River Watch 2016: Managing high water along the Mississippi

River Watch: Managing high water along the Mississippi

(WAFB) - Covering more than 1.25 million square miles, the Great Mississippi, along with its tributaries including the Red River, drain more than 40 percent of the United States, and all of that water passes right through southern Louisiana.

The U.S. Army Corps of Engineers is tasked with managing that massive flow and maintaining the Mississippi flood protection system through the Bayou State. And no doubt about it, it is a daunting task.

In fact, the Corps' assignment is further complicated by the mandate to divert a portion of the Mississippi River's flow to join the Red River as they head down the Atchafalaya.

This partial diversion of the Mississippi flow is accomplished through the Old River Control Complex, located on the Mississippi about 14 miles northeast of Simmesport. Old River Control is actually a system of floodgates designed to precisely regulate the amount of Mississippi water diverted into the Atchafalaya.

The task is to take the combined flow of the Red and the much larger Mississippi and divert enough Mississippi River water to maintain a 70-30 percent flow distribution between the Mississippi and the Atchafalaya.

As the Mississippi rises and falls, the flow through Old River Control must be adjusted to maintain this ratio. That is why flooding on the Mississippi can produce flooding for both the lower Mississippi and the Atchafalaya.
 
Flow rates on the Mississippi River below the Old River Control are measured at Red River Landing, located just east of Simmesport, and at the Port Allen Locks across the river from Baton Rouge.

The levee system from Red River Landing all the way to the Bonnet Carre Spillway is designed to carry a flow of up to 1.5 million-cubic-feet-per-second (1.5Mcfs). In round numbers, that flow is roughly equivalent to a river stage of approximately 62.5 feet at Red River landing and roughly 43 to 44 feet at Baton Rouge.

A flow significantly larger than that along this stretch of the levee system would exceed the engineering design. Therefore, when the river flow is expected to exceed that flow rate, the Corps turns to the Morganza Spillway to divert additional Mississippi River flow into the Atchafalaya.

The Corps opens the Morganza Spillway sufficiently to maintain the design flow of 1.5Mcfs down river and past Baton Rouge. However, opening Morganza means a substantially greater amount of water being sent into the Atchafalaya Basin, increasing flood impacts there.

Indeed, the two largest floods ever for the lower Atchafalaya, in 1973 and 2011, were both a result of the Corps' use of the Morganza Spillway to control the flow along the Mississippi.

The maximum flow down the Mississippi past Baton Rouge is maintained at 1.5 Mcfs during a flood event like the current one, and that flow is continued all the way down to St. Charles Parish, the location of the Bonnet Carre Spillway.

For New Orleans, the Corps maintains a peak flow of 1.25 Mcfs on the Mississippi, producing a river stage of 17 feet for the Crescent City. The Bonnet Carre Spillway is used to reduce the Mississippi River flow from 1.5 Mcfs to 1.25 Mcfs for New Orleans by diverting the excess into Lake Pontchartrain.

So in the end, even when the Mississippi is exceptionally high, the Corps of Engineers maintains the flow down the main-stem within the levee system's safety margins.

Unfortunately, during floods like we saw in 2011 and with what we may be dealing with again in the coming days, the Morganza Floodway and the middle and lower Atchafalaya Basin serve as the buffer for that excess water.

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