On Sunday, February 12, 2017 residents of Oroville, California and nearby towns were ordered to immediately evacuate as the giant Oroville Dam emergency spillway began to crumble and the towns were threatened with flooding. The Oroville Dam is the tallest dam in the United States at 770 feet. The Oroville Dam and complex is part of the California State Water Project (SWP), the largest state-owned water storage and delivery system in the United States. The SWP is owned and operated by the California Department of Water Resources (DWR), which is part of the California Natural Resources Agency. The SWP provides water to about 25 million people and 750,000 acres of irrigated farmland in California. The design and construction of most of the SWP was completed in the 1960s and early 1970s. The Oroville Dam project was completed in 1968.
After years of drought, when the spillway had not been used, in January and February 2017, the spillway had its first significant discharges since 2011. From February 1, 2017 through the morning of February 3, 2017, service spillway discharges were generally about 15,000 cubic feet per second; then discharges were increased to about 25,000 cubic feet per second and maintained at that level until mid-day on February 6, 2017, at which time the release flow increased to between 42,000 and 45,000 cubic feet per second and held in that range until the morning of February 7.
As water flow from the winter storms increased, release flow was increased to and reached about 52,500 cubic feet per second on February 7th. Workers noticed significant disturbance in the service spillway chute flow and on-site Department of Water Resources (DWR) workers contacted DWR headquarters in Sacramento. An order to close the spillway gates was issued that same day. After the gates were closed, it was found that a significant section of the service spillway chute slab was missing, and as you can see below the damage to the spillway was very serious and had happened over a short period of time.
The flow that destroyed the spillway was a small fraction of the 300,000 cubic feet per second that the spillway was designed to handle. The chute failure happened in 2017 at a spillway discharge of about 52,500 cubic feet per second. The spillway chute had not failed previously during higher discharges, most recently a discharge in excess of 70,000 cubic feet per second in 2006, and historically discharges up to as much as about 160,000 cubic feet per second in 1997.
Back to last February, once damage to the spillway began, dam operators closed the gates and allowed the reservoir's water level to rise up to a concrete lip, known as a weir, on an emergency spillway. Responding to the damage to the service spillway chute, workers allowed the reservoir level to rise above the emergency spillway without a full understanding of relative uncertainties and consequences. It was the first time in the dam's history that the emergency spillway was used, and it quickly eroded the bare hillside under the weir and endangered the communities downstream that lead to the evacuation order.
After the Oroville Dam spillway incident in February 2017, the Federal Energy Regulatory Commission (FERC) required the California Department of Water Resources (DWR) to engage an Independent Forensic Team (IFT) to investigate the incident. Their report was released this month and casts a shadow over the integrity of the Oroville complex and possibly more of the California State Water Project infrastructure.
The forensic investigation found that water that penetrated through cracks and joints that lifted entire sections of the spillway and eroded the underlying soft rock. Also playing a role were thin concrete, weak anchors into the underlying rock and corroded steel reinforcement.
Although the poor foundation conditions at both spillways were well documented in the original geology reports, these conditions were not properly addressed in the original design and construction. The forensics team found that one of the key Oroville spillway designers was hired as a postgraduate with no engineering experience in spillways. After construction 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.
Not long after the spillway was completed in the 1960s, cracks began developing that indicated problems, but the department soon considered those to be normal and never questioned the original design or construction. The report found that periodic inspections over the 50 year history of the spillway failed to identify the original design flaws and the subsequent deterioration of the spillway's integrity.
The seriousness of the inferior construction and lack of repair durability was not recognized. Over time, chute flows and temperature variations led to progressive deterioration of the concrete and corrosion of steel reinforcing bars and anchors. Indications of deterioration were ignored. The forensics team noted that a comprehensive review of the original construction and design and whether it meets modern standards should have been conducted long ago. Such a review is warranted for the entire Oroville Dam, since the engineering shortcomings in the spillway could be present in the earthen embankment as well.
The forensic investigation found that water that penetrated through cracks and joints that lifted entire sections of the spillway and eroded the underlying soft rock. Also playing a role were thin concrete, weak anchors into the underlying rock and corroded steel reinforcement.
Although the poor foundation conditions at both spillways were well documented in the original geology reports, these conditions were not properly addressed in the original design and construction. The forensics team found that one of the key Oroville spillway designers was hired as a postgraduate with no engineering experience in spillways. After construction 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.
Not long after the spillway was completed in the 1960s, cracks began developing that indicated problems, but the department soon considered those to be normal and never questioned the original design or construction. The report found that periodic inspections over the 50 year history of the spillway failed to identify the original design flaws and the subsequent deterioration of the spillway's integrity.
The seriousness of the inferior construction and lack of repair durability was not recognized. Over time, chute flows and temperature variations led to progressive deterioration of the concrete and corrosion of steel reinforcing bars and anchors. Indications of deterioration were ignored. The forensics team noted that a comprehensive review of the original construction and design and whether it meets modern standards should have been conducted long ago. Such a review is warranted for the entire Oroville Dam, since the engineering shortcomings in the spillway could be present in the earthen embankment as well.
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