(For more photos, skip the words and scroll down.)
On February 7th, 2017, someone noticed unusual turbulence in the freeway-wide main spillway at the United States’ tallest dam. Shutting the headgates and stopping the flow revealed a massive gaping hole in the concrete spillway. Too big to fix quickly, dam operators decided to leave the gates closed to avoid further damage. But, after years of record drought, this year’s exceptionally wet winter had nearly filled the reservoir, leaving little room to spare. Meanwhile, the next major storm system was headed for Northern California, bringing with it yet more rainfall. The rivers that feed the reservoir continued pouring water in and within days the reservoir had filled completely. People around the world were glued to the helicopter live-cams as water inched up and began to overtop the emergency spillway.
The emergency spillway, also called the “auxiliary spillway”, a thirty-foot high concrete weir between the headgate of the main spillway and the boat launch parking lot to the north, had never been used in the history of the dam. Unlike the main spillway which is solid concrete from the dam’s top to its base in the Feather River canyon, the emergency spillway has no stabilization below the concrete top. Within hours, the overflowing water became a deep torrent, gouging away the unstabilized soil and bedrock and undermining sections of the weir. Fearing the emergency spillway’s imminent failure and the resulting thirty foot wall of water that would inundate the nearby town of Oroville, authorities ordered 180,000 residents to evacuate on a one-hour notice.
As people frantically jammed highways attempting to escape, engineers and emergency planners were faced with a very difficult decision: allow the water to continue flowing over the emergency spillway and risk its complete collapse; or open the headgates and let water run down the main spillway, risking further damage to the spillway and possibly even its total failure. I would have liked to have been a fly on the wall in the incident command center at that moment.
The first option would have led to certain disaster. Option two, opening the headgates on the main spillway to lower water levels, presented a smaller risk, especially since the spillway had been holding up reasonably well under limited outflows. The main spillway is built on top of bedrock and the damaged part is well below the spot where the spillway steepens, known as the inflection point. Below that point, the water speeds up, increasing its destructive power the farther it falls. Even if the process of cavitation and collapse had continued to migrate up the spillway—called headcutting—the rate would have likely slowed, since the water would have less and less cutting power the higher up it got. Imagine the difference between pouring a bucket of water slowly across soft soil compared to pouring it onto the same spot from a rooftop.
Engineers chose to open the main floodgates further, increasing flows from 55,000 cubic feet per second (cfs) to 100,000 cfs. While a large segment of the lower spillway disintegrated under the crushing impact of so much falling water (see photos below), the upper section held and the lake level fell to the target of fifty feet below the top of the emergency spillway. Repairs were initiated immediately to stabilize the emergency spillway as yet another massive storm aimed itself at the 25-square-mile reservoir’s watershed.
The gamble of opening the main spillway paid off and the reservoir, as of today, February 21st, has enough surge capacity to absorb the runoff from approaching storms and spring snowmelt as long as the main spillway holds steady. Once the inflow to the reservoir slows as colder, drier weather moves in later this week, engineers hope to close the headgates and do emergency stabilization on the main spillway. Assuming it holds through the remainder of the winter wet season, the lower part of the main spillway will require a complete rebuild. Not only is there an enormous gash in the middle, but the water blew out the side and ripped an enormous canyon into what had been a forested hillside days before. The eroded rock debris is now blocking the Feather River channel, causing water to back up toward the base of the dam, submerging the tailraces of the turbines and threatening to inundate the powerhouse. This forced the shut down of water through the turbines, which could have otherwise provided another way to release water safely. A separate operation is now underway to clear some of this sediment (see photo below).
To comprehend what happened and what the dangers were requires understanding the difference between the spillways and the dam. If you walk along the dam from the south to the north, you’ll get to the parking lot atop the bedrock ridge anchoring the dam’s north end. Next to that is the main spillway and headgate structure. A rough Google Earth measurement puts that at about five hundred feet across. Continuing northward away from the dam, you’ll cross the top of the emergency spillway and eventually get to the boat launch parking lot about nine hundred feet from the headgates. The area around the parking lot and north end of the emergency spillway is where I believe, based on news reports, some of the worst erosion was taking place. That’s a long way from the dam. More importantly, for erosion to get to the dam itself, it would have to take out the headgate structure and a bunch of bedrock at the top of the ridge. The existing hole far down in the main spillway was no threat to that, but a collapse of the emergency spillway would be hitting closer to home in that regard.
The purpose of the spillways, both of which are lower elevation than the top of the dam, is to keep excess water from overtopping the dam itself, much like the bathtub overflow prevention drain keeps the tub from overtopping and flooding your whole house if you leave the water running. With the spillways being lower elevation and separated from the dam by the bedrock ridge, the dam itself was never in immediate danger, despite the misinformation (still) circulating on social media. That is not to say that a complete failure of either spillway wouldn’t have had dire, long-term consequences. The failure of the emergency spillway would have allowed thirty feet or more of water from the entire twenty-five square mile reservoir to drain uncontrollably down the Feather River for the rest of the rainy season, possibly overtopping levees well down into the Sacramento River delta area, forcing long-term evacuation of many hundreds of thousands of people and flooding vast areas of cropland with few options but to wait till the summer dry season.
I visited the site on Saturday, February 18th with a journalist friend. The evacuation order had been downgraded to an advisory and people were returning, though parts of downtown Oroville were still deserted and sandbags remained around the doors of many businesses in the lower part of town. While we weren’t allowed to go to the worksite up on the dam, we were given access to a media viewpoint at the rock staging area above the dam and to an otherwise closed access road that gave a good view of the lower spillway.