Testing the waters

• By JOE GYAN JR.
• Advocate New Orleans bureau
• Published: Apr 2, 2008 - Page: 1B - UPDATED: 12:30 a.m.

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NORCO — The Bonnet Carré Spillway in St. Charles Parish has been opened eight times since the 1930s to divert Mississippi River water into Lake Pontchartrain, but a ninth opening is unlikely during the current high-water event on the river, the U.S. Army Corps of Engineers said Tuesday.

But just in case the 7,000-foot structure needs to be opened this spring, corps workers put a small section of the concrete structure — and themselves — through a test, removing and replacing 10 of the vertical wooden “needles’’ or Southern pine timbers inside one of the 350 20-foot-wide bays.

That allowed muddy river water to gently pour into the 8,000-acre floodway for about an hour.

“We just want to be ready if the call comes in,’’ the corps’ Chris Brantley, project manager for the spillway, said during the annual test.

That call would come from Brig. Gen. Michael Walsh, president of the Mississippi River Commission and commander of the corps’ Mississippi Valley Division in Vicksburg, Miss.

The spillway was last opened because of high river flows in 1997 from March 17 to April 18. Some 298 bays were opened, allowing river water to flow through the structure at a rate of 243,000 cubic feet per second,

Brantley said the high-water season on the river runs from March to May.

When open, the spillway can divert water from the river to the lake — a 7-mile trip — at a rate of up to 250,000 cubic feet per second, which relieves pressure on local levees, lowers river stages and reduces the velocity of the river current south of the spillway.

New Orleans is about 33 miles downstream from the spillway.

Brantley said the National Weather Service is forecasting the river to crest at New Orleans at 16‰ feet April 8 because of heavy rain and melting snow upriver and along the Ohio River Valley. Flood stage at the Carrollton gauge in New Orleans is 17 feet. Flooding is not expected in the city.

The Mississippi at Baton Rouge is expected to crest at 39 feet — 4 feet above flood stage — by early next week, but city-parish officials say the river would need to reach 47 or 48 feet to top the levee. Flood stage is the level at which the river would be out of its banks if it did not have its massive levee system.

The U.S. Coast Guard is limiting the number of barges a towboat can tow on the lower Mississippi because of high water conditions.

Brantley said operation of the Bonnet Carré Spillway is determined by water flow and velocity, not river stage. The spillway structure is opened when river flows exceed 1.25 million cubic feet per second, he said. It takes two crews of four to five workers apiece about 36 hours to open the entire structure, he added.