Water & Drought

Spillway threat asserted: A state agency seeks relicensing to run Oroville Dam, but a challenge sees design flaws.

A 2005 Bee graphic shows the Oroville Dam and its two spillways.
A 2005 Bee graphic shows the Oroville Dam and its two spillways. nlevine@sacbee.com

This story was originally published on November 27, 2005. It is being republished in light of damage at the Oroville dam.

Oroville Dam contains a flaw, some critics assert, one that could damage the structure during a major flood and threaten downstream communities.

That flaw is the dam's emergency spillway, which empties onto a bare dirt hillside adjacent to the earthen-fill dam.

If the emergency spillway had to be used to help quickly drain the reservoir during a major flood, the force of water rushing over the spillway lip would violently erode the hillside, washing out roads and power lines below, according to both the critics and the state agency that operates the dam.

It also could undermine the foundation of the spillway, a potential disaster, said Ronald Stork, a senior policy advocate at Friends of the River.

Video images recorded from the air on Monday, Feb. 13, 2017, show the chewed-up and denuded route taken by water coming over Oroville Dam's emergency spillway when damage to the main spillway required the first-ever use of the alternative path.

"Then you have what some people might say would be a dam failure, " Stork said. "That's why you don't have unarmored spillways unless you're really sure you're not going to use that."

Stork is among those calling for the spillway to be re-engineered and paved with concrete so water would not spill directly onto the hillside.

The California Department of Water Resources owns and operates Oroville Dam as part of the State Water Project. It is seeking approval from the Federal Energy Regulatory Commission to relicense the dam for another 50 years.

Stork's group, along with the South Yuba River Citizens League and the Sierra Club, have applied to intervene in the relicensing process. This would allow them to present evidence at commission hearings and argue for spillway improvements.

So far, state officials are not planning to upgrade the spillway, and they object to the issue being raised during relicensing.

"Our facilities, including the spillway, are safe during any conceivable flood event, " said Raphael Torres, acting deputy director of the State Water Project.

Oroville Dam was built on the Feather River in Butte County in 1968. It holds 3.5 million acre-feet of water, and is used both for flood protection and to store water for farm and urban water users.

The spillway issue dates back to 1970, when the operational manual for the dam was updated with the expectation that Marysville dam would be built on the Yuba River, a tributary of the Feather.

This new dam was authorized by Congress in 1966, but never was built.

Nevertheless, Oroville operations were designed to work in concert with the Marysville dam to ensure Feather River flows would not exceed the holding capacity of downstream levees.

This goal required water to flow over the emergency spillway at Oroville Dam under certain extreme conditions, as well as through the dam's primary spillway, which is controlled by movable gates.

The two spillways work together like the drains in a bathroom sink.

The sink has a primary drain in the bottom that operates all the time, with a stopper to control the flow.

If someone should walk away while the basin is filling, the sink also has an overflow hole on the side so the bathroom doesn't get flooded.

Similarly, in regulating upstream runoff through Oroville Dam, operators open the main spillway gates first.

If inflow exceeds these releases, the lake continues to rise until it reaches the emergency spillway lip.

At that point, water can flow freely over the emergency spillway.

This was considered acceptable in 1970, because it was assumed the Marysville dam would keep the Yuba River from flooding into the Feather.

Without the Marysville dam, flows out of Oroville Dam could exceed the holding capacity of downstream levees if the emergency spillway were put to use.


"We have consistently asked for Oroville Dam operations to be changed to take into account there is not a Marysville dam, " said Larry Combs, Sutter County administrator.

"The state and feds do not want to do that. We are not enough population to count politically, and basically it's OK if we flood."

In 1997, a record rainfall year, Oroville Dam operators exceeded the dam's normal release capacity through the main spillway gates, fearing the reservoir might fill too fast.

These high flows contributed to the evacuation of 100,000 people downstream in Yuba and Sutter counties, and played a role in numerous levee breaks.

But Torres said the dam cushioned a major flood and prevented much worse damage downstream.

"The facility saved the community, " he said.

The emergency spillway was not overtopped in 1997. But some believe those events called into question Oroville Dam's ability to handle a major flood.

Water flowing over the emergency spillway would wipe out two roads and two power lines built on the hillside below, and wash an estimated 70 acres of soil and rocks downstream. It also could erode the foundation of the spillway lip, in the same way that floodwalls in New Orleans were toppled by overflowing canals after Hurricane Katrina.

Still, Torres said investing money to upgrade the spillway may not be worth it.

"The emergency spillway has never been used. ... It's possible that it will be used in the future, but it would take an extremely large event, " he said. "You have to do an analysis of the economics of being able to build some projects vs. the frequency of risk."

It probably would cost "tens of millions of dollars, " Stork said, to armor the spillway. By law, that cost would have to be shared by the state's water contractors, which include powerful agricultural interests in the San Joaquin Valley.

"We're talking about the linchpin of the State Water Project, the tallest earth-fill dam in the United States, " Stork said.

"It really shouldn't be designed according to Third World standards. It's time to spend the money to do it."

The Federal Energy Regulatory Commission could order the spillway improvements during relicensing.

The state plans to submit its relicensing application by Jan. 31, with a commission ruling expected by the end of next year.

Water levels dropped Monday at California's Lake Oroville, stopping water from spilling over a massive dam's potentially hazardous emergency spillway after authorities ordered the evacuation of nearly 200,000 people from towns lying below the lake