By Alisha Renfro, Staff Scientist, Restore the Mississippi River Delta Coalition, National Wildlife Federation
As the Mississippi River high water event continues, the U.S. Army Corps of Engineers will soon open the Bonnet Carré Spillway, and potentially the Morganza Floodway, to help relieve pressure on river levees and prevent catastrophic flooding. During high-water events like this one, the river contains more water and carries more sediment than usual. Without restoration projects like sediment diversions in place to capture sediment, much of this essential component for restoring our coast is lost. In the future, when sediment diversions are in place, we’ll be able to utilize the increase in sediment carried by the river during high water events and capture it for coastal restoration. This blog is the first in a series that examine management of the Mississippi River for flood protection and the opportunities that exist to do so for coastal restoration.
Unusually heavy winter rainfall throughout much of the Mississippi River’s drainage basin has led to early flood conditions on the Mississippi River. Deaths of at least 29 people and loss of property in Illinois, Missouri, Arkansas and Oklahoma are a stark reminder of how dangerous floods can be.
On the lower Mississippi River, the Mississippi River and Tributaries (MR&T) project uses levees and floodways, among other measures, to manage river floods, like the one happening now, to protect people and property.
Prior to the disastrous 1927 flood, attempts to prevent flooding along the lower Mississippi River relied on levees along the river built to withstand the previous flood of record. The MR&T project, authorized through the Flood Control Act of 1928, is a more comprehensive U.S. Army Corps of Engineers’ flood control program. It relies on levees along the river to control flood flows, floodways to lessen pressure on critical points in the river levee system, improvements and stabilization of the river channel for navigation, and improvements to major tributary drainage basins, such as dams and reservoirs. Combined, the features of the MR&T system are designed to handle the largest flood that is reasonably expected to occur, known as “project flood.”[i],[ii]
In Louisiana, river floods rely on protection provided by levees along the Mississippi and Atchafalaya Rivers, the Old River Control Complex, the Bonnet Carré Spillway and the Morganza and West Atchafalaya Floodways.
- The Old River Control Complex is designed to send 30 percent of the combined flow of the Mississippi and Red Rivers down the Atchafalaya River, and the remaining 70 percent of the flow down the main stem of the Mississippi River. During this current river flood, the U.S. Army of Engineers began operating the Old River Overbank Structure on December 30 to reduce the risk of damage to Old River Control Complex sill structures.
- The Morganza Floodway is operated during flood events to shunt excess floodwater from the Mississippi into the Atchafalaya Basin. This structure has only been operated twice, 1973 and 2011, since construction was completed on it in 1954. The U.S. Army Corps of Engineers is monitoring the river gage at Red River Landing to determine if this structure will be needed during the current high water event.
- The Bonnet Carré Spillway is located upriver of New Orleans. When opened, this structure can shunt up to 250,000 cubic feet per second of sediment-laden water from the river into Lake Pontchartrain, to reduce pressure on the river levees by keeping water flow below 1.25 million cubic feet per second. The U.S. Army Corps of Engineers will open this structure during this flood event beginning on Sunday, January 10.
- The West Atchafalaya Floodway is the last feature of the flood control system and has not been used to date.
The MR&T project has been successful in managing river flood risk during several flood events, most recently in 2011. The water levels in the river are expected to continue to rise over the next week and features of the project, such as the Bonnet Carré Spillway, will be used to manage the floodwaters and protect people and property along the lower Mississippi River.
Up next in this blog series, Alisha Renfro will examine sediment levels in the Mississippi River during high flood conditions and what implications this could have for coastal restoration.