Federal inspectors have recommended that California’s Oroville Dam, the tallest earthen dam in the nation, get a thorough earthquake safety inspection, but the state Department of Water Resources (DWR) doesn’t agree that is necessary.

The Sacramento Bee reported last week that a 2010 inspection by contractors for the Federal Energy Regulatory Agency (FERC) didn’t find any significant flaws in the 742-foot-high dam located on the Feather River in Butte County, approximately 130 miles northeast of Sacramento. But the FERC report said new information about earthquake hazards in the area should be incorporated into a “finite element analysis” that would use computer modeling to illustrate earthquake scenarios.

That kind of study could cost hundreds of thousands and take years to complete. DWR officials told the Bee that the expense is prohibitive, but their reluctance to back a new study is based on the dam’s safety. “The dam is essentially overbuilt,” DWR chief of dam safety David Panec said. “Even with today’s understanding of seismicity and ground motions, Oroville still would meet the criteria that would be set today.”

If he’s wrong and a quake brought down the dam, the result would be catastrophic. Water from the second largest reservoir in the state would flood the city of Oroville and other communities downstream, including Yuba City and parts of Sacramento. The dam is a critical part of the State Water Project that millions of Californians rely on for drinking water and agriculture.

The dam, which was first filled in 1968, was built near a fault line and underwent a 5.7 earthquake in 1975. It came through with flying colors, but that prompted a study by the U.S. Geological Survey that raised questions (pdf) about whether the dam itself, and its fluctuating water level, contributed to the earthquake. They considered the possibility but made no final determination. Scientists recognize that water in dams can put pressure on rock formations beneath them. The pressure on existing fault lines can cause slippage and contribute to earthquakes.

That quake revealed the existence of the Cleveland Hills Fault. But the USGS report cites two other potential quake sources—the Foothill Fault System and Prairie Creek Fault, which have not been extensively studied.

Part of the reluctance to embark upon a lengthy, costly seismic study of the dam is the timing. The state is locked in debate over the Brown administration’s $25 billion proposal for a building a massive water diversion system using tunnels around the Sacramento-San Joaquin Delta, and is negotiating contracts with myriad parties to water resource development in the area.

The state is also grappling with costs that resulted from two mishaps at the Oroville Dam. Valves at the dam were left inoperable in 2009 after a steel bulkhead blew out, and a fire last year reduced its energy-producing capacity. The Bee said that any repairs or reports would be paid for by the 29 big water contractors that buy water from the State Water Project.

Although the DWR doesn’t want to do the study, it may end up having little choice if FERC or the department’s own Division of Safety of Dams orders it.   

–Ken Broder

To Learn More:

Oroville Dam Earthquake Investigation May be Needed (by Matt Weiser, Sacramento Bee)

Oroville Dam Earthquake Study Recommended (Associated Press)

Scientists Link China's Dam to Earthquake, Renewing Debate (by Gautam Naik and Shai Oster, Wall Street Journal)