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Insight | Inside Oroville Dam spillway design woes

Oroville main spillway damage 3x2

Oroville Dam’s failed main spillway was designed by a graduate engineer with no prior experience, knowingly built on weak rock and hit by cost pressures during construction, a critical report has revealed.

The claims are made in the Independent Forensic Team’s (IFT) report on the crisis which hit the tallest dam in the US in February last year. The report blames systematic failures within the California Department for Water Resources (DWR) and the US dam industry at large and highlights a raft of poor design and construction practices. It adds that no one individual can be blamed for the failure.

The IFT, which was formed by the US Association of State Dam Safety Officials and United States Society of Dams and comprised a team of world-renowned experts in dams and spillways, said its findings should act as “a wake-up call” for the US dam industry.

“Challenging current assumptions on what constitutes ‘best practice’ in our industry is long overdue,” its says in the report.

Last February heavy rainfall led to erosion in the dam’s main spillway, a concrete chute designed to take away excess water. Flows were temporarily halted while engineers assessed the damage before water was slowly reintroduced.

However, water levels had crept up in the dam and the emergency spillway, a channel cut into the hillside, came into operation for the first time in the dam’s 48-year history. Erosion soon occurred at the head of the emergency spillway and resulted in more than 180,000 residents being evacuated.

British Dam Society chair Tracey Williamson said the DWR should be commended for its report, and the lessons should be shared across the global dam industry.

She added: “It is vital that high risk dams in the UK, which have an average age in excess of 100 years, are inspected, upgraded and maintained to current best practice standards.”

An interim report published in April last year revealed that poor design and construction of the 1968 spillway was partly to blame for its failure.

Oroville

Oroville Dam crisis

Now the IFT report reveals how the principal spillway designer was a post-graduate engineer with no prior experience designing spillways, whose work was not overseen by a more experienced DWR engineer.

In addition, a lack of communication between the designer and geologists meant that original design and construction plans assumed better quality rock than existed in reality. As a result, all subsequent reviews assumed the spillway rested on good foundations and erosion of the spillway was not anticipated.

Even by the standards of the day the design was sub-standard, according to the report. The IFT found that “considering the height and slope of the Oroville Dam service spillway chute and the resulting hydraulic conditions, it would have been reasonable to expect the design to reflect the best practices of the day, which it did not.”

Standards appear to have been relaxed during construction of the second part of the spillway, making the situation worse. According to the design drawings, the chute slab was supposed to be built on moderately weathered rock which had been well prepared to best support the concrete slab and its anchors.

However, the report claims the foundation requirements “were substantially relaxed during construction”, which meant parts of the chute rested on pockets of “soil-like material”. The initial erosion hole on the spillway occurred at a point with a weak rock foundation.

During construction there was also poor communication between the designer and construction team. “There appears to have been little, if any, communication with the spillway designer(s) during construction,” says the IFT in the report.

Changes to the design were made during construction without approaching the design team. It meant the diameter of drainage pipes running beneath the slab was changed from 100mm to 150mm, which lead to a thinner layer of concrete being poured introducing a greater risk of cracking across the slab.

Cracks were first sighted in 1969 soon after the spillway opened, but these were soon assumed to be “normal” and were repaired over the years, which the report claims could have made the situation worse.

Recently hairline cracks have appeared in the repairs to the spillway chute, although the reason for these is not known. In November DWR chief engineer Jeanne Kuttle told New Civil Engineer that all repairs to the main spillway will be finished by April 2019.

Kg oroville repair 16066

Oroville Dam main spillway repairs

The IFT believes cost and time pressures were also a big issue for the project, evidenced by the fact that the bid for spillway construction was 10% below the engineer’s original estimate.

“It would not be atypical if the engineer’s estimate served as an ‘anchor’ relative to which the actual construction cost was judged, thus creating pressure to control escalation of costs to within the engineer’s estimate, and substantially influencing decision-making during construction,” says the report.

HR Wallingford principal dams engineer Craig Goff said the report provided “food for thought” for the UK dam safety community.

“It is sobering that ‘inadequate priority for dam safety’ within the organisation (DWR) was one contributory factor identified in this incident,” he said, adding there will be discussion over the coming months within the dam industry about lessons to be learned.

Severn Trent Water dams and reservoir manager Ian Hope added: “It is interesting to note that this was not caused by one extreme event which we conventionally design and construct for, but an ‘unlikely combination of likely events’ which has challenged our thinking as an industry in more recent years.” 

The DWR has agreed with the IFT’s recommendation that all dam owners need to review current practices.

“We strongly supported having an independent assessment of the spillway failure and take the findings very seriously,” said DWR director Grant Davis.

Key quotes from the report

  • “The Oroville Dam spillway incident was caused by a long-term systemic failure of the California Department of Water Resources (DWR), regulatory, and general industry practices to recognize and address inherent spillway design and construction weaknesses, poor bedrock quality, and deteriorated service spillway chute conditions.”
  • “While the Oroville chute slab design was within the range of design practices for rock foundations – as apparently assumed by the principal spillway designer – it would not meet the typical practice of the day for spillway chutes on highly weathered or soil-like foundations.”
  • “It appears that the DWR construction team was making decisions regarding chute design and construction without any significant consultation with the principal spillway designer.”
  • “Like many other large dam owners, DWR has been somewhat overconfident and complacent regarding the integrity of its civil infrastructure and has tended to emphasize shorter-term operational considerations. Combined with cost pressures, this resulted in strained internal relationships and inadequate priority for dam safety.”

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