Mississippi River Mouth Must Be Abandoned to Save New Orleans from Next Hurricane Katrina

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 in 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 lays?
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. 
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 till 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 massive quantities of silt and freshwater to the brackish marshes, which is 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