Bonnet Carre Spillway

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Mississippi River's Bonnet Carre Spillway Opens For 13th Time Since 1931

Dr. Jeff Masters  ·  February 27, 2019, 12:00 PM EST



At 10 am CST February 27, 2019, 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 marks the first time in the 88 years of the spillway's existence that it needed to be opened in consecutive years.

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. The latest National Weather Service forecast predicts that the river will rise to minor flood stage in New Orleans on March 19, but opening of the Bonnet Carré Spillway should prevent that from occurring.

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 plans to open 38 of the 350 bays to start, then increase this number to as many as 200 bays when the flood crest arrives later in March. The spillway is scheduled to operate for about a month.

The spillway has been opened just thirteen times since 1937, but with increasing frequency in recent years—five of the thirteen openings have occurred since 2008, including three of the past four years. The spillway was last opened in March 2018, when it was operated at approximately half of its capacity. Although the spillway drains water into Lake Pontchartrain through uninhabited land that is primarily used for recreation, operation of the spillway does come with a cost: the excess fresh water that drains into the Gulf of Mexico is harmful to the oyster industry. The prolonged opening of the spillway in 2011 likely cost the Mississippi oyster industry $22 - $46 million and the loss of several hundred jobs during the period 2011 – 2013, according to an analysis by Mississippi State University.

Water levels on the Mississippi RIver are not high enough to force the Corps 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 to open compared to the Bonnet Carré Spillway. The value of property at risk of flooding in the Morganza Floodway is up to $2.2 billion, including over 21,000 homes.



Figure 2. Wednesday'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 16, at 61'. This would be the river's fourth highest flood on record, and similar in height to the ones in 2018 and 2016. Including 2019, five of the top seven floods on record at Red River Landing--where records extend back to 1851--will have occurred since 2008. 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. Record flooding along the Lower Mississippi caused $3.3 billion in damage that year. See our 2011 post, America's Achilles Heel: the Mississippi River's Old River Control Structure, for more information on why it is necessary to protect the Old River Control Structure. Image credit: USGS.