California officials worked frantically into the night Sunday to evacuate thousands of residents downstream from the Oroville Dam after a hole on an emergency spillway raised fears of flash floods.
"There was significant concern that it would compromise the integrity of the spillway, resulting in a substantial release of water," Kory Honea, the Butte County Sheriff, told reporters at a press conference Sunday evening. "I couldn't risk the lives of thousands of people so we took this rather significant step."
The emergency spillway at the nation's tallest dam was activated Saturday for the first time ever in the dam's 48-year history after the swollen dam reached above its capacity following a deluge of rain in the Northern California region.
However, state engineers on Sunday discovered significant erosion had occurred back towards the face of the emergency spillway at Oroville Dam after huge water outflows, meaning the structural integrity of the dam's auxiliary spillway was at risk. The emergency spillway is an unlined channel that is essentially a natural hillside with rocks, brush and other debris.
Normally the dam would use its primary spillway but that too was found to have significant concrete erosion earlier in the week.
"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," the California Department of Water Resources said Sunday. "Those potential flows could exceed the capacity of downstream channels."
If there were uncontrolled outflows from the dam it could result in rising waters in the Feather River and tributaries and flood communities in eastern Sacramento Valley.
As of 10 p.m. local time on Sunday, Cal Fire said the evacuations due to the flash flood warning were still underway and to an area affecting approximately 35,000 residents – from the city of Oroville in Butte County down about 40 miles south to the Sutter County line. Also, parts of Yuba County near the Feather River were under an evacuation order. AP later reported that at least 188,000 people remained under evacuation orders.
"Cal Fire is mobilizing engines, crews, helicopters and swift water crews and sending in equipment from other parts of the state to the affected areas as part of a contingency plan," said Cal Fire Capt. Dan Olson, a spokesman for the Oroville incident.
An evacuation center for residents was set up at the fairgrounds in Chico, located about 23 miles northwest of Oroville.
State engineers were able to resume significant outflows from the primary concrete spillway on Sunday as a way to ease pressure on the emergency spillway.
Also, Sunday evening helicopters were being used to drop bags of rock and boulders into crevices of the emergency spillway erosion to prevent any further erosion.
Honea said stabilizing the emergency spillway and reducing pressure on it are measures that will "hopefully prevent it from a complete failure."
Meantime, more rain is forecast as early as Wednesday from incoming storms. The arriving storm is also expected to be significant and could bring rainfall for several days.
The mountains surrounding the Oroville Dam received between 10 and 20 inches of rain from Wednesday to Friday, according to the National Weather Service.
There have been concerns about Oroville Dam safety since the erosion was discovered Tuesday at the primary spillway at Oroville Dam. The crippled primary spillway last week was unable to release a sufficient amount of water to keep up with inflows from the plentiful rains in the surrounding area.
Oroville Dam is located in the foothills of the western Sierra Nevada mountain range. The pressure is on state officials to resolve the spillway crisis at Oroville since the heavy snowfall in the Sierras will be melting in the spring and bring more water to area reservoirs.
By Saturday, the reservoir reached its elevation capacity of 901 feet, which automatically triggered the emergency spillway. At noon on Sunday the elevation topped 902 feet but by 10 p.m. it was at 900 feet and declining, according to the DWR data website.
DWR reiterated Sunday that "Oroville Dam itself is sound," explaining that the dam built in the late 1960s is actually "a separate structure" from the emergency spillway.
DWR said Saturday the cost to repair the primary spillway was estimated to be as much as $200 million. With the significant damage to the emergency spillway, the price tag on repairs could go much higher.