ppears 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 transmission tower they deemed vital to dam operations.
Meanwhile, with runoff from a wet, warm Sierra storm pouring into the reservoir, lake levels continued to climb. And DWR, in effect, was forced to choose between two bad, untested options:
Speed up releases over the damaged main spillway and possibly sacrifice the transmission tower. Or, let lake levels rise to the point that they crested the lip of an adjacent emergency spillway that had never been used in the dam’s 48-year history. Officials wanted to avoid the emergency spillway. They knew the spills would be uncontrolled and send mud and timber rolling down the unlined hillside beneath the structure into the Feather River below.
On Feb. 11, the water from the swollen reservoir rolled over the emergency spillway lip. Although the flows peaked at just 12,000 cubic feet of water per second – a fraction of the 250,000 cfs the structure was supposed to handle – the effect was far worse than imagined. On the afternoon of Feb. 12, the hillside directly beneath the spillway was eroding fast and on the verge of collapse.
The hillside looked “like flour being blown off by a fire hose,” J. David Rogers, a dam expert with the Missouri University of Science and Technology, recalls being told by an engineer at the scene.
In one area, the erosion came dangerously close to the emergency spillway’s concrete apron. If the water were allowed to eat away those remaining yards, officials feared it would undercut the emergency structure. That would have sent an uncontrolled blast of deadly, fast-moving water down the Feather River, flooding a vast swath of the eastern Sacramento Valley from Oroville to Yuba City.
The crisis was averted when dam operators gambled on the broken main spillway. They sent an explosion of water, at 100,000 cfs, nearly twice as strong as what they had been dumping onto the fractured chute just hours before. The move paid off. The main spillway held. The transmission tower stood. And lake levels dropped below the emergency spillway. The mass evacuations in