So long, '500-year flood'?

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As the Missouri River flood of 2011 unfolded, federal officials said the slow-motion disaster was caused by a once-in-500-year combination of rain and snow.

While some found comfort in that number, many who live along the river have seen too many big floods in too few years to trust that statistic.

They are on to something.

World-Herald interviews with experts show the bases of such terms as "500-year flood" are constantly evolving and increasingly questioned.

"There is a large amount of uncertainty in the process by which we ... characterize flood frequency," said Robert Holmes, national flood hazard coordinator for the U.S. Geological Survey. "The period of record is so short ... no one really knows the true rarity of this flood."

Federal officials themselves are moving away from the 100-year and 500-year descriptions because people take the labels too literally, he said.

Such terms are shorthand for the way experts prefer to describe the likelihood of floods. They use probability: a 1 percent chance for a 100-year flood in a given year and 0.2 percent chance for a 500-year flood.

Along the river, people say they are seeing more floods.

"We're having a lot more problems with water than people realize," said Mike Crecelius, emergency manager in Fremont County, Iowa. "Every time we get a flood, somebody says it's a 100-year event, even a 500-year event, but lo and behold, we turn around and it happens again."

In Crecelius' neck of the woods this second of two consecutive years of flooding did what the first could not: breached levees, inundated Percival, encircled Hamburg and washed away more than 75,000 acres of crops.

Farther south at Rulo, Neb., four of the top six recorded crests on the Missouri have occurred since 2007.

Based on interviews with hydrologists, climatologists and geologists, it's clear that communities along the Missouri face questions beyond whether the Army Corps of Engineers mismanaged its dams this year.

Among them:

* The statistical record used to calculate flood risk is so short — a half-century to a century — that no one really knows what the Missouri is capable of doing.

* Precipitation patterns in the Missouri basin are changing in a way that could dramatically alter future flooding.

"As humans we are too tied to preparing for what we have observed or witnessed before. This year may not be as bad as it gets," said Mark Anderson, director of the USGS South Dakota Water Science Center. "Much larger floods are possible, especially if we look beyond the historical record."

It's the corps that for now calculates this year's flood as a 500-year runoff season.

"It doesn't mean it can't happen again next year," said Jody Farhat, chief of water management for the Omaha district of the corps. "It's not 2512 the next time we get an event like this ... . We feel it is a rare event and the chance of getting another ... next year is very remote."

Over the past 50 years, three seasons of remarkably high runoff — 1978, 1997 and this year — have changed the corps' analyses of flood frequency, said Monique Farmer, a corps spokeswoman.

Until this year, 1997 was the record for runoff — at the time calculated as a 200-year event, she said. As more years have passed, analyses now indicate that that level of runoff could occur once every 100 years, Farmer said.

The corps' calculations for this season's runoff were developed to help manage its six reservoirs in the immediate months ahead, said Brig. Gen. John McMahon, who oversees the corps' Northwestern Division.

The corps this summer faced a pressing question: What should it do to best protect people and property next year?

Their options:

* Getting water off the levees and dams sooner to lessen ongoing damage and allow more time for repairs.

* Or discharging more water to free up space in the reservoirs in the event of extraordinary flooding next year.

The corps chose to focus first on levee repairs, a decision based, among other things, on its statistical analysis that a repeat was unlikely.

Victor Baker, one of the country's foremost experts on the geologic study of flooding, said he would like to see the corps and others shift away from their reliance on statistical analyses. A geologic study of flooding would examine flood patterns over several thousand years, instead of extrapolating from data gathered over about a century or less.

"We design (dams and levees) so we can say they're providing protection against floods, but that statement is sort of a lie," said Baker, a professor of geosciences at the University of Arizona. "What they are doing is protecting against little floods."

Baker says a safer approach would examine sediment in the surrounding landscape for clues of past flood levels.

"The issue of where and when ... floods occur is relatively easy science," he said.

A geologic study of the Colorado River near Moab, Utah, found that a statistically predicted 500-year flood had actually occurred, on average, every century.

"That showed that all those (statistical results) were completely absurd," Baker said. "They had no relationship to how natural systems work."

Anderson said the USGS did a geologic study of the Rapid Creek in South Dakota after a 1972 flood killed more than 200 people.

In layman's terms, that analysis found a 500-year flood, on average, every 140 to 200 years.

Corps officials say the existence of the dams alters the usefulness of paleo-analysis.

Anderson, who sees value in geologic study of the Missouri, said statistics are better than nothing at all.

Limited records on the Missouri extend back 113 years. However, that includes only about 60 years of extensive data from some 200 gauges.

Those gauges portray a clear division in runoff patterns from west to east, Anderson said. Over time, runoff has decreased in the Missouri's mountain west and increased in the east, he said.

These changes have implications for the corps, he said.

The data, at least until this year, had made the case that more, not less, water should held back in reservoirs.

Why?

With less water flowing in, there appeared to be a trend toward less threat of flooding from upstream runoff.

With more water flowing into the Missouri below the reservoirs, there also was increased risk of downstream flooding.

"This, I'm sure, has complicated management of the reservoirs for the corps," he said.

Then came 2011, with heavy snowpack and record spring rains.

"This has been a bizarre year," Anderson said. "It will be interesting to see what happens in the next year or two to see if we indeed ... are headed toward a wet cycle."

Like the USGS, the National Oceanic and Atmospheric Administration is noticing changes in the watershed that feeds the Missouri.

Anderson said a changing climate only compounds the shortcomings of using existing data to estimate the frequency of floods.

"If the climate is shifting, all those earlier records may not be as relevant today," he said. "The point is, the future isn't always a reflection of the past."

Contact the writer:

402-444-1102, nancy.gaarder@owh.com

Video: Midlanders at the Bob Kerrey Pedestrian Bridge give their views of the ongoing flooding:

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