SIZING IT UP
The Boxelder Creek Basin covers more
than 265 square miles, extending from
just north of the Wyoming border to
the Poudre River to the south – with
Coal Creek and Indian Creek feeding
into Boxelder Creek near the Town of
Wellington.
an overflow channel upstream of Prospect Road, large
culverts beneath Prospect Road, and an outfall channel
downstream of Prospect Road to the Poudre River to
convey large flood flows.
The Boxelder Creek Stormwater Master Plan identified that
a regionally sized detention facility was needed to reduce
the powerful flows on Boxelder Creek. Ayres Associates
conducted a siting study, evaluating 14 different locations
to potentially put the flood control facility, said Andrea
Faucett, who managed the project for Ayres through the
design process.
“That was a lengthy process, going through the pros and
cons of each of these locations,” Faucett said.
What also proved challenging were the monthly board
meetings held for years, the political struggles to balance
the goals and desires of so many separate entities, and
the state and federal regulatory processes involved in
designing and building a high-hazard dam.
“FEMA uses completely different hydrologic models than
the State Engineer’s office does; FEMA’s is based on the
100-year storm, and the State Engineer’s is based on the
probable maximum flood. We needed to make sure that
this dam met both criteria,” Faucett said.
Conditions at the site itself brought additional challenges.
“It was a very large project using on-site material to build a
30-foot-high embankment, and the embankment needed
to be very specifically built with certain materials and
certain properties,” Pletcher said, explaining that doing
so helped cut costs. “It wasn’t like we could buy those
materials off-site and bring them to the project. We had to
create the materials from what was available at the project
site, so we would dig holes in one spot to get material.
STATISTICALLY SPEAKING
W
hat’s the difference between a 10-year, 100-year, and 1,000-year flood? We’ll break it down. A 10-year storm has a 1
in 10 chance (10%) of occurring in any given year. Likewise, a 100-year event has a 1 in 100 chance (1%) of happening,
and a 1,000-year flood has a 1 in 1,000 chance (0.1%) of taking place.
A probable maximum flood, also known as PMF, refers to the worst case scenario flood that could conceivably occur in a
location, typically estimated from rainfall records and other National Oceanic and Atmospheric Association data.
14│TRENDS