Bonnet Carre Spillway Explained

Why do we open the spillway and what does it affect?

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Picture of a previous Bonnet Carre Spillway opening courtesy of the Army Corps of Engineers.

This week, the Army Corps of Engineers will send Mississippi River floodwaters flowing into Lake Pontchartrain at as much as 250,000 cubic feet per second. That's nearly enough water to fill three Olympic sized swimming pools every second.

 

Sediment-laden floodwaters from the Mississippi will first flow through the Bonnet Carre Spillway control structure and six miles of levee-lined floodway before reaching Lake Pontchartrain. Located in St. Charles Parish, the spillway structure was designed to help prevent the Mississippi River from overtopping levees adjacent to populated communities like New Orleans. The river levees near New Orleans were only constructed to withstand a flow of 1.25 million cubic feet per second- anything faster and the levees might fail.

Side Effects

 

When the river reaches its flood stage due to rain and snowmelt throughout the Mississippi River Valley, the opening of the Bonnet Carre Spillway is necessary to protect communities downstream.

Although changes in salinity caused by spillway floodwaters will not cause permanent damage to the estuarine environment of Lake Pontchartrain, recreation on the lake may suffer. Recreators should be mindful of risks due to possible algal blooms in the lake.


During this time of change, while the spillway is open and afterward, the scientists at the Lake Pontchartrain Basin Foundation will be keeping a close eye on any harmful short term effects. 
Locals should expect more woody debris, alligators, snakes, and other wildlife to be present in and around the lake. After the spillway is closed, it will take about four to six months for Lake Pontchartrain to return to its usual habitat.

Sidebar on Sediment

 

Sediment is a crucial resource for our coast and the landloss crisis we are facing. Similar to the coast, the marshes that line the western shore of Lake Pontchartrain suffer from landloss and continue to weaken. Levees built along the Mississippi, as well as the guide levees along the Bonnet Carre Spillway, starve these marshes from the replenishing flow of sediment-laden river water.

Fortunately, in the current Coastal Master Plan, there are two projects that plan to address this. One will dredge a conveyance channel in the western guide levee of the Bonnet Carre Spillway, to allow floodwater and sediment to flow into the adjacent wetlands. The other will erect a new levee to help to protect neighboring LaPlace communities from the flow of the planned channel.

A collection of satellite imagery from the January 2016 Bonnet Carre Spillway opening, showing the movement of sediment from the river, through the spillway and Lake Pontchartrain, and out to the gulf. See more here.

Bonnet Carre Floodway Structure and History

 

This year, the Bonnet Carre Spillway flood control structure celebrates its 88th birthday. Built in just two years, the flood structure was engineered in response to the devasting 1927 flood of the Mississippi River Valley. It's made up of 350 concrete bays which are pinned with 7,000 wooden needles or slats. Despite being entirely closed, the structure can leak if the river is at a high flood stage. Last year, right before the spillway was opened, the structure was leaking about 5,200 cf/s of river water, enough to fill about 920 bathtubs every second.

When the decision is made to open the Bonnet Carre Spillway structure, two cranes move along the top of the concrete bays removing one needle at a time. Floodwaters and the sediment suspended in them are then directed by guide levees through six miles of floodway that leads to Lake Pontchartrain. If fully open, the spillway can divert as much as 250,000 cf/s of fresh river water and sediment into the lake.

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