In breaking news that occurred over the weekend, the auxiliary spillway at Oroville Dam suffered erosion during high flows, leading to the evacuation of a reported 188,000 nearby residents.
According to the California Department of Water Resources, evacuation orders were delivered to residents surrounding Lake Oroville at about 4:30 pm on Sunday, Feb. 12.
The concern is that erosion at the head of the auxiliary spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville that potentially could exceed the capacity of downstream channels. Lake Oroville is the largest reservoir in the State Water Project, with a capacity of 3.5 million acre-feet. The auxiliary (emergency) spillway has not been used since the dam was finished in 1968.
DWR says it has been monitoring conditions at the main and auxiliary spillways around the clock for signs of erosion that could threaten the integrity of the emergency spillway and allow large, uncontrolled flows to the Feather River.
Water from the auxiliary spillway has eroded the roadway below the spillway.
To lower the lake level, thus reducing flows and the potential for erosion at the top of the emergency spillway, DWR increased flows down the main spillway to 100,000 cubic feet per second from 55,000 cfs. This is the spillway that HydroWorld.com reported on Feb. 10 was damaged, when a cavity opened on the concrete due to erosion. DWR says current releases remain within the capacity of downstream channels.
For comparison, DWR said the morning of Feb. 11 that inflow to the lake was about 95,000 cfs. However, on Feb. 9, DWR reported inflow to the reservoir was about 192,000 cfs, thanks to “a storm that stalled over the watershed.”
Hampered by the erosion to the main spillway, Lake Oroville exceeded the elevation of 901 feet above sea level shortly after 8 am on Saturday, Feb. 11, at which point water began to flow over the concrete weir of the auxiliary spillway.
On Saturday, DWR reported it was focusing on ways to get its 645-MW Hyatt Power Plant, at Oroville Dam, back in operation because 14,000 cfs can be discharged from the plant when it is operating. Power generation was halted when the water levels in the channel that leads from the power plant became high enough to compromise operation.
Oroville Dam is a separate structure from the emergency spillway, and DWR says it remains sound. Oroville is the tallest dam in the U.S. at 770 feet and is a zoned earthfill embankment.