Report on dam crisis: Flaws missed

No sign thorough review was ever done at Oroville

The most detailed report yet on what went wrong at Oroville Dam last winter when 180,000 people fled amid fears of flooding found that state and federal officials failed to uncover long-standing construction and maintenance issues at the nation’s tallest dam.

Water released from Lake Oroville for decades via the dam’s primary spillway seeped through cracks on the concrete chute, weakening the halfmile-long channel, wrote the team of experts behind Tuesday’s independent report. A comprehensive inspection of the spillway and its history could have identified the flaw, the team said.

The problems came to a head Feb. 7 when a section of spillway gave way as large amounts of water poured out of the lake. Dam managers closed the damaged chute as it continued to erode, and when a backup spillway failed days later, they ordered temporary evacuations of towns downstream out of concern that water might pour uncontrollably out of the lake.

“The independent forensic team has not seen any indication that such a (comprehensive) review for the service spillway chute at Oroville Dam has ever been conducted since original construction” in the 1960s, the report’s authors wrote. “Such a review would likely have ‘connected the dots.’ ”

Inadequate drainage beneath the spillway, an absence of water stops at joints and too little anchorage of the concrete, all of which contributed to water seepage, would have been discovered by examining the dam’s original design documents, according to the report.

While the authors said signs of the spillway’s potential failure were clear, they acknowledged that looking at old construction plans and comparing them with modern-day standards is time-consuming and not the industry norm.

“Hopefully, this can be an impetus for dam safety programs to get more resources, and they can do more,” said John France, an engineer who is leading the investigation into the dam’s neardisaster. “This kind of review really needs to be done for all the features of a dam and its pertinent structures.”

The report’s authors speculated that the large amount of water released down the Oroville Dam spillway during last winter’s big storms supplied the final bit of stress needed to cause the collapse. Not only did more water probably seep into the ground beneath the spillway, but more pressure from water below also lifted the chute’s slabs, allowing water releases from the dam to get hold of the concrete and loosen it.

The new report was ordered up by the California Department of Water Resources, which manages the 770-foot-tall dam and the 3.5 million acre-foot capacity reservoir.

It’s the team’s second report on the likely causes of February’s scare, narrowing the list of some two dozen possible problems that the experts first presented in May. A final report is expected this fall.

The Department of Water Resources responded by promising to improve inspections.

“DWR agrees with the independent forensics team that dam owners need to reassess current procedures, as visual inspections (alone) would not have caught the February failure,” said spokeswoman Erin Mellon. “Additional spillway evaluations already under way are the start of that process.”

Both the Department of Water Resources and the Federal Energy Regulatory Commission have historically performed routine inspections of California’s reservoirs, including Oroville Dam. But the independent team of safety experts reported that the review process appeared to be unable to uncover more than superficial problems.

“Physical inspections, while necessary, are not sufficient to identify risks and manage safety,” the team’s report said.

Besides missing the design flaws, the inspections failed to identify problems posed by shoddy repairs of spillway cracks, the report said. They also missed erosion that probably opened up small cavities beneath the spillway and corrosion of rebar anchoring the chute, all of which could have contributed to water seepage, according to the report.

The Department of Water Resources is rebuilding much of the dam’s spillway and making improvements to the emergency spillway, essentially a barren hillside that began to erode when lake overflow was directed to it in February.

State officials said Tuesday that the upgrades are on track to be completed by Nov. 1, around the time the first rains are likely to fall.