The Bonnet Carre Spillway is located in Norco, Louisiana, which is about 32 miles upstream of downtown New Orleans. The spillway acts as a relief valve for the great river in the event of a major flood. The levees around New Orleans cannot handle the volume of water that the Mississippi River can potentially carry. In addition, we know that is not a good idea to stress the levee system, let alone press it to its limits. During periods of high water, the spillway is opened, allowing a large fraction of the Mississippi River water to flow into Lake Pontchartrain upstream of New Orleans. This reduces the amount of water that has to flow past New Orleans, which reduces the chance of a levee failure and flood in the urban area.

A secondary function of the Bonnet Carre Spillway is to prevent the Mississippi River from changing course. Both the Mississippi River and Lake Pontchartrain are at sea level, and the majority of the land in the area varies between one and three feet above sea level. The only thing holding back the Mississippi River is a ridge that runs along the east bank of the river that is 12 to 16 feet high. That ridge and the levee system are all that keeps the great river in check. In fact, in past floods, the Mississippi River would overflow this ridge and drain part of its flow into Lake Pontchartrain. In the mid-1800's, the Mississippi River flowed into the Lake for 6 months following a flood. The result is that a number of small channels had been cut over the years between the river and the lake. The spillway normally blocks the Mississippi River from flowing into Lake Pontchartrain, but provides a controlled outlet for this water when needed.

Update—the Bonnet Carre Spillway was opened on Friday, April 10, 2008, at shortly after noon due to high water in the Mississippi River channel. This was the first time that the spillway has been opened since 1997, and only the 9th time in history that the spillway has been used. The first time was in 1937. In the 2008 opening, 160 of 350 bays of the spillway were opened, with the structure remaining open for 31 days.

Update—the Bonnet Carre Spillway was opened again on Monday, May 9, 2011, as 28 gates opened. 44 gates were opened by the end of the day on Tuesday, with 330 gates in operation by May 18. Due to the advance notice that the structure was opening, spectators crowded the observation deck at the south end of the structure to watch the spillway being opened.

The photo above is the east face of the spillway looking towards the southwest. The spillway runs east and west, with the west end being upriver, and the east end being downriver. The water flows north when exiting the Mississippi River. The photo below is the US Army Corp of Engineers project sign.

The photo above is a close view of several of the spillway gates. The gates are made out of pressure treated wood. To open the spillway, a crane lifts each wooden plank out and puts it on top of the structure for storage. The gap in the middle of each gate allows a small flow of water through the spillway in the event of moderately high (but not flood level) water. The photo below is the northern end of the spillway structure. The structure connects into the levee system that follows both sides of the river.

The photo above is the front side of the spillway structure taken from the south end, while the photo below is the rear side of the structure. These cranes travel the top of the structure and are used to remove and reinstall the timbers that form the gates. There are 7,000 timbers in all, filling 350 bays. It can take 36 hours for these two cranes working in concert to fully open the spillway.

The row of concrete blocks running parallel to the spillway are in place to calm the flow of water. Normally, water rushing through the spillway and falling ten feet would cause a lot of turbulence, which could lead to scour holes and failure of the dam. The blocks calm the water, preventing the turbulence.

The photo above is looking towards the Mississippi River. The spillway is located in a natural swale depression in the terrain. Notice that the levee that normally runs along the river does not exist in this area. As a result, when the river level rises, water is free to flow up to the spillway structure.

The photo below is looking east towards Lake Pontchartrain, which is nearly six miles in the distance. When not in operation, the spillway is used as a recreation area. Given that it is used about once every 10 years, there is no other use for the land. A railroad crossing can be seen in the distance. In addition, US-61 crosses the spillway just past the railroad, and I-10 crosses further east right along the edge of the lake. Both highways are on elevated structures.