California dam's risks were known for decades

WASHINGTON - Engineers have known for decades that if water ever spilled onto Lake Oroville's unpaved auxiliary spillway, it would cause serious erosion, possibly compromising the earthen structure that holds back the California reservoir and threatening communities downstream.

But California water districts that helped pay for Oroville resisted calls to armor the backup spillway, which would have required construction outlays in the tens of millions of dollars. Environmentalists, meanwhile, opposed an earlier proposal to install gates atop the structure to raise the dam's elevation and prevent water from topping it during a flood.

The resulting stalemate contributed to Oroville's near-catastrophe on Sunday, when nearly 200,000 people were ordered to evacuate after officials detected erosion on the unlined hillside.

Water receded on Monday behind the dam, as engineers raced to drain the rain-swollen reservoir and shore up the crumbling overflow channel before new storms sweep the region later this week.

Authorities said they had averted the immediate danger of a catastrophic failure - one capable of unleashing a wall of water three stories tall on towns below.

But evacuation orders for some 188,000 residents remained in effect indefinitely, Butte County Sheriff Kory Honea said.

The aim is to lower the reservoir's overall water level by 50 feet - and prevent further spillover down the emergency hillside channel - before more rain arrives in the coming days and snowmelt runoff begins in the spring, acting state water resources director Bill Croyle told reporters on Monday. He said he hoped to achieve that goal within two weeks.

On Monday afternoon crews began dropping large bags filled with rocks into a gap at the top of the emergency spillway in a bid to rebuild the eroded hillside.

Congressional representatives said Monday they were stunned to learn that Oroville did not have a backup spillway paved with concrete that could be safely used if the main one was damaged.

"Some hard questions have to be answered about why this facility was apparently neglected in a way that left it vulnerable to these problems," said Rep. Jared Huffman of California, the top Democrat on the House subcommittee with oversight over dams. "Clearly there were warning signs, there were people saying, 'we need to fix this.'"

With the tallest dam in the United States, Lake Oroville is the major water source for the State Water Project, which provides water for 23 million people and farmers in the San Joaquin Valley. It was financed with a $1.75 billion bond that California voters approved in 1960. Some 34 laborers died during its construction.

The dam was designed with a main spillway, which was gated and lined with concrete. The ungated auxiliary spillway was added to handle a flood so big that "no one could imagine it," said Joe Countryman, a former engineer with the U.S. Army Corps of Engineers. As a result, he said, California and the water contractors "didn't want to put a lot of extra resources" into lining that spillway, which is basically an earthen hillside.

Many groups knew that, if the reservoir were ever hit by a major flood, water toppling over the emergency spillway would cause serious erosion. In 2002, the Yuba County Water Agency - which owns transmission lines and other infrastructure in the area - highlighted these concerns in a technical memorandum.

In 2003 through 2005, three environmental groups - Friends of the River, the South Yuba Citizens League and the Sierra Club - urged the federal government to require the lining of the auxiliary spillway as part of the dam's licensing process.

"At present, the ungated spillway at Oroville Dam consists of a spillway lip only - and utilizes a hillside as the project spillway," the groups wrote in 2003 to the Federal Energy Regulatory Commission. "Utilizing such a spillway has the potential to cause severe damage to the downstream hillside, project facilities and downstream environments located in the path of the flood release."

Despite such concerns, FERC ultimately decided not to require lining of the spillway, as the San Jose Mercury News reported Sunday. A senior engineer told his manager that it would take a "rare event" to utilize the emergency spillway and that using it "would not affect reservoir control or endanger the dam."

Late last week, engineers discovered a gaping hole in the concrete of Oroville's main spillway, following heavy releases from the reservoir. To stem the damage, they reduced flows going down the spillway, allowing the reservoir level to rise and spill over the auxiliary spillway for the first time in the dam's 48-year history.

The result was severe erosion at the base of the spillway, along with "severe damage to the downstream hillside" - just as the three environmental groups had predicted.

Oroville provides water to the State Water Contractors, which includes the powerful Metropolitan Water District of Southern California, as well as water districts in Silicon Valley and Kern County. Documents from 2006 show that lawyers for the State Water Contractors downplayed the risk of the unlined spillway, and argued that relicensing was the wrong forum to address such issues.

Countryman, a flood-control consultant formerly with the Corps of Engineers, said that the 2005 FERC proceedings were the second time in a decade that Oroville's backup spillway was a focus of debate. Following devastating 1997 floods in northern California, Countryman said he urged the California Department of Water Resources to add special gates atop Oroville's auxiliary spillway. Such gates, he said, would have allowed the reservoir to rise an extra 10 feet.

But, according to Countryman and others, environmental groups opposed the gates, because they would have allowed water to back up into tributaries of Oroville that are protected by federal wild and scenic status. The proposal died.

Reuters contributed to this report.