Water Vapor Almost Busts Dam

A strange phenomenon was shredding Glen Canyon Dam. Here's how it was saved.
HOW AERATION WORKS Rushing water skips over an air supply slot, creating a low pressure zone under the water's bottom surface. This draws air into the water column from the upper portion of the slot, softening the impact of cavitation. Illustration by Jason Lee

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.