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 Oroville Dam spillway, federal regulators have ordered the state to figure out what went wrong.
Earlier inspection reports offer potential clues, including cracks on the spillway surface that if not properly repaired could let water tear through the concrete. In recent years, construction crews patched cracks – including in the area where water burrowed a huge pit last week.
Damage to the main spillway triggered a series of problems culminating with the first use of the emergency spillway, which quickly began eroding and 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 both operates and checks the dam, the nation’s tallest at 770 feet, walked the half-mile-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.