Hurricane Katrina demolished New Orleans 10 years ago, a grim anniversary to be marked next week. Huge earthen levees dissolved and concrete floodwalls toppled over. But the real culprit when the tropical cyclone made landfall was outside the city. Thousands of square miles of wetland marshes and swamps that had once provided a buffer between the city's coastline and the ocean had been badly tattered from decades of human damage. Thick, robust wetlands would have absorbed much of the surge of water that Katrina pushed up from the Gulf of Mexico. But levees had starved the wetlands of needed nutrients, making plants weak, and thousands of miles of manmade canals had torn the vegetation apart, allowing Katrina’s onrushing storm surge to flow right into New Orleans.

Extensive studies done after Katrina verified what lifelong residents of southeastern Louisiana already knew: Unless the rapidly disappearing wetlands are made healthy again, restoring the natural defense, New Orleans will soon lay naked against the sea (see satellite image, below).

So, how does one reengineer the entire Mississippi River delta—one of the largest in the world—on which New Orleans lies?

Three international engineering and design teams have reached a startling answer: leave the mouth of the Mississippi River to die. Let the badly failing wetlands there completely wither away, becoming open water, so that the upper parts of the delta closer to the city can be saved. The teams, winners of the Changing Course Design Competition, revealed their detailed plans on August 20. Graphics from each plan are below.

Scientists worldwide agree that the delta’s wetlands disintegrated because we humans built long levees—high, continuous ridges of earth covered by grass or rocks—along the entire length of the lower Mississippi River. The leveed river rims the southern boundary of New Orleans and continues another 40 serpentine miles until it reaches the gulf. The levees, erected almost exclusively by the U.S. Army Corps of Engineers, prevented regular floods from harming farms, industries and towns along the river’s course. However those floods also would have supplied the brackish marshes with massive quantities of silt and freshwater, which are necessary for their survival.

Silt carries nutrients that grasses and mangroves need to stay lush, and it provides new material to build up the soft substrate beneath those plants, which subsides naturally under its own weight. Incoming freshwater mixes with the delta's saltwater to create the reduced salinity required by the region's vegetation. This soup also prevents pure ocean water from intruding further inland, which kills grasses and trees from the roots up.

Instead, hundreds of miles of navigation channels, cut by the Corps for more than half a century through the wetlands have torn the wetlands apart from within. So have thousands more miles cut by industry during the same period to build and maintain oil and gas pipelines running in from the Gulf.

The studies done by university experts, engineering firms and the Corps itself since Katrina concur that the only realistic way to reconstitute healthy wetlands is to make cuts in the levees, install gates, and open those gates periodically to allow sediment and freshwater to once again flow into the marshes. The three winning design teams rely heavily on that strategy, yet they also differ in where and how to use the so-called diversion structures.

The river nowadays only carries perhaps half of the sediment it used to, because communities on its banks for hundreds of miles siphon off water for irrigation, industry and many other uses. There is simply not enough sediment to rebuild the entire delta, according to the winning teams, which operated independently. Rather than try that and fail, the teams found it is better to essentially end the river many miles north of the current mouth, where much sediment is sent like a shot out into the deep ocean and lost. Then engineers could redirect all the sediment to portions of the delta closer to New Orleans. “Capture every grain,” is one team’s slogan.

The need to let the end of the delta, known as the bird’s foot because of its shape, die is also assumed in the official Louisiana Comprehensive Master Plan for a Sustainable Coast, although not necessarily called out in detail. The plan took seven years to develop, after significant political wrangling among state, federal and local authorities.*

The master plan would tap about half the river’s sediment for diversions, and try to restore as much of the delta as possible. Founders of the Changing Course competition, led by the Van Alen Institute and the Environmental Defense Fund, and supported by the Rockefeller Foundation, Kresge Foundation and other large institutions, saw that approach as a weakness, and announced the competition to seek alternative ideas.

The competition also encouraged a 100-year outlook for the delta, instead of the 50 years outlined in the master plan. In the end, the three winning blueprints, chosen from 21 entrants, complement the state plan well, says Steve Cochran, director for Mississippi River Delta Restoration at the Environmental Defense Fund, who oversaw the competition.

Cochran also hopes the winning designs will prove valuable to other delta regions around the world: “Every place is different, but the kinds of innovations needed are similar.”

Only one of the three groups, the Baird Team, included a cost estimate: between $4.3 billion and $5.7 billion. But it also cited savings of up to $2 billion in eliminating the need to replace certain aging flood control structures now on the river. The other two plans are larger in scope and would likely be more expensive. Cochran says his committee did not require cost estimates “because the state and the Corps would decide which aspects of the plans to implement, and do their own estimates.”

The winning teams received neither prize money nor other rewards. The teams got involved primarily to gain notoriety for potential large contracts in the future. “The coming work in southeastern Louisiana is huge, even on a global scale,” Cochran notes. The real goal for Changing Course was to educate the state, the Corps and other industries and authorities that will be involved in reengineering the region about how to best exploit the Mississippi River to save the region. “The teams have been explaining their ideas to all these people along the way,” Cochran says.

Additional details of the three plans are shown below. The full set of designs can be obtained online from Changing Course.

*Editor's Note (8/20/15): This paragraph was edited after posting to clarify its description of the Louisiana Comprehensive Master Plan.

A satellite image of the lower Mississippi River (winding dark line) shows that south of New Orleans (white, at center) the wetlands (green) are severely tattered, allowing hurricanes and other storms to push surges of water from the surrounding Gulf of Mexico right into the city, largely unimpeded. 
Credit: USGS and NASA
A map included in the revitalization plan by the Baird Team shows the hard truth: the Mississippi River (blue) no longer carries enough sediment to rebuild the entire delta. The river should be cut off (north of the number 5) to better save wetlands closer to the city (red “Sustainable” line), and the rest must be left to wither away, becoming open water (brown “Historic” line). New Orleans is indicated by the symbol near the center.
Image courtesy of Changing Course
In a plan from the Moffatt & Nichol team, a greatly expanded Port Sulphur (white area at lower right, #7) would mark the end of the enhanced wetlands region, and a larger, deeper navigation channel (green line) from there south into the Gulf of Mexico (bottom) would be used. The rest of the Mississippi River delta (off to the bottom right, not shown) would be abandoned. New Orleans is the white area at the top.
Image courtesy of Changing Course
The Studio Misi-Ziibi team plan, the most detailed and extensive, would attempt to save the entire delta (all green colors) with numerous cuts in the levees. It would also rebuild the barrier islands (outer ring at bottom and right) and add new bay islands (dots behind the barriers) to break down the surge of water driven in by hurricanes. But the potential cost, which the team did not estimate, could be extremely high. 
Image courtesy of Changing Course