Hydrologists and forest managers warned for over a year that severe burns in the Colorado Rockies have left the region vulnerable to flooding. A week ago, those warnings materialized with devastating effect.
Monsoonal rains came hard and fast to the Pike National Forest last week, dumping water down the burn scar left by the Waldo Canyon fire a year ago. Churning up soil and debris along the denuded slope, the flow formed a flash flood that swept through the small town of Manitou Springs, about 6 and a half miles west of Colorado Springs at the foot of Pikes Peak.
Two people died and more than 30 buildings were severely damaged as a result.
Much of the flood's ferocity can be attributed to the burn, said Fernando Rosario-Ortiz, a professor of environmental engineering at the University of Colorado, Boulder.
"Fire changes the landscape, so that more water can move through, and more quickly," he said. In an untouched ponderosa forest, much of the ground below the tree's canopy is covered with a litter of pine needles, downed branches and saplings -- "all the materials that would allow the water to percolate into the soil," according to Rosario-Ortiz. But fire tends to clear this ground cover, leaving exposed earth, he said.
The Waldo Canyon fire burned through the rugged terrain of the Pike forest for several days before breaching the outskirts of Colorado Springs, Colo., and destroying more than 200 structures. Accumulated fuels from decades of fire suppression allowed the fire to climb into the canopy, burning at a much higher temperature than a normal ponderosa fire.
That, in turn, caused chemical changes in the soil that added to its hydrophobicity, or ability to repel water, Rosario-Ortiz said.
Under extreme temperature, the waxy surface of pine needles can gasify and diffuse into the soil, coating granules of earth that then cool into a water-repellent surface.
"The hydrophobicity of the soil depends a lot on the temperature of the fire," said Diane McKnight, co-director of hydrologic sciences at CU Boulder. "If you think about a brush fire in the lower stories, they don't get that hot. But once the fire's climbed into the canopy, the temperature shoots up."
Fire, flood and then drinking water problems
Conditions on -- and in -- the ground paired with steep terrain around Pikes Peak to create the flash-flood conditions that hit Manitou last week. The flood washed mud, ash and debris through the downtown.
The dangers were well known. City and forest service personnel have worked on the Waldo Canyon burn area since last year, seeding the area with mulch, rebuilding slopes and debris traps to try to head off the worst of the monsoonal downpour.
In the end, however, there just wasn't enough time to take all the steps needed, forest managers said.
Even after the flood's aftermath was cleared away, the floods may have a lingering effect on the region's water supplies.
"What you often see after these flood events is a legacy impact on water quality," said McKnight. "All that ash and sediment finds its way into a stream or a river, and that affects everything, from microorganisms to insects to fish."
Humans are not immune, either, as significant depositions of ash can render drinking water unpotable and cannot be filtered out by conventional treatment methods.
That is a recurring problem for Colorado, most famously after the Hayman Park fire outside of Denver in 2002. That 140,000-acre fire surged around the city's oldest reservoir, the Cheesman, and for years thereafter, summer rains inundated it with silt and ash.
This summer, flooding has occurred several times in the area of last-year's High Park fire along the Cache la Poudre River.