OMAHA, Neb. (AP) -- Several hundred thousand acres of rich Midwestern farmland and even some urban areas near the Missouri River are at risk of flooding this summer during months of historically high water that experts fear will overwhelm some levees, especially older ones.
Engineers who have studied past floods say the earthen levees in rural areas are at greater risk.
"Most of the levees are agricultural levees. They're not engineered. They're just dirt piled up," said David Rogers, an engineering professor at Missouri University of Science and Technology.
So far, most levees have held along the 811 miles the Missouri travels from the last dam at Gavins Point in South Dakota to its confluence with the Mississippi River near St. Louis. The flooding thus far has covered more than 560,000 acres of mostly rural land, including nearly 447,000 acres of farmland. The water has forced some evacuations, but the extent of the damage to may not be clear until it recedes.
That's not expected to happen until the fall as the Army Corps of Engineers says it needs to continue releasing substantial amounts of water from upstream reservoirs inundated with heavy spring rains and melt from an above average Rocky Mountain snowpack.
The Corps predicts that the river will eventually rise high enough to flow over some 18 to 70 levees, mostly in rural areas of southeast Nebraska, southwest Iowa and Missouri. Other levees will become saturated, and water can erode their foundations, seep underneath or find other flaws to exploit.
A saturated levee may lose stability, potentially causing it to crumble, as one did in June near Hamburg, Iowa, allowing floodwater to cover several miles of farmland and threaten the town. Flaws in levees, such as animal burrows, can allow water to flow through and eventually destroy the structure.
"At times like these, this is when we find out where the weak spots are," said Erik Loehr, an associate professor of engineering at the University of Missouri.
Rural levees, experts say, are likely to be older, privately maintained and not tall or strong enough to stand up to such a long-running flood.
Corps officials and engineering experts are more confident that city areas such as Omaha, Kansas City and St. Joseph are well protected by substantial floodwalls that have been maintained.
Omaha's main floodwall, for instance, is built to contain a river 40 feet deep -- which is 4 feet higher than the river is expected to reach. But several of the rural levees in northwest Missouri are more than 2 feet shorter than the river's expected crest, and some have already been exceeded.
Even stronger urban floodwalls and levees can falter against the destructive force of floodwaters. As a precaution, officials in Omaha and across the river in Council Bluffs, Iowa, have developed plans to evacuate roughly 40,000 people from areas near the river in case a levee fails.
A key factor in levee performance is what material was used to build it.
Levees were often constructed using whatever material was near or could be delivered cheaply. Omaha's levees outside the city's main floodwall are built almost entirely out of clay. But barriers around the city's airport and in Council Bluffs were designed with a clay cap over a different filler material.
Many levees along the Missouri were built with a clay cap over sand or gravel to combat seepage, but any damage to the clay cap can cause them to fail.
"The designers at the time felt they were as good, with less expense, than the solid clay layer levees -- as long as they are maintained properly," said civil engineer John LaRandeau, with the corps' Omaha office.
The standards for levee construction have changed, with modern designers concerned more about fortifying the foundation material. Newer levees are likely to be built entirely out of clay or another nonporous material.
Still, Jud Kneuvean, who serves as the emergency management chief for the corps' Kansas City district, said he feels good overall about the flood protection along the Missouri River, especially in downriver stretches.
"I believe here in the lower Missouri, we have some of the best levees in the nation," Kneuvean said.
Most levees in Missouri were rebuilt after the devastating 1993 flood that damaged thousands of homes, farms and businesses and set flooding records in the state. But that flood wasn't as bad in Nebraska and Iowa so fewer of the levees in those states were improved recently.
Southern Illinois University professor Nicholas Pinter, who studies river system dynamics, said the corps re-examined and modernized levees after flaws in many structures near the Gulf of Mexico became apparent in 2005.
"After (Hurricane) Katrina, the government realized some of the shortcomings of its levee system," Pinter said.
The flooding along the Mississippi and Missouri rivers this year has parallels to the catastrophic flooding of New Orleans during Hurricane Katrina. In each case, levees and flood protection systems overseen by the corps and maintained by local levee districts proved inadequate against the massive amount of water.
But they're not an exact comparison. The levees that failed around New Orleans were protecting against the fierce, but short-lived, assault of hurricanes. The flooding taking place in the Missouri River valley has been building for weeks and the high water is set to last all summer.
It's important to remember that levees and floodwalls only reduce the flooding risk, LaRandeau says, but don't eliminate it.
"The whole system is going to be under pressure," he said.
(Copyright 2011 by The Associated Press. All Rights Reserved.)