page_title: Oroville Dam crisis
section_title: Background
hierarchical_section_title: Oroville Dam crisis / Background
caption_attribution_description: English: Oroville Dam face, with spillway. The dam is located near Oroville, in Butte County, California.
context_page_description: In February 2017, Oroville Dam's main and emergency spillways were damaged, prompting the evacuation of more than 180,000 people living downstream along the Feather River and the relocation of a fish hatchery.
  Heavy rainfall during the 2017 California floods damaged the main spillway on February 7, so the California Department of Water Resources stopped the spillway flow to assess the damage and contemplate its next steps. The rain eventually raised the lake level until it flowed over the emergency spillway, even after the damaged main spillway was reopened. As water flowed over the emergency spillway, headward erosion threatened to undermine and collapse the concrete weir, which could have sent a 30-foot wall of water into the Feather River below and flooded communities downstream. No collapse occurred, but the water further damaged the main spillway and eroded the bare slope of the emergency spillway. Many schools closed due to the floods.
context_section_description: 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 earthen dam in the United States, at 770 feet (230 m). It impounds Lake Oroville, the second largest man-made lake in the state of California, capable of storing more than 3.5 million acre feet (4.3 cubic kilometers). 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. It is not yet clear if the outdated manual was a significant factor in the February 2017 crisis.
  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 16,950 cu ft/s (480 m³/s).
  Through a river outlet (also known as the bypass valve) that has a capacity of 5,400 cu ft/s (150 m³/s).
  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 3,050 ft (930 m) 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 270,000 cu ft/s (7,600 m³/s) when the lake elevation is 905 feet. Since the sill elevation of the spillway is 813.6 ft (248.0 m), it can only be used at lake elevations above that level.
  Over the top of an emergency spillway, consisting of a 1,730-foot (530 m) long, 30-foot (9.1 m) high concrete weir beside the main spillway and the adjacent earthen slope of the abutment. Once the lake reaches an elevation of 901 ft (275 m) above mean sea level—21 feet (6 m) 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 s
caption: Partial view of the emergency spillway (left) and the concrete structure containing the gates for the main service spillway (right)
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