ONLY IN THE WORLD-HERALD
For the era in which it was built, the levee system protecting much of Omaha is first class.
The levees that protect Council Bluffs and Eppley Airfield are a notch down. That system has less clay standing between the swollen Missouri River, thousands of homeowners and a regional transportation hub, but it is far better than some of the dirt-only levees that protect rural areas.
The question of whether the metropolitan levees will hold hangs over Omaha and Council Bluffs like a foreboding storm. Only time will answer, but — without a doubt — the levees will be put to a historic and an unprecedented test in this summer of floods.
If they fail, thousands of people could flee their homes. In the event of a massive failure, up to 30,000 could be evacuated in Council Bluffs, 8,000 in Carter Lake and 2,700 in Omaha.
The levees have some history on their side. Both systems are federally designed and survived the 1952 storms, when the Missouri rose higher than it is expected to rise this summer.
But the levees are old. And it's questionable whether they were built to handle three or more months of high water.
“Engineers 60 to 70 years ago did not anticipate the long-duration floods we're experiencing today ... (but) there is every reason to believe they will hold,” said Nicholas Pinter, a levee expert and geology professor at Southern Illinois University in Carbondale, Ill.
The metro area's mantra this summer should be “Pray the clay will stay.”
Clay is an essential component of both cities' levees. It is one of the best building blocks for earthen levees because it is relatively impermeable to water.
“It's the material of choice,” Pinter said.
Much of Omaha's 13 miles of levees are made entirely of solid clay — top-of-the-line when built in the early 1950s. The levees differ in elevation, but all have 10-foot tops on broader bases.
Less clay was used to build the levees in Council Bluffs and around one of Omaha's most vulnerable and vital pieces of infrastructure: Eppley Airfield.
In Council Bluffs and around the airport, the levees are made of dirt with a 5-foot clay cap on the river side.
It is the same type of levee that recently failed near Hamburg, Iowa. The cause of that failure has yet to be identified.
One levee expert, who has spent time studying breaches, is not a fan of such clay-capped levees.
Robert Bea is a professor emeritus of civil engineering at the University of California, Berkeley. He studied levee breaches after the 2008 floods on the Mississippi River and said the levees that collapsed often were clay capped.
“The clay caps sometimes disappeared,” said Bea, who in the 1950s worked for five years with the U.S. Army Corps of Engineers.
In some instances, Bea said, it appeared water had eroded the levees' dirt interior from the bottom. “A levee is like a chain. It will break at its weakest link,” Bea said.
Neither city officials nor officials with the corps know why most of Omaha's levees were built entirely of clay while Council Bluffs' were built with a clay cap. At the time it was believed a clay cap offered as much protection as levees entirely of clay, Bea said.
A lot has changed in 60 years.
The best levees that engineers can design are being built in Great Britain and other parts of Europe. Sometimes they are made of concrete. Other times they are made with clay but designed wider and higher than America's older levees, Bea said.
About one mile of Omaha's 13-mile levee system is concrete, along the city's downtown riverfront. The wall was built because there was not enough room to build an earthen levee, said John Bertino, chief of the engineering division for the corps in Omaha.
A key question for the Omaha-area levees will be time. The longer they are under stress, the riskier they become.
This is the first time the levees will be asked to hold back water from a roaring Mighty Mo for up to three months. During the 1952 flood, the levees withstood high water for about two weeks.
Bertino said both kinds of levees were designed to withstand a steady flow of seepage under their base, which occurs after a levee is saturated. “The design is based on water being on it for a long time,” Bertino said.
But Bertino acknowledged that the longer the levees are under stress, the greater the risk of a breach. He also stressed that levees offer no “guarantees” against flood.
Bea said the fact that the levees were designed 60 years ago to handle a steady flow of seepage gave him no “comfort” the levees would hold. The tests and research used to design levees have dramatically changed.
“A levee well-designed back in the 1950s for quote ‘long-term saturation' would be pretty close to what I would call junk science today,” Bea said.
The fate of the levees could come down to monitoring. Omaha and Council Bluffs both have mounted large-scale surveillance of the river on a 24-hour basis, with the help of National Guard units.
They will be looking for soft spots on the levees and monitoring wells on the back side of the levee for erosion at the base.
There are numerous pressure wells drilled along the levees to help relieve water pressure. In addition, sand boils could erupt — essentially, nature's homemade relief wells.
If the water that bubbles up through the relief wells and the sand boils is clean, there is no problem. But if that water is filled with sand and dirt, the levee could be threatened, said Greg Reeder, director of public works for Council Bluffs.
“Once you start seeing the sand move, that means the stability of the levee may be threatened,” he said.
Corrective measures can be taken. One step would be to ring the wells or the sand boils with sandbags to allow the water to rise. The weight of the water will then put pressure on the water flow and, hopefully, stop the flow of dirt, Reeder said.
If there is a breach, city officials will work to contain the floodwaters. One breach does not mean entire cities will be flooded.
For example, a breach in south Council Bluffs could conceivably be contained to that area, though even that flooding could be monumental.
“We are doing as much as we can to prepare for any event,” Reeder said.
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View video of the Missouri River near Lewis and Clark Landing.