Thousands of homes sit abandoned in the Feather River Valley below California’s Lake Oroville Dam, as officials race to repair the dam’s damaged spillways.
Nearly 200,000 people were forced to flee their homes Sunday due to fears one of the damaged spillways would fail and unleash a wall of water on the communities below. On Tuesday, engineers raced to drain the rain-swollen reservoir and dropped sacks of rocks into holes in the spillway in hopes of stabilizing it.
What happened to the dam’s spillways?
Dams are designed to handle overflow using a “spillway.” Engineers can release excess water through these chutes in a controlled manner in order to prevent flooding.
The potential disaster began last week after officials noticed unexpected erosion in the main spillway after heavy rains, sending chunks of concrete flying and creating a 200-foot-long, 30-foot-deep hole that continues growing. Engineers don’t know what caused the cave-in, but a Department of Water Resources spokesman told the Associated Press it appears the dam’s main spillway has stopped crumbling even though it’s being used for water releases.
Over the weekend, water began spilling down the unpaved emergency spillway — for the first time in the dam’s 50-year history — for nearly 40 hours. Emergency spillways are usually reserved for last-resort situations.
This left the emergency spillway badly eroded with a gaping hole.
READ MORE: 200,000 people forced to evacuate as California dam threatens floods
According to the Los Angeles Times, the emergency spillway began eroding when peak flows reached 12,600 cubic feet per second; however, the Department of Water Resources said the spillway was designed with a capacity of 450,000 cubic feet per second.
The erosion happened quickly enough for officials to call for evacuations to begin.
Were officials warned about the potential for erosion?
In 2005, at the start of the dam’s still-unfinished relicensing process, environmental groups asked federal regulators to require that the California Department of Water Resources “armour” the hillside — or reinforce it, typically with concrete or boulders — to prevent potentially catastrophic erosion from water escaping when the reservoir was cresting over full capacity.
The groups said rocks and other debris could be swept into the river, damaging highway bridges and power plants downstream. In the worst case, they warned, a major breach would unleash floods that could take lives and destroy property.
Back when environmentalists and local officials were raising their concerns, the water resources department dismissed the need to fortify the hillside, insisting it would not be in danger if water cascaded down it.
Bill Croyle, acting head of the Department of Water Resources, said he was “unaware” of the 2005 report stating, “I’m not sure anything went wrong. This was a new, never-having-happened-before event.”
The hillside was designed to handle the “probable maximum flood,” and annual dam inspections include a review of the spillway’s structural integrity, according to a May 2006 filing by the Metropolitan Water District of Southern California. The cost of reinforcing the hillside was not immediately clear.
In the filing, the water agencies told federal regulators that environmentalists and local officials did not show that the emergency spillway posed a public risk.
However, as the New York Times reported, because emergency spillways aren’t often used, they are not built with the same specifications as main spillways.
Is the situation still dangerous?
Officials have been working around the clock to try to stabilize both spillways with more rain in the forecast this week. Giant white bags are being filled with rocks, and at least two helicopters will fly them and then release them in the spillway’s erosion. Dump trucks full of boulders also are dumping their cargo on the damaged spillway.
READ MORE: Water level drops behind California dam, easing flood fears
A spokesperson for the State Department of Water Resources said lake levels are also dropping at a rate of eight feet per day. The goal is to see the level at 860 feet by Thursday when inflows should begin from the expected storms.
However, officials still urged residents to continue evacuating in the event that repair efforts fail during the next rain storm.
“We’re doing everything we can to get this dam in shape so that they can return and they can live safely without fear. It’s very difficult,” said California Governor Jerry Brown.
— With files from The Associated Press and Reuters