The Mississippi River is rising because of heavy rains to the north, with forecasts suggesting that it might swell enough to require opening of the Bonnet Carre Spillway later in March.
The U.S. Army Corps of Engineers and local levee authorities have already begun twice-weekly inspections of the levees in the New Orleans area as they brace for a river that could crest at about 17 feet in New Orleans by the third or fourth week of March.
“When you have a forecast that reflects reaching that level here in New Orleans, we definitely have to look at the possibility of operating the spillway,” Corps spokesman Ricky Boyett said.
The high water levels are a result of 5 to 15 inches of rain over the last two or three weeks in the Ohio and Mississippi river basins, said Jeff Graschel, a hydrologist with the National Weather Service. That rain is now working its way into the waterways and moving downriver.
Exactly how high the river — which was at 12 feet in New Orleans on Wednesday — will rise will depend largely on the weather over the coming weeks.
“Certainly, the rainfall over the next couple weeks will play a significant role in what we see in the New Orleans and Baton Rouge areas,” Graschel said.
The inspections underway are aimed at ensuring there are no weak points in the levees and that water is not seeping through them at any point, which can undermine the protective structures.
Daily inspections will begin once the river rises to 15 feet at the Carrollton gauge, a height it is expected to reach on March 10 or 11.
It is expected to continue rising after that, eventually cresting at 17 feet in New Orleans and 43.2 feet in Baton Rouge.
The spillway is opened anytime the river flows at more than 1.25 million cubic feet per second, which roughly corresponds to a river level 3 feet below the 20-foot height of the levees in New Orleans.
The Bonnet Carre Spillway was built in St. Charles Parish after the great 1927 flood to allow some of the water in the river to be diverted into Lake Pontchartrain, keeping the level in the New Orleans area low enough not to be a threat to the levees.
The Corps traditionally has needed to open the spillway about once a decade but has had to use it far more frequently in recent years.
The last time the spillway was opened was in January 2016, when it was kept open for 22 days. Rising water levels just narrowly missed hitting the trigger early last year. And high water required it to be opened in both 2011 and 2008.
No major problems with the levees have been found in the New Orleans area so far, though the Corps has had to do some work in areas upstream.
Crews have been working to repair damage from the 2016 flood near Angola, north of Baton Rouge, and the rising water means that area has had to be shored up with sandbags, Boyett said.
Farther north, in Concordia Parish, the Corps is adding sand and gravel to the levee near La. 15 to protect the levee from seepage and prevent the materials that make it up from shifting.