Fri, Feb 17, 2017 - Page 7 News List

Cracks may offer clues to California dam’s troubles

‘PATCH AND PRAY’:Problems such as cracks in a spillway are common, the question is whether they have been repaired carefully, experts said

AP

People watch the gushing waters of the Feather River from the town’s fish hatchery a day after an evacuation was lifted on Wednesday in Oroville, California.

Photo: AP

Six months before rushing water ripped a huge hole in a channel that drains a northern California reservoir, state inspectors said the concrete spillway was sound. As officials puzzle through how to repair it, federal regulators have ordered the state to figure out what went wrong at Oroville Dam.

Earlier inspection reports offer potential clues, including cracks on the spillway surface that could either be cosmetic or indicate deeper problems.

In recent years, construction crews patched cracks — including in the area where water burrowed a huge pit last week. If past repairs were not done properly, water could infiltrate and eventually tear through the concrete.

Damage to the main spillway triggered a series of problems that threatened to unleash a torrent of water on cities downstream.

On Tuesday, officials said the immediate danger had passed and allowed nearly 200,000 residents to go home after evacuation orders scattered them for nearly two days.

Inspectors with the state agency that operates and checks the dam, the nation’s tallest at 234m, went into the 800m-long spillway in 2014 and 2015 and did not find any concerns.

“Conditions appeared to be normal,” the inspector wrote in reports from both years.

Getting into the channel affords both a closer view of cracks as well as a chance to tap it with a special hammer, with the sound telling a trained ear whether the concrete is solid or there may be erosion in the earth below.

In August last year, a team of inspectors only checked the channel from vistas around it, not inside. They concluded that everything looked fine.

The inspection came as California was enduring a five-year drought, and the channel rarely was used to relieve pressure on Oroville Lake, which is more than 110km north of Sacramento. An extraordinarily wet subsequent six months changed that.

Dam managers were draining water last week from the fast-filling reservoir into the Feather River below when the pit appeared. They temporarily stopped the releases and the reservoir kept rising — pushing water over its lip and down a hillside, where erosion prompted concerns that a broader failure was imminent.

Experts said problems like the cracks in the concrete spillway and spots in nearby areas where water seeped from the reservoir through a hillside were common issues with dams.

What mattered was whether dam operators dealt with the problems carefully — patching cracks so they were watertight, and dealing with spots where water was leaking through so they did not grow to undermine the concrete, said John Moyle, New Jersey’s director of dam safety and flood control.

The California Department of Water Resources declined to answer specific questions about the repair work, saying engineers were focused on ensuring public safety.

Robert Bea, professor emeritus of civil and environmental engineering at the University of California, Berkeley, said it was “obvious those repairs didn’t work.”

“We don’t have details on the repairs, but they put cement into the cracks and troweled it over,” Bea said. “I call it ‘patch and pray.’”

On Monday, federal regulators told the department it must enlist a group of independent consultants to assess what went wrong and to recommend long-term fixes.

Documents and interviews show that crews were patching cracks in 2009 and 2013.

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