Danger building up behind aging, vulnerable dams

dams in the United States, but the seepage at Lake Isabella was especially severe — it is what prompted the corps to perform a full-scale study of the dam.

Water seeping through a dam can erode it from the inside out, to the point where the dam may fail. Engineers have learned to build structures into dams like drains and filters, to stop erosion and allow infiltrating water to drain safely away. The Lake Isabella dams were constructed before such features became commonplace.

“It was built with the best available knowledge and technology at the time,” said Veronica V. Petrovsky, who is managing the project for the corps.

That knowledge, or lack of it, extended to the understanding of the large and complex watershed, which includes the slopes of Mount Whitney, the tallest peak in the contiguous United States. To determine how big the spillway needs to be, it is critical to know how much water might be impounded behind the dam each year.

Calculations show that in an extreme year with a “probable maximum flood,” the spillway would be far too small. “We could not release the water fast enough,” Petrovsky said. “It would overtop.” An overtopped dam can fail quickly as the water erodes the downstream side.

Concerns about seepage, in particular, prompted the corps to restrict the lake level, because less water creates less hydrostatic pressure that would force water through the dam. Earlier this winter, the lake was so low that water did not even lap up against the auxiliary dam. The corps has been monitoring the heavy rains and snowfall that California has experienced this winter and says that in the spring and summer it may be necessary to divert water through the spillway to maintain the safer lake level. Overtopping, however, presents only a “small concern,” the corps said.

With both seepage and overtopping there would be plenty of warning that the dam was in jeopardy, allowing Lake Isabella and Bakersfield residents to evacuate. An earthquake would be a more immediate disaster, although Bakersfield would still have about seven hours before a wall of water made its way down the canyon, according to the corps.

The auxiliary dam was built, knowingly, on the Kern Canyon fault, one of many in the region. At the time the corps brought in seismologists and geologists who concluded that the fault was not active.

Only recently have scientists been able to accurately detect and measure ancient earthquakes, a field known as paleoseismology. Serafini and others determined that there have been three significant earthquakes on the fault in the past 10,000 years. “We have got a fairly active fault on our hands,” Serafini said. The last quake occurred about 3,400 years ago, he added.

NYT notes that it is possible to construct a safe earthen dam on an active earthquake fault, by using the proper materials to minimize settlement or slumping when shaken, and including drains and filters to help stop the inevitable cracks from growing through erosion. Not only do the Lake Isabella dams lack those features, but the auxiliary dam was built on sediments that could turn into a virtual liquid in a quake, leading to even greater damage.

While Serafini and his team are still working on proposals, the likeliest solutions include blasting a much bigger spillway out of bedrock adjacent to the main dam and using the excavated rock to build a buttress — essentially an entirely new dam — downstream from the auxiliary dam. The old dam could still move in an earthquake, Serafini said, but the buttress would have the necessary drains and filters to prevent failure.

While the proposals are being fleshed out, the corps team has been holding meetings in the area to let people know what the possibilities are.

“We don’t hear much from the people of Bakersfield,” Petrovsky said. “It’s one of those ‘out of sight, out of mind’ things. You forget there’s a dam up here holding back a lot of water.”

Not so in Lake Isabella, however, where the dam, and its potential for failure, are harder to ignore.