A strange phenomenon was shredding Glen
Canyon Dam.
Here's how it was saved.March 2003
by Brian Fortner
THE SITUATION
Late spring, 1983. Heavy snowmelt and steady rains create the worst flooding
in nearly a century in the Colorado River basin. Lake Powell, a
185-mile-long reservoir on the Utah-Arizona border, is the hardest hit. Both
spillways at the reservoir's 710-foot-high Glen Canyon Dam must be opened
for the first time to prevent the reservoir from breaching its top.
On June 6, rumbling sounds begin emanating from the left spillway. It's the
calling card of cavitation, a little-understood phenomenon involving the
formation of vapor cavities in high-velocity water columns. These cavities
are short-lived, imploding with enough force to scour concrete from the
spillways. If they eat into the dam abutments, the structure could give way,
unleashing 17 billion cubic meters of water on the canyon below.
Bureau of Reclamation engineers temporarily shut down the left spillway to
assess the damage. Philip Burgi, then a hydraulic engineer with the federal
agency, notes "five holes in a Christmas tree pattern," starting small at
the top and getting larger as they go down. The only positive: They are
ripping into the mountain below rather than toward the dam abutments on the
side.
THE RESPONSE Operators install
4-foot-high wooden flatboards on top of the dam to give Lake Powell
additional capacity. These are replaced by 8-foot-high metal flashboards as
the reservoir continues to rise. By mid-June, however, concrete chunks are
blasting 60 to 80 feet in the air out of the bottom of the left spillway,
along with sandstone-colored water. Operators reduce flow through the left
spillway and crank it up on the right. In July, the water level peaks just
inches from the top of the flashboards.
LESSONS LEARNED When engineers finally
enter the left spillway to begin repairs, they find a crater 32 feet deep
and 180 feet long at its elbow, and the holes Burgi discovered in June are
now cavities 10 feet deep and 20 feet long. What's more, nearly 300 cubic
yards of concrete, reinforcing steel and sandstone have been deposited in
the deflector bucket at the base of the spillway. The right spillway suffers
similar, if less severe, damage. "The spring runoff would come again in
1984," says Burgi. "We had to get this thing up and operational, and we only
had one year to do it."
Contractors blast away damaged concrete, fix tunnel linings and fill holes
with 3,000 cubic yards of concrete. Engineers, meanwhile, begin their own
race to retrofit the dam with aeration slots, a new technology that
introduces small amounts of air into rushing water, cushioning the blow of
imploding vapor cavities. The plan works. The '84 runoff sets more records,
but the spillways show no sign of cavitation.
This success leads the Bureau of Reclamation to retrofit aerators to two
other large dams, Hoover and Blue Mesa. "It was a defining moment in dam
design," says Burgi. "The world was watching how we were going to solve this
problem." As it turns out, the world did more than watch -- aeration slots
are now standard from the Tarbela Dam in Pakistan to the Infiernillo Dam in
Mexico.
