The emergency spillway at California's swollen Oroville Dam was activated Saturday, as water levels from heavy rains this week caused the reservoir to rise above its capacity during an unusually plentiful rain season.
Saturday marked the first time the emergency spillway has been used in the dam's 48-year history. Normally the dam would use its concrete-lined spillway to discharge water, but that primary channel is severely damaged. However, utilizing the emergency spillway—essentially an unlined hillside—is likely to send mud and other debris into the water of rivers and channels downstream.
The California Department of Water Resources said in a release that the "the volume of water is expected to pose no flood threat downstream, and should remain well within the capacity of Feather River and other channels to handle."
Even so, the state agency cautioned that "the rate of flow into the ungated emergency spillway may change quickly."
The use of the emergency spillway follows a series of so-called atmospheric river storms that have dropped huge amounts of rain into Northern California—a region tracking to have its wettest year ever recorded.
The mountains surrounding the Oroville Dam received between 10 and 20 inches of rain from Wednesday to Friday, according to the National Weather Service in Sacramento. The dam is located in the foothills of the western Sierra Nevada mountain range.
"This is tracking to be perhaps the wettest season in Northern California ever," DWR spokesman Doug Carlson said Friday.
According to Carlson, the Northern California region as of Friday was at 228 percent of normal for this time year. This year's wet season, which began on October 1, is on track to be wetter than the 1982-83 season, which was the wettest.
Oroville Dam, the state's second-largest reservoir, suffered damage to its primary concrete-lined spillway due to erosion. The primary spillway was designed to divert rising water out of the dam, but damage to the channel was discovered Tuesday just as major storms were approaching.
"Oroville Dam itself remains safe, and there is no imminent threat to the public," DWR said Saturday. "DWR is coordinating closely with state and federal wildlife and dam safety officials at Oroville Dam."
The average annual rainfall in Northern California is 50 inches, based on data the state tracks going back to the 1920s. As of Saturday morning, the region was tracking at 68 inches of precipitation and trending higher than 1982-83 when the wet season produced a whopping 88.5 inches.
Late Friday, California Gov. Jerry Brown announced that he requested a presidential disaster declaration for the state. It comes after a series of damaging storms in January "that caused flooding, mudslides, erosion, power outages and damage to critical infrastructure across California."
In a letter to President Donald Trump, Brown said "preliminary assessments indicate the most severe impacts were to public infrastructure, including roads and bridges, flood and water control structures, and other public facilities."
At a press briefing Saturday, state officials estimated it could cost between $100 million and $200 million to repair the damaged Oroville Dam spillway. A DWR spokesman said later there's also a possibility they may decide to build an entirely new spillway at the dam.
The failure of the concrete base at Oroville Dam's primary spillway has put the spotlight on the state's aging infrastructure at a time when Trump is talking about increasing U.S. spending on infrastructure. Oroville Dam was dedicated in the late 1960s when Ronald Reagan was the state's governor.
Inflows into the Oroville reservoir—located about 70 miles north of Sacramento—were rising faster than outflows late Thursday as the storm system brought heavy rain to the Oroville area. And Friday it appeared the inflows were moderating from earlier levels.
Indeed, state officials Friday at a press conference indicated they were unlikely to use the emergency spillway since the damaged primary channel still was able to release sufficient water levels. Yet, things changed by Saturday as more water continued to flow into the reservoir as a result of the heavy rains.
The state began preparations Thursday to use the emergency spillway. The auxiliary outlet is designed to kick in automatically if Oroville—the nation's tallest dam—reaches the 901 feet elevation. As of 11pm on Friday the elevation stood at 899.16 feet according to the state's website. By Saturday morning at 7am the level was at 900.89 feet and rising, according to the DWR data website.
The emergency spillway was always considered a backstop option if the existing 3,000-foot-long spillway could not be utilized for outflows. Engineers were able to use the damaged spillway to a limited extent although water releases into damaged spillway caused further erosion of the channel.
The erosion to the Oroville Dam spillway was originally estimated to be a 200-foot-long strip, and about 30-foot depth. But by Friday the length of the erosion had grown by at least 50 percent due to more concrete crumbling.
Ahead of possible use of the emergency spillway, there were frantic efforts over the last few days, as teams worked to prepare the hillside for a deluge of water. Heavy earth-moving equipment was brought in and there were helicopters assisting with efforts on the ground.
Cal Fire crews cleared a hillside area near the dam's emergency spillway of trees, rocks and other debris to reduce potential debris flows downstream. Crews from the local power company, PG&E, removed several electrical lines using crews carried by helicopters and there also were plans to move transmission towers from the path of the emergency spillway.
Also, the state's Department of Fish and Wildlife on Thursday began to evacuate fish from a hatchery located downstream.