Damage to the main and emergency spillways of the Oroville Dam in California, triggered in February 2017 by heavy rain that was part of Northern California’s wettest winter in almost 100 years, has brought to light several lessons that hydropower and dam engineers can learn from the disaster. The cost to repair the spillway has reached $870 million, according to the state’s Department of Water Resources (DWR), as repair work has discovered numerous defects in the original construction from the 1960s.

Oroville (Figure 1) is the tallest (770 feet) and largest (6,920 feet in crest length) U.S. dam. It is home to the Edward Hyatt Powerplant, which draws water from Lake Oroville behind the dam. The Hyatt plant, which is located in bedrock in the left abutment near the axis of the dam, was built between 1964 and 1967. It has six turbines with a total generating capacity of 819 MW, although one of the units has been offline since 2015.

This aerial view looks east toward Oroville Dam and Lake Oroville, showing the damaged spillway with its outflow of 100,000 cubic feet per second (cfs) at the Butte County site. The California Department of Water Resources has a goal to lower the lake level by 50 feet to handle the next round of winter storms. Photo taken February 15, 2017. Dale Kolke / California Department of Water Resources
1. Spilling over. This aerial view looking east toward Oroville Dam and Lake Oroville was taken on February 15, 2017, following heavy rains in Northern California. The outflow was reported to be 100,000 cubic feet per second. Oroville is the tallest dam in the U.S. and has the largest crest length. Source: California Department of Water Resources

Several groups issued recommendations about future dam and hydropower projects in the wake of the 2017 incident, based on the lessons learned from the disaster, which forced the evacuation of about 200,000 residents downstream from the dam. They include better flood modeling; construction of emergency spillways that do not risk erosion of surrounding soil and rock; relocation of power transmission towers and lines away from potential flood and slide areas; and having the Federal Energy Regulatory Commission (FERC) address dam safety issues as a part of relicensing hydropower projects.

FERC in January 2018 issued a report on its independent forensic analysis of the Oroville incident. It found “Inherent vulnerabilities in the spillway designs and as-constructed conditions, and subsequent chute slab deterioration,” and “poor spillway foundation conditions in some locations” to be the two main physical factors in the incident. It added that “numerous human, organizational, and industry factors led to the physical factors not being recognized and properly addressed, and to the decision-making during the incident.”

FERC listed several factors involving the DWR, including that the “dam safety culture and program within DWR, although maturing rapidly and on the right path, was still relatively immature at the time of the incident and has been too reliant on regulators and the regulatory process.” FERC also said “DWR has been a somewhat insular organization, which inhibited accessing industry knowledge and developing needed technical expertise.”

Erin Mellon, assistant director of public affairs for the DWR, told POWER that the spillway incident “caused previously scheduled Hyatt Powerplant turbine maintenance to be pushed back three months.” She said repair and reconstruction of the main (Figure 2) and emergency spillways “is not impacting current scheduled maintenance at Hyatt. Five out of the six turbines are operational. Refurbishment of the sixth generator/turbine is part of routine maintenance at the Powerplant and unrelated to the February [2017] incident.” Mellon said that turbine is scheduled to be back online this year.

Draw down and cut off of releases from the damaged spillway at Oroville Dam. Taken on Feburary 27, 2017. Dale Kolke/California Delpartment of Water Resources.
2. Major damage to spillway. Damage to the Oroville Dam’s main spillway during the 2017 rain event resulted in its closure. An independent investigation of the dam and spillway after the incident discovered construction defects, including that the structure was on weak rock, it was not anchored properly to the underlying rock, and the concrete in place was too thin, making it structurally unsound. Source: California Department of Water Resources

Flows from Lake Oroville through the hydropower plant were shut down for three weeks—from February 10 to March 3—last year due to the incident, and have been reduced from their previous levels over the past year as repair work on the spillway has continued. As the event unfolded, the FERC report notes the high level of concern that the amount of water flowing onto the emergency spillway could affect operations at Hyatt, which led to the decision to shut down the plant and de-energize a high-voltage transmission line. Sandbags were deployed in the plant, which did not flood.

Mellon said that protecting the Hyatt plant from flooding “became one of the many risks DWR managed” during the 2017 Lake Oroville spillways incident. She added that “at the start of the incident, debris from the eroded main spillway blocked the flow of water out of the Diversion Pool Tailrace. The Tailrace is the channel directly in front of the dam where Hyatt releases reservoir flows. DWR staff and outside contractors performed on-site engineering and flood control mitigation to protect the facility from damage. This included sandbagging and PVC routing of any water to the plant sump pump system.”

Reconstruction of the dam’s spillway is designed to safely allow flows of 100,000 cubic feet per second (cfs) when completed. Dam managers can release about 17,000 cfs to the Hyatt plant when all six turbines are operating; at the time of last year’s event, with one turbine out of service, the maximum discharge rate was about 14,000 cfs. The water released to the power plant is important for managing reservoir inflows and outflows through the spring runoff season in Northern California.

The January FERC report said “the incident can be viewed as a ‘textbook’ case of a major dam incident, in terms of typifying the extent of the contribution from human factors. The incident was preceded by decades of somewhat complex interactions and effects of human and physical factors, through which numerous warning signs of the impending spillway failure were missed, and many barriers, which were intended to provide ‘checks and balances,’ were overcome to eventually produce the spillway failure.”

FERC in the report said “Appurtenant structures associated with dams, such as spillways, outlet works, power plants, etc., must be given attention by qualified individuals. This attention should be commensurate with the risks that the facilities pose to the public, the environment, and dam owners, including risks associated with events which may not result in uncontrolled release of reservoirs, but are still highly consequential.”

Kiewit, an Omaha, Nebraska-based building contractor, was the winning bidder for the contract to repair the spillway; it bid just more than $275 million, but in October the DWR said the construction cost would reach $500 million, a figure that did not include the agency’s costs. Mellon, the DWR spokeswoman, in a late January 2018 report said the state agency had spent $160 million in its emergency response to the dam’s failure, and another $210 million to remove debris, replace power lines, and for staff time and technical consultants. The DWR said the Federal Emergency Management Agency (FEMA) is expected to cover 75% of the project’s costs, with the state of California paying the rest.

The additional costs for construction include a nearly 80% increase in the amount of concrete needed to restore the spillway. Initial designs called for 485,000 cubic yards; that level has since increased to 870,000 cubic yards. The DWR report in late January said three of the 234 concrete slabs placed on the spillway last year have surface imperfections that may require fixes later this year. It said the concrete did not cure correctly due to an outage at the concrete plant that led to delays, causing winds to dry the concrete too quickly. Other problems noted by the DWR include pooled water on the adjacent hillside, causing seepage through the spillway walls, which were roller-compacted concrete. Those walls will be replaced with structural concrete later this year. The report also said a section of new wall is 1% out of vertical alignment and engineers will need to determine whether it should be fixed.

Darrell Proctor is a POWER associate editor.