huge thundercloud parked itself over the City of Cheyenne, Wyoming, on
the evening of August 1, 1985, dumping a record 6 inches of rain in three
hours. Dry Creek, normally a small stream at its wettest, quickly turned into a
surging river. When the night was over, 12 people died – including seven
children – in the surging floodwaters. Beyond the devastating loss of life,
damage totals exceeded $61 million.
Cheyenne, which normally averages 15 inches of rain a year found itself
unequipped to handle such an overpowering flood. The storm set state precipitation
records. Eleven of the 12 people who died drowned in the Sheridan Reach of
Dry Creek, which runs along a primarily residential area in the Buffalo Ridge
subdivision in the north part of the city. During the storm, crossings were
overtopped, and motorists who drove around barricades were swept into the
surging floodwaters. The water pressure removed tops of sewer lines,
supercharging the system and washing raw sewage into people’s homes.
Cheyenne City Engineer Doug Vetter, PE, was living in Laramie, a city
west of Cheyenne, when the storm hit. “During the storm, I happened to be up
on a mountain between Laramie and Cheyenne. I saw the large cloud and knew
it didn’t look good. But I had no idea what was happening,” recalled Vetter,
who ended up helping friends and family clean up their damaged homes. “I
wasn’t there for the storm, but I saw the aftermath.”
Determined not to suffer similar destruction caused by the 1985 storm – or
even flooding from a lesser storm – the City set out to devise solutions to its
flood control issues. In the short term, washed-out crossings and roads were
repaired. In 1987, the City developed a master drainage plan that identified $60
million in infrastructure needs. But nothing was done. “There just wasn’t money
in the budget for that,” Vetter said.
Now, more than two decades later, an innovative solution combined with a
$3 million grant from the Federal Emergency Management Agency (FEMA) has
made flood control in Dry Creek’s Sheridan Reach a reality. The idea for the
project began in 2002, when Gene MacDonald, PE, CFM, now manager of
Ayres Associates’ Cheyenne office, was the surface water drainage engineer for
the City of Cheyenne. At that time, the City was revisiting its 1987 master
drainage plan to reprioritize problem areas. Dry Creek’s Sheridan Reach was
identified as the No. 1 hazard.
At first, the proposed solution was to fix the conveyance system by improving
crossings to handle a 100-year flood. “The problem is, you can never build (the
conveyance system) big enough,” MacDonald said. “Continued urbanization,
combined with better precipitation records, generally result in higher peak
discharges.” This solution also would have cost the City approximately
$10 million.
This kind of fix was in line with conventional engineering solutions.
MacDonald said all recommendations in the master drainage plan involved
getting water out of the City quickly. “This typically was the solution of choice
for the last generation of engineers,” he said. Now other solutions involve water
quality and attenuation of peak discharges through use of detention features.
“It’s no longer a good idea just to divert the water and get it out of the way as
fast as you can.”
One day, MacDonald had an epiphany while staring at a huge map of the
City’s drainage basins in the City’s municipal building: “I was looking at the
map and I said, ‘You know, we have all this open area here; we can just divert
it.’ ” Long ago the federal government granted a 14-square-mile area comprising
A
Going
flow
with the
Diverting creek through undeveloped area
keeps Cheyenne residents protected from floods
By Tawny Quast
TRENDS
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