[Update 2] Morganza Spillway opened, water flowing. Live video at wwltv.com.
[Update] The U.S. Army Corps of Engineers has confirmed the Morganza Spillway will be opened on Saturday.
The crest of the Mississippi River flood will reach New Orleans some time in the next few days. The U.S. Army Corps of Engineers has several tools at their disposal to ease the height of the river and the pressure exerted on the levees before it reaches New Orleans.
There is another, long-term purpose for controlling the flow of the lower river: Without control, hydrologists are fairly certain that the Mississippi would change course (as it has many times throughout history) and take the more direct and steep course to the Gulf through the Atchafalaya River Basin. The consequences of such a course change would obviously be dire for New Orleans and Baton Rouge, but it would also be an economic calamity for the country as a whole because there would be no suitable port where the river meets the Gulf of Mexico.
Many people believe nature will prevail eventually anyway, and the course change is inevitable.
This diary is intended to serve as a basic discussion of lower Mississippi River control for people who may be reading a lot about cresting Mississippi in the upcoming days. I've just been learning about this topic over the last couple of days, so feel free to add information or correct anything that I get wrong.
There are three main tools that the U.S. Army Corps of Engineers uses to control the flow of the Mississippi River north of New Orleans.
The Old River Control Structure
The Old River connects the Mississippi to the Red River, which had already been captured by the Atchafalaya. River water flows from the Mississippi, to the Old River, to the Red River, and then to the Atchafalaya. In the early 1900's, more and more of the Mississippi river was diverting to the Atchafalya River through the Old River.
In the 1950's it was decided that unless something was done quickly, the Mississippi would redirect entirely through this tributary - leaving the river below that point as a slack bayou. It was decided that they would build a structure that would keep the flow at 70% in the main river channel and allow 30% to flow through the channel.
The floodgates, which could open and close to varying degrees to control the flow, were completed in 1963. The main purpose of the structure is to hold the present river channel, rather than flood control in New Orleans and other areas below the junction.
After a bad flood compromised the structure in 1973, an additional structure was built: The Old River Control Auxiliary Structure. This structure can be used to alleviate pressure on the original structure during times of high river levels.
The Bonnet Carre Spillway
The Bonnet Carre Spillway is a series of floodgates and a spillway from the river to Lake Pontchartrain. The gates can be opened individually to allow water to flow from the east bank of the Mississippi, through the spillway, into Lake Pontchartrain, and eventually into the Gulf. The gates are currently more than half open due to the present river levels.
No homes or business are affected by water traveling through the Bonnet Carre and into the lake
The Morganza Spillway
The Morganza Spillway was built with the sole purpose of avoiding catastrophic flooding by relieving pressure on levees both above and below the structure. Normally dry on both sides, it is only designed to be used during very bad floods. When opened, it allows much more water to flow into the Atchafalaya River Basin than the Old River Control Structure.
The Spillway has only been opened once, in 1973. When opened, as it will likely be some time this weekend, residents along the Atchafalya Basin downstream will experience flooding. At risk are farmland, various small towns, and portions of Morgan City. The degree of the flooding will depend on how much of the spillway is opened.
Residents that live in the spillway are aware of the risk, and are given annual written notices from The Corps reminding them that they are in a floodway.
The Current Scenario
It is expected that as the crest approaches New Orleans, the Bonnet Carre Spillway will be opened to 100%, and the Corps will need to open the Morganza Spillway anyway from 1/4 to 1/2 open. Some residents of Morgan City will experience flooding as a result, but it expected that the action will prevent the river from overtopping or severely compromising levees in New Orleans.
For up to date current information, the best source is The Times-Picayune online site, nola.com
This is my first diary. I would have liked to add pictures and diagrams, but just as started to learn how, I realized that my lunch hour is over and it's time to get back to work! Please excuse the plainness.