Storm could test rebuilt spillway of Oroville Dam

With a pounding storm headed for California, state water officials said Tuesday that Oroville Dam’s crumbled spillway could get its first test since being rebuilt in the wake of last year’s near-catastrophe.

The 700-foot-deep Lake Oroville is just 36 feet below capacity this week, officials said, and its water may have to be released through the restored half-mile-long concrete chute, which partially collapsed in February of 2017. The spillway’s failure triggered concerns that a wall of water would pour uncontrollably from the dam.

Work crews have been laboring to reconstruct much of the spillway since then. The planned $710 million, two-year fix isn’t expected to be completed until after a second phase of construction this year, but officials say the chute is in working order after last year’s first phase was finished.

“Hopefully we don’t need to use the spillway,” said Erin Mellon, spokeswoman for the California Department of Water Resources, which operates the reservoir. But if it is needed, “we’ve done a lot of hydraulic monitoring. We’re confident in the construction.”

If the spillway has to be used, state officials said their concern is not so much the structural integrity of the dam, but rather the flow of water down the chute. Much of the newly built spillway consists of temporary roller-compacted concrete, which has a much rougher surface than the permanent concrete and is likely to make for more turbulent discharges.

The temporary concrete could wear amid prolonged water releases, officials said. They expect cutoff walls that were built around the temporary concrete during last year’s construction to soften the blow of the streaming water.

Inspectors will be on site around the clock to monitor the performance of the spillway and correct any problems should the floodgates open.

To lessen the chances of having to use the outlet, dam operators began releasing water more aggressively from the reservoir this week through the power plant, which has served as the primary means of discharging water during the recent construction. The dam also has a handful of deep outlets in the lake that can be used to drain water, which managers said they may employ.

Forecasters say a giant weather system from the central Pacific will probably make landfall by Friday. State officials say that they don’t think Lake Oroville will fill to its rim, but they’re taking precautions should they have to use the spillway late this week or early next.

Problems at the dam emerged Feb. 7, 2017, when the main spillway partially collapsed as large amounts of water were being released from the lake. When managers closed the chute to take inventory, water was let out from an emergency spillway — essentially an unpaved hillside — which also began to erode.

Fears that a blast of water would gush out of the reservoir prompted authorities to evacuate nearly 200,000 residents downstream. Before any problems occurred, however, dam operators were able to put the main spillway back into use, at least enough so that the reservoir level dropped.

Lake Oroville, which is about 75 miles north of Sacramento, is the state’s second-largest reservoir. It provides drinking water to parts of the Bay Area and Southern California.