As massive releases of water pummeled its surface all night Sunday and all day Monday, the main spillway at Oroville Dam showed no new signs of erosion, reducing the chances of a catastrophic collapse that would flood vast expanses of Butte, Yuba and Sutter counties, state officials said.
But serious threats remain. A new storm is brewing. Officials are frantically releasing water from Lake Oroville to keep levels below the emergency spillway. And no one can guarantee that crippling erosion on the dam’s emergency and main spillways won’t once again threaten tens of thousands of homes and businesses downstream.
In addition, state officials are unable to answer key questions: Why did the emergency spillway – the dam’s safety valve – approach the brink of failure when handling flows that should have been a minor strain? Why did state officials state with confidence Sunday afternoon that the situation had stabilized and the emergency spillway would hold, only to predict its failure and order the evacuation of nearly 200,000 people a few hours later?
“How can they assure us that it’s safe?” asked Maribel Cervantes, 35, of Yuba City as she sat in a Yolo County evacuation shelter on Monday. “How can we be 100 percent sure when one minute they’re saying the spillway was about to collapse?”
The engineering crisis facing Oroville Dam – and repeated swings in strategy – started last week, when Department of Water Resources engineers discovered a cavernous hole in the lower section of the dam’s main spillway following a series of strong storms. The main spillway, a 3,000-foot concrete span with gate controls, acts as the dam’s primary flood-control outlet during California’s rainy season.
Fearing the spillway would further erode and become inoperable, dam operators stopped the flows, then gradually and cautiously reactivated releases. With stormwater and runoff from the snow-packed Sierra still rushing in, the lake climbed to the point that water threatened to overtop the emergency spillway adjacent to the damaged main spillway for the first time in the dam’s 48-year history.
Unlike the main spillway, which is lined in concrete and controlled via release gates, the emergency spillway is designed to dump water in uncontrolled sheets down an unlined wooded hillside. DWR worked feverishly to avoid the scenario, concerned about the effects of sweeping mud, trees and debris into the Feather River below.
A flurry of announcements followed. First, officials warned that the reservoir level likely would hit 901 feet, sending water over the spillway’s 1,700-foot concrete lip. Then they said it wouldn’t. Then they reversed message again, saying the emergency spillway would be topped, but the flows should pose no risk downstream.
Saturday morning, water began streaming over the emergency spillway. “It’s not a lot of water; it’s a relatively small spill,” DWR spokesman Eric See said later that day.
Acting Director Bill Croyle said Sunday he believed the situation, while complex, remained manageable, particularly with a brief dry spell ahead. “We made it through the hump,” he said.
But late Sunday afternoon came another sharp veer: Engineers spotted serious erosion on the emergency spillway, warned it could collapse within a matter of hours, and authorities began their frantic efforts to evacuate hundreds of thousands of people downstream.
Left with no good options, DWR essentially crossed its fingers and ramped up releases on the main spillway to 100,000 cubic feet per second, taking the chance the fracture in the main spillway’s midsection would not grow and take out the top of the chute. Erosion spreading up the main spillway would threaten the release gates at the top of the structure – a development experts said could lead to problems in the main earthen dam holding back the massive reservoir.
On Monday, DWR officials once again shared good news, but remained cautious. The main spillway, albeit damaged, had survived the explosive releases. Erosion had not spread. Lake levels had dropped enough that water was no longer flowing over the fragile emergency spillway. And the 100,000 cfs releases from the main spillway would continue.
At that pace, reservoir levels will fall by as much as 25 feet by the time the next storm system arrives Wednesday night. Will it be enough to avoid water hitting the spillway lip? That’s not clear.
During storms that spanned Monday through Friday last week, lake levels increased by 50 feet. But Wednesday’s storm is not expected to be strong. National Weather Service forecasts are calling for about an inch of rain in Oroville from Wednesday night through Thursday night.
It rained about that amount – after a dry spell – in Oroville on Jan. 18. That day, inflows into the lake were just below 20,000 cubic feet per second, far below the outflows currently coming out of the lake.
The recent warm spell complicates the picture. Temperatures will be about 10 degrees warmer than that January day, causing more snowmelt from the mountains above the dam to drain into the lake. On Monday, inflow into the lake stood at 37,000 cubic feet per second.
Even as they worked to right the ship, DWR officials had no clear answers Monday on why the spillways hadn’t performed as designed. Asked why the emergency spillway had destabilized when handling only a fraction of the 250,000 cfs it was rated to accommodate, Croyle suggested it was difficult to predict the reliability of something built 48 years ago that had never been used.
“That system’s been installed since the early 1960s,” he said. “It’s been looked at. It’s been monitored. This is the first time it’s ever taken water over the system.”
Chris Orrock, a DWR spokesman, said slower flows sometimes can create more erosion than swifter flows. Why? Because a faster flow sometimes skims over the top of the landscape, while a slower flow gets into the nooks and crannies.
Joe Countryman, a member of the Central Valley Flood Protection Board and a former engineer with the U.S. Army Corps of Engineers, said the slope at the start of the emergency spillway is steep, and engineers may have underestimated the power of water rushing down it to create erosion.
“The fact that it was eroding right at the foot of the concrete section, that could be a weak spot,” he said.
On Monday, the Federal Energy Regulatory Commission – the agency that issued the license for the dam – ordered DWR to convene an independent panel of five experts to assess the reasons for the damage and issue recommendations on emergency repairs.
As the day progressed, a giant parking lot overlooking the dam was turned into a congested prep area for the effort to fix the erosion that jeopardized the integrity of the emergency spillway’s concrete apron. An armada of trucks bought giant chunks of aggregate to the south end of the lot, where they were ground into smaller boulders and deposited into oversized sandbags. Shortly after 8 a.m., two heavy-duty helicopters landed at the lot, kicking up a furious dust storm.
Several hours later, the copters joined dump trucks carting heavy, unbagged boulders to the road atop the wall of the dam and dropped the aggregate on the eroded hillside.
DWR officials said they were still strategizing about how best to repair the area below the emergency spillway if it has to be used again. There are still months left in the traditional rainy season, and the Sierra is loaded with snow. To prevent flooding for the rest of the season, officials said they would release water from Lake Oroville at a rapid pace until levels fall at least 50 feet below the emergency spillway, a process that could take up to two weeks.
“We expect to continue to make significant progress,” Croyle said.