California orders closer look at 93 dams after Oroville crisis; including in Merced

California officials have ordered owners of 93 dams, including two operated by Merced Irrigation District, to reinspect their flood-control spillways following the Oroville Dam crisis, saying the spillways need a closer look following a preliminary review.

The list released by the Department of Water Resources includes some of the largest dams in California, such as the New Exchequer Dam on the Merced River, New Bullards Bar on the Yuba River, and Lake Almanor Dam on the Feather River in Plumas County. Each holds back reservoirs roughly the size of Folsom Lake, which can store about 977,000 acre-feet of water.

Also on the list is Don Pedro Dam, on the Tuolumne River, which is about twice the size of Folsom and contains the sixth largest reservoir in California.

The 93 dams represent less than 1 percent of the 1,250 dams overseen by the Department of Water Resources’ dam safety division.

DWR said the probes were ordered in recognition of the emergency at Oroville, which prompted a mass evacuation, and the fact that California’s dams are 70 years old on average.

Preliminary assessments showed each of the spillways on the list “may have potential geologic, structural or performance issues that could jeopardize its ability to safely pass a flood event,” according to letters the dam-safety division sent to the dam owners. “Therefore, we are requesting that you perform a comprehensive condition assessment of the spillway as soon as possible.”

DWR officials have been notifying the owners of the affected dams since June. The complete list was released Thursday by the agency.

“It will not be known which spillways, if any, will need repairs until the comprehensive assessments are completed and reviewed by (the dam-safety division),” DWR said in a note accompanying the list. “Dam owners of these spillways have been directed to perform any needed maintenance repairs prior to the next flood season. (The division) has already received immediate responses from many dam owners in compliance with the notice.”

Merced Irrigation District, which manages both the New Exchequer and McSwain dams, has complied with the DWR’s requests, said Mike Jensen, a spokesman for the district.

Although the New Exchequer gated spillway wasn’t used this past winter, faced no threat and hasn’t been needed to manage flood flows for the past 50 years, MID regularly operates the gates for maintenance and inspection, Jensen said.

In May, MID began work to bolster the channel downstream of Lake McClure’s gated spillway and concrete area below the spillway.

Just this week, MID safely made controlled, voluntarily releases of water through its gated spillway at Lake McClure into downstream Lake McSwain in order to evaluate the Lake McClure emergency spillway system, Jensen said.

So far, the efforts have cost MID about $1.5 million, Jensen said.

Federal and state officials have openly talked for months about toughening inspection programs following the near disaster at Oroville.

“One thing we all have in common is that we didn’t predict this happening,” said Frank Blackett, a regional engineer at the Federal Energy Regulatory Commission, in a speech in Sacramento in May.

David Gutierrez, a DWR consultant and former chief of the dam-safety division, said routine annual inspections generally aren’t able to detect the types of problems that were lurking at the Oroville spillway. “We’re trying to find the obvious issues,” he said in an interview in June. “It’s a visual inspection. You’re climbing things. You’re not X-raying.”

Oroville’s crisis began when a giant crater opened in the main spillway Feb. 7. DWR officials throttled back water releases to limit the damage, but a heavy rainstorm filled Lake Oroville and water began spillway over the adjacent emergency spillway for the first time in the lake’s 48-year history. When it appeared the emergency spillway might fail, officials ordered the immediate evacuation of 188,000 downstream residents. DWR quickly ramped up releases over the main spillway, arresting the flow of water over the emergency structure, and lake levels fell. The evacuation order was lifted two days later.

The exact cause of the crater has yet to be determined. An independent forensic team has cited two-dozen possible factors but won’t finish its investigation until this fall.

This story was originally published July 28, 2017 at 10:03 AM with the headline "California orders closer look at 93 dams after Oroville crisis; including in Merced."