Water levels in the Mississippi River upstream from New Orleans have fallen enough to allow the U.S. Army Corps of Engineers to begin closing bays at the Bonnet Carré Spillway.

The spillway is designed to ensure that a maximum river flow of 1.25 million cubic feet per second (cfs) passes through what is known as the Mississippi River and Tributaries system at New Orleans. Now that the river has fallen below the maximum flow at Red River Landing northwest of New Orleans, the diversion of water into Lake Pontchartrain is no longer needed.

The Bonnet Carré Spillway, part of the Mississippi River and Tributaries project (MR&T) is the southernmost floodway in the MR&T system. This spillway was first opened during the flood of 1937, and nine times through 2011 to lower river stages at New Orleans. For the first time, it was opened in back-to-back years in 2018 and 2019. Source: G.A.Volb, USACE, New Orleans DistrictThe Corps began by closing 10 of the 168 bays that are currently open. The Corps said it will continue to assess river levels and close additional bays as conditions allow.

Bonnet Carré Spillway is 28 miles above New Orleans and can divert a portion of the river's floodwaters into Lake Pontchartrain and then into the Gulf of Mexico. The structure has a design capacity of 250,000 cfs, the equivalent of roughly 1,870,000 gallons per second.

The facility consists of a control structure and a floodway. The control structure is a concrete weir that parallels the river for a mile and a half. It consists of 350 gated bays, each holding 20 timber "needles," for a total of 7,000 needles. When needles are removed, river water flows into the floodway and flows nearly six miles between guide levees to the lake.

To open the structure's bays, two cranes, moving on tracks on top of the structure, lift timbers from their vertical position in the weir and set them horizontally across the top of the structure. River water then flows into the spillway.

July 22 marked the 74th day the structure had been open. The current operation was the first time the structure had been operated twice in one year and the first time it had been operated in back-to-back years.

That sinking feeling

Civil engineers watched southern Louisiana’s system of levees and hurricane defenses in early July as heavy rain flooded New Orleans streets, Hurricane Barry threatened to dump massive amounts of rain on the region and Mississippi River levels remained high, a condition that had persisted since last fall.

Concerns over the health of the protection system were heightened in April after the U.S. Army Corps of Engineers warned that the Greater New Orleans Hurricane and Storm Damage Risk Reduction System (HSDRRS) “will no longer provide 1% level of risk reduction” as early as 2023.

In a notice published in the Federal Register, the Corps warned that “Absent future levee lifts to offset consolidation, settlement, subsidence and sea level rise, risk to life and property in the Greater New Orleans area will progressively increase.”

“We have been in a flood fight since October and November,” said Ricky Boyett, chief of Public Affairs for the New Orleans District of Corps of Engineers in an interview with Engineering360 shortly after the Federal Register announcement was published. “This is the longest we’ve maintained water levels that high.”

After the 2005 hurricane season, which included Hurricanes Ike and Katrina, the Corps began a $14 billion infrastructure project that included restoration, accelerated construction, improvements and enhancements of a number of risk reduction measures in southeastern Louisiana. The series of levees, floodwalls, gates and pumps that were upgraded and installed brought 100-year level of hurricane and storm damage risk reduction to areas that previously had not been offered that level of protection.

But a combination of rising sea levels and ongoing ground subsidence that is exacerbated by the 150-year-old levee system itself is challenging flood control systems. Those challenges were evident in July with the trifecta conditions of high water flows on the Mississippi River, heavy rain showers and the season's first hurricane.