How an 85-year-old engineering marvel protects New Orleans and shapes the region

Aug 8, 2017

New Orleans, La. — Though it was a wet and cold January morning, Gerald Donaldson, 66, rode his bicycle through the traffic jam outside his house in tiny Norco, Louisiana.

Something about “all that water” drew Donaldson there, along with a few thousand other Louisianans in wool hats and big coats who were similarly drawn to the small Mississippi River town to see the spectacle.

Not often is the mighty Mississippi River unleashed deliberately from its banks.

Once standing on the levee, parents lifted children onto shoulders. All eyes focused on the Bonnet Carré Spillway, a Works Progress Administration-era structure that, from the air, looks like a colossal concrete-and-wood zipper. For a 1-½-mile stretch of the Mississippi, about 30 miles upstream from New Orleans, the weir serves as the river’s east bank, holding back water.

Occasionally, when the water gets too high, the Bonnet Carré unzips. That’s only happened 11 times over the structure’s 85-year history. And most openings are much later in the year, as the spring snowmelt begins upriver.

So the crowd was especially big in January, to see the rare winter opening, driven by heavy December rainfalls across the central United States that had already caused historic, recording flooding along many of the nation’s waterways.

As a few people popped champagne corks on the levee, Maj. Gen. Michael Wehr of the U.S. Army Corps of Engineers used a red crane to lift up one of the zipper’s 7,000 wooden “needles,” 11-foot-long creosoted timbers about a foot wide that stand upright in the weir. The removal of each subsequent 400-pound timber opened a new gap in the zipper where river water gushed through, entering a 6-mile inclinedfloodway that empties into Lake Pontchartrain and, eventually, the Gulf of Mexico.

When completely open, this watery shortcut can divert one-fifth of the river’s water from New Orleans and the rest of lower Louisiana.

Yet few of the WPA-era engineers who designed the spillway purely for flood control would have imagined its impacts on research. Each time it opens, scientists rush in, to track the river sediment and water and measure its effects on creatures and marshland. “We’ve gotten an awful lot in bits and pieces, over the past few decades,” said Loyola University biologist David White, who studies the river and its effects on riverine wetlands.

The Bonnet Carré was built in response to the great river flood of 1927 on expropriated land that included four plantations, two small cemeteries, and a town called Montz. Almost instantly, it was promoted as a new marvel of the modern world.

A 1930s WPA photo caption captures the era’s triumphant tone. “Forever relieving New Orleans of the fear of Mississippi River floods, this great concrete bypass, the only structure of its kind in the world, was completed in December 1935. … With twice the flowage capacity of Niagara Falls, it will keep the water in the river at New Orleans from ever rising above the 20-foot flood level.”

Eight decades later, New Orleans Mayor Mitch Landrieu felt similarly about the massive weir, describing it “an engineering miracle” able to protect his city from record-breaking winter floodwaters. Upriver in Missouri this year, high water had killed 14 people and engulfed river towns, highways, and farmland.

The spillway’s opening is triggered whenever the Mississippi reaches the “near-flood” level of 17 feet at its storied, crescent-shaped bend in New Orleans.

Alleviating pressure on the city’s levee system is critical, since New Orleans is crisscrossed with canals that carry industrial traffic to the river and help drain the city. In 2005, a number of shoddily built floodwalls failed along those canals, flooding 75 percent of the city.

So, the spillway is seen as a crucial safety valve for this swampy region, which bracketed by the lake and the river and dotted with swampland, says Misty Trahan, 31, who drove a few hours with her teenage children to see the spillway open. During the drive, she explained to her children how the river in Louisiana is responsible for carrying rain and snowfall from 31 states.

“This part of the river drains water from everywhere up north,” Trahan says. “I have absolute pride in that.”