Mandatory evacuation orders remained in effect Monday morning for nearly 200,000 residents downstream from Northern California’s Oroville Dam as state engineers started to look at repairing a crippled emergency spillway.
“We have inspections teams that are on the ground inspecting the erosion that was occurring below the auxiliary spillway,” Eric See, a California Department of Water Resources spokesman said in an interview this morning.
Officials were scheduled to hold a press briefing at noon local time.
California officials worked frantically into the night Sunday to evacuate thousands of residents downstream from the dam after a hole on an emergency spillway raised fears of flash floods. Oroville Dam — California’s second-largest dam — is located about 70 miles north of Sacramento.
Also, the state official denied allegations there had lax safety at the Oroville Dam despite a report of previous warnings about the emergency spillway at the nation’s tallest dam.
Erosion to the unlined emergency spillway — essentially a natural hillside of soil, rock and brush — was halted after state engineers discovered a major hole Sunday from erosion that raised the risk of failure of the auxiliary channel. The main spillway is damaged too from significant concrete erosion but was being utilized to release water from the swollen dam Monday, according to DWR.
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.
State and federal officials failed to heed safety warnings about Oroville more than a decade ago, according to the Mercury News. The report Sunday said three environmental groups warned the Federal Energy Regulatory Commission about the vulnerability of the hillside emergency spillway.
The Oroville Dam was completed in the late 1960s when Ronald Reagan was governor of California.
FERC confirmed last week it had several engineers onsite at the Oroville Dam but Monday morning the agency didn’t respond to requests for comment.
But DWR spokesman See denied Monday that the state had ignored earlier concerns about the Oroville Dam’s emergency spillway or had been lax in inspections there.
“We have a very rigorous schedule of inspections that is determined by state and federal regulators,” said See. “We actually do those inspections annually.”
The emergency spillway at Oroville Dam was activated Saturday for the first time ever in the dam’s 48-year history after the 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. That led state and local authorities Sunday to order an immediate evacuation of communities downstream from the dam.
If there were uncontrolled outflows from the dam it could result in rising waters in the Feather River and tributaries and flood communities in the eastern Sacramento Valley. Overall, the evacuations affected nearly 200,000 people, according to the Butte County Sheriff’s Office.
Cal Fire was mobilizing engines, crews, helicopters and swift water crews Sunday evening and sending in equipment from other parts of the state to the affected areas as part of a contingency plan.
An evacuation center for residents was set up at the fairgrounds in Chico, located about 23 miles northwest of Oroville.
State engineers resumed significant outflows of 100,000 cubic-feet per second down the damaged primary concrete spillway on Sunday evening as a way to ease pressure on the emergency spillway.
On Saturday, Oroville Dam reached its elevation capacity of 901 feet, which automatically triggers the emergency spillway. At noon on Sunday the elevation topped 902 feet and by Monday morning after major outflows from the primary spillway the elevation was down to 897 feet, according to the DWR data website.
“The primary spillway can actually handle quite a bit more water,” said DWR spokesman See. “It’s just that we know when we go above the 150,000 cubic-feet per second that the channel capacity downstream can have some problems.”
Also, Sunday evening helicopters had been expected to drop bags of rock and boulders into crevices of the emergency spillway erosion to prevent any further erosion. But See said the helicopter drops didn’t happen after all.
“There hasn’t been a repair effected yet,” said See. “Right now they need to do an inspection first to identify the best plan to proceed.”
The DWR official said the first priority at the dam itself was to bring the water level down “because it’s the water that is providing the erosion” to the emergency spillway. “They shut the spigot off basically. It abated the immediate issue of erosion that was going now so now they are going in and assessing.”
Butte County Sheriff Kory Honea told reporters at a press conference Sunday that stabilizing the emergency spillway and reducing pressure on it are measures that will “hopefully prevent it from a complete failure.”
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.
Also, rain is forecast in the Oroville area as early as Wednesday from incoming storms. The arriving storm systems are expected to be significant and remain in the Northern California region through next week.
“This is a series of storms coming in and we could see potentially 4 to 8 inches of precipitation,” said Idamis Del Valle, a meteorologist with the National Weather Service office in Sacramento.
The mountains surrounding the Oroville Dam received between 10 and 20 inches of rain from Wednesday to Friday, according to NWS.
Hydrologists are currently modeling the incoming storm system, according to See. “We’re tracking it very closely,” he said. We’re bringing the lake down to essentially…create storage so when this next storm comes later on this week and flows start to increase we’ll be able to absorb that water.”
DWR said in a release on 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.
The cost to repair the primary spillway was estimated to be as much as $200 million, state officials said over the weekend. With the significant damage to the emergency spillway, the price tag on repairs is likely to much higher.