“This is NOT a drill.” That is the emergency alert about 188,000 residents from Butte County and areas to the south along the Feather River near Oroville, Calif., received when they were ordered to evacuate Sunday night.
A hole in the main spillway measuring 250 feet long by 170 feet wide and 50 feet deep emerged last week. At that time officials said the spillway was secure but around 3 p.m. Sunday, authorities learned that the dam’s emergency spillway was also eroding and threatened to flood the surrounding area.
While the dam itself remains intact, erosion damage to the emergency spillway raises the potential of the structure failing and unleashing an uncontrolled torrent of floodwaters. The erosion of the emergency spillway is dangerous because “when you start to erode the ground, the dirt and everything else starts to roll off the hill,” said Kevin Lawson, California Fire incident commander.“It starts to undermine itself. If that is not addressed, if that’s not mitigated properly, essentially what we’re looking at, is approximately a 30-foot wall of water.”
Trying to head off a disaster, state officials increased the flow down the dam’s main spillway on Sunday night. Their efforts to deal with the crisis could determine not only whether entire towns are inundated but also whether the state’s second-largest reservoir emerges with manageable damage or something much worse.
As of 5 a.m. Monday morning, water was at 898.19 feet, a little more than a foot below the top. The emergency spillway is only used if water levels real 901 feet - which hasn't happened in 48 years.
The dam holds back Lake Oroville, the state's second largest reservoir. This year's snowpack in the northern Sierra and record setting rainfall are causing flood-control releases around the state.
According to KGO-TV, three environmental groups warned federal and state officials 12 years ago the spillway in Oroville could erode during heavy winter rains and cause a catastrophe. The San Jose Mercury News reports the environmental groups, including the Sierra Club, filed a motion with the federal government in 2005, urging federal officials to require that the dam's emergency spillway be armored with concrete, rather than remain as an earthen hillside. The Federal Energy Regulatory Commission rejected the request after the state department of water resources and other water agencies said the upgrades were unnecessary.