Flood-control system a complex marvel

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GREENVILLE, Miss. (AP) - 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.

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