(CNN)Up to 5 inches of rain is expected over the weekend in Northern California, causing the state's Department of Water Resources to make preparations to use the partially repaired main spillway at the Oroville Dam if necessary.
Rain may put Oroville Dam's partially repaired spillway to the test
The spillway caused a potential crisis in February 2017 during another period of heavy rain when erosion created a big hole in the concrete structure. Authorities then discovered a massive hole in the emergency spillway -- essentially a large embankment -- and evacuated about 180,000 people out of fear that an uncontrolled release of water could flood downstream communities.
The first phase of repairs on the primary spillway was finished in November, but work on the middle section is still underway, the department said
But the agency announced it may have to use the spillway soon because of the weather forecast, even though the "objective for the year has been to minimize use of the main spillway while it is still under construction."
If the spillway is put into full use, the Department of Water Resources says it will closely monitor water flow and keep local authorities in the loop. No evacuations have been ordered yet.
The Oroville Dam, the nation's tallest at 770 feet, is about 75 miles north of Sacramento and provides flood control for the region.
Two to 5 inches of rain is expected in that part of Northern California, with the heaviest amount peaking Friday night and into Saturday, CNN meteorologist Dave Hennen said.
More than 8 million people are under floor or flash flood watches in that area.
The rain comes courtesy of an "atmospheric river" -- a long, narrow channel that transports water vapors outside the tropics.
The one that's saturating California is known as "the Pineapple Express," because it brings moisture from the tropical Pacific near Hawaii and can wallop the West Coast with rain and snow.
The National Weather Service in Sacramento said winds could reach 20 mph Friday night and Saturday. It should turn sunny on Sunday in Sacramento.
Last year's evacuation near the Oroville Dam was chaotic. Officials warned that a 30-foot "wall of water" could be sent rushing downstream if the eroding spillway were to collapse. People loaded up their cars and left the area.
"Everyone was running around," Oroville resident Maggie Cabral told CNN affiliate KFSN at the time. "All of the streets were immediately packed with cars, people in my neighborhood grabbing what they could and running out the door and leaving."
Sean Dennis told CNN the situation was "pretty scary, just because of how fast everything was developing. Me and my wife managed to throw as much stuff as we could into garbage bags, whatever we could find. We got both of our cars loaded down pretty well."
"It was pretty stressful to move everything we could in 25 minutes," Oroville resident RaeLynn Jones said.
During the rains in February 2017, erosion caused a hole to appear in the concrete spillway. A Department of Water Resources official said it was 250 feet long, 170 feet wide and about 40 to 50 feet deep.
The first phase of the repair was finished in November to handle outflows of 100,000 cubic feet per second, the department said. The upper and lower sections of the spillway were covered with structural concrete and cutoff walls installed on the sides. The middle section was temporarily repaired with roller-compacted concrete.
The second phase, which will include covering the middle section with structured concrete, is scheduled to start May 1, the department said.
Because heavy rain is expected this month, the agency ordered the main spillway be used if the reservoir elevation reaches 830 feet.
The reservoir elevation as of Thursday was 793.2 feet, and forecasts call for the elevation to near the 830-foot mark, the department said. In February 2017, the reservoir topped 900 feet.
The Department of Water Resources ordered more aggressive outflow from the Hyatt Powerplant in recent days to handle the expected rain.
The emergency spillway was repaired last year. Dump trucks and cement mixers shuttled in material. Workers dropped huge white bags filled with rocks into the hole that had opened up in the spillway and deposited large chunks of concrete to protect it from future erosion.
The atmospheric river is causing problems elsewhere in California.
Yosemite National Park announced that Yosemite Valley will be closed to park visitors after 5 p.m. Friday because of "significant impacts" from the storm. The park in the Sierra Nevada is more than 200 miles southeast of Oroville Dam.
All campgrounds and visitor lodging will close Friday afternoon, and reservations for Friday and Saturday have been canceled.
Last month, officials in Santa Barbara and other fire-ravaged communities in Southern California warned the atmospheric river could cause mudslides and debris flow in areas ravaged by wildfires.