The 180,000 people downstream of Lake Oroville are once again asked to trust the state water agency and federal regulators. Residents fled with barely time to put on shoes last February when first-time use of the emergency spillway, claimed to be safe, resulted in erosion that threatened to undercut the slope and release a wall of water. Now they are being asked to trust the same water officials’ statements that the $500 million newly constructed spillway at Lake Oroville is safe. The spillway’s safety, however, had been questioned even before construction reached its November 1st completion deadline.
In early October, the Federal Energy Regulatory Commission asked California Department of Water Resources to explain cracks in sections of the 2,270 foot long newly rebuilt spillway. When concrete curing coverings were removed from the new concrete slabs, small cracks in the surface were discovered.
FERC’s letter outlined the problem and requested four actions be accomplished by November 2nd.
We understand that DWR has at least initially expressed some concern about these cracks and has taken a number of actions to adjust the concrete mix design and contact some concrete material experts to identify possible actions. Based on the observed cracking, DWR should complete the following:
1. Map the observed cracks in each of the completed slabs to establish a baseline condition.
2. Evaluate and identify the potential cause(s) of the cracking.
3. Identify possible remedial actions, if necessary.
4. Document the findings in a report.
(Bold added)
DWR’s response was five days late. Their November 7th cover letter to a classified memorandum the public cannot read states that it’s no problem, just part of “the design elements included to restrain the slabs and produce a robust and durable structure.” They also asserted that cracking “was anticipated and is not expected to affect the integrity of the slabs.” In a November 21 response letter FERC said okay, we looked over your classified assessment and agree the cracks “do not warrant repair at this time.” But keep an eye on it, okay?
KQED’s report noted that the DWR spokesperson said hairline cracks are “something you expect to see in concrete slabs as massive as the those in the rebuilt spillway, which measure 30 feet by 37.5 feet [each slab]. These cracks are not abnormal, nor do they cause a concern….” Nevertheless, the FERC letter claims that DWR did express concerns.
Robert Bea, a professor emeritus of civil engineering at UC Berkeley and a veteran analyst of structure failures, said that DWR’s letter leaves “a lot of uncertainties regarding the implications of the reported micro-cracking.”
Bea, who heads a Berkeley-affiliated group that has issued several reports this year highly critical of DWR’s management of the Oroville facility, added that cracks in the concrete surface are potentially serious and require urgent attention.
“Cracking in high-strength reinforced concrete structures is never ‘to be expected,’ ” Bea said in an email. Even small cracks could increase stresses in the concrete when it is under “service loading” — for instance, when large volumes of water hurtle down the structure at speeds approaching 90 mph.
The cracking also “develops paths for water to reach the steel elements embedded in the concrete and accelerate corrosion,” Bea said. “Such corrosion was responsible for the degradation and ultimate failure of the steel reinforcing in parts of the original gated spillway.”
(Bold added.)
Last winter’s disaster that resulted in the lower two-thirds of the spillway falling to pieces and leaving a massive 80-foot-deep crater in the hillside was said to have begun due to pre-existing cracks found in the former spillway. They were documented in 2009 and 2013 and repaired in what Brea called “patch and pray.” Additionally, the spillway slab was too thin for the water load, and the steel rods anchoring the slabs to the bedrock were corroded by water under the slab.
The prospect is simple, yet terrifying and has been the culprit in a number of near disasters at dams across the globe since engineers discovered it about 50 years ago. In a process called “cavitation,” water flowing fast and in large volumes can rumble over small cracks, bumps or other imperfections in concrete dam spillways as they release water during wet years. The billions of gallons of water bumping off the surface at 50 miles an hour create enormous turbulence that can form tiny water vapor bubbles that collapse with powerful force, and like jackhammers, chisel apart concrete.
“It starts with small holes, but it can break off big chunks of concrete,” said Paul Tullis, a professor emeritus of civil engineering at Utah State University and cavitation expert. “It’s like a big grinder. It causes concrete to be torn apart.”
(Bold added)
The 180,000 people urgently evacuated last winter in case Lake Oroville flooded downstream are again asked to trust DWR and FERC this winter. It was these agencies who said past spillway maintenance and design issues brought up during the relicensing process were not important. They ruled against any action to improve the Emergency Spillway, an unreinforced hillside that, when used in February, resulted in the flood threat and evacuation.
A FERC civil engineer wrote in a July 27, 2006, memo: “The emergency spillway meets FERC’s engineering guidelines for an emergency spillway. The guidelines specify that during a rare flood event, it is acceptable for the emergency spillway to sustain significant damage.”
Less than eleven years later, in February 2017, “a rare flood event” happened and the emergency spillway sustained significant damage that was not acceptable. The eroded slope came close to collapsing and releasing the reservoir’s water.
Now, DWR claims construction met the objectives for this season’s work and the spillway can handle 100,000 cubic feet per second flows this winter. FERC concurs. As of Tuesday evening, the reservoir held almost 702 feet of water (full capacity is 900 feet and after that water flows over the emergency spillway). Inflow was 5,978 cfs and outflow was 4,513 cfs. The rainy season has just begun. Last year at this time, the lake level was 729 feet.
If the new spillway is unable to handle sufficient flows, DWR’s ability to release water and keep the lake level below the emergency spillway is limited to releases through the power plant at the base of the dam. The 3,607 square mile watershed extends over the Sierra crest and includes areas that often receive precipitation as snow during winter. One of the reasons for last February’s high water level was an unusually warm rain that didn’t fall as snow in the upper watershed. Rain also melted some of the existing snow, increasing inflows above what outflows could handle. During this storm, the lake level was too high to cope with the inflows because primary spillway releases were shut down so DWR could inspect the damaged chute. Releases from the power plant also were curtailed because the eroded spillway material blocked the channel at the base of the spillway and trapped deep water against the dam base.
Once again this rainy season, DWR and FERC ask the public to trust their decisions and believe that the repaired spillway will hold up to flows. Cracks in the new concrete were “anticipated” but are “not a concern” and “not expected” to affect spillway integrity. But 180,000 people evacuated last winter will spend rainy season scrutinizing Lake Oroville’s water level and spillway. There’s more cracked than the new construction. Fractured trust is difficult to repair.