This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
More than six years after Hurricane Katrina plowed into New Orleans and the Mississippi River delta, a plan has finally emerged to protect the area from future storms. It relies heavily on the restoration of wetlands to cut down high surges of ocean water like those that flooded the city in 2005—somewhat of a surprise, considering past efforts focused on levees and seawalls.
Last week, after prolonged deliberations over competing plans between state and federal agencies, the U.S. Army Corps of Engineers, and cities and parishes (counties), the state’s Coastal Protection and Restoration Authority released the Louisiana Comprehensive Master Plan for a Sustainable Coast. If all its provisions are carried out, the work would require $50 billion over 50 years.
The plan includes maps of what the state’s refurbished delta would look like from the air by 2061. It also shows maps of the wetlands that would disappear by 2061 (see image below), as well as the extent of flooding that storms such as Katrina would bring, if the projects aren’t built. Southern Louisiana has lost 1,883 square miles of wetlands during the past 80 years, an area three-quarters the size of Delaware, largely because of erosion that has been catalyzed by hundreds of miles of manmade navigation channels and oil and gas pipeline canals. Most of that land will not be regained. But if the plan’s projects succeed, by 2042 the state would begin to gain more land annually than it loses, and by 2061 it would gain an average of about 2.5 square miles a year.
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Several major strategies make up the bulk of the plan (see example below). Along the outer edge of the torn-up coast, furthest from New Orleans, former barrier islands that have been worn to thin wisps of land would be broadened with sandy sediment, mostly dredged from the ocean bottom and conveyed through pipelines. Natural ridges of land along the coast would be strengthened in similar fashion. Together, the islands and ridges would form a dotted line around southeastern Louisiana that can cut down storm surges. They would not all connect, so wind-driven water could still find its way through, but the many segments would break up the incoming wavefront into chaotic eddies flowing in conflicting directions that would at least partially cancel out one another.
Closer inland, large areas of wetlands that are severely tattered or nearly gone would be reconstituted. Large openings, called diversions, would be cut in the levees that line the winding Mississippi River, as well as the Atchafalaya River to its west. Gates would be inserted, which would allow freshwater and sediment—the lifeblood of marshy terrain—to wash down into the wetlands when the river is running high. Decades ago the delta had thick, robust marshes and swamps that began behind the barrier islands and ran back for miles and miles to where towns and cities had sprouted. The vast marshes could absorb large storm surges, turning them into the equivalent of mild high tides by the time they reach metropolitan areas. Healthy wetlands also gradually dilute the salt from seawater, so it doesn’t kill plants that grow in fresher water closer to firm land, a mechanism that has further eroded today’s struggling regions.
Close to New Orleans, of course, levees would continue to be raised and connected, and breakwaters would also be erected along certain shorelines that are close to populated areas. Numerous homes and businesses would be raised or floodproofed. And some houses in areas that were destroyed by Katrina and are at the greatest risk for future flooding would simply be bought and removed, and the land left vacant.
These strategies strongly echo three different protection plans that experts had recommendations back in early 2006, which Scientific Americandetailed in an article before the infighting between stakeholders widened. As it was then, restoring wetlands remains a controversial strategy, yet the Coastal Protection and Restoration Authority is clearly relying on it; the biggest chunk of money designated in the plan is $17.9 billion to improve thousands of acres in numerous locations.
Sediment and freshwater are needed to build and maintain wetlands; spring flooding by the Mississippi River is largely what built the vast stretches to begin with, until levees raised along the river prevented the annual overflows. Much of the initial rebuilding will be done by dredging sediment from nearby channels and pumping it into needed spots, but the diversions are important for supplying new sediment, freshwater and nutrients to the areas year after year.
Some interest groups, notably fishers, have already expressed opposition to the diversions, most recently on Monday during the first of three consecutive days of public meetings about the plan (the full comment period ends February 25). They claim that the inflows of freshwater will chase shrimp, crabs and certain fish that prefer brackish water further out to sea, harm spawning grounds or oyster beds, or impede the fishers’ ability to harvest the seafood. They also claim that two small, experimental diversions that have been running for at least a decade have failed to actually rebuild land. Studies by scientists have shown improvements in those places, however, although land has not always be regained at the rates initially predicted. Even if the planned diversions do work, it will be many years before large, healthy marshes return—years during which, proponents hope, no Katrinas come blowing in.
In the meantime, lessons learned while rebuilding the Mississippi delta could prove valuable across the U.S. The country has more than 30,000 miles of levees, and as much as 70 percent of them can no longer be trusted because of long-term erosion or poor construction, according to a 2010 report by the Federal Emergency Management Agency.
Images: Courtesy of the Coastal Protection and Restoration Authority
