Allen Best does the math and science behind the phrase in this report running in The Denver Post:
Do you know how to make a meteorologist squirm? Ask for hard numbers immediately after a flood or a big rainfall, especially something like the September deluge that drenched many parts of Colorado’s Front Range with 10 inches of rain in just a few days. In some places, up to 18 inches of rain fell, most of it within the space of 36 hours.
Almost immediately there came a report that this was a 100-year flood in Boulder. Well, no, said a later report; it was more like a 50-year flood, and possibly less. Maybe it was a 100-year flood somewhere else. Check with us in a few months.
Others — including meteorologists at the National Oceanic and Atmospheric Administration — announced that it was a 1,000-year rainfall. Really? A thousand years is an awful long time. What about the massive flooding of 1938, others wanted to know? Where does that rank in the scheme of things?
If the great deluge of 2013 in Colorado revealed anything, it was that the science of rainfall, flooding and even global warming is still imprecise. We all want clear answers and instant tabulations, the way that a website can report page visits and even the locations of viewers. Most of the time, however, the hard sciences can’t give us the hard numbers and precise explanations that we crave.
“Be very careful about historical frequencies, because what gets published the first day gets remembered forever,” said Nolan Doesken, Colorado state climatologist, at a recent forum organized by the government consortium, Western Water Assessment.
Doesken pointed to the Big Thompson flood of 1976, in which 12 to 14 inches of rain fell in just a few hours, creating a giant surge of about 32,000 cubic feet of water per second. Some 143 people died as a result. At the time, it was described as a 100-year flood. But an expert in evaluating flooding, who later methodically examined the canyon, concluded that nothing comparable had occurred since the Ice Age ended 10,000 years ago.
Historical records have been kept in Colorado for only a little more than 100 years. And yet we know that giant storms have occurred many times over the centuries. Every year, Colorado has remarkable deluges somewhere within its borders.
“Throughout history, we have had monsters pretty often somewhere in the state,” Doesken said. “I think there are 50 100-year floods each year somewhere in Colorado.”
Scientists would like something other than the simple description of “100-year” floods in favor of a more complicated evaluation that emphasizes the actual frequency of major incidents. Too often, a 50-year flood is assumed to mean that such an event happens every 50 years, almost like clockwork, though in fact two so-called 50-year floods can occur two years in a row.
“We’re trying to get away from all those aging atlases,” said Kelly Mahoney, a research scientist with the Cooperative Institute for Research in Environmental Sciences, based at the University of Colorado. “But after a day or two of staying away from (the hard numbers), those numbers (still) flow to the top.”
Sometimes, floods occur in places that have never before been identified as floodplains. Such was the case in 1997, when a deluge swamped Fort Collins, killing several people. The water ran in “entirely new channels — very subtle channels — that had not been identified,” noted Doesken. The basement of Morgan Library at Colorado State University got swamped, for example, though it’s nowhere near a creek. “Flash floods can happen anywhere.”
Martin Hoerling, a research meteorologist who specializes in climate dynamics at NOAA’s Earth System Research Laboratory in Boulder, made the same point, in a more general way.
“Caveats can get in the way of sound bites,” he said. “As scientists, caveats is all we do.”
Mapping of flood plains is also imprecise, in part because the construction of buildings and other alterations over time have changed the flow of water. Mahoney said identified flood plains are “constructs, and not determinations of truths for all time.”
Can global warming explain at least part of this deluge? The answer seems to be a definite “maybe.” As Hoerling put it, “I am skeptical that you can include or exclude climate change. Our models just aren’t good enough.”
Globally, the atmosphere contains an estimated 5 percent more moisture than it did 50 years ago. In the Boulder area, just prior to the recent storm, there was a great deal of water vapor — 150 to 200 percent more than normal. But how much did that added water vapor contribute to making it a 1,000-year event, at least in some places?
“It is a factor, but still a very small factor,” said Hoerling. In other words, weird weather has been with us for a very long time.