again the delta forms and reforms. By roughly four thousand years ago, the St. Bernard delta lobe was building up to the east of New Orleans and cut off three bays from the sea; those became lakes Maurepas, Pontchartrain, and Borgne [see map above]. The lobe terminated at the Chandeleur Islands, to the east. Geologically, though, that drainage did not last long. By roughly three thousand years ago, the Mississippi was probably also entering the Gulf via the Lafourche lobe, which lies southwest of present-day New Orleans. That delta lobe formed the region around the present-day communities of Houma, Golden Meadow, and Grand Isle.
The path the river takes to the Gulf is constantly changing, because the river is continuously drawn along the most efficient path to the Gulf.