Federal and state engineers were on site at the damaged Oroville Dam spillway in Northern California Friday as the region tracked to have its wettest year ever recorded.
Oroville Dam, the state’s second-largest reservoir, suffered damage to its concrete spillway due to erosion. The 3,000-foot-long 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.
Mountains surrounding the dam have received between 10 and 20 inches of rain over the last three days, 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,” Doug Carlson, a spokesman for the California Department of Water Resources 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 (starts Oct. 1) is on track to be wetter than the 1982-83 season, which was the wettest.
The average annual rainfall in Northern California is 50 inches, based on data the state tracks going back around 100 years ago. As of Friday morning, the region’s was tracking at 67.4 inches of rain and trending higher than 1982-83 when the wet season produced a whopping 88.5 inches of rainfall.
Meantime, inflows into the Oroville reservoir—located about 70 miles north of Sacramento—have been rising faster than outflows as the latest storm system continued to pound the Oroville area. Yet it appears the inflows are starting moderate from the levels they were at late Thursday.
Even so, preparations started Thursday to use an emergency spillway that has never been used before in the dam’s 48-year history. The emergency spillway is a hillside and could cause potential debris flow downstream into the Feather River where a fish hatchery is located.
The emergency outlet is designed to kick in automatically if the reservoir reaches the 901 feet elevation. As of Friday at 2pm local time, the elevation stood at 896.8 feet, according to the state’s website.
State officials continued to insist Friday there was no danger to the public due to the gaping hole in the spillway at Oroville. They also said the use of an emergency spillway or hillside to relieve the swollen reservoir may not be needed after all.
“Right now there is no threat to the public at this point,” Robb Mayberry, a spokesman for the California Governor’s office of Emergency Services, said Friday. He said the emergency agency is supporting other state agencies and local officials to coordinate efforts at the dam’s spillway and “monitoring it very closely.”
The emergency spillway was considered a back-up option for as early as Saturday if the existing spillway could not be utilized for outflows. However, engineers have been able to use the damaged spillway.
Engineers conducted two test runs with water flows on Wednesday, checking to see if the spillway can handle the strength of about 20,000 cubic feet per second of water. They later increased the outflows to 35,000 cubic feet per second, and with the deluge of rain the outflow level by Friday morning were releasing 65,000 cubic feet per second over the existing spillway, according to the state data.
“It would appear if we can maintain the flow at 65,000 cubic feet per second, the use of the emergency spillway may be avoided,” said Carlson.
Yet releases from the damaged spillway are worsening 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 Friday the length of the erosion had grown by at least 50 percent due to more concrete crumbling.
While the existing spillway may work through the current storm, there are no assurances it could withstand another series of storms.
Indeed, engineers are taking no chances and still want to have the option of using the emergency spillway.
As a result, Cal Fire crews on Thursday and Friday were clearing a hillside area near the dam’s emergency outflow of trees, rocks and other debris to reduce potential debris flows downstream.
Also, the state’s Department of Fish and Wildlife on Thursday began to evacuate the fish and was still exploring options for the remaining fish eggs.
As of Friday, the state still had no estimate on repair costs for the broken spillway. Officials also indicated that any repair work may need to wait months until the rainy season ends.