Dam spillway near SLO County has significant cracks, is ‘unsafe for use,’ state says

Remember when the concrete spillway of the nation’s tallest dam developed a giant hole, and downstream communities were evacuated out of fear that a wall of water would charge down the river?

It turns out that the same structural problems that caused the failure at Oroville Dam in February 2017 also exist at the spillway of San Antonio Dam, just two miles north of Lake Nacimiento and above the community of Bradley.

Those problems have been known “for quite sometime,” according to the current manager of the dam operator. But the state didn’t take notice or downgrade the dam’s safety condition until after it revamped its oversight process following the massive failure in Northern California.

Now, Monterey County has five years to completely overhaul the spillway or replace it at a cost that will likely reach above $40 million, some of which San Luis Obispo County might have to pay. In the meantime, managers of the dam have vowed not to use the spillway until those repairs are made.

“It’s the responsible reaction,” said Brent Buche, manager of the Monterey County Water Resources Agency. He added that the state classified the dam as “extremely high hazard,” because failure would likely result in loss of life.

Dam is in ‘poor condition’

An audit of the 60-year-old dam found significant cracking in the concrete surface of the spillway, and the integrity of the understructure has been compromised by erosion and clogged drainage pipes — a “poor condition” that the state Division of Safety of Dams said in an April 2019 letter is “unsafe for use under high flow conditions.”

Failure of the spillway would lead to massive flooding downriver, first hitting the community of Bradley. Backwater in the Salinas River could also cause inundation at Camp Roberts, including in San Luis Obispo County. The dam holds back 350,000 acre-feet of water.

san antonio upper portion of chute.JPG
Concrete degradation due to groundwater seepage has undermined the integrity of the spillway at the San Antonio Dam in Monterey County. Monterey County Water Resources Agency

A spillway failure is extremely unlikely, however.

“We will not allow that situation to happen,” Buche said.

The dam rarely reaches capacity, and water has only flowed over the spillway four times in its history.

Buche and his team plan to monitor the water levels in the reservoir to prevent it from filling and tipping over into the spillway. That could mean increased releases from a drainage pipe at an elevation lower than the spillway.

They can’t predict rain flow, Buche said, but they can control the lake levels.

Some repairs are already underway, both at San Antonio Dam and at to the Nacimiento Dam. A recent audit of the massive infrastructure there found rusted pipes, cracks in the spillway and voids under the surface caused by erosion of soils.

Those repairs have been overdue as well, and a different audit pointed to a lack of funding and number of staff as the reason they weren’t addressed earlier.

How will repairs be funded?

Who will pay for the repairs is the big question.

Ratepayers in the Salinas Valley who are the beneficiaries of the water storage facility will likely pay for some.

And, while San Luis Obispo County doesn’t receive any water from Lake San Antonio, it will likely have to foot a portion of the bill, too, because of a decades-old agreement.

Monterey County is hosting a financial workshop on the issue in September, and the discussions will likely go on for years.

In the meantime, reservoir managers will be tasked with balancing water levels with needs while closely watching weather forecasts for the next five winters.

This story was updated 10:20 a.m. Aug. 1 to clarify how San Miguel and Camp Roberts might be affected by dam failure.

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Monica Vaughan reports on health, Cal Poly, San Luis Obispo County, oil and wildlife at The Tribune. She previously covered crime and justice in the Sacramento Valley, is a graduate of the University of Oregon journalism school and is a sixth-generation Californian. Have an idea for a story? Email: mvaughan@thetribunenews.com