Corps Closes Bonnet Carré Spillway, River Continues Slow Fall

The New Orleans Engineer District closed out the month of March by ceasing operation of the Bonnet Carré Spillway in St. Charles Parish, La., about 33 river miles upriver from New Orleans.

The Corps opened the initial bays of the spillway on March 8 to divert water from the flooding Mississippi River to Lake Pontchartrain and on to the Gulf of Mexico. At its high point, 183 of the spillway’s 350 bays were opened, diverting a peak flow of 196,000 cubic feet per second (cfs.) from the Mississippi River. The spillway was in operation a total of 23 days.

The Corps activates the Bonnet Carré Spillway to prevent the Lower Mississippi River at the Carrollton Gage in New Orleans from getting above a stage of 17 feet or a flow rate of 1.25 million cfs. This was the 12th time the Corps has operated the 87-year-old Bonnet Carré Spillway. Prior years include 1937, 1945, 1950, 1973, 1975, 1979, 1983, 1997, 2008, 2011 and 2016.

In all, the Carrollton Gage remained above 15 feet a total of 24 days, from March 8 until March 31. As of late last week, the river in New Orleans was just above 14 feet. The National Weather Service forecast the river to hover around 14 feet through at least April 18. While current National Weather Service long-range forecasts do not indicate the Mississippi River will trigger a second operation of the Bonnet Carré Spillway this season, plenty of question marks remain regarding rainfall throughout the Mississippi River Basin and the potential effects of a snow melt on the Mississippi Valley. Regardless, the Corps remains vigilant and the Bonnet Carré remains ready should extreme high water return.

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“We have never operated the structure twice in the same year, but fortunately, the design of the spillway allows multiple operations without change in the process,” said New Orleans District spokesman Ricky Boyett. “While a second operation would certainly be historic, I don’t believe the overall impact would differ greatly from that of a long continuous operation.”

Boyett pointed out that, while the spillway was open this year for a total of 23 days, it was open continuously for 75 days in 1973 at a much higher flow rate.

Boyett added that the Corps saw no major areas of concern along the river levees in South Louisiana last month and fewer “hotspots” than in 2016.

Five Dredges Working

“Our greatest challenge is more tied to navigation than flood risk,” Boyett said. “Whenever we have a high river, we experience a lot of shoaling as a result of the increased sediment. We currently have five dredges working in Southwest Pass in an effort to mitigate the impacts this sediment has on the navigation industry.”

As of last week, the Associated Branch Pilots, who guide ships from the Gulf to Pilottown, La., recommended a 44-foot maximum draft for vessels transiting Southwest Pass, which is typically maintained to at least 45 feet.

Elsewhere in Louisiana, all five locks on the Red River Waterway were open as of last week following closures due to high water in March. With the river dropping, though, concerns over shoaling and the loss of the waterway’s 9-foot channel led to the dredge Integrity mobilizing to Lock 1 near Mile 44.8 on the Red River, with the dredge standing by until flow rates subside.

On the Atchafalaya River, all locks were open last week, with the exception of Bayou Boeuf Lock near Morgan City, La., which was closed to navigation because of a high head differential. Coast Guard Marine Safety Unit Morgan City continued to maintain high water safety measures in the Berwick Bay area. The Atchafalaya River in Morgan City was forecast to remain above 6 feet—in minor flood stage—at least into this week.