The U.S. Army Corps of Engineers anticipates opening the Bonnet Carre Spillway on Thursday morning as the level of the Mississippi River in the New Orleans area continues to rise.
That's weeks ahead of earlier forecasts, thanks to heavy rains upriver.
The spillway, which diverts water from the river into Lake Pontchartrain, is designed to keep the river's level in New Orleans below 17 feet. Right now, it's at 14 feet and is expected to continue rising.
Col. Michael Clancy, the Corps' commander and district engineer in New Orleans, said Monday that he has requested the spillway be opened about 10 a.m. Thursday.
Forecasts predict that by then the Mississippi will be flowing at a rate of about 1.25 million cubic feet per second, the point at which officials begin opening the spillway's 350 individual bays to keep water levels in New Orleans well below the 20-foot levees that line the river.
The spillway stretches for more than a mile through St. Charles Parish, holding back the Mississippi's waters with about 7,000 wooden "needles." Those needles — actually, heavy wooden beams — are removed to divert water from the river into the spillway, which runs for about six miles to the lake.
Water is already leaking around some of the needles.
Thursday's opening will likely involve the removal of a small number of needles with cranes; additional ones will be taken out as the water level rises, Clancy said.
The spillway will likely be kept open for about three weeks, Clancy said. When the water level is at its peak, the Corps anticipates opening about half of the structure's bays to allow about 130,000 cubic feet of water per second to flow into the lake.
The Corps, which last week began conducting levee inspections twice a week, will likely switch to daily inspections Tuesday, when the water level at the Carrollton gauge in New Orleans is predicted to reach 15 feet.
That height will also trigger a ban on any construction activity within 1,500 feet of the area's levees, to protect them from damage.
That includes work on a floodgate near Dumaine Street in New Orleans. Officials have a plan for closing the opening created by that work, though Clancy said that likely will not be necessary.
With water flowing fast and high, the U.S. Coast Guard may also put limitations on how fast ships can travel on the river.
The weekly inspections have not turned up any serious problems so far, though crews have found areas where water is already seeping through the levee, Clancy said. Inspectors will watch those areas to ensure that sand boils — a sign that the levees may be weakening — do not develop, he said.
With the river swollen by heavy storms upriver, the Corps was anticipating opening the spillway later this month. But rain has continued in the basins that feed the Mississippi, causing water levels to rise faster than expected.
The spillway was built more than 80 years ago, and officials originally expected they would need to use it only about once a decade. But the diversion has already been used in 2008, 2011 and 2016 and was almost needed last year.