Built in the aftermath of the great Mississippi River flood of 1927, the current levee and spillway system has prevented disastrous floods in Louisiana and kept the river on its current course. But it has also ensured that the massive quantities of silt borne by the Mississippi have been carried straight out the chute of the river's mouth and into the deep waters of the Gulf.
Previously, the Mississippi largely wandered wherever it liked, with floods replenishing marshlands up and down the coast with the sediments they carried. Deprived of those sediments for the past 80 years, the marshy Louisiana coast has steadily subsided, sinking into the Gulf of Mexico.
Now, with the Mississippi having been loosed through the Morganza spillway into the Atchafalaya Basin, scientists are using the great flood of 2011 as an opportunity to study how these waters might replenish wetlands when they reach the coast. Moreover, they are promoting the idea of building more such spillways, which would not only help contain future flooding, but also would help strategically replenish wetlands along more of the Louisiana coast.
“In the Delta region, we trade off preventing regular, small-scale floods with having infrequent but catastrophic large floods,” says Douglas Jerolmack, professor of earth sciences at the University of Pennsylvania. “And by preventing the river from flooding most of the time, we’re also preventing the natural deposit of silt in the marshes needed to maintain the wetlands.”
Protecting New Orleans
Without the Old River control station and the Morganza spillway north of Baton Rouge, La., it is possible that sometime in the past 30 years the Mississippi could have changed course completely – captured by the Atchafalaya River, which offers a steeper, more direct route to the Gulf, Dr. Jerolmack says.
As of Wednesday, the Mississippi had crested at 17.1 feet at New Orleans and was expected to remain at that level through the rest of the week. While the metro New Orleans area seems to be safe, however, as many as 25,000 people who live near the Atchafalaya River Basin west of the city have evacuated in anticipation of flooding from the opening of the Morganza spillway.
The Mississippi River is forecast to fall below flood stage at New Orleans on June 5, according to the National Weather Service. “This is not a natural crest, but an artificial crest that has been controlled by the Army Corps [of Engineers] by opening the spillways,” says Danielle Manning, a forecaster with NWS.
The Bonnett Carre spillway north of New Orleans, which empties river floodwaters into Lake Pontchartrain and does not threaten population areas, has by far carried the heaviest load. As of Wednesday, 330 of the spillway’s 350 gates were open, diverting 316,000 cubic feet of flood water per second, well above the spillway’s designed capacity of 250,000 cfs. But “so far everything looks good,” says Jerolmack.
Putting sediment 'to use'
Partly to relieve pressure on the system in such large floods, he and other scientists and engineers are proposing construction of new spillways in the levee system, which could also divert sediment to the wetlands.
“There’s a huge amount of sediment in a flood like this one that could be put to use,” says Clinton Willson, a civil engineer at Louisiana State University in Baton Rouge. “We’re looking at large diversion projects in the New Orleans area that could augment the current system. It would add to the flow rate diverted from the river, protecting the public and maintaining river navigation, but would also send sediment into the marshes.”
Two existing diversions below New Orleans, the Davis Pond and Caernarvon diversions, could be opened now to channel flood sediment into wetlands, but Louisiana state regulations prohibit such openings when water salinity levels are low, to protect oysters in the marshes. The diversions were opened during last year’s Gulf oil spill to block oil from entering.
Calls for new diversion projects to rebuild Louisiana’s coast also came at a meeting of the America’s Wetland Foundation in New Orleans earlier this week. A recent scientific study commissioned by the foundation found that stronger and more frequent hurricanes, degradation of wetlands, and rising sea levels could cost the region $350 billion during the next 20 years.
Where the sediment goes
The US Geological Survey is conducting a variety of studies during this year’s high-river event to determine how much sediment is carried by the floodwaters and where it goes.
“The opening of the Morganza spillway is simulating a natural flow in the Atchafalaya River Basin that could have a net benefit for the region’s wetlands, allowing water to go over levee banks where it hasn’t in the past 40 years,” says Jerolmack. “But scientists still don’t understand very well how sediments from river floods help build up wetlands.”
While most flood sediment will end up in the Gulf, some is expected to build new coast in south Louisiana. “We’re looking at satellite photos that show big sediment plumes in the Gulf from this flood that are drifting back toward the coast, so not all is lost,” Jerolmack says.