100-year storms happen more frequently than name suggests

2013-11-18T00:00:00Z 2013-11-18T16:56:06Z 100-year storms happen more frequently than name suggestsBy CHRIS CIOFFI ccioffi@billingsgazette.com The Billings Gazette

The summer of 2013 was one of the stormiest that Marv Anderson can remember.

“I’ve been around close to 76 years,” he said. “I’ve never seen that much damn rain in my whole life.”

His home is situated in a low area of Watson Peak Road sitting below new homes being constructed in the Twin Oaks subdivision in the Heights.

The runoff from those empty lots from the severe rain turned his street into a river and his crawl space into a lake.

“I had to pump it out three times this summer,” he said. “I must have pumped out 10,000 gallons.”

Anderson’s crawl space is where his furnace and hot water heater are located. Any one of the floods could have been very expensive.

“It was close, it was real, real close,” he said.

On Sept. 7, a rainstorm dropped 1.3 inches of rain in about 15 minutes, flooding basements and pummeling Billings with hail. That’s two times more powerful than a 100-year storm.

Good thing it won’t happen again for another 200 years, right?

Not quite.

“It’s like playing the lottery and someone gives you the odds of you winning,” said Kelsey Jensco, Montana state climatologist.

A 100-year, 200-year or even 500-year storm is just a measurement, he said.

“It’s our best guess based on conditions,” Jensco said. “It may happen once, or it may happen five times.”

A 100-year storm has a 1 percent chance of occurring during any given year. A 200-year storm would have a 0.5 percent probability.

In Billings, about three inches of precipitation falling on Billings in 24 hours is a 100-year storm.

In some cases, a 100-year storm isn’t even based on 100 years of weather measurements.

“It’s not always based on observed events,” said Dan Borsum, a meteorologist with the National Weather Service in Billings.

In Billings, the data only go back to 1934.

Weather experts fill in the other 20 years with careful guesswork and complex computer models.

“It takes a look at the known data sets and pushes it out to the extreme to predict the 100-year climate,” Borsum said.

So if a 100-year storm can occur more frequently than once per century, why bother calling them 100-year storms?

Borsum said the NWS is trying to get away from that term to avoid confusion.

Calling storms 100-year storms may not be a good idea but defining severe weather is important.

“A 100-year storm attaches a numeric value to storms that have a 1 percent chance of occurring,” said Rick Leuthold, an engineer and chairman of Sanderson Stewart.

They give an actual value to engineers who can design systems that keep people safe when huge storms hit, Leuthold said.

“And you can imagine the difference from getting that water over 24 hours versus two hours,” Leuthold said.

Engineers know it’s not possible to channel water away as fast as it is coming down.

They solve the problem by building systems that slow the flow of water and retain it until it can slowly drain off into storm drains.

“Even if you have a big stormwater drain or channel in front of their house,” Leuthold warns, “The chances are that some time in the future, every place has the potential to flood.”

For homeowner Anderson, he’s hoping that his stormwater problems are solved.

Recently, he and his children changed the grade of his property in hopes that the water will flow away from his foundation.

“We spent a lot time getting the lot filled up,” he said. “So it doesn’t happen again, hopefully.”

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