Louisiana OLD RIVER CONTROL COMPLEX and Mississippi river flood protection

Note:  This section discusses flooding by and from the Mississippi and Atchafalaya rivers.  To learn about flooding from hurricanes through the non-river levee system in Louisiana, see the section on Hurricanes Katrina and Rita.

CONTROLLING THE RIVER: MAINTAINING THE MISSISSIPPI RIVER FOR NATIONAL COMMERCE.

HISTORY OF THE OLD RIVER AREA AND DEVELOPMENT OF THE OLD RIVER CONTROL COMPLEX (ORCC). 

As mentioned earlier, the Atchafalaya River is a distributary of the Mississippi, but it has not always been this way.  The two rivers have an interesting history that includes people trying to merge the two.  Let's take a look at their area of contact, called the Old River Area since, as we will see, one of the early links was called Old River.

• BEFORE THE 15th CENTURY:  The Red River and Mississippi River were separate rivers, more or less parallel.

• 15th CENTURY:  The MR turned west and a loop, later called Turnbull's Bend, formed.  It intercepted the RR, which became a tributary of the MR and the Atchafalaya River (AR) was formed as a distributary of the MR.

• BY 1778:  The entrance to AR was occluded by a log jam.

• 1831:  Capt. Henry M. Shreve, founder of Shreveport and a world renowned river engineer, dug a canal through the neck of Turnbull's Bend, thus shortening river travel time.

• Over time, the north section of Turnbull's Bend filled in with sediment.  The lower half remained open and became known as Old River (OR) and linked the three rivers.

• 1839:  Locals burned the 30 mi long logjam of the AR to the level of the water surface.  In 1840, the state removed the rest.  Periodic clearing efforts were required until the 1860s.
• As time passed, AR became deeper and wider and began to capture more of the MR.  During high water in the MR, water flowed west in OR and down AR.  When RR was high and MR low, water flowed down AR and east through OR.
• 1880:  AR was now large enough that there was only occasional eastward flow.  AR continued to grow and capture more of the MR.
• 1953: The U.S. Army Corps of Engineers concluded that MR could change course to AR bed by 1990 if it were not controlled.  This observation came from studies that monitored latitude flow over the years.

Latitude flow is an important term if one is to understand the operation of the ORCC. At the latitude of the "Red River Landing" (located at 30° 56' 20.4" about five miles downstream of the inflow channel of the Old River Auxiliary Structure), all the water passing the imaginary line that crosses the Mississippi and Atchafalaya rivers represents latitude flow. It is NOT the flow just in the Mississippi River, nor is it the flow just in the Atchafalaya. It is ALL the water, regardless of source or destination, that crosses the latitude of 30° 56' 20.4".

Percentage of Latitude Flow Entering the Atchafalaya River
1850-1950

The decision was made to control the river so that year-round the Mississippi River would handle 70% of latitude flow and the Atchafalaya River would handle 30%.

• 1963:  The Old River Control Complex (ORCC) was completed .

New channels dug include the outflow channel and the navigation channel, the latter having a lock to allow for the passage of shipping.  The following table shows that the Old River Control Structure is working as designed:

Percentage of Latitude Flow Entering the Atchafalaya River
1973-1985

• 1973: A large flood partially undermined the Low Sill Structure.  If it had failed, the MR probably have shifted to AR and possibly not been able to be shifted back.

Flood of 1973 showing the flooded overbank structure.

Lowsill structure showing the collapsed wing wall.

Potential scouring beneath the low sill structure. Had this happened, the structure may have collapsed. In fact, deep pits were scoured on each side, but did not unite underneath.

Repair done to prevent future scouring.

• 1985: Construction begins on Sidney A. Murray, Jr. Hydroelectric Plant.  It is situated just north of the Low Sill Structure with affluent from the Mississippi River and effluent to the Outflow Channel.

• 1986: The Auxiliary Structure (see slide above) with its new channel opened to relieve pressure at other sites.

One of the reasons that people suggested that the Atchafalaya would eventually capture (that is, the main flow of water through our state would exit Morgan City instead of its present location at the mouth of the Mississippi) is that the distance is so much shorter and steeper to the Gulf via the Atchafalaya than the meandering Mississippi:

• ORCC to Gulf via Atchafalaya:  142 mi
• ORCC to Gulf via Mississippi:  335 mi

If the events of 1973, as described above, happened, how would life on the lower Mississippi and Louisiana coast change?  Would the present-day Mississippi River suddenly dry up?  Would a fisherman sitting by the river see it go glub, glub, glub, with fish flopping around in the mud?  Would ship traffic stop on the river?  Would there be any impact at all?  The following description of possible life after the change is excerpted from Kazmann and Johnson (1980:10-16).

Long term, we would adapt.  Once the drinking and sanitation water issues were resolved, tourism would return.  Coastal erosion could be reversed on the west side of the present-day Mississippi River.  Shrimp, oysters, and other fisheries would probably flourish after a number of years due to new marshes being produced and nutrient rich sediments being redistributed.
This would obviously place a lot of stress on at least two generations of residents.  We would survive, but it would be a new Louisiana and Mississippi River delta. 

 What condition might potentially lead to this scenario? Experts predict that the ORCS might fail if the snowfall between Saskatchewan and New York exceeds that of the winter of 1972-73.

HOW THE FLOOD PROTECTION SYSTEM WORKS ON THE LOWER MISSISSIPPI RIVER.

For ages, the Mississippi River followed a similar pattern year-after-year.  As the water began to rise in the spring, the river would leave the depths of its channel and fill the batture (the area between the levee and the water) of the natural levee.  All backwater areas would slowly fill.  When non-native Americans came on the scene, they began to build levees that protected them from floods during most years.  Occasionally, higher water would result in flooding, so the local folks would build the levees higher.

After the great floods of 1927, the public mandated that Congress solve the flooding problems of the lower Mississippi River basin.  They did so by constructing a levee and "relief valve" (spillways and related structures) system:

In the slide above, all the dark lines represent levees that were constructed for flood control.

The Bonnet Carré Spillway in 1984.  Note the guide levees on each side.  The control structure is the obvious straight line on the river side in the photo.  See the leakage of water running into the spillway?  Lake Pontchartrain is at the other end.

Bonnet Carré open in 1997, looking toward the river.

Bonnet Carré open in 1983, looking toward the lake.

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.

Morganza Flood Control Structure and crane used to open steel doors on the bays.

Morganza Flood Control Structure bays.

Morganza Flood Control Structure.

Morganza Flood Control Structure bay with its steel door.

Morganza Flood Control Structure wing wall.

c.  Fuse Plug Levee:  If all the above procedures are not enough to handle the flood stage of the river, the last resort is the Fuse Plug Levee.  This is a 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 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.

PROJECT DESIGN FLOOD
Everything will eventually happen given infinite time.  Hitchhiker’s Guide to the Galaxy.

Below is a schematic that shows flow rates that the Corps of Engineers considered, in 1958 (this version has a couple of updated flows), to be the Probable Maximum Flood possible in the Mississippi River.  As the comment from the Hitchhiker’s Guide says above, there is no way of predicting the maximum flood.  Mother Nature is always full of surprises and will always find a way to surpass what humans consider the worst case scenario.

Today, the Corps focuses on events with joint probabilities that yield a given return period.  As mentioned in the definitions below, they select certain thresholds (such as a river height at a certain point) and an action plan that the threshold activates.

TERMS:

Joint probabilities:  the probability of multiple events occurring at the same time.  An example might be unusually high volumes in the Mississippi, but relatively low water coming from the Red and Ouachita rivers (such as happened in 2011), vs. a year when all tributaries of the lower Mississippi River are flowing at enormous rates.  Since many combinations of events can result in the same flood height at any given place on the river, the Corps plans for specific flood heights.  In the 2011 event, the river height reaching 17 ft at the Carrollton Gauge was an example of their present technique.

Return Period:  The theoretical average amount of time between two events of the same magnitude.  An example is a flood event that has a 1% chance of occurring in a given year.  Such an event would be expected to occur on average once every 100 years.  Such an event used to be called a 100-year event, but the fact that one can get 100-year events back-to-back, or five years apart, invalidates the use of this phrase.  Note that such an event is now referred to as “the storm (or other event) with a 1% chance of occurring in any given year.”

The diagrammatic map below graphically shows how the flood protection system is supposed to work.  It is called the Project Design Flood.

Flow rates updated 2011.

WHAT ARE KEY ISSUES TO CONSIDER ABOUT CONTROLLING THE PROJECT DESIGN FLOOD?

• The elaborate flood control system began design immediately after the Flood Control Act of 1928 and was basically completed in 1956.  The bulk of its visible infrastructure includes the Old River Control Complex and Lower Mississippi Flood Control system, both described in detail above.  Other important elements extend as far north as Cape Giraudeau, Missouri, and as far south as Head-of-Passes, Louisiana, on the Mississippi River.  It was well designed and constructed, protects the nationally important operations of the lower Mississippi River system, and it works.
• Another important function of the system is to protect people who live along the Mississippi and Atchafalaya rivers south of the ORCC. 

1. Those who live along the Mississippi are either high above expected flood levels (Baton Rouge, St. Francisville, etc., except for those who decide to move down by the water) are behind levees built to handle the estimated probable maximum flows in the river.
2. Those who live in the Atchafalaya Basin and have government protection are the people who live in Morgan City.  People who have built camps throughout the basin, or who have built in places like Butte La Rose, are in an official federal flood plain and know that they may have to abandon their homes if the flood control system (Morganza Spillway is opened, or the Fuse Plug Levee opens) comes into use.

• An examination of the Project Design Flood schematic shows that the flood control system is designed to allow 1.25 million cfs of water to safely flow past Greater New Orleans.  In 2011, many feared testing the existing river levee system with that flow rate.  In fact, it was reached and the levees did just fine.  This flow rate has been reached at least 10 times since 1927, so the levees have been tested many times.  This knowledge should help people cope with rising river water each spring, especially as the reports indicate that the magic 1.25 million cfs flow rate is not being surpassed.

updated 6/1/2012