SACRAMENTO (CN) — Nearly 190,000 Californians were evacuated this weekend from areas below the nation’s tallest dam at Lake Oroville, as erosion threatened the dam’s main spillway and a backup, emergency spillway was opened for the first time in the dam’s history.
To stem erosion near the linchpin of California’s water system, California officials doubled flows down the crumbling Lake Oroville spillway on Sunday and sent 188,000 evacuees scrambling for higher ground.
With water levels threatening to eclipse the 770-foot Oroville Dam Saturday morning, the California Department of Water Resources was forced to use an emergency spillway for the first time in the dam’s history.
Burdened by a deficient and obliterated main spillway that couldn’t keep up with the reservoir’s rapidly rising water, officials sent water over the concrete lip of the emergency spillway, down into the heavily wooded hillside.
For more than 30 hours officials sent flows cascading over the crude spillway, in a desperate attempt to supplement the dam’s stymied main spillway, which suffered severe structural damage earlier in the week.
But the backup strategy didn’t relieve enough pressure on the 48-year-old dam, and officials warned that the untested spillway could quickly give out and send a 30-foot wall of water tumbling southwest toward the Feather River and the already saturated Sacramento Valley floor.
Oroville is 69 miles north of Sacramento and 23 miles southeast of Chico.
“Officials are anticipating a failure of the auxiliary spillway at Oroville Dam within the next 60 minutes,” the department said in an evacuation order at 5 p.m. Sunday. “Residents of Oroville should evacuate in a northward direction.”
The announcement interrupted a sun-baked Sunday afternoon and caused a rare mass evacuation in the Golden State. Gov. Jerry Brown broached the topic of federal emergency relief and directed state resources to the “very serious situation” in Oroville.
“I’ve been in close contact with emergency personnel managing the situation in Oroville throughout the weekend and it’s clear the circumstances are complex and rapidly changing,” Brown said late Sunday.
Finished in 1968, Lake Oroville is the state’s second-largest reservoir and the most important in terms of water storage and delivery capacity. Much of Southern California relies on water from Lake Oroville.
Evacuation orders were quickly issued in several counties downstream from the compromised dam, including Butte, Sutter and Yuba counties. An estimated 188,000 people were given evacuation notices; hotels and makeshift evacuation centers were overwhelmed.
Butte County Sheriff Kory Honea ordered residents downstream of the dam to evacuate, reiterating that the command was “NOT a drill.”
The dramatic orders jolted residents from the rural farming communities to much larger Chico and Sacramento. The few routes south to Sacramento were quickly jammed and the California National Guard stepped in to help direct the evacuation.
Sunday’s chaos was the culmination of problems spawned by the collapse of a portion of the dam’s main spillway last week.
Officials announced on Tuesday, Feb. 7 that heightened discharges caused by a torrential series of winter storms had punctured a 200-foot-long, 30-foot-deep hole in the spillway. Video captured chunks of concrete flying from the weakened spillway as officials ramped up flows above 50,000 cubic feet per second.
After temporarily slowing releases to assess the spillway’s damage, officials were forced to continue using the damaged spillway as a potent storm crashed into Northern California on Thursday. Initial estimates pegged the spillway damage at more than $100 million.
By Saturday morning, water began to spill over the backup spillway and state officials said that both of the dam’s main release avenues were in jeopardy. Helicopters dropped bags of rocks into the spillway’s cavity and the dam’s power plant was temporarily shut down because debris had clogged the turbines.
Despite the turmoil, water had stopped spilling over the backup spillway by 10 p.m. Sunday. Officials remained adamant that the main dam structure was not compromised and said they would continue using the broken spillway throughout the week in preparation for another storm forecast for later this week.
First District Congressman Doug LaMalfa warned his constituents that the dam was not in the clear yet.
“Water has stopped flowing over the top of emergency spillway but there is no time to relax, there is still 20 feet of pressure of the entire lake on the structure,” LaMalfa tweeted.