Catastrophic engineering expert asks: Is Oroville Dam leaking?

One of the country’s foremost experts on catastrophic engineering failures released a new report Thursday on the troubled Oroville Dam that asks a disturbing question: Is the country’s tallest dam leaking?

State dam managers have insisted for months that there’s no problem, and that persistent green wet spots near the top left abutment of the nearly 770-foot-tall earthen dam are nothing more than natural vegetation growth caused by rainfall.

In response to persistent questions about the wet spots since the February spillway crisis, the Department of Water Resources even has a section of its website devoted to the wet spots. The agency says they’re “caused by rainfall on the face of the dam, allowing vegetation to grow,” and that the spots have been there since before the reservoir was first filled in the late 1960s.

“Oroville Dam is sound and safe,” the website reads.

But in his 124-page report, Robert Bea of the Center for Catastrophic Risk Management at UC Berkeley says he led a team of 14 volunteers, including a group of retired state dam safety and water officials, who reviewed historical photos, videos, and state and federal inspection reports and concluded that something more serious may be going on.

The report warns that the dam may be “facing a breach danger from a serious and a dangerous form of a slow-motion failure mode” from persistent leaks in the main dam, perhaps caused by internal shifting of dam fill. The report notes that the sensors embedded in the dam to detect such problems quit working years ago.

“That’s scary. But it pays to be afraid,” Bea said in an interview. “That doesn’t mean you tremble and quake and crawl in the closet and suck on your thumb, but you have to understand there’s something here that’s potentially very harmful.” Bea is a retired engineer whose credentials include conducting an independent investigation into why the levees around New Orleans failed in 2005 during Hurricane Katrina.

Diagram of “wet spot” from Robert Bea’s report. Click to see entire report.
Another view of “wet spot” area from Robert Bea’s report. Click to see entire report.

On Thursday, Bea called for an independent investigation into the soundness of Oroville Dam.

Oroville Dam spokeswoman Erin Mellon said the state would make sure Bea’s report is shared with an independent team of experts that has been hired by the state to conduct a forensic study of what caused the spillway to collapse.

Bea’s report doesn’t just sound alarms about potential leaks. It also points to possible problems with the dam’s main spillway that may loom hundreds of feet above where a massive crater formed in the concrete chute in early February. The hole in the spillway set off of a chain of events that eventually led to the two-day evacuation of 188,000 people living below the dam.

The authors say they have evidence of broken and cracked “anchor tendons” that help support the structure that raises and lowers the spillway gates, allowing for water to gush down the chute. The report says two 50-year-old steel anchor tendons have already failed and DWR has data showing that 28 more have “crack indicators” in the steel. There are 384 of the anchor tendons in total, the report says.

The report’s authors also were troubled by a 14-foot crack growing in a massive five-foot-thick concrete pier attached to the spillway’s headworks. The crack has been well-documented in state inspection reports, but the reports don’t address whether or not it poses a safety risk.

“If the structural support and anchorages are inadequate to support the gate loadings, catastrophic failure of the gates could occur with catastrophic effects,” Bea’s report reads.

The report, meanwhile, restates many of the conclusions Bea made in April that shoddy design, construction work and maintenance caused the crater to form in the spillway.

Bea contends the spillway concrete was too thin, there was a lack of reinforcement between concrete slabs, and the spillway had a poor drainage system built on top of weak soils and rock – problems that were exacerbated by poorly repaired cracks and the state’s failure to remove trees and bushes growing too close to the structure. The roots, Bea contends, plugged the drainage system.

Bea’s findings have dovetailed with the early conclusions made this spring by consultants advising the state on Oroville’s repairs, and an internal analysis by the U.S. Army Corps of Engineers.

Department of Water Resources officials have declined to say what they think happened to the spillway. The state is awaiting the results from the forensic study being conducted by the outside group of engineers. That report is due this fall and will be made public.

The state estimates that the February crisis and ensuing repairs will cost more than $500 million. The ongoing spillway repairs are expected to span two years.


Read more about Robert Bea’s report “Preliminary Root Causes Analysis of Failures of the Oroville Dam Gated Spillway” in this KQED article.

Article by: Ryan Sabalow

Published: July 20, 2017 by Sacramento Bee

Read original here.