Failed Oroville Dam Spillway designed by inexperienced grad student in the 1960s

California’s Department of Water Resources was blasted in an independent report for having a culture of complacency and incompetence that contributed to last year’s near-disaster at Oroville Dam.

The full 584 page independent forensic team report is here.

The agency’s largest water storage site and the nation’s tallest dam at Lake Oroville fell into disrepair. In February, pounding rain and large water releases caused the reservoir’s spillway to collapse. A back-up spillway also failed. Fears that water would pour uncontrollably downstream prompted the evacuation of 180,000 people.

The independent panel of safety experts said the dam was badly built from the start in the 1960s. The principal designer of the spillway told the dam-safety team that he had just completed post-graduate work at the time he worked on the Oroville project decades ago, had had no previous engineering employment beyond two summer stints, and had never designed a spillway before.

Fifty years of incompetence and complancency

The seriousness of the weak as-constructed conditions and lack of repair durability was not recognized during numerous inspections and review processes over the almost 50-year history of the project. Over time, chute flows and temperature variations led to progressive deterioration of the concrete and corrosion of steel reinforcing bars and anchors, with likely loss of slab strength and anchor capacity. There was likely also some shallow underslab erosion and some loss of underdrain system effectiveness, which contributed to increased slab uplift forces. The particularly poor foundation conditions at the initial service spillway chute failure location contributed to likely low anchor capacity and shallow underslab erosion.

Due to the unrecognized inherent vulnerability of the design and as-constructed conditions and the chute slab deterioration, the spillway chute slab failure, although inevitable, was unexpected. Once the initial section of the chute slab was uplifted, the underlying poor quality foundation materials were directly exposed to high velocity flows and were quickly eroded. Undermining and
uplift of other portions of the chute slab resulted in further removal of slab sections and more foundation erosion.

Although the poor foundation conditions at both spillways were well documented in geology reports, these conditions were not properly addressed in the original design and construction, and all subsequent reviews mischaracterized the foundation as good quality rock. As a result, the significant erosion of the service spillway foundation was also not anticipated. Following the unexpected chute slab failure and erosion, and subsequent closure of the service spillway gates to examine the damage, delicate and difficult risk tradeoffs, involving myriad considerations, were necessary over the next few days in order to manage the incident.

There were many opportunities to intervene and prevent the incident, but the overall system of interconnected factors operated in a way that these opportunities were missed. Numerous human, organizational, and industry factors led to the physical factors not being recognized and properly addressed, and to the decision-making during the incident. The following are some of the key factors which are specific to DWR:
• The dam safety culture and program within DWR, although maturing rapidly and on the right path, was still relatively immature at the time of the incident and has been too reliant on regulators and the regulatory process.
• 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.
• DWR has been a somewhat insular organization, which inhibited accessing industry knowledge and developing needed technical expertise.
• DWR’s ability to build the appropriate size, composition, and expertise of its technical staff involved in dam engineering and safety has been limited by bureaucratic constraints.