What We Learned From The September 2013 Flood
Sept. 9, 2014
Kelly Mahoney
What started out as a welcomed drizzle on Sept. 9, 2013, developed into a
torrential deluge that eventually saturated some places on the Front Range of
Colorado with nearly 20 inches of rain, causing widespread flooding, damaging
homes, businesses, roads and bridges.
For climatologist, it was a chance to experience a very raire rain event. For
Kelly Mahoney, an atmospheric research scientist at CU-Boulder and NOAA who
studies flash floods and other historical flooding events, this storm was unique for a
number of reasons.
CUT 1 “The thing that really came out of that September 2013 event is that wasn’t
flash flooding the way that we tend to think of it. When I came to CU and NOAA to
study flash flooding I was looking at the Big Thompson Canyon. I was looking at Fort
Collins in 1997 -- these very small - scale - short duration events done in six hours
or less. (:21) The thing about September 2013 was that, all told, was a five-day event
and it covered a huge region.” (:26)
Mahoney is currently looking into how climate change might influence flash
flooding in the future. She says right now scientists do not have enough data to say
how a warmer and wetter atmosphere associated with climate change will impact
flash flooding since it takes more than a moisture-laden atmosphere to create a flash
flood.
CUT 2 “In the future we expect things to be warmer and moister overall. The huge
caveat on top of that, though, is the competing effects. Moisture is, strangely, really
only one fairly small part of what goes into making a flash flood. It’s a really
important part but there’s a number of off-setting factors that could take away from
that. (:20) In the Front Range we need upslope flow to get moisture to rain out. If the
weather pattern shifts and you don’t have upslope flow you can have all the
moisture in the world but you’re not really going to see flooding impacts.” (:31)
But Mahoney says during the flood scientists saw a few things out of the
ordinary that might indicate how a warmer, wetter atmosphere may impact similar
events in the future. For instance, she says, heavy rainfall was recorded at high
elevations in the mountains where it typically doesn’t happen.
CUT 3 “Usually any kind of heavy precipitation that we see falls as snow in the
mountains or maybe small hail. This event was one of the first where we actually
had adequate observations in places eight, nine thousand feet and higher that
showed heavy rain falling there and all of the impacts. (:19) And so you can look to
places like Jamestown and higher where we don’t typically expect to see flash
flooding occur and it did in this event. So I think that’s going to be another area of
research going forward piecing out how that happened for this event and whether
that is a factor that might be affected by climate change.” (:32)
Another aspect of last year’s flood that Mahoney says she and her colleagues
are going to research is how extreme events like this can be influenced by stream
flow. As the climate warms spring runoff from the melting snowpack is happening
weeks earlier than it was decades ago. She says if what happened last September
were to occur in May when streams are full it could have been much worse than it
was.
CUT 4 “And just linking back to that same climate change question -- as snowmelt
shifts in warmer climates and our river flows shift in warmer climates, just coupling
that piece of how does the hydrology feed back into flood risk is going to be
something else that we need to piece out. Because there may be certain months
where the atmosphere becomes more probable of generating a lot of rainfall. (:21)
We might see in the spring more rain-driven events when the streams are already
full. Then you’re looking at a really heightened flood risk.” (:29)
Last year’s rains began on Sept. 9. By the night of Sept. 11 – 12 the situation
intensified. Boulder County was worst hit with over 9 inches of rain recorded that
night. By Sept. 15 rainfall totals exceeded 18 inches in some place in Boulder County.
The floodwaters impacted a range of almost 200 miles from north to south along the
Front Range, affecting 17 counties.
-CU-