For three weeks, Oroville Dam’s fractured main spillway and the surrounding hillsides have taken a nearly nonstop pounding. The stunning waterfall crashing down what’s left of the 3,000-foot concrete span has split the spillway in two and carved massive canyons on either side.
The Department of Water Resources, which operates the dam, has had little choice. The state agency had to force water down the damaged spillway to avert a potentially catastrophic “wall of water” from pouring out of Lake Oroville when the reservoir filled to the brim Feb. 12 – and dam operators watched in horror as the hillside below the never-before-used emergency spillway started to wash away. That sparked the frantic two-day evacuation of 188,000 downstream residents.
In the weeks since, the state has continued to blast water onto the battered main spillway, albeit in reduced volumes, to reduce water levels in the immense reservoir that sits behind the earthen dam. Lake Oroville – the state’s second-largest reservoir – has receded to the levels required this time of year to absorb a huge rainstorm.
So, with at least five more weeks to go in California’s rainy season, and billions of gallons of water waiting to rush into the reservoir when the snowpack blanketing the Sierra Nevada melts this spring, the question on the minds of many living in the Sacramento Valley is this:
Just how long can the badly damaged spillway hold up?
In practical terms, DWR officials are hoping the spillway can operate in its hobbled state through the rainy season, which this year is on pace to break records. Typically, the season stretches into April. According to several independent experts contacted by The Sacramento Bee, that appears to be a realistic goal.
Even with a fissure as long as a football field cutting across the midsection of the structure, they say the damaged chute appears strong enough to make it through spring. The top two-thirds of the span, including its critical release gates, appears to be undamaged. And the bottom third, while broken up, appears to sit on stable rock.
The jagged tear in between has misdirected a portion of the powerful flows cascading down the spillway onto the adjoining hillsides, carving deep ruts on either side. But that erosion, while dramatic, isn’t undermining the top of the spillway or posing a threat to the main earthen dam itself, according to geologist Jeffrey Mount of the Public Policy Institute of California. The spillway and dam are separated by a large triangle of hillside sitting on bedrock.
The experts said the bedrock underneath the main spillway has proved more durable than the crumbly material that washed away so dramatically below the adjacent emergency spillway Feb. 12.
But they also said it’s clear the state is in uncharted territory at Lake Oroville: DWR has ad-libbed its response from the start of the crisis, and can’t be sure exactly what will evolve as the weeks progress.
“That’s the No. 1 question,” said Joe Countryman, a flood expert and former engineer with the U.S. Army Corps of Engineers in Sacramento. “They’ve got 24-hour surveillance on how it’s operating, and so far, so good.”
For his part, DWR acting director Bill Croyle now speaks with confidence about the ability of the main spillway to last through spring. But the agency wasn’t always so sure.
After the crater formed in the lower third of the main spillway on Feb. 7, DWR closed the spillway gates to check the damage. A day later, engineers resumed the flow of water, but gingerly, conducting test runs at low volumes of water to see how the spillway would stand up. The tests sent tons of concrete and soil down the chute in a gush of water the color of chocolate milk. Water also flowed over the sides, eroding the spillway sidewall and carving into the hillsides.
Within a day or so, DWR officials said the water was pouring out clear and that the erosion on the spillway itself seemed to be in check. But they resisted ramping up the outflows and at one point dialed back slightly, to make sure the hillside erosion wouldn’t topple an electrical transm