BONNET CARRE SPILLWAY: At the Bonnet Carre Spillway, two cranes are used to individually lift timbers from bays of the spillway and place them horizontally across the structure. A complete opening of all 7,000 timbers from the 350 bays in the spillway structure requires about 36 hours. Image: COURTESY OF US ARMY CORPS OF ENGINEERS, VIA FLICKR
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Rain continues to fall (as it has for the past month) in record-breaking amounts across the middle Mississippi and Ohio river valleys, swelling the two waterways and their tributaries. As some residents evacuate and others await word on whether they must flee, the U.S. Army Corps of Engineers is considering its increasingly limited options for containing a major catastrophe already washing away homes and farmland.
Most of those options depend on the volume of water and the length of time that water stresses the man-made infrastructure of levees, dikes and spillways built along the Mississippi.
The Corps partially opened Louisiana's Bonnet Carre Spillway earlier this week to reduce the volume of water heading toward New Orleans. By the end of Friday a total of 264 spillway bays (out of 350) will be opened, according to the Corps, which plans to open additional bays as needed. They are also considering opening some portion of the 125 bays at Morganza Spillway near Baton Rouge as early as this weekend—for the first time in 38 years—to divert Mississippi River floodwaters heading down from the Midwest and Ohio River into the Atchafalaya River Delta, according to The Times-Picayune. If Morganza is opened even partially several thousand homes as well as farms, a wildlife refuge and a small oil refinery in the Atchafalaya Basin would be at risk of flooding in order to prevent a deluge from hitting densely populated Louisiana state capital Baton Rouge and New Orleans.
Heavy rains aside, the current flood conditions can be traced back to a number of factors, including the buildup of shallow flood barriers atop loose soil along the Mississippi and its tributaries over time that narrowed their pathways. This has led to more water running faster through narrower spaces, thereby placing more pressure on the region's levees.
To gain a better understanding of the precarious situation along the Mississippi as well as how such flooding might be mitigated in the future, Scientific American spoke with J. David Rogers, a professor of engineering at the Missouri University of Science and Technology, who also teaches flood control courses for the Corps. Whereas floods are inevitable, their impact can be mitigated through well-designed and constructed flood barriers, proper land management along major bodies of water, and a better understanding of how floods work, according to Rogers.
[An edited transcript of the interview follows.]
How serious is the situation facing communities along the Mississippi (in particular, Memphis, Vicksburg, Baton Rouge, New Orleans)?
The forecasted flows should be within the design capacity of the Mississippi River and Tributaries [MR&T] flood control project constructed by the Army Corps of Engineers between 1931 and 1980. That doesn't mean there won't be any levee failures or other problems, but that it should be within the design capacity to handle it.
Levees and other man-made attempts to regulate water levels often become an issue when the Mississippi threatens to flood its banks. Are such floods an inevitable fact of life along the river, or is it possible to protect all of the communities that have sprung up along the river over time?
We have probably diked off too much of the river's active flood plain; in many cases close to 90 percent of the historic flood plain. We probably shouldn't have diked off more than 75 percent, because the dikes located close to the low-flow channel (pdf) [designed to concentrate flows and increase channel velocity and depth during low-flow periods] are those most prone to foundation problems, due to under-seepage through pervious materials. Floods are an inevitability, and we would need 1,000 years of hydrographic records to accurately estimate a flood with a 100-year recurrence frequency. We have about 160 years of records, so there is a lot of guesswork involved.