Oroville Dam crisis

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Water flows over the emergency spillway eroding the hillside on February 12, 2017. This erosion threatened to cause the catastrophic failure of the emergency spillway.

The Oroville Dam Crisis was an event in February 2017 which forced the evacuation of 188,000 people[1] in the Feather River Basin of northern California. The first sign of trouble came when the main spillway of the Oroville Dam developed a crater in its concrete deck[2] on February 7, 2017. The spillway was shutdown for inspection,[3] while Lake Oroville continued to swell with rain and snowmelt from the Sierra Nevada mountains. On February 11, 2017, the reservoir reached 901 feet (275 m) above mean sea level (MSL), sending water over the emergency spillway for the first time ever.[4] After water managers repeatedly assured residents that there was no danger, suddenly they began to worry that the emergency spillway structure could fail due to the erosion of the ground below it. At 4:42 PM on February 12, 2017, the California Department of Water Resources tweeted that the emergency spillway was expected to fail within the hour.[5]Residents were given one hour to clear the evacuation zone, but many were stuck for hours in traffic jams.[6] After water managers managed to lower Lake Oroville's level, the evacuees were allowed back in their homes on February 14, 2017, but officials warned that future evacuations were still possible.[7]

Background

Oroville Dam, Hyatt Powerplant, and main spillway. The structure immediately left of the main spillway head gates is the emergency spillway.

The Oroville-Thermalito Complex consists of several interrelated structures along the Feather River in the Oroville, California area. The Oroville Dam itself is an earthen embankment dam which rises 770 feet (230 m) from its base,[8] making it the tallest earthen dam in the United States.[9] Its reservoir, known as Lake Oroville, is the largest reservoir in the California State Water Project with a capacity of 3.5 million acre-feet (4.3×109 m3)[8] of water storage. Downstream is the Thermalito Diversion Dam, which directs outflow from Oroville Dam into either the Feather River or the Thermalito Power Canal. The portion of the Feather River between the two dams is called the Thermalito Diversion Pool. The Thermalito Power Canal leads to the Thermalito Forebay, which in turn leads to the Thermalito Afterbay, which can feed water not needed for irrigation back into the Feather River via the Thermalito Afterbay River Outlet.

Water can be released from Lake Oroville in four ways:

  1. Through the Hyatt Powerplant.
  2. Through the river valve outlet system.
  3. Through the main spillway.
  4. Through the emergency spillway.

The Edward Hyatt Powerplant has three conventional turbines and three pump-generators which can either generate electricity or pump water back up into Lake Oroville. With all six turbines generating it can release up to 16,950 cu ft/s (480 m3/s)[10] into the Feather River, but at the time of the crisis one turbine was down for maintenance reducing capacity to approximately 14,100 cu ft/s (400 m3/s). In order for the plant to run, the water level at the plant's outlet pipes in the Thermalito Diversion Pool must be below 224 feet (68 m) above sea level to avoid unsafe back pressure on the turbines. Additionally, it is unsafe to run the plant when the turbines do not have an electrical load on them, e.g. if the plant is not connected to the power grid. The high-voltage power lines connecting the powerplant to the power grid cross above the main and emergency spillways.

The river valve outlet system is designed to be used when the reservoir is low to provide a bare minimum water release to keep the Feather River fishery alive. It consists of two 35 ft (11 m) diameter pipes inside a tunnel dating to the construction of the dam. Prior to a 2009 accident, the system had a capacity of 5,400 cu ft/s (150 m3/s).[9] In the period 2012-16, California DWR spent $18 million to repair the system.[11] State officials have given contradictory statements about whether the repairs brought the system back to its original capacity[11] or if it still has a reduced capacity of 2,000 cu ft/s (57 m3/s).[12]

The main spillway is a concrete chute 3,050 feet (930 m) long[9] going down the canyon slope. It is located on the dam's abutment wall, a short distance to the northwest of the dam. It is used at times of high water to release large volumes of water from Lake Oroville. It has eight control gates at the top to regulate the amount of water entering the chute. It has the capacity to release up to 150,000 cu ft/s (4,200 m3/s).[9]

The emergency spillway (sometimes called the auxiliary spillway) is a concrete weir structure 30 feet (9.1 m) tall atop a natural hillside just to the northwest of the main spillway. The emergency spillway has no control structures, but allows water to flow over the top of it if the surface elevation of Lake Oroville reaches 901 feet (275 m) above sea level.[9] It exists as a fail-safe mechanism to prevent water from overtopping the earthen dam, and is only to be used as a last resort. It had never been used prior to February 11, 2017, though the lake had come within 1 foot (30 cm) of the top in January of 1997.[4]

Timeline of Crisis

After several years of drought, the winter of 2016-17 brought abundant rain and snowfall to the Feather River basin, refilling Lake Oroville. By February 2, 2017, Lake Oroville held 21% more water than its historical average.[13] At the same time, a survey showed abundant snowpack in the mountains, with Phillips Station showing a snow water equivalence of a healthy 28.1 inches (71 cm).[13] In early February, a warm storm system, known locally as a pineapple express, stalled out over northern California, dumping 12.8 inches (33 cm) of rain on the Feather River basin between February 6 and February 10.[14]

Water flow over the main spillway (foreground) and emergency spillway (background) on February 11, 2017.

February 7

The California Department of Water Resources (DWR) increased the flow rate on the main spillway of Oroville Dam to 54,500 cu ft/s (1,540 m3/s)[14] in order to make room in the reservoir for expected inflows from the then ongoing storm. Workers noticed the flow down the spillway was behaving erratically. Around noon, engineers halted the flow down the main spillway in order to inspect it.[3] They discovered the water had eroded a crater into the concrete lining of the spillway, measuring about 180 ft × 250 ft × 40 ft (55 m × 76 m × 12 m).[15] The main spillway remained shut down the rest of the day while DWR consulted outside experts about whether or not it was safe to continue operating the damaged spillway. Meanwhile, inflows into Lake Oroville varied between 80,000 and 130,000 cu ft/s (2,300 and 3,700 m3/s).[14]

February 8

DWR conducted two test runs of a combined six hours duration down the damaged main spillway to see how additional flows would affect it.[16] After the two tests, the crater in the spillway grew by about 50 feet (15 m).[16] The DWR also began making preparations for using the emergency spillway,[14] such as clearing trees and brush from the hillside[17] which could wash down and choke the river below. Meanwhile, inflows into Lake Oroville varied between 80,000 and 120,000 cu ft/s (2,300 and 3,400 m3/s).[14]

February 9

With lake levels surging, DWR officials reopened the damaged spillway, gradually ramping it up to 35,000 cu ft/s (990 m3/s). In the afternoon, DWR began preparing the public for the likelihood the emergency spillway would have to be used, but reassured residents that "Oroville Dam itself is sound and there is no imminent threat to the public."[4] Downstream, the Feather River Fish Hatchery began evacuating young fish that could be choked by muddy water. By evening, DWR increased flows down the damaged spillway to 40,000 cu ft/s (1,100 m3/s) with an addition 7,000 cu ft/s (200 m3/s) passing through the power plant.[4] Meanwhile, inflows into Lake Oroville never dropped below 100,000 cu ft/s (2,800 m3/s) and spiked as high as 190,435 cu ft/s (5,392.5 m3/s).[14]

February 10

In the early morning hours, the Hyatt Powerplant was forced to shut down,[15] because erosion debris backed up water in the diversion pool and blocked the plant's outlet pipes. DWR increased main spillway flows to 65,000 cu ft/s (1,800 m3/s) to compensate for the loss of outflows through the powerplant. Erosion to the southeast side of the main spillway threatened to topple a tower holding high-voltage power lines from the Hyatt Powerplant, which would be needed to get the powerplant back running. With the Hyatt Powerplant not operating, Pacific Gas and Electric crews quickly cut the powerlines and removed them from the section over the two spillways. In the evening, DWR reduced main spillway flows to 55,000 cu ft/s (1,600 m3/s) to save the tower.[18] Inflows into Lake Oroville gradually fell from 180,000 cu ft/s (5,100 m3/s) to 100,000 cu ft/s (2,800 m3/s).[14]

February 11

KCRA-TV chopper footage from February 12.

A little before 8:00 AM, Lake Oroville's elevation reached 901 feet (275 m) above sea level, sending water over the emergency spillway for the first time ever.[7] Inflows into Lake Oroville gradually fell from 100,000 cu ft/s (2,800 m3/s) to 70,000 cu ft/s (2,000 m3/s).[14]

February 12

At 3:00 AM, Lake Oroville crested at 902.59 feet (275.11 m),[14] at which time flow over the emergency spillway peaked at 12,600 cu ft/s (360 m3/s).[12] Headward erosion on the emergency spillway proceeded faster than anticipated, and opened three large holes near the top of the hill, just below the concrete weir. The hillside "eroded to within several feet" of the concrete spillway itself[5], according to DWR engineer Kevin Dossey. This opened up the possibility of a nightmare scenario where the ground underneath the emergency spillway might give way under the pressure of Lake Oroville pushing against the concrete structure.[19] The resulting rapid and uncontrolled release would overwhelm downstream flood control measures.

The Butte County Sheriff's Office issued a mandatory evacuation order at 4:21 PM, telling residents the emergency spillway could fail within the hour.[20] Yuba and Sutter counties soon issued their own evacuation orders. This sent 188,000 evacuees onto the roadways all at once. The resulting gridlock snarled traffic for hours, and some openly wondered if they would reach high ground in time.[20]

DWR increased the flow down the damaged main spillway to 100,000 cu ft/s (2,800 m3/s) to take pressure off the rapidly eroding emergency spillway.[14] Inflows into Lake Oroville gradually fell from 60,000 cu ft/s (1,700 m3/s) to 40,000 cu ft/s (1,100 m3/s).[14] By 8:45 PM, the lake level had dropped enough to halt flows over the emergency spillway.[5]

February 13

Contractors began repairing erosion damage on the emergency spillway in case it needed to be used again. This work continued around the clock, and would continue seven days a week for weeks to come.

Lake Oroville dropped to 892 feet (272 m) above sea level by midnight.

February 14

As lake levels continued to fall, in the early afternoon officials downgraded their evacuation order to an evacuation warning, meaning that residents could return to their homes, but had to be ready to leave again should conditions at the dam deteriorate.[7] The evacuation warning continued until March 22.[21]

Lake Oroville dropped to 882 feet (269 m) above sea level by midnight.

Post-Crisis Phase

The lifting of the evacuation order on February 14 marked the transition from the immediate crisis phase to a post-crisis phase. The damaged spillways were still damaged, and the Hyatt Powerplant was still offline. There was still a huge snowpack in the Sierras, and northern California still had 6-8 weeks left in its rainy season.

Clearing diversion pool

A massive debris pile sits at the foot of the main spillway on February 27, 2017.

Over the period February 10-26, DWR maintained at least 50,000 cu ft/s (1,400 m3/s) flowing down the main spillway at all times. This flow destroyed much of the lower half of the spillway. With the lake's elevation reduced to 839 feet (256 m), and a break in the weather, the flow down the main spillway was halted midday on February 27, 2017. By then, an estimated 1.7 million cubic yards (1,300,000 m3) of debris[22] had washed down into the Thermalito Diversion Pool, creating a dam and trapping water against the Hyatt Powerplant, preventing it from operating.

At a press conference on February 27, DWR Acting Director Bill Croyle announced that the level of the Thermalito Diversion Pool at the outlet pipes of the Hyatt powerplant was 254 feet (77 m) above MSL, which is 30 feet (9.1 m) higher than the level at which the powerplant can function normally. With the spillway shut down, crews could get to work clearing the debris in the diversion pool to get the powerplant back running. DWR estimated they had about a week before the lake rose to 860 feet (260 m), at which time they would need to reopen the spillway to drop the reservoir.

Contractors worked around the clock to clear the diversion pool, both from shore and from barges. The Hyatt Powerplant tested running one turbine for several hours on March 4 before shutting down again on March 5. It restarted on March 6 and gradually built up to releasing 12,900 cu ft/s (370 m3/s) by March 10. Lake Oroville reached the arbitrary cutoff level of 860 feet (260 m) on the evening of March 9, by which time crews had cleared approximately 650,000 cubic yards (500,000 m3) cubic yards of material from the diversion pool. On March 9, DWR announced that with outflows through the powerplant nearly matching inflows, they would allow Lake Oroville to rise to 865 feet (264 m) before reopening the main spillway on March 17. This gave the contractors more time to complete their work. By the time the spillway reopened on March 17, more than 1.25 million cubic yards (960,000 m3) of debris had been removed from the diversion pool.[23]

Shoring up main spillway

While the main spillway was shut down, other contractors attempted to stabilize the spillway itself. The chief concern was to prevent headward erosion up the hill. They sprayed shotcrete under the lip of the crater to armor loose material which could be carried away. They also walked the spillway above the crater, repairing any cracks in the concrete. Finally, they used rock bolts to anchor the spillway to the ground below.[24]

With the reopening of the spillway, crews focused on stabilizing access roads and building an on-site concrete batch plant,[23] which will be needed for permanent repairs once the spring flood season ends. DWR expected to announce their permanent repair plans by the end of March.[23]

Armoring the emergency spillway

Repairs just below the emergency spillway began on February 13. Night and day, trucks dumped boulders into the large holes and workers covered them with concrete. During the daytime, helicopters carried bags of boulders to some of the harder to reach areas.[7] Workers built benches and check dams to slow floodwaters, should the emergency spillway ever be used again.

Aftermath of the Crisis

Butte County Sheriff's Office updated their evacuations plans after the crisis, with the goal was of avoiding a repeat of the gridlock that occurred on February 12.[25] The new plan has better early-warning capabilities, better-defined geographic limits and directs people to the most efficient evacuation routes. It also incorporates new communication tools like automated alerts sent to the smartphones of enrolled users.

Investigations

The Federal Energy Regulatory Commission (FERC) ordered the California DWR to assemble a team of outside experts to assess the dam and spillway structures and issue recommendations[26]. On February 17, DWR selected five experts to assess the current condition and operations of the dam and associated structures.[27] On March 15, DWR announced a separate panel of six to conduct a forensic investigation of the circumstances leading up to the crisis.[28]

Working independently, Robert Bea of the Center for Catastrophic Risk Management at the University of California conducted his own forensic investigation into the main spillway failure. Bea released his preliminary findings on April 17, 2017. Bea identified five design flaws, two construction flaws, and two maintenance flaws as contributing to the failure. Among the most serious findings, Bea concluded that the spillway was composed of slabs of concrete that were not sufficiently connected either to each other or to the ground beneath them to prevent the slabs from shifting under hydraulic loads.[29]

The official forensic team released their preliminary findings on May 5, 2017. These findings largely agreed with Robert Bea's work. The forensic team did add that neither groundwater pressures nor cavitation appeared to play a significant role in the failure.[30]

Photo Gallery

Main Spillway Comparison
March 30, 2011
February 7, 2017
February 27, 2017
At left, undamaged spillway in 2011. At center, the crater when first discovered. At right, spillway after weeks of continued use.


  • Muddy water from emergency spillway enters the diversion pool.

  • Water from emergency spillway cuts a deep ravine through access road.

  • Floodwaters from the emergency spillway cut a deep ravine through access road.

  • Crews work around the clock building erosion control measures on the emergency spillway.

  • Workers place sandbags inside the Hyatt Powerplant to keep out water backing up in the Thermalito Diversion Pool on February 16, 2017.

  • Contractors construct benches and check dams to slow erosion should the emergency spillway be used again.

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ 3.0 3.1 Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ 4.0 4.1 4.2 4.3 Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ 5.0 5.1 5.2 Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ 7.0 7.1 7.2 7.3 Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ 8.0 8.1 Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ 11.0 11.1 Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ 12.0 12.1 Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ 13.0 13.1 Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ 14.00 14.01 14.02 14.03 14.04 14.05 14.06 14.07 14.08 14.09 14.10 Lua error in package.lua at line 80: module 'strict' not found.
  15. ↑ 15.0 15.1 Lua error in package.lua at line 80: module 'strict' not found.
  16. ↑ 16.0 16.1 Lua error in package.lua at line 80: module 'strict' not found.
  17. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  18. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  19. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  20. ↑ 20.0 20.1 Lua error in package.lua at line 80: module 'strict' not found.
  21. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  22. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  23. ↑ 23.0 23.1 23.2 Lua error in package.lua at line 80: module 'strict' not found.
  24. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  25. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  26. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  27. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  28. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  29. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  30. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External Links

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