Oroville Spillway Incident: The Atmospheric Impetus
Winters in northern California are normally wet, but not as saturated as they were in 2017. January and February of that year were among the wettest months on record for the entire 110-year history of Feather River hydrology. As the days and weeks passed during those months, rain poured, raising levels in California’s lakes, rivers and reservoirs to capacity – some going above.
Over the next week and a half, the California Governor’s Office of Emergency Services (Cal OES) and the California Department of Water Resources (DWR) would come to understand intimately the impacts this wet weather would have on our nation’s tallest dam.
The Oroville Reservoir Dam in Butte County began to show signs of compromise and inadequacy. According to DWR, the Feather River watershed above the Oroville reservoir received an entire year’s average runoff – 4.4 million acre-feet – in 50 days during those two months. During routine visual inspections on February 7, 2017, officials discovered damage to Oroville’s main spillway.
As a large atmospheric river storm settled over the Feather River basin, massive inflows and higher than expected precipitation increased lake levels rapidly. For the first time in the Oroville spillways history, the lake reached 901 feet and activated the emergency spillway, an uncontrolled concrete weir that allows water to pour over onto a bare hillside.
Then on February 12, Butte County Sheriff’s Office gave the evacuation order for the city of Oroville and multiple downstream communities along the Feather River due to concerns about downhill erosion threatening the emergency spillway structure. It’s estimated 188,00 people evacuated throughout the Feather River Basin.
On February 13, Cal OES coordinated with the National Guard to put 23,000 guards on standby to be ready for immediate deployment if the dam spillway should fail to help with evacuation and relief efforts. The next day, at 2:45 p.m., the evacuation order above was reduced to an evacuation watch by Cal OES officials and DWR and could return to the evacuated areas.
The Catalyst for Cal OES and New Requirements for Emergency Action Plans for Dam Safety
A significant result of the Oroville Spillway Incident was the creation of the Dam Safety Planning Division at Cal OES. Jose Lara was involved from the beginning of its inception.
In 2018, the main spillway was fully reconstructed, and the emergency spillway was completed. A concrete buttress to further bolster the emergency spillway weir and an underground secant pile wall and splashpad on the hillside were constructed to prevent uphill erosion if the emergency spillway is ever used again.
More than 1,000 people worked more than 2 million hours to rebuild the Oroville spillways to ensure the safety of downstream communities.