Those aluminum panels on Burlington's flood wall, the ones that will block everyone's view of the river this week during Steamboat Days, are supposed to hold back a 500-year flood.

No, that's not a flood lasting 500 years, or a flood that happens once every 500 years, although the latter is where the name originates.

A 500-year flood, according to National Weather Service hydrologist Jessica Brooks, is a flood with a 0.2 percent chance of happening each year. Brooks, who studies and monitors local waterways at the National Weather Service's Quad Cities office, said a 100-year flood is a flood with a 1 percent chance of happening each year. That's it.

Many are frustrated when close-together years bring multiple 100- or 500-year floods, Brooks said, but it's just a matter of statistics. If a 100-year flood occurs one year, it still has a 1 percent chance of happening the next year, at least according to the common model.

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"A 500-year flood is a very unlikely event to happen, but every year there is still a 0.2 percent chance that it happens," Brook said.

So what does that mean in Burlington? The flood of 2008 reached a 25.73-foot crest, exceeding the 25.19-foot level labelled a 500-year flood. 

The new flood wall actually goes beyond that figure to contain a 28-foot flood. They stopped there, according to Public Works Director Nick MacGregor, because it contained the 500-year mark and was a standard size. A taller wall would have meant a pricier wall.

Markings for 100- and 500-year floods are done for virtually the entire stretch of the Mississippi River. The Federal Emergency Management Agency maintains maps that show where such floods hit along the full length of the river.

At the end of May, MacGregor called the wall project “85 percent” complete.

It’s already functional, though not as functional as it will be once remaining construction stages are finished. The wall took flooding concerns out of the picture for Steamboat Days, but temporary barriers will continue to be needed at its north and south ends for years.

Work was hoped to finish by Steamboat Days, but construction will continue throughout the festival to the north of the Port of Burlington.

MacGregor said it was weather, not spring flooding, that impacted construction the most.

Terry Phillipes, the project manager on the construction site, said the early days of the project were some of its toughest. He recalled his crews pumping the river out of the construction and working "in mud up to their knees."

"The public's not going to see the biggest part of this project," Phillipes said. "It's all underground."

Looking out over a near-finished project, he said he was proud of the work.

The new riverfront has a few new amenities. New trees and grass on the east edge offer a green break from old the parking lot.

Red brick paving gives another splash of color.

The wall has several walkout points where, in times of low river, visitors will be able to walk out onto awnings overlooking the river.

The removable panels went up as part of the city's contract with S & G Construction. The panels all must be installed and formally inspected at the end of the construction before work can be considered finished. MacGregor said panels will go down after this year's project is complete.