The Army Corps of Engineers is facing a huge test as record high flood waters course down the Mississippi River towards the Gulf. As I’ve followed this story, I’ve learned about some of the man-made structures that have been engineered to keep the mighty Mississippi channeled.

First, let’s take a look at the Mississippi in Southern Louisiana:

Again, this time with the river crudely hilighted:

As you can see, the river flows SE from the Mississippi state line, passing Baton Rouge and New Orleans before emptying into the Gulf.

Structure #1

The first major structure to impact the flow of the river is the Old River Control Structure, located in the Three Rivers Wildlife Management Area:

Overview (A):

Detail:

According to Wikipedia,

The Old River Control Structure is an edifice built by the U.S. Army Corps of Engineers at the divergence of the Mississippi and Atchafalaya Rivers in order to maintain the water distribution between the two, at 70% and 30%, respectively. This was done in response to the increasing amounts of water flowing from the Mississippi into the Atchafalaya, due to the latter’s shorter and increasingly steeper course to the Gulf of Mexico. The floodgate system was completed in 1963. The complex is located at river mile 315 on the lower Mississippi—315 miles (507 km) up the river from Head of Passes, where the river’s main stem breaks into three branches that soon flow into the Gulf of Mexico.

Here is a satellite image of the Old River Control Structure:

And this is how the 70/30 (Mississippi/Atchafalaya) divergence plays out:

Here is a look at the Old River Control Structure’s three floodgates:

The Atchafalaya River takes a relatively straight course to the Gulf when compared with the Mississippi River. The lower half of the Atchafalaya is the Atchafalaya Basin, an area comprised of swamps, wetlands and delta:

Here’s a satellite view of the Atchafalaya River (green) leading into the Basin, with the Mississippi (blue):

Structure #2

The second structure of note is just below the Old River Control Structure and is called the Morganza Spillway. The Spillway was constructed to divert waters from a flooded Mississippi into the Atchafalaya Basin.

Overview (B):

Closeup:

This next image shows that when the Morganza Spillway is opened, water will flow from the Mississippi River into the Atchafalaya River and Basin. The red lines are the levees that define the perimeter of the spillway:

Structure #3

The third structure of note is the Bonnet Carré Spillway, which is located downriver, about 13 miles West of New Orleans. The Bonnet Carré Spillway operates in a similar manner to the Morganza Spillway, but here, it diverts floodwaters of the Mississippi to Lake Ponchetrain.

Overview (C):

Here’s a closer look at the Bonnet Carré:

Water released from the Bonnet Carré Spillway into Lake Ponchetrain ultimately exits the lake into the Gulf:

SO, WHY DOES THIS ALL MATTER?

The Army Corps of Engineers must divert the flooding Mississippi River away from New Orleans (and, as we’ve seen, other vulnerable upriver cities and towns). To do so, it will have to: 1) adjust the flow at the Old River Control Structure, 2) open the Morganza Spillway, and 3) open the Bonnet Carré Spillway.

This is pretty big news down here; the Morganza has not been opened since 1973, and while the Bonnet Carré has been opened more frequently, all of its 350 bays have not been opened since 1983.

Now, even if the Bonnet Carré is opened fully, unless the Morganza is opened, the Mississippi will crest at 19.5′ in New Orleans. New Orleans’ levees protect up to 20′. Therefore, it seems clear that the Corps will have to open both the Morganza and the Bonnet Carré in order to avoid a potential breach of New Orleans’ levees.

The consequences of this historic flood for SE Louisiana are not yet known. Communities surrounding the Atchafalaya Basin could be flooded, and the New Orleans levees could be put to the test. The Mississippi River will crest in New Orleans on Tuesday May 24th; at that time, I will be heading to my bar exam prep course in above-sea-level Boston. Nonetheless, for my friends in and around New Orleans, I hope (and do believe) that the Corps will get this right.

Link:
Article in NOLA.com