This time of year, those living on the Mighty Mississippi can’t help but fret the water’s annual, inevitable rise.
Yet, it is a fear confronted year-round by those in charge of the elaborate flood-control system that has been erected along the river and its principal tributaries the past 86 years.
After the Great Flood of 1927, which stripped bare any mislaid perceptions of security fostered by the crude levees in place at the time, the Congress passed the Flood Control Act of 1928, thereby creating the Mississippi River & Tributaries System under the direction of the U.S. Army Corps of Engineers.
Those engineers, charged with developing the complex and still evolving system of locks and reservoirs, floodgates and levees, backwaters and cutouts, pump stations and dikes, “didn’t have supercomputers, they didn’t have computer modeling,” said Peter Nimrod, the chief engineer of the Mississippi Levee Board. “They had to rely on slide rules and calculations done by hand.”
They based those calculations on what they hoped would never occur, an unprecedented inundation they dubbed the Project Design Flood.
While their math and engineering skills, and faith in calculations produced by both, were tested repeatedly and without fail throughout the 20th century — most significantly in 1937 and again in 1973 — it wasn’t until 2011, when massive and rapid snowmelt after a particularly prolific winter combined with rainfall no less than 600 percent above what falls in a typical year in the Mississippi’s watershed threatened to overwhelm the complex system.
The Corps of Engineers has calculated that the 2011 flood brought down the Mississippi fully 85 percent of the water anticipated by the 1928 Corps engineers’ apocalyptic Project Design Flood.
The flood topped reservoir spillways and, closer to home, created potentially disastrous sand boils up and down the 278-mile stretch of the Mississippi River that falls under the purview of the Corps’ Vicksburg District.
The Corps has claimed, and no one has disputed the calculation, that the rivers and tributaries system during the 2011 flood prevented some $230 billion in damage, much of it economic — thereby realizing a savings of $16.4 for every $1 invested in the cumulative $14 billion system since 1928.
Crews today continue working to complete repairs at critical spots where levee damage from the 2011 flood was so severe it threatened aspects of the structure’s integrity.
In December 2011, “Congress authorized $802 million to fix the levee system from Cairo to the Gulf of Mexico,” Nimrod said. “Five of the repairs have been completed, one is half done, and the other five are about to begin. All will be fixed by some point this summer.”
The Mississippi River drains the world’s third-largest watershed — trailing only those of the Amazon and Congo rivers — an area comprising fully 41 percent of the nation and stretching from New York to Montana and points south.
By law, the Corps of Engineers must maintain “a 9-foot-deep channel 300-feet wide and 500-feet wide in the bends” along the Mississippi’s 2,300-mile run from northern Minnesota to the Gulf of Mexico, said Greg Raimondo, spokesman for the Corps’ Vicksburg District office.
The Corps of Engineers also is charged with laying revetment mats — 140-foot-wide blankets of articulated concrete slab along river banks to reinforce them against the water’s inexorable urge to reroute itself.
All told, the Corps’ Vicksburg District is responsible for managing waterways coursing through a 68,000-square-mile land mass, an area a fourth again as large as the state of Mississippi.
The first levee along the Mississippi was begun in 1717, as that nation’s colonists sought to protect New Orleans: a 3-foot high barrier just more than a mile long. Over the years, the effort expanded, yet remained largely ineffectual.
After the Mississippi overflowed its banks during a particularly destructive two-year period _ in 1849 and 1850_ Congress passed a law calling for federal funding of levee construction and mandating that states bordering the river establish levee boards.
The seemingly foresighted Swamp Act, however, led to unintended consequences.
“When you go back in time to pre-1927 flood, Mississippi had a levee board; Arkansas had a levee board; Louisiana had a levee board,” Nimrod said. “The race was always to who had the highest levees.”
If anyone was going to get flooded, the thought went, it would be the guy on the opposite bank.
That all changed as a result of the federal Flood Control Act of 1928.
“The Corps took over with the 1928 act and established a uniform height” for levees, Nimrod said. As a result, “levees today are, on average, 30-feet tall.”
Moreover, “nowadays, we don’t build a levee right up by the river. It might be a mile from it, it might be three miles,” he said.
In between the river’s banks and the setback levees, lie what are known as batchers — areas occupied by, for the most part, fishing and hunting camps and little else, all susceptible, by design, to flooding.
“It takes a certain amount of water to fill a river to its banks,” Raimondo said.
He said once the river goes over flood stage, past bankfull, and with room to expand laterally across the batchers, it rises much more slowly.
There are along the Mississippi River, and at the mouths of its various major tributaries from the Ohio south, some 50 gages that measure the water’s height and flow as it approaches flood stage. The gages were put in place decades ago without coordination among the various municipalities that installed them.
As a result, in Greenville, flood stage is 48 feet on the local gage while in Vicksburg, flood stage is 43 feet.
As it is, Raimondo said, “flood stage is defined as bankfull, which affects hunting and fishing camps on the river side of the levees. They see flood stage is nearing, they know they’ve got to start moving whatever they have out of those camps.”
“Batchers give the river room to spread out and rise more slowly,” Raimondo said. “The levees are set back far enough so the river doesn’t tear away at them.”
Cutouts dug through the river’s natural bends, mostly in the 1930s, straightened the river and to a significant degree tamed its destructive nature.
The 2011 flood was the result of the watershed channeling “600 percent of normal rainfall — that system just sat over the Ohio Valley and dumped rain after rain,” Raimondo said.
“The National Weather Service is telling us this should be an average year for flooding, but everything can change,” he said. “I wouldn’t want to make a living predicting the weather.”