The Bonnet Carre Spillway will be opened Wednesday for an unprecedented third time in four years to siphon water from the Mississippi River following an extremely wet winter across the regions that feed it.

The decision to open the spillway comes four months after the Mississippi first rose above 11 feet at the Carrollton gauge in New Orleans, high enough to trigger additional levee inspections.

Swollen by recent heavy rainfall, particularly in the Ohio River Valley, the river is now above 15.5 feet and rising toward the 17-foot height that the levees in the New Orleans area are designed to handle. The heights are measured against sea level.

In Baton Rouge, the river is already considered to be at moderate flood stage with a height there of more than 39 feet. It is expected to crest there around March 18 at 43 feet, about 4 feet shy of its record level. 

022619 Bonnet Carre Spillway openings

It will be only the 13th time in the nearly 90-year history of the spillway that the U.S. Army Corps of Engineers has used it to keep the river level low enough to avoid threatening the levees in New Orleans. But the increasing frequency and intensity of storms driven by a changing climate mean that such measures may be necessary more frequently, experts said.

“The trend is clear in recent years that the (amount of) rain is getting bigger, and the science indicates that trend will continue in a warming world,” said Deke Arndt, chief of climate monitoring at the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information.

The Bonnet Carre, built after the historic 1927 floods, is one of two spillways in the state the Corps uses to keep the river below the 17-foot height in New Orleans; it works by diverting water into Lake Pontchartrain. Its 350 bays are typically kept closed by 7,000 wooden “needles.”

The spillway is opened whenever the flow of the river reaches 1.25 million cubic feet per second, a rate that would fill the Mercedes-Benz Superdome in less than two minutes and roughly corresponds with the 17-foot river height.

Col. Michael Clancy, who oversees the Corps’ office in New Orleans, said about 38 of the 350 bays will be opened Wednesday morning. The spillway is expected to remain open for at least a month, and the Corps may open as many as 200 bays when the river is at its maximum height, Clancy said.

Wednesday will be the first time in the spillway’s history that it has been used in two consecutive years; the opening also marks its most frequent use over a four-year period. The Corps opened the spillway last March and in January 2016, with each opening lasting a little more than 20 days.

That corresponds with the five wettest years on record in the upper Midwest, where many of the river’s tributaries are located, Arndt said.

“We’re on the extreme end of the scale” when it comes to precipitation, he said.

The immediate cause of the high river levels is heavy rainfall in the upper Midwest and Ohio River Valley over the past two months, said Jeff Graschel, a hydrologist with the Lower Mississippi River Forecast Center in Slidell.

The Ohio River is the Mississippi's biggest single tributary. Areas that feed the Ohio have seen anywhere from 10 to 30 inches of precipitation over the past two months, Graschel said. Typically, those regions would receive 5 or 6 inches over that time, he said.

The Tennessee River, another major tributary, also had record amounts of rainfall as well, he said.

“There’s a higher probability of more extreme rainfall across the United States based on the climate studies that the climate experts have come up with,” Graschel said.

The opening of the spillway is accompanied by daily inspections of the levees to look for problems; construction and other potentially damaging activities are banned within 1,500 feet of the earthen structures.

So far, inspections since November have turned up about 150 minor problems, ranging from seepage to boats tied up to close to the levees, most of which have been fixed within a day, Corps official Heath Jones said.

At this point, the Corps does not expect to open its second spillway, the Morganza Control Structure, which has been used only twice in history — both times when the Bonnet Carre could not divert enough water on its own. The Morganza structure is upriver from Baton Rouge.

This year’s high water comes unusually early. Three-quarters of previous openings have been later in the year.

That could potentially present more challenges down the line, as much of the Midwest has seen weeks of heavy snow and freezing temperatures. The spring thaw is usually expected at the end of March or beginning of April, which would add more water to a river that will likely still be above normal levels.

On its own, snowmelt does not typically cause a problem for river levels in New Orleans, Graschel said. However, combined with heavy rainfall, it could once again swell the river to levels where action would be required, he said.

At this point, forecasters cannot tell whether that would be necessary. But Jones said the Corps would be able to reopen the spillway if needed.

“We’re doing it this time. If it happens again, we’ll do it again,” he said.

Follow Jeff Adelson on Twitter, @jadelson.​