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Learn about events that led to the Oroville Dam spillway damage and remediation efforts

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Seven months have elapsed since spillway erosion occurred at Oroville Dam in the U.S. Learn about the events that led to the damage, how California DWR responded, and remediation efforts under way.

When the evacuation order for residents below Oroville Dam was lifted on Feb. 14, 2017, more than 100,000 people were safe to return to their homes. And personnel with the California Department of Water Resources, and the agencies DWR was working with, were able to breathe a sigh of relief that a potential disaster had been averted.

However, the sense of relief would be short-lived, as California DWR was already in the midst of substantial work to remediate the problems at the site, ensure the continued safe operation of the facility and determine a future course of action to get the gated spillway and 762-MW Hyatt Powerplant operating reliably as quickly as possible. In the long term, California DWR is working with agencies and a team of experts to determine what went wrong during that fateful period seven months ago.

Timeline of events

On Feb. 2, the water content of the northern Sierra snowpack was 144% of the multi-decade average for the date. The central Sierra reading was 174% of average and southern Sierra was 200% of average. In addition, during the first four months of Water Year 2017 (Oct. 1, 2016, to Feb. 2, 2017), total precipitation passed the average annual amount on Jan. 21, just 112 days into the water year. Lake Oroville was holding 121% of its historical average, compared with just 68% at the same time in 2016.

On Feb. 7, water releases from the gated spillway ramped up to 52,250 cubic feet per second (cfs) in anticipation of inflow expected from the largest forecast storm of the winter, and DWR employees noticed an unusual flow pattern. Spillway outflow was stopped for inspection, and engineers found a large area of concrete chute and foundation erosion, too significant to repair. DWR began ongoing consultation with the Federal Energy Regulatory Commission and California Division of Safety of Dams (DSOD) to consider the steps forward.

On Feb. 8, after a decision with FERC and DSOD, DWR ran short-duration 20,000 cfs flows down the spillway and then re-inspected, to find the initial damaged area had essentially doubled in size. As a contingency, crews from Cal Fire and DWR began preparing for possible use of the emergency spillway by clearing the hillside of trees, rock and debris, as well as grouting and shotcreting some areas. DWR activated four 24/7 emergency interagency operations centers to study and implement responses to the flood control spillway and related structures, monitor weather forecasts and coordinate operations. As gated spillway releases were resumed and initially ramped above 20,000 cfs, significant erosion occurred headward and downslope toward the Thermalito Diversion Pool (DP). DWR, FERC and DSOD engineers and geologists were uncertain how far upslope the erosion would go, which was the primary concern to public safety as the gated spillway would have to be shut down if erosion got too close to the gate control concrete superstructure on the right abutment. In this scenario, Oroville Dam and Lake Oroville would have become run-of-river over the emergency spillway, which was not a situation anyone wanted in the midst of the wettest year in 112 years of Feather River hydrologic record.

On Feb. 9, DWR ramped up releases to 35,000 cfs and then 45,000 cfs. Releases were being balanced to be just high enough to avoid spilling over the emergency spillway while using all of the available flood reservation space in Lake Oroville. (DWR had its full flood reservation space of 750,000 acre-feet available at the onset of this storm.) By 8 pm, the forecast peak of 2017’s largest storm had increased more than 25% from the forecast just six hours earlier. DWR, FERC and DSOD agreed to higher gated spillway releases, and outflow from the Hyatt Powerplant was halted because debris below the damaged spillway caused water to back up into the DP, precluding safe plant operation due to high tailrace water surface elevations. An additional concern emerged as the upslope erosion along the gated spillway was within about 100 feet of Tower 4 of PG&E’s Table Mountain 230-kV transmission line, which serves Hyatt.

On Feb. 10, releases down the damaged spillway were increased to 55,000 cfs after midnight and then to 65,000 cfs. This operation was intended to reduce the likelihood of using the emergency spillway while also using all of the available Lake Oroville storage space below the emergency spillway crest. This precise operation balanced two risks: potential spill over the emergency spillway and the risk of severely damaging all controlled outlet capabilities of the dam with higher gated spillway releases (through the spillway chute and Hyatt Powerplant). The risk associated with both was perceived to be roughly equal. As a safety contingency, emergency spillway preparations continued. In addition, as tailrace water elevations reached never-before-seen levels, DWR crews shifted work at Hyatt to emergency flood fighting preparations. This was critically important as flooding would have rendered the Hyatt plant inoperable for a year or more, significantly limiting DWR’s ability to manage lake elevations and meet all beneficial water use releases from Lake Oroville. The trailing edge of the storm stalled over the Feather River watershed, and by late in the day it was apparent water would flow over the emergency spillway for the first time in the dam’s operational history.

Oroville Dam (far right) is equipped with a main gated spillway (center) and an emergency overflow spillway (at left).
Oroville Dam (far right) is equipped with a main gated spillway (center) and an emergency overflow spillway (at left).

On Feb. 11, as the runoff from the largest storm in 20 years (which would total more than 1 million acre-feet since Feb. 6) slowly receded, Lake Oroville’s level rose above 901 feet. At about 8 am, water flowed over the lip of the 1,700-foot-long concrete weir, engaging the emergency spillway. Engineers and geologists on the top of Oroville Dam and in other safe vantage points monitored the emergency spillway and flows down the hillside to the DP. Drones also were used for monitoring.

On Feb. 12, in the mid-afternoon, anticipated erosion began progressing faster than previous observed rates near the top of the slope beneath a portion of the emergency spillway crest. The Butte County Sheriff’s Office conferred with DWR, Cal Fire, FERC and DSOD on the risk this erosion posed to a portion of the emergency spillway crest and issued mandatory evacuation orders for the Oroville area, as did cities in Yuba and Sutter counties. To more quickly cease flow over the emergency spillway, the gated spillway outflow was increased to 100,000 cfs. The lake level dropped below 901 feet at about 8 pm, and flows over the emergency spillway stopped. Erosion to the emergency spillway hillside was assessed using aerial and direct inspection.

On Feb. 13, DWR crews began working around the clock to fortify the emergency spillway hillside slope. A temporary flight restriction was enacted to allow crews to survey and conduct their work, including the use of helicopters to bring in rock for revetment. Acting DWR Director William Croyle announced his goal to reduce the lake level to 850 feet or lower.

On Feb. 14, as the lake level continued to drop, the mandatory evacuation order was reduced to an evacuation advisory warning. Crews continued working around the clock to repair the emergency spillway, placing 1,200 tons of material per hour in the erosion gullies below the crest. Sustained 100,000 cfs releases from the gated spillway added significantly to the debris mass in the DP and raised the tailrace elevation to even further record high levels. Flood fighting inside the plant increased, with extraordinary measures including sandbagging, grouting, pumps and even more personnel.

On Feb. 15, the lake level continued to drop as DWR worked day and night to reinforce the emergency spillway. State and federal engineers performed continuous monitoring of the construction area, dam, spillways and other structures. Barges and cranes were stationed to remove debris and sediment from the DP.

On Feb. 16, outflow from the flood control spillway was reduced to 80,000 cfs to facilitate clearing of debris, but the effectiveness of excavating from accessible DP shore areas was limited. Extraordinary flood fighting measures continued inside the Hyatt Powerplant, and the tailrace elevation reached a record 256 feet, or 4 feet above the level at which the plant would otherwise be flooded.

Understanding the context

The State Water Project, or SWP, is a system of 32 storage facilities, 21 pumping plants, four pumping-generating plants, eight conventional hydroelectric plants and about 700 miles of canals and pipelines. Its main purpose is to store water and distribute it to 29 urban and agricultural water suppliers in North California, the San Francisco Bay area, the San Joaquin Valley, the Central Coast and Southern California, and its multiple purposes also include flood control, fish and wildlife preservation and enhancement, and recreation. The SWP makes deliveries to two-thirds of California’s population and irrigates more than 750,000 acres of highly productive agricultural lands. The SWP was designed and constructed, as well as maintained and operated, by DWR.

Oroville Dam and Lake Oroville lie in the foothills on the western slope of the Sierra Nevada mountain range, 1 mile downstream from the junction of the Feather River’s main tributaries. The lake stores winter and spring runoff, which is released into the Feather River to meet local agricultural senior water rights, Feather River in-stream flows and temperature for fish and their habitat, water quality requirements in the Sacramento-San Joaquin Delta, and SWP water supply deliveries. It also provides pumped-storage capacity, 750,000 acre-feet of flood control storage, and recreation, with the Lake Oroville State Recreation Area operated by the California Department of Parks and Recreation as DWR’s SWP recreation management and operating partner.

The main spillway suffered erosion damage and was quickly removed from service.
The main spillway suffered erosion damage and was quickly removed from service.

Work on the site began in 1961 and the embankment was topped out in 1967. At 770 feet, Oroville is the tallest dam in the U.S. and is a zoned earthfill with an embankment volume of 80 million cubic yards. The dam has a crest length of 6,920 feet and crest elevation of 922 feet. The dam has a concrete core gravity structure that is more than 150 feet high. Maximum operating storage of Lake Oroville is 3,537,580 acre-feet with a water surface elevation of 900 feet at maximum operating storage, comprising about 16,000 acres at full pool.

The gated spillway is located on the right abutment (looking downstream) and discharges into the Thermalito Diversion Pool (DP) regulating reservoir. The emergency spillway to the right (north) of the gated spillway also discharges to the DP. The Hyatt powerhouse, in rock in the left abutment of the dam, discharges to the DP via two 35-foot-diameter tunnels.

Behind the scenes

Calling Oroville Dam the “Perfect Storm” incident of the dams community may not be too far off the mark. The hydrologic record surrounding the double spillway damage was entirely unprecedented. Annual unimpaired runoff from the Feather River exceeded every year in the 112-year hydrologic record of the river’s watershed, reaching more than 10 million acre-feet. The next closest year on record was 1907 at about 9.5 million acre-feet.

In the period from Jan. 1, 2017, through Feb. 25, 2017, Lake Oroville received more than 4.5 million acre-feet of runoff, which is equal to the long-term annual average for the 3,600-square-mile Feather River watershed. The previous record runoff for any two-month period was 3.5 million acre-feet. The 15-day hydrograph flood volume, which included the Feb. 6 to 9 storm, also exceeds any in the Feather River history, including those of February 1986 and January 1997, which are the two largest single storms in Feather River history. The watershed’s antecedent wetness condition for the February 2017 storms was two to four times greater than that of the 1986 and 1997 storms.

By the time the radial gates were closed on May 19, for the summer and to commence the construction to repair the gated spillway concrete chute, DWR’s impaired gated spillway had discharged 5.2 million acre-feet of floodwater without loss of life or significant property damage. So, two-thirds of the spillway originally constructed released more than 2.5 times the volume in any of the previous 49 years of operation of the entire spillway and an astonishing 20% of all flood releases from Oroville Dam in its history.

In addition, as of mid-May, water year 2017 was the wettest year in the 97-year history of the Northern Sierra 8 Station Index, with annual precipitation, totaling 92 inches, at 209% of the average for that date. In this 97-year record, only five years have exceeded 80 inches in annual precipitation and 2017 is now the first to exceed 90 inches.

Another key indicator of wetness and storm or flood intensity is the number of Atmospheric River Storms (ARS). ARSs typically have greater precipitation totals than other storms hitting California from the Pacific Ocean owing to their tropical component that greatly increases atmospheric water vapor content. The 2016 to 2017 water year had 49 ARSs reaching land in California, while the previous annual record was 28, with the long-term average at 12. In addition, of these three were ranked as extreme and two occurred back to back in January and February, also a first. Between 1980 and 2015, California had only received four extreme ARSs, including the aforementioned February 1986 and January 1997, so to receive three of these intense storms in one winter and two in essentially 30 days from early January to early February 2017 punctuates the extreme nature of this past winter, particularly in northern California.

Allowing water to flow down the emergency spillway caused erosion damage to the slope.
Allowing water to flow down the emergency spillway caused erosion damage to the slope.

It is worth noting that while more than 180,000 people were evacuating their homes and businesses, personnel with DWR, Cal Fire, Butte County Sheriff, FERC and other emergency responders were staying at the site, with more personnel coming to the site. Many of these dedicated public servants live downstream of the dam and left their families to handle the evacuation while they helped deal with the emergency situation. Additionally, through it all, the DWR communications personnel were working overtime to keep a steady stream of information available to concerned onlookers, including the media.

One immediate need was to protect electrical equipment in the Hyatt powerhouse. A massive amount of debris from the eroding gated spillway and hillside clogged the channel downstream from the powerhouse. This caused a significant rise in tailwater level, to levels never before seen in the 50 years of Oroville Dam operation, and the Hyatt powerhouse was under intense flood risk for 17 consecutive days from Feb. 9 to Feb. 27, when DWR was able to shut down the gated spillway to clear the DP channel. The powerhouse is five stories tall and the floodwater would have entered on the top level through the turbine draft tube stop log wells, and the muddy water would have cascaded down through all plant levels, thereby rendering it likely a total loss. There are six Francis units, three traditional turbine-generator and three pump-generating units.

DWR personnel put a concerted effort into keeping water out of the electrical equipment. They used sandbags and grouting to keep it at bay, as well as sealing metal plates over the top of the turbine draft tube stop log wells. For the entire 17-day period, the DP flood water was 2 to 4 feet above the plant’s top level turbine deck, but the extraordinary flood fighting efforts kept all the vital plant equipment dry.

A non-functioning Hyatt Powerplant would have been an environmental and economic calamity for northern California and the entire state. DWR would have been solely dependent on the low-level dam outlet once Lake Oroville falls below the gated spillway sill at elevation 812 feet, which would have significantly restricted releases for all of the SWP purposes.

Work to clear the DP debris fan commenced in earnest on Feb. 27 once the gated spillway releases were temporarily halted and was a massive excavation effort, with 36 land- and barge-based excavators working. The channel below the dam is 75 feet deep and about 200 feet wide and was nearly completely filled with rock, dirt and concrete. On May 19, crews had removed 1.7 million cubic yards of debris from the channel, with more than 1 million yards being excavated in the first eight days of this effort. Hyatt Powerplant was returned to operation on March 5. Most of the rock and concrete debris recovered has been moved to one of four spoil areas and is being used in concrete production for the spillway repair.

Planning for recovery

Per the requirements of its operating license for the Oroville facility, DWR almost immediately began working with FERC. The California Water Code requires a board of consultants (BOC) for modifications to any dam DWR owns, and FERC requires a BOC to review and comment on repairs to dams. On Feb. 14, FERC Acting Chairman Cheryl A. LaFleur issued a statement saying, “The Commission has a team on site at the Oroville Dam spillway in California and is closely monitoring the situation. Our top priority is the safety of residents and property downstream from the dam.” DWR named the BOC on Feb. 18 and FERC approved the board on Feb. 21.

A massive quantity of debris washed into the Feather River below the dam, requiring extensive excavation.
A massive quantity of debris washed into the Feather River below the dam, requiring extensive excavation.

FERC also tasked DWR to convene a forensic team to be approved by FERC to determine the root and contributory factors that led to this past winter’s damage to the two Oroville Dam spillways and the ensuing emergency. This team has been working to ascertain the causational factors. In addition, the BOC has produced several technical reports on the original gated spillway design and construction, as well as the design of the replacement spillways.

DWR’s objective for recovery at Oroville Dam is to get systems in place by Nov. 1 - the date by which DWR’s U.S. Army Corps of Engineers-governed flood manual requires flood storage space be available behind the dam. To accomplish this, DWR must return the gated flood control and emergency spillways to operational readiness. In April, DWR awarded a $275 million contract to Kiewit Infrastructure West to perform repairs at Oroville Dam. The complete recovery or replacement of both spillways will require work in the summer of 2018 as well, and it is anticipated the gated spillway will have a restricted discharge capability during the winter of 2017 to 2018, along with an increased flood reservation that is yet to be determined.

Check HydroWorld.com for continuing updates on Oroville Dam.


Elizabeth Ingram is managing editor of Hydro Review.

Implications for dam safety

The spillway erosion incident at Oroville Dam has had a ripple effect on the attitude toward dam safety, both within the state of California and at the federal level in the U.S.

Very shortly after the event, on Feb. 24, California Governor Edmund G. Brown announced a series of immediate and longer-term actions to bolster dam safety, improve flood protection, and fix the state’s aging transportation and water infrastructure. This included a plan to invest $437 million in near-term flood control and emergency response actions, as well as require emergency action plans and flood inundation maps for all dams and enhance the state’s existing dam inspection program.

Since the incident, several other states have announced plans designed to help ensure the safety of their dams. For example, in early April Oregon introduced a bill requiring owners or operators of dams with a “high hazard” rating to develop an emergency action plan, including determining the frequency for conducting emergency response exercises. In early May, Maryland Governor Larry Hogan signed a bill into law that requires all dam owners to upgrade their facility’s emergency action plans by Aug. 1. The state has 81 high-hazard and 114 significant-hazard dams.

And, back in California, in mid-June the California Division of Safety of Dams requested that dam owners in the state assess dam appurtenant structure, including spillways, to confirm they meet minimum safety standards. This is part of the Spillway Re-Evaluation Program, which was established in 2017.

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