In early February 2017, Oroville Dam's main and emergency spillways both sustained damage during a period of sustained outflow from Lake Oroville, leading to the evacuation of areas downstream. Severe rainstorms and snowstorms impacted California, which, after being in a state of drought for five years, was not environmentally capable of absorbing the excess precipitation into the ground. The lake levels began rising rapidly following another wet storm in early February, and soon began flowing out of the main spillway. Damage to the spillway was first noted on February 7; as the spillway continued to degrade, the decision was made to reduce its outflow by using the emergency spillway on February 11, 2017, which had not been used since the dam's construction, which consisted of a barren hillside. The hillside began eroding much faster than had been anticipated, particularly at its top, and this headward erosion threatened to undermine the concrete weir at the top of the emergency spillway, which could have sent a 30-foot (10 m) wall of water into the Feather River below and flooded communities downstream. On February 13, as authorities oversaw the evacuation of more than 180,000 people living in the threatened areas, mainly those in Butte County, Yuba County, and Sutter County, emergency reinforcements were added to the emergency spillway and the outflow through the main spillway was increased again, which significantly exacerbated the damage to the main spillway and the adjacent hillside. On February 14, amidst declining outflows and the cessation of flow through the emergency spillway, the evacuees were allowed to return. Repairs to the dam were substantially complete by 2019.
Background Oroville Dam, an important part of the California State Water Project, is an earthen embankment dam on the Feather River, east of the city of Oroville in Northern California. The dam is used for flood control, water storage, hydroelectric power generation, and water quality improvement in the Sacramento–San Joaquin River Delta. Completed in 1968, it is the tallest dam in the United States, at . It impounds Lake Oroville, the second largest man-made lake in the state of California, capable of storing more than . The adjacent Edward Hyatt Powerplant has six power-generating turbines with a total installed capacity of 819 megawatts (MW) of electricity.
For flood control purposes, some space in Oroville Reservoir has to be kept dry to capture floodwaters, a practice that has caused controversy at other dams of California over the amounts of water wasted. Dam operators were required to discharge water based on charts contained in the Oroville Dam Reservoir Regulation Manual, a flood-control manual developed by the U.S. Army Corps of Engineers. At the time of the incident, the Oroville Dam Reservoir Regulation Manual had last been updated in 1970 and the discharge charts were based on climatological data and runoff projections that did not account for climate change or significant floods in 1986 and 1997. In the immediate aftermath of the incident, it was not clear if the outdated manual was a significant factor in the February 2017 crisis. The Final Report of the Independent Review Board, released 11 months after the crisis, did not cite the outdated manual as a significant factor.
The dam complex was designed with four routes for water to pass from Lake Oroville: Through the hydro-electric generators, which have a combined maximum flow rate of . Through a river outlet (also known as the bypass valve) that has a capacity of . Through the main (service) spillway located on the northwest abutment of the dam, which is used to control the height of the reservoir by quickly releasing large amounts of excess water down a concrete-lined channel that extends to the river below. Water flow onto the main spillway is controlled by eight radial (Tainter) gates that have a combined maximum discharge capacity of when the lake elevation is 905 feet. Since the sill elevation of the spillway is , it can only be used at lake elevations above that level. Over the top of an emergency spillway, consisting of a long, high concrete weir beside the main spillway and the adjacent earthen slope of the abutment. Once the lake reaches an elevation of above mean sea level— below the height of the main dam structure—water flows, uncontrolled, over the weir and down the earthen slope until it reaches the river below.
In 2005, the dam underwent re-licensing by the Federal Energy Regulatory Commission, during which three environmental advocacy groups filed a motion with the FERC raising concerns about use of the emergency spillway. The environmental advocacy groups stated a 2002 technical memorandum by the Yuba County Water Agency that noted that significant erosion of the hillside would occur if the emergency spillway is used, which could cause significant downstream damage and impair operation of the Edward Hyatt Powerplant. The groups urged the FERC to condition renewal of the Oroville Dam's license on the lining of the slope of the emergency spillway with concrete and to update the Oroville Dam Reservoir Regulation Manual.
According to FERC engineering guidelines, "Emergency spillways may be used to obtain a high degree of hydrologic safety with minimal additional cost. Because of their infrequent use it is acceptable for them to sustain significant damage when used and they may be designed with lower structural standards than used for auxiliary spillways." The FERC determined that the emergency spillway structure was compliant with its engineering standards and that the erosion concerns raised by the environmental advocacy groups were overblown. The commission also determined that the emergency spillway could handle of discharge and renewed the dam's license without requiring the concrete lining of the emergency spillway, estimated to cost over $100 million. A 2018 study by the Association of State Dam Safety Officials discovered that the commission relied on original geology reports from the 1960s, stating that the hillside below the emergency spillway was bedrock; it was actually weathered rock subject to deep erosion if the spillway were to be put into use.https://damsafety.org/sites/default/files/files/Independent%20Forensic%20Team%20Report%20Final%2001-05-18.pdf pg53
Prior to February 2017, the main spillway had been last inspected in August 2016. During that inspection, the spillway was visually inspected without inspectors entering the spillway chute. Inspectors entered the spillway chute for inspections in 2014 and 2015 without finding any concerns. Cracks in the concrete spillway chute were repaired in 2009 and 2013.
Timeline of crisis Timeline of Oroville Dam crisis, February 2017Oroville Dam normal operations 2005: Upgrade proposal rejected7 Feb 2017: Main spillway fails+ raw example of images below text1. The lake level is controlled using the main spillway gate, which releases water down the concrete spillway to get to the river below. 2. The emergency spillway, which has a high concrete wall at the top of a hill, is unused.Despite concerns that the emergency spillway is vulnerable to erosion, a $100 million request by environmentalists to upgrade it to a concrete-lined auxiliary spillway is rejected by the federal regulators.Craters appear in the main spillway. To avoid increasing the damage to the spillway, water releases are slowed allowing the lake to rise.11 Feb 2017: Emergency spillway used13 Feb 2017: Repairs madePotential risksWater flows over the emergency spillway causing erosion and damage. This is by design and prevents water going over the top of the main dam. However the ground erodes faster than expected.Rocks and concrete (1) are placed under the emergency spillway weir to repair erosion damage (2). The release of water into the main spillway is increased, to lower the lake in preparation for more rain. This erodes the adjacent hillside considerably, generating a debris dam (3) that blocks the river and forces the closure of the hydroelectric plant. if the erosion on either spillway reaches the top, it would cause the weir or gate (respectively) to collapse, causing a large uncontrolled water release and life-threatening floods. version 3 – February 28, 2017 public domain OK to share and use
Main spillway damage In early February 2017, high inflows to Lake Oroville caused dam operators to start using the concrete main spillway to control the lake level. Between February 6–10, 2017, of rain fell on the Feather River Basin. Inflow of water into Lake Oroville rose from on February 6 to over at mid-day on February 7, before subsiding to about the following day. Dam operators began increasing the flow down the main spillway to on February 7 but soon noticed an unusual flow pattern. The flow of water down the spillway was stopped for investigation, revealing a crater about halfway down the spillway where the concrete lining was eroded through and water was escaping the concrete chute. The DWR consulted with FERC and other dam safety agencies about the issue and prepared for the possibility of using the emergency spillway.
During two test flows down the spillway on February 8–9, the length of the crater increased from to . Meanwhile, the level of Lake Oroville was rising as inflow into Lake Oroville skyrocketed from about on February 7 to a peak of late on February 9. The DWR was then confronted with two choices: use the main spillway, knowing it would likely be further damaged, or allow Lake Oroville to rise until it overtopped the emergency spillway.
On February 9, the main spillway was reopened. DWR hoped that using the damaged spillway with a limited flow could drain the lake enough to avoid use of the emergency spillway, which would potentially damage powerlines servicing the hydroelectric plant. The following day, discharge on the main spillway was reduced from to , but this flow was not enough to prevent the lake from rising. On February 11, the lake level reached above mean sea level and water began flowing—as designed—over the concrete weir along the top of the emergency spillway, cascading onto the emergency spillway for the first time in the dam's history.
Emergency spillway damage
As the lake level rose, measures were taken to prepare the emergency spillway for use. On February 10, 2017, workers began clear-cutting trees on the hillside below the emergency spillway.
Shortly after 8:00 am on February 11, 2017, the emergency spillway began carrying water for the first time since the dam's construction in 1968. Because the spillway was a separate structure from the dam, officials stated that there was no danger of the main embankment being breached, and evacuation of Oroville was not considered at that time, as officials stated that there was no threat to public safety. The uncontrolled flow over the weir topped out at .
Erosion at the base of the weir—which was expected—progressed much faster than anticipated. The headward erosion of the emergency spillway threatened to undermine and collapse the concrete weir, in which case a wall of water would be sent into the Feather River below and flood communities downstream. Fearing a collapse, the Butte County Sheriff's Office issued an evacuation order of the Oroville area. To ease pressure on the emergency spillway and prevent a possible collapse of the weir, the DWR nearly doubled the discharge volume of the main spillway from to . Several hours later, water stopped flowing over the weir onto the emergency spillway and officials began assessing the state of the emergency spillway.
Evacuation of the Feather River Basin