NEW ORLEANS (AP) — Congress needs to act decisively and set up a commission to oversee of the 100,000 miles of U.S. levees to prevent the next catastrophic flood, according to a new government report.

About $1.2 billion should be spent over the next five years to set up the commission and fix aging and inadequate levee systems that protect tens of millions of Americans, the report by the National Committee on Levee Safety said.

The congressional committee’s warning is poignant because it bookends a period marked by levee failures, most prominently New Orleans in 2005 and Cedar Rapids, Iowa, last year.

‘‘As a nation, our call to action is not predicated on if the next levee system fails and causes catastrophic damage but when and where it fails,’’ the report said. ‘‘The current levee safety reality for the United States is stark.’’

The report, commissioned by Congress in 2007 and overseen by the Army Corps of Engineers, was released late Thursday.

To improve levees, Congress should set up a commission similiar to the Federal Energy Regulatory Commission, which oversees electricity, natural gas and dams.

‘‘We have 100 years of experience with levees and we have not paid proper attention to them,’’ said Eric Halpin, a senior adviser on levees and dams for the Corps of Engineers and committee vice chairman.

The report said levees are in such bad shape that a wide range of steps need to be taken to make them safer and to change public attitudes about the threat posed by living near rivers and water. The report did not assess levee systems, but laid out policy recommendations for Congress.

Generally, the threat of flooding from levees has been ignored because people and policymakers do not understand the risk, the report said. The complacency is rooted in Congress’ 1968 National Flood Insurance Program, which designated areas as particularly hazardous if they had a 1 percent chance of flooding each year, or were vulnerable to a so-called 100-year flood.

That standard had the unintended consequence of encouraging communities to build just enough flood protection to offset that risk, the report said. In reality, someone who is protected against a 100-year flood has a 26 percent chance of having their house flooded over the span of a 30-year mortgage.

‘‘These are actually pretty high levels of risk considering that playing one round of Russian Roulette is comparable to a 17 percent chance of disaster,’’ the report said. ‘‘It is not until we reach a 500-year level of flood protection that the chance of flooding starts getting down to a relatively small chance.’’

Congress should require ‘‘mandatory risk-based flood insurance’’ to encourage more people to buy flood insurance and to persuade communities to build better levees, which would cost a lot of money, the report said.

The committee estimated that it would cost about $315 million over the first five years to establish the commission, inspect levees and set up state programs. In addition, the committee recommended setting aside $923 million to repair levees, build stronger levees and move people out of floodplains over the next five years.

The investment is worth it because Corps of Engineers studies have shown every dollar spent on flood protection is more than $6 in damage prevention.

‘‘It may appear at first blush to be costly, but we have to include the cost of not taking action,’’ said Sam Riley Medlock, policy manager of the Wisconsin-based Association of State Floodplain Managers. ‘‘We cannot afford to ignore this hazard any longer.’’

Still, levee experts say funding will be a major obstacle.

‘‘The big question that has to be answered is who is going to pay for this,’’ said George Sills, a Vicksburg, Miss.-based geotechnical engineer and former Corps of Engineers levee expert who reviewed the report.

He said one of the more important elements was the report’s emphasis on changing the public’s understanding.

‘‘If I went to my 80-year-old mother and said you have a 1 percent chance of getting flooded out here she’d probably say that’s not that bad,’’ he said. ‘‘But we need to say, ’Look, potentially you could get the water up to this yellow mark on this street pole. Now, does that bother you?’’’

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On The Web:

http://www.iwr.usace.army.mil/ncls/index.cfm