The sprawling network of levees — built over many years to protect the Upper Mississippi basin from the sort of disastrous flooding...

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The sprawling network of levees — built over many years to protect the Upper Mississippi basin from the sort of disastrous flooding that has claimed homes, lives and millions of acres of farmland in the past week — was never designed to withstand the magnitude of a 500-year flood.

And so residents of towns such as Gulfport, Ill., and La Grange, Mo., have watched as waters spilled over the tops of levees that were supposed to keep them dry.

The flooding has raised questions about the adequacy of the patchwork system — in which little information is known about where levees exist, who maintains them and what their condition is — as towns downstream hurry to fortify their levees in preparation for the cresting floodwaters heading south.

“Nature has its way of upping the ante,” said Eric Halpin, special assistant for dam safety and levee safety with the U.S. Army Corps of Engineers. “This storm proved that even if we had built levees to the floods of record, they would have been overwhelmed by this event.”

Severe storms and flooding in the Midwest have killed 24 people and injured 148 in six states and forced tens of thousands to evacuate their homes. As of Thursday, the Federal Emergency Management Agency and other federal agencies had distributed water enough for 1.1 million people, 12.8 million sandbags, 2,500 tarps and 4,000 rolls of plastic sheeting.

Though the flood has receded in many places, the Mississippi River continues to rise. So far, at least 30 levees along the Mississippi have failed and 10 to 20 more are at risk.

The flooding seems likely to equal or exceed the 1993 floods that killed 48 people and caused more than $20 billion in damage.

Experts said levee failures aren’t surprising, especially given the low standards to which most levees are built.

To qualify for the National Flood Insurance Program, structures simply need to be behind a levee built to a so-called 100-year standard, meaning there is a 1 percent chance in any given year that a flood will rise above the levee. In the Netherlands, levees for ocean flooding are built to a 10,000-year standard, and inland levees are designed at least to a 250-year standard and usually in excess of 1,250 years.

“Around the world, the 100-year standard is a joke,” said John Barry, author of “Rising Tide,” a book about the Mississippi River flood of 1927, and a member of a flood-control authority that oversees six levee districts in metropolitan New Orleans. “We invest on the cheap.”

Numerous things can cause a levee to fail, experts said: The wrong material may have been used, a channel could have been created by a rotting log or the foundation may fail due to unstable ground.

Putting sandbags atop a levee to increase its height — as dozens of riverside communities are doing — can weaken a levee, as people and bags add strain and create weak spots. Once a levee has been overtopped, breaching is fairly common, as the rushing water washes away the backside.

Re-engineered landscape

In addition, humans have radically re-engineered the landscape to take away its water-absorbing functions. Plowed fields have replaced tallgrass prairies. The basic hydrology of Iowa has been changed since the coming of the plow. By early in the 20th century, farmers had installed drainage pipes under the surface to lower the water table and keep water from pooling in what otherwise could be valuable farmland. More of this drainage “tiling” has been added in recent years. The direct effect is that water moves quickly from the farmland to the streams and rivers.

“We’ve lost 90 percent of our wetlands,” said Mary Skopec, who monitors water quality for the Iowa Department of Natural Resources.

Crop rotation may also play a subtle role in flooding. Farmers who may have once grown a number of crops are likely to stick to just corn and soybeans, annual plants that don’t put down deep roots.

Another potential factor: sediment. “We’re actually seeing rivers filling up with sediment, so the capacity of the rivers has changed,” said Lyle Asell, a special assistant for agriculture and environment with the Iowa DNR.

The landscape wasn’t ready for the kind of deluge that hit Iowa in May and early June. Central and eastern portions of the state received 15 inches of rain. That came on top of previous rains that had left the soil saturated. Worse, the rain came at the tail end of an unusually cool spring. Farmers had delayed planting their crops. The deluge struck a nearly naked landscape of small plants and black dirt.

Lessons ignored

After the 1993 floods, the Corps of Engineers repaired 23 miles worth of levee breaks, said Ron Fournier, a spokesman for the corp’s Rock Island District. But the levees remain much the same. “It’s repaired, but it’s not a … system that manages flood control, it’s a patchwork of different systems,” he said, noting that significantly improving the system could be costly.

Several lessons and recommendations emerged from the 1993 floods, but few have been acted upon, said Gerry Galloway, a retired brigadier general with the Army and a University of Maryland engineering professor who led an acclaimed study of the 1993 floods.

One key lesson: The loose amalgam of federal and nonfederal levees wasn’t sufficiently monitored or maintained. No one knows how many levees exist or what their condition is, and most are turned over to communities after being built.

Galloway recommended that the government develop an inventory and inspection system for levees similar to the one it created for dams in the 1970s. Congress agreed after Hurricane Katrina to start such a program, but lawmakers have yet to appropriate sufficient money, Galloway said.

“We also indicated that critical infrastructure” — hospitals, nursing homes, fire and police departments — “was not supposed to be in the flood plain,” Galloway said, a recommendation that was ignored.

Some environmentalists and engineers pointed out that any structural system such as levees is always going to be inadequate, and it interferes with the river’s natural ability to deal with flooding by overflowing into wetlands and floodplains.

“This type of structural work creates a false sense of security for those relying on it to protect them from these waters,” said Erich Pica, director of domestic programs for Friends of the Earth, which wants federal money used to help move people out of the 100-year floodplain. “We need to face the reality that these floods are going to occur, and maybe the best solution is to move out of the way.”

Material from The Washington Post, Gannett News Service and the Los Angeles Times is included in this report.