A preliminary root cause analysis of the failures of the Oroville Dam gated spillways says their origins are "deeply rooted in pervasive design defects and flaws" developed by the California Department of Water Resources (DWR).
The analysis was done by Robert Bea, a retired civil engineering professor for the Center for Catastrophic Risk Analysis at University of California at Berkeley.
The analysis says these design defects and flaws included the following:
1. Spillway base slabs of insufficient thickness for the design hydraulic conditions: 4 to 6 inches thick at minimum points;
2. Spillway base slabs not joined with "continuous" steel reinforcement to prevent lateral and vertical separations;
3. Spillway base slabs designed without effective water stop barriers embedded in both sides of joints to prevent water intrusion under the base slabs;
4. Spillway base slabs not designed with two layers of continuous steel reinforcement (top and bottom) to provide sufficient "flexural strength" required for operating conditions; and
5. Spillway base slabs designed with ineffective "ground" anchors to prevent significant lateral and vertical movements.
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Construction Defects and Flaws
The report says that the design defects and flaws were propagated by DWR during construction of the spillway in the 1960s. Bea says these construction defects and flaws included the following:
1. Failure to excavate the native soils and incompetent rock overlying the competent rock foundation assumed as a basic condition during the spillway design phase, and fill the voids with concrete, and
2. Failure to prevent spreading gravel used as part of the under-slab drainage systems and "native" soils to form extensive "blankets" of permeable materials in which water could collect and erode.
(Read "Highest Dam in the U.S. Faces Threat.")
Maintenance Defects and Flaws
The design and construction defects and flaws were propagated by DWR during maintenance of the spillway, Bea says in the report. These maintenance defects and flaws included the following:
1. Repeated ineffective repairs made to cracks and joint displacements to prevent water stagnation and cavitation pressure intrusion under the base slabs with subsequent erosion of the spillway subgrade; and
2. Allowing large trees to grow adjacent to the spillway walls whose roots could intrude below the base slabs and into the subgrade drainage pipes resulting in reduced flow and plugging of the drainage pipes.
By the time of the February 2017 spillway releases, Bea says that the gated spillway had become heavily undermined and the subgrade eroded by previous flood releases. The first spillway release completed the undermining of the spillway slabs, allowing water cavitation and stagnation pressures to lift the "weak" slabs and break them into pieces .
After the almost catastrophic water release over the un-surfaced auxiliary spillway, the subsequent water releases down the gated spillway propagated the initial spillway breach until spillway releases ceased, the report says.
Root Causes Analysis
Currently available information indicates the Root Causes of the gated spillway failures are founded primarily in "Extrinsic" uncertainties (human and organizational task performance and knowledge development and utilization) developed and propagated by DWR during the gated spillway design, construction, and maintenance activities.
The Los Angeles Times says that Bea is a member of the National Academy of Engineering, and was a pioneer in engineering risk analysis during his days as a Shell Oil Co. vice president, overseeing offshore platforms in the Gulf of Mexico. He later helped conduct independent investigations into the New Orleans levee system that failed during Hurricane Katrina in 2005.
Bea says that a key question that cannot be answered at this time is “why did DWR and the responsible State and Federal regulatory agencies (California Water Commission, Federal Energy Regulatory Commission) allow these Root Causes to develop and persist during the almost 50 year life of the gated spillway?”
He says that one answer that has been offered is that the spillway was designed and constructed according to the standards of the time. "While that answer may or may not be factual or true, current evidence indicates the original spillway design and construction does not meet current guidelines and standards," he says in the report.
Another answer that Bea says has been offered is that the spillway operated for almost 50 years and was subjected to water discharges that exceeded those that occurred during 2017 without failure.
Recent inspections indicated that the spillway was in "satisfactory condition," he says. The conclusion prior to the February 2017 discharges was that the gated spillway consequently was "suitable for service."
The experience prior to the DWR attempt on February 11 to use the emergency spillway indicated that conclusion "was not valid," Bea says. He says the gated spillway failed during discharges that were much less than the design conditions.
Bea says that based on his previous experiences with investigations of failures of public infrastructure systems leads to a conclusion that it is "likely that the wrong standards and guidelines are being used to requalify many critical infrastructure systems for continued service."
He says that the "majority of these standards and guidelines were originally intended for design, not re-qualification or reassessment of existing aged infrastructure systems that have experienced ‘aging,’ ‘technological obsolesce,’ and increased risk (likelihoods and consequences of major failures) effects."
Inappropriate standards and guidelines are being used to re-qualify these infrastructure systems for continued service. The currently available information indicates this is one of the primary Root Causes of the failures of the Orville Dam gated spillway.