Levees are as varied in the United States as the people they guard. They're shaped like snakes, rings and spurs that can be tough, flimsy or a century old.
No one knows for sure where all of these earthen walls are, who built them or what type of rocky mixture lies shrouded inside their bulk. Some help protect homes from flooding, while others ring industrial zones containing chemical plants and refineries. Many stop rivers from turning into lakes on farms that abut the riverfront.
They shadow the diverse landscapes they protect. But these widely disparate levees do share at least two common traits -- they are all steadily weakening from water's efforts to go under, over or straight through their earthy embankment. Very few were built to protect against the more powerful storms expected by climate scientists and the flooding that will accompany them.
Now, after years of treating suspect levees with remarkable caution, federal flood officials are recalibrating the way they view the risk to communities lying behind these walls. Government officials say it's driven by better scientific methods used to measure precise flood hazards. But critics believe it could result in still more homes being built in the path of runaway water.
"I do think it's a huge concern," said Shana Udvardy, who directs flood management policy for American Rivers. "The risk is, people are going to believe they're safe when they're not."
At last count, there were roughly 30,000 miles of levees across the country. String them together and they would wrap around the world, and then some. Here's another way to harness their ubiquity: For every McDonald's restaurant in America, there are more than 2 miles of levees.
And most of them, almost 70 percent, are not trusted by government flood officials to do their job. The Federal Emergency Management Agency (FEMA) said in 2010 that 20,350 miles of levee walls are "not accredited" -- meaning that those who live behind the barriers are assumed to be threatened by a "high risk" of flooding.
Which levees are safe?
"without levee." It's a decision based on strict criteria: If a town can't prove that the engineering of its walls is able to repel a flood with a 1 percent chance of occurring each year, then FEMA doesn't consider it to be there.
That's a sober position, many believe, at a time when the arc of development is growing in dangerous flood plains. The "without levee" provision is akin to a red alert. Everyone behind the unaccredited wall is living in a race course for water, FEMA assumes.
That sparks requirements to build homes and businesses that are resilient to flood damage. They might have to be constructed higher, stronger and out of the most dangerous flow ways. Flood insurance is also mandatory behind iffy levees for homeowners with federally guaranteed mortgages; some private banks also demand it.
But taming a flood's potency doesn't come for free. Building to stricter standards adds cost and difficulty to projects that many communities see as vital economic progress. Proposed subdivisions, commercial developments and family homes face higher hurdles than those behind proven levees. In some cases, strips of flood-prone acreage become unavailable -- and less valuable to towns -- for construction.
Treating unaccredited levees as if they've been "completely wiped off the map," as a bipartisan group of 27 senators told FEMA last year, "may be unnecessarily devaluing property and hurting the economies of cities, towns, counties and businesses."
So, now federal flood insurance officials are preparing to scrap the "without levee" approach. Rather than assuming these unproven barriers will do nothing to inhibit the rush of floodwater -- resulting in widespread inundation -- FEMA will begin modeling a variety of dangerous options, which are likely to cause flooding in smaller and more specific areas behind the levee, rather than all over.