Mercury News provides a possible answer.
How did a giant, gaping hole tear through the massive Oroville Dam’s main concrete spillway last week, setting in motion the chain of events that could have led to one of America’s deadliest dam failures?
Dam experts around the country are focusing on a leading suspect: Tiny bubbles.
The prospect is simple, yet terrifying and has been the culprit in a number of near disasters at dams across the globe since engineers discovered it about 50 years ago. In a process called “cavitation,” water flowing fast and in large volumes can rumble over small cracks, bumps or other imperfections in concrete dam spillways as they release water during wet years. The billions of gallons of water bumping off the surface at 50 miles an hour create enormous turbulence that can form tiny water vapor bubbles that collapse with powerful force, and like jackhammers, chisel apart concrete.
“It starts with small holes, but it can break off big chunks of concrete,” said Paul Tullis, a professor emeritus of civil engineering at Utah State University and cavitation expert.
“It’s like a big grinder. It causes concrete to be torn apart.”
It’s still too early to investigate the cavity on the spillway while dam operators at the nation’s tallest dam desperately drain billions of gallons of water down the damaged chute ahead of coming storms.
But the same phenomenon nearly caused the collapse of one of America’s other largest dams, Glen Canyon, a 710-foot tall behemoth on the Colorado River, in 1983.
Heavy snowmelt and rains that winter flooded the Colorado River basin, filling the 185-mile-long Lake Powell to the brim. Glen Canyon Dam — completed in 1966, just two years before the 770-foot Oroville Dam went into operation — opened its two spillways for the first time ever to lower the lake levels.
On June 6, 1983, rumbling sounds could be heard from the left spillway — which is a tunnel, different than Oroville’s 3,000-foot long concrete chute — and the dam began to shake violently. Bureau of Reclamation engineers shut off the spillway and found a series of five holes being torn into the rocks on the dam’s side.