|Above: Extracting a needle--very carefully--from the Bonnet Carré Spillway during its last opening on January 10, 2016. Each of the 350 concrete bays holds 20 needles. Image credit: Bob Henson.|
At 10 am CST March 8, 2018, the U.S. Army Corps of Engineers opened the gates on the Bonnet Carré Spillway in St. Charles Parish, Louisiana to allow flood waters from the swollen Mississippi River to flow into Lake Pontchartrain. This is just the 12th time since it became operational in 1931 that the Spillway has been used, and the first time since January 2016. Opening of the spillway is expected to keep the Mississippi River below its 17-foot flood stage in New Orleans; levees there protect the city against floods three feet higher than that. When all 350 bays are opened, the spillway can divert one-fifth of the river’s water into Lake Pontchartrain. The last time all 350 bays were opened was in 1983. This year, the Army Corps is anticipating opening about half of the bays, and the spillway is scheduled to operate for 2 – 4 weeks.
|Figure 1. Crowds line the slopes of the outlet between the Mississippi and Lake Ponchartrain downstream from the Bonnet Carré Spillway during its last opening in January 2016. Image credit: Bob Henson.|
It is not expected that the Corps will also be forced to open the Morganza Floodway in Pointe Coupee Parish northwest of Baton Rouge, which would divert water from the Mississippi River down the Atchafalaya River. That floodway has been opened only twice--in 1973 and 2011--and has a much higher cost of being opened than opening of the Bonnet Carré Spillway, due to the large amount of agricultural lands that flood below the Morganza Floodway. Thursday’s forecast from the NWS River Forecast Center predicted that the Mississippi River would crest at Red River Landing, just above the Morganza Floodway, on March 19, at 60.0'. This would be the river’s seventh highest flood on record, and similar in height to the 60.71’ crest on January 19, 2016. The all-time record crest there was 63.39' on May 18, 2011, when the Corps was forced to open the Morganza Floodway in order to relieve pressure on the Old River Control Structure.
|Figure 2. Statewide rankings for average precipitation during February 2018, as compared to each February since 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 124 the wettest. Image credit: NOAA/NCEI.|
Heavy February rains to blame for the opening of the spillway
A swath from Michigan to Louisiana set February records for the highest amount of water vapor observed in the atmosphere, helping trigger the wettest February on record for Arkansas, Illinois, Indiana, Missouri, Pennsylvania and Tennessee. It was also a top-ten-wettest February for nine other nearby states. The heavy February rains primarily fell in the watersheds of the Mississippi and Ohio Rivers, and the Ohio River suffered its worst flood in 20 years in late February. The surging flood waters from the Ohio River have now emptied into the Lower Mississippi River, swelling the river to moderate flood stage just below the river’s confluence with the Ohio River at New Madrid, Missouri. Additional rains of 1 – 2” are expected over portions of the Lower Mississippi River watershed over the next week. As a result of all these heavy rains, moderate to major flooding is expected for most of the Lower Mississippi, from Illinois to Louisiana, for most of March. However, this flooding is not expected to be at a level capable of causing a billion-dollar disaster, such as occurred in 2011. Record flooding along the Lower Mississippi caused $3.3 billion in damage that year.
Weather.com published an excellent long-form piece on the Bonnet Carré Spillway back in 2016, which has a very detailed look at its history, function and ecological impacts.