High Waters Offer Sediment-Laden Lessons Amidst Flooding Tragedy

As Mississippi River spillways open, sediment deposition and land-building may restore natural defenses

05-13-2011 // Craig Guillot
Bonnet Carre opening the spillway

As the rising waters of the Mississippi River continue to impact communities along its banks, the U.S. Army Corps of Engineers and flood control managers are desperately trying to mitigate the impacts. In Louisiana, the solution has been to open spillways that relieve pressure on the levees and divert some of the river's waters to the Gulf of Mexico.

The human consequences of this flood will be catastrophic and long-lasting. Massive flooding has already hit parts of Tennessee and Mississippi and tens of thousands more homes are at risk of flooding. New Orleans is carefully monitoring the rising waters.

But experts say there's a valuable lesson to be learned in this historic event: sediment deposits from floods like these could actually help build land along Louisiana's rapidly eroding coastline. While the opening of spillways and diversions may help prevent further flooding, experts say the river could be put to even greater use to help both people and natural systems.

Natural Process Interrupted

It is part of the natural process for the Mississippi River to overflow its banks every spring. That sends water inland, but with each passing year the sediment deposited in those floods adds elevation to the land. Left untouched, the annual events would continually build the ground higher until they became less susceptible to the impact of annual floods.

People built levees to prevent annual flooding of homes and property – and the unfortunate unintended consequence was to halt the natural overflow process that built and sustained the land.

NWF Louisiana State Director David Muth said the French began confining the river with artificial levees as early as 1718. By 1840 permanent distributaries began to be pinched off and after the Great Mississippi Flood of 1927, even larger levees were constructed. Not only did they stop the land from forming along the banks, they prevented the process of land-building south along the marshes and distributaries of the river.

"Ironically, early primitive engineering, while stopping most annual overbank flooding, led to a period of perhaps 150 years of more infrequent but bigger and more catastrophic flooding," said Muth.

Climatologists say climate change is further fueling the river’s historic floods.

Flooding the Atchafalaya to save New Orleans and Baton Rouge

In the 1930s, the U.S. Army Corps of Engineers began to construct "spillways," areas of land ringed by levees that could intentionally be flooded during high river periods to divert some water to the Gulf of Mexico. These included the Bonnet Carre and Morganza Spillways.

To combat the recent rising river levels near New Orleans, the U.S. Army Corps of Engineers began opening the 350 bays of the Bonnet Carre Spillway on May 9. Guided by levees, the six-mile floodway can channel waters through Lake Pontchartrain to the Gulf of Mexico. It has been opened only 10 times since it was built in 1937.

Two of the massive spillgates of the Morganza were opened on Saturday May 14, which will funnel water up to 600,000 cubic feet/second to the Gulf through the Atchafalaya Basin.

In the coming weeks, the opening of the Morganza could impact more than 11,000 structures, 25,000 people and more than 3 million acres. Impacts will likely be felt from Krotz Springs, where new levees are being quickly built, all the way to Morgan City, which is hoping its seven-foot high seawalls will hold.

The Morganza opening was a last resort effort to relieve pressure on levees from Baton Rouge to New Orleans, where millions of residents would be threatened. The Corps recently released a map showing that without opening Morganza, metro New Orleans could have been  engulfed in 10 to 40 feet of water.

"There are going to be serious problems for people inside and outside of the levee system. Hopefully it holds and there are no catastrophic failures anywhere in the system to make things even worse," Muth said.

Using Sediment to Combat Louisiana’s Top Ecological Problem

Wax Lake Delta

There is nothing good to come of nearly 25,000 people having their homes flooded. Since the Morganza floodway has only opened once, in 1973, many incorrectly assumed it would never open again and built homes and businesses in the floodplain. But by this point all residents could do is gather their belongings, move to higher ground and hope for the best.

But within every disaster lies opportunity: the Morganza has the potential to carry a tremendous amount of sediment to the wetlands and Louisiana's coast. Muth calls the event a "once-in-a-lifetime opportunity" to create new land and help fight the state's ongoing battle with coastal erosion.

"The land-building that will occur is going to be unprecedented. Some of the sediment is going to end up at Wax Lake or the Atchafalaya Delta," he said.

Much of the sediment that was carried through the Atchafalaya in 1973 can still be seen on satellite images today.

Paul Kemp, Ph.D., vice president of the National Audubon Society's Louisiana Coastal Initiative, said that during these kinds of flooding events, the river may move as much sediment in a couple of weeks as it may in 50 years during normal flow times.

"This will be the first fully intended use of it and a huge amount of sediment will travel through the Atchafalaya to help build the deltas," said Kemp.

Experts say this year's historically high river levels will serve as a model for how we can work with the river’s natural systems for land-building purposes in the future. Scientists from NWF, the National Audubon Society, and the Environmental Defense Fund will be monitoring the coast to learn exactly how much sediment is deposited.

There are lessons hidden in this flooding tragedy. If government takes the initiative to use diversions to channel the water to the coast, it can create both better flood protection and help to fight Louisiana’s top environmental problem. With more strategic use of the river’s power, Mother Nature can help fight coastal erosion.

Otherwise, rare flows of sediment that could potentially build thousands of acres will drift away to the mouth of the river where it is usually dredged and pushed off the continental shelf.

“I hope this changes things. We need to start learning lessons from the river. We need to adapt the way we manage it and take advantage of it at a time like this. The river can be used,” Muth said.

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