Saturday, August 24, 2013

Why didn't Manitou flood on Thursday?

Posted By on Sat, Aug 24, 2013 at 2:50 PM

click to enlarge How did Manitou avoid another scene like this? - BRADLEY FLORA
  • Bradley Flora
  • How did Manitou avoid another scene like this?
Thursday night’s storm certainly looked like the big one.

In Manitou Springs, which saw mass destruction and a house afloat in the Aug. 9 storm, police blocked roadways. A siren wailed. Fountain Creek rose with muddy water.

But the storm turned out to be anticlimactic in the mountain town.

The next morning, the creek was high, but everything was pretty much back to normal. In fact, Indy handyman George Davis, who lives in the mouth of Williams Canyon and saw his garden wall and cars float away Aug. 9, says Williams creek only had “a little water” during the worst part of the storm.

But here’s what’s weird: The Aug. 9 storm mostly hit just Williams and Waldo Canyon watersheds. At its worst point, it dropped perhaps an inch of rain in 15 minutes in Upper Williams Canyon. By comparison, National Oceanic and Atmospheric Administration Warning Coordination Meteorologist Tom Magnuson estimates that Thursday's storm dropped an inch of rain on most of the burn scar.

Meanwhile, upstream, Woodland Park saw about 2 to 3 inches of rain in an hour. Just outside of Woodland Park, as much as 4 inches in an hour may have fallen.

Yet, Manitou didn’t flood.

There were two keys, according to Magnuson. First, the inch of rain on the burn scar took about two hours to fall, giving that water a chance to soak into the ground. Second, the huge amount of water coming down from Woodland Park didn't just shoot quickly through Fountain Creek, as some feared would happen; the creek, with all its twists and turns, did manage to slow it somewhat. By the time the water arrived in town, it wasn’t so much a tidal wave as a steady stream. 

A major lesson to take from this: One of the biggest factors in flooding is how fast rain falls on vulnerable terrain. But Magnuson warns that lighter rain can still cause flooding, if there’s enough of it to oversaturate the ground.

“What we worry about,” he says, “is another heavy rain on top of [Thursday night's], because none of it will go into the ground.”

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