Old River Control Structure

The Old River Control Structure (ORCS) is a floodgate system located in a branch of the Mississippi River in central Louisiana. Completed in 1963, the complex was built by the U.S. Army Corps of Engineers in a side channel of the Mississippi known as "Old River," between the Mississippi's current channel and the Atchafalaya Basin, a former channel of the Mississippi.

Purpose

The Old River Control Structure was built to prevent the main channel flow of the Mississippi River from altering its current course to the Gulf of Mexico through the natural geologic process of avulsion. Downstream of the facility, the Old River channel eventually merges with the Atchafalaya River which in turn enters the Gulf of Mexico at Morgan City, Louisiana. This alternative path to the gulf is much shorter and steeper. The increased drop allows for greater erosion of the river bottom, while the decreased flow in the main channel allows for more sediments to build up where the river enters the Gulf of Mexico. Left unchecked, more and more water would take the steeper and shorter path to the Gulf of Mexico until eventually this would become the new main channel of the Mississippi. Historically, this natural process has occurred about every 1000 years, and is overdue. Accordingly, between 1900 and 1975, more and more water from the main channel was taking the Atchafalaya River path to the sea. In 1975, artificial control took effect. Some researchers believe that the likelihood of this event increases each year, despite artificial control efforts.

If the Mississippi diverts its main channel to the Atchafalaya Basin and the Atchafalaya River, it would develop a new delta south of Morgan City in southern Louisiana, greatly reducing water flow to its present channel through Baton Rouge and New Orleans. Integrity of the Old River Control Structure, the nearby Morganza Spillway, and other levees in the area is essential to prevent such a diversion; see "Mississippi River: Future course changes" for further discussion. Jeff Masters of Weather Underground noted that failure of that complex "would be a serious blow to the U.S. economy, and the great Mississippi flood of 2011 will give its most severe test ever."

Components

A waterway between the Mississippi into the Atchafalaya-Red River system was first dug in 1841. It parallels Old River, a naturally occurring waterway at the same site.

The present Old River Control Structure was completed in 1963 and expanded in 1990. The first two floodgates are the Low Sill Control Structure, which regulates routine flow in the waterway, and the Overbank Control Structure, in use only when the Mississippi exceeds its banks. A navigation channel and lock were also part of the original facility design. Subsequent expansion created what is now known as the Old River Control Complex, when the Auxiliary Structure, which became operational in 1986, was added to reduce pressure on the original floodgates after extensive damage caused by the flood of 1973. The Sidney A. Murray, Jr. Hydroelectric Station, completed in 1990, also provides an additional measure of control at the site.

Operation

Water from the Mississippi is normally diverted into the Atchafalaya Basin only at Old River, where floodgates are routinely used to redirect the Mississippi's flow into the Atchafalaya River such that the volume of the two rivers is split 70%/30%, respectively, as measured at the latitude of at Red River Landing, Louisiana..

Water diverted at Old River flows into the Atchafalaya Basin, first entering the West Atchafalaya Floodway, then continuing down the Atchafalaya River to the Gulf of Mexico, bypassing Baton Rouge and New Orleans (see diagram).

The Morganza Spillway, between the Mississippi and the Atchafalaya Basin nearby downstream, is normally closed, but can be opened in an emergency to relieve water levels and water-pressure stress on various levees and other flood-control structures, including the Old River Control Structure, by diverting additional water from the Mississippi into the Atchafalaya. The Morganza Spillway was never used before the construction of ORCS, and has only been opened twice (as of 2011) for flood control since ORCS was built.

Building Activity

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