Explanation of the Spillway system

This is an excerpt from the article, which contains graphics also:
Louisiana River Control

Thanks to Capt. Don Houghton of the NATCHEZ for sharing this. The NATCHEZ and the ferries will continue running, unless/until the floodwalls are closed.

After the great floods of 1927, the public mandated that Congress solve the flooding problems of the lower Mississippi River basin. Congress intended the US Corps of Engineers to tackle the challenge, and they did so by constructing a levee and "relief valve" (spillways and related structures) system.
Each spring, as the water in the river rises, there is a sequence of events that occur that protect the lower river basin from flooding.

1. The water begins to pool into a large back swamp area in Avoyelles and Concordia parishes. This area is a vital part of the system because it holds so much water and gradually drains as the waters recede through the summer months. If the back swamp is ever lost (through development), the lower basin will be in grave danger.

2. As the back swamp fills, water rises in the river channel and covers the batture (The area between the levees and the normal river channel). In the Atchafalaya, this occurs between the internal levees that are along the margins of the river above the latitude of Baton Rouge, then the water spreads to cover the entire floodway between the guide levees that extend southward all the way to the Atchafalaya Bay.

3. The flow rates of the Mississippi and Atchafalaya increase, but remain in the 70%/30% ratio, as controlled by the ORCC.

4. If the predictive model used by the Corps of Engineers indicates that the above three steps will not prevent flooding, then they go to the human operated structures:

Bonnet Carré Flood Control Structure: When the water reaches a critical stage (it was originally designed to keep the Carrollton Gauge, located at the Corps of Engineers headquarters at the river on River Road, below the 20 ft mark - which kept the river 5 ft below the top of the levee), this flood control structure is opened to allow up to 250,000 cfs of water to flow into Lake Pontchartrain through the Bonnet Carré Spillway. It has been operated ten times (1937, 1945, 1950, 1973, 1975, 1979, 1983, 1997, 2008, 2011).

b. Morganza Control Structure: After the Bonnet Carré Flood Control Structure is opened, and if the river continues to rise to the next critical stage, this structure is opened. It shunts up to 600,000 cfs into the Atchafalaya River through the Morganza Spillway. Since this is a very rare occurrence (it has only been opened once, in 1973), the rich soils of the spillway are allowed to be used for farming, especially for cattle and soy beans. If it has to be used, the lessors are notified and, if they cannot remove their animals and crops, they lose them when the waters are released through the structures.

c. Fuse Plug Levee: If all the above procedures are not enough to control the flood stage of the river, the last resort is the Fuse Plug Levee. This is an east-west running levee located between the west guide levee and the west internal levee along the Atchafalaya. To its north lies the great back swamp area; to its south lies the West Atchafalaya Floodway. It works much like a fuse in your car. The fuse consists of a piece of wire that will tolerate an electric flow of a certain level (e.g., 15 amps). If a surge of higher electricity hits the fuse, the wire melts before the surge damages the electronics of the car. The Fuse Plug Levee is lower than the adjacent west guide and west internal levees. If the water in the back swamp is not contained by all the above steps (1-3 & 4a,b), then water begins to flow over the fuse plug levee rather than over adjacent levees where it would flood human habitations. Once water begins to flow over the top of the Fuse Plug Levee, it quickly tears it down until it carries a maximum of 250,000 cfs. This is designed to work on its own, but if extremely critical, it can be dynamited. The Fuse Plug Levee has never been needed.

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