The Geology of Flooding in Manitou
Heavy rain has fallen several times this summer over the Waldo Canyon burn scar, but Manitou Springs hasn't always seen flooding after a heavy rain. So what causes flooding like the kind that has devastated parts of the city on Friday?
Eric Billmeyer, an environmental science and geography instructor at UCCS explains: "It's the rainfall rate."
If half an inch of rainfall falls in three and a half hours then there usually isn't any problem. But if that same amount of rainfall falls in half an hour - watch out.
Another issue is the topography.
"You're talking about an elevation change of 3000 feet very, very quickly," said Dana Butler, a hydrologist with the U.S. Forestry Service.
Last year's Waldo Canyon fire made the problem worse than it would be otherwise.
Butler said, "there's no vegetation there so it's just like a parking lot. So the water ends up moving very quickly off this wartershed and entering town very quickly."
Another issue is that the soils have changed due to the fire. Billmeyer says that hydrophobic particles are present just below the top soil. That forms a layer that water can't penetrate. Stripped of much of its absorption power, the soil runs off and becomes that dark color that is present in the water that flows - and overflows - creeks like Fountain Creek.
One thing is for sure. Flash flooding on the burn scars is going to be with us for a while, which means that people in Manitou Springs are going to have to pay attention to the clouds for years to come.
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