By Dr. Robert Thorson
Boulder, Colorado. Both of these names derive from torrential river flows. So why the media hysteria about the floods there last month?
The city of Boulder was named for Boulder Creek, which was named for the abundance of large boulders. How did they get there? By torrential stream flows giving rise to enough power to carry them in turbulence. Colorado was named for the presumed headwaters of the Colorado River, which was named for the reddish-brown tinge of its waters, imparted by the sediment suspended within it.
How did it get there? By torrential stream flows eroding loose soil typically chock full of iron-stained grit and clay.
Though damaging and lethal, the floods of last month in Boulder were so-so. According to a preliminary report by John Pitlick of the University of Colorado, the peak discharge on Boulder Creek was 5,070 cubic feet per second, or 144 cubic meters per second. This was less than six times the average annual flood (25 cubic meters per second) and well below what might be expected in a 100-year flood.
“Areas in Boulder inundated by the flood,” Pitlick writes, “are consistent with maps developed for floods with return periods of 50 years or less.” In other words, flooding at this scale can be expected every half-century. And the city was well-prepared because flood mitigation “undoubtedly reduced damages to roads and buildings.” Yes, people died. And yes, the city needs federal assistance to repair the damage. But in Boulder, this event was not the catastrophe portrayed by the nightly news for nearly two weeks.
The real story involves a higher power. I refer to the great mass of water vapor that precipitated as much as 16 inches of rain at some sites in the Front Range. From the dark, gray sky, over the span of four days and four nights, fell an extraordinary mass of water. Though an order of magnitude below biblical proportions, the deluge exceeded what was expected for a thousand-year rainfall.
This conclusion was published Sept. 17 by the Hydrometeorological Design Studies Center of the National Weather Service of the National Oceanic and Atmospheric Administration. Technically, we’re dealing with a parameter called the Annual Exceedance Probability, which takes into account the total amount of rainfall and the duration of the storm. For durations of less than one hour, the rainfall was merely torrential: that which would be expected once every 10 years. After two hours, the rainstorm exceeded the expectation for a 50-year event, putting it on par with the river flooding in Boulder. After 24 hours, the storm exceeded the expectation for a millennial event. After three additional days, this storm became a home run that flew far beyond the statistical fence of the climatic ballpark.
This brings up the elephant-in-the-room issue of climate change. There are two possible interpretations. First, the rainfall patterns haven’t changed, and the recent storm was equivalent to winning the lottery against nearly all odds. Second, rainfall patterns have changed, giving rise to a “new normal” within which the recent Front Range event is nothing unusual. Not being a gambler, this is the interpretation I favor.
The second elephant in the room is what happened to the deluge after it fell. Though much of it drained off the land in raging streams, an enormous amount soaked into the ground. There it saturated the soil, added weight to loose sediment on slopes and raised what’s called the pore water pressure. This precipitated landslides, mudflows and rockfalls, which took out roads, isolating dozens of communities. Though perhaps more important, these geological consequences were under-reported by the media because the sites were hard to reach and the events were short-lived.
Residents of Boulder successfully mitigated against the flood hazard because their stream geographies were so well defined. Much harder to mitigate against is the slope hazard, because nearly all of the landscape is sloped and because the mechanisms causing mass movements are more covert. Impossible to mitigate against is the unexpected excess of rain, over which Colorado residents have no control because it involves climate.