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Oroville Dam crisis

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View of the damaged spillway, February 11, 2017. The flow of water has carved a large gully to the right of the spillway.

The 2017 Oroville Dam crisis is a major and ongoing threat to lives and property in Northern California. At 770 feet (235 m), Oroville Dam is the tallest dam in the United States. Located in Butte County about 70 miles (110 km) north of Sacramento, the dam impounds Lake Oroville, and controls the flow of the Feather River. Storms in early February 2017 caused heavy damage to the dam's spillway, hindering the safe release of floodwater and threatening the structural integrity of the spillway. Due to the flood threat, more than 180,000 people living along the Feather River have been evacuated.

Summary of crisis

Partial view of the spillway structure next to Oroville Dam with the main service spillway (right) and emergency spillway (left) (2008)

There are four intended routes by which water can pass the dam, in order from greatest preference to least:

  1. Through the hydro-electric generators, which utilize a maximum flow rate of 16,950 cu ft/s (480 m3/s).[1]
  2. Through the river outlets or bypass valve, which has a capacity of 5,400 cu ft/s (150 m3/s).[2] The outlet was damaged during an accident in 2009 and has not been used since.[3]
  3. Through the main (service) spillway, which regularly carries water in excess of the needs of the generators. The service spillway is controlled by gates and has a capacity of 150,000 cu ft/s (4,200 m3/s).[2]
  4. Over the top of an ungated auxiliary (emergency) spillway, built several feet below the height of the main dam. Once the lake reaches an elevation of 901 ft (275 m) it will naturally flow over the auxiliary spillway.[2]

When a large crater developed in the main spillway, it was shut off for inspection, but was later restarted. However, the 55,000 cu ft/s (1,600 m3/s) flow was not enough to prevent the lake from rising. The water eventually rose high enough that it began flowing over the top of the nearby auxiliary spillway—as per design—for the first time since the dam was constructed in 1968. When it appeared that the earth just beneath the lip of the auxiliary spillway was starting to erode, an evacuation order was issued out of fear that a collapse of the auxiliary spillway would send a 30-foot (9 m) wall of water into the Feather River below, flooding communities downstream.[4]

Background

After a period of heavy rain, on February 7, 2017, during ongoing flood control release of about 50,000 cubic feet per second (1,400 m3/s), a crater appeared in the Oroville Dam spillway.[5] High inflows to Lake Oroville forced dam operators to continue using the damaged spillway, causing additional damage. By February 10, the spillway hole had grown to 300 feet (91 m) wide, 500 feet (152 m) long and 45 feet (14 m) deep.[6] Meanwhile, debris from the crater in the main spillway was carried downstream, and caused damage to the Feather River Fish Hatchery due to high turbidity.[7] State workers began evacuating fish and eggs from the hatchery in an attempt to mitigate the damage.[8][9]

Although engineers had hoped that using the damaged spillway could drain the lake enough to avoid use of the auxiliary spillway,[10] they were forced to reduce its discharge from 65,000 cu ft/s (1,800 m3/s) to 55,000 cu ft/s (1,600 m3/s) due to potential damage to power lines.[11][12] In addition, a large volume of debris eroded out of the damaged spillway clogged the channel of the Feather River below the dam, preventing release of water from the hydroelectric plant, thus putting further pressure on the spillway.

Emergency spillway use and evacuation of the Feather River Basin

Water overflowing the auxiliary spillway in the background, February 11, 2017

Measures were taken to prepare the emergency spillway – officially known as the auxiliary spillway – for use for the first time. On February 10, 2017, power transmission lines were moved, and workers began clear-cutting trees on the hillside below the auxiliary spillway.[13]

Shortly after 8:00 am on February 11, 2017, the auxiliary spillway began carrying water for the first time since the dam's construction in 1968.[14] Because the spillway was a separate structure from the dam, officials stated that there was no danger of the main embankment being breached, and evacuation of Oroville itself was not considered at that time, as officials stated that there was no threat to public safety. Once the lake rose to the level of the auxiliary spillway top, an uncontrolled overflow began that topped out at 12,600 cu ft/s (360 m3/s)[15][16], and water flowed directly onto the earthen hillside below the concrete crest of the auxiliary spillway.

On February 12, 2017, evacuation was ordered for those in low-lying areas along the Feather River Basin in Butte, Yuba and Sutter counties, due to an anticipated failure of the auxiliary spillway.[17] Specifically, erosion on the hillside was growing uphill toward the concrete lip of the auxiliary spillway, leading to the fear that it would collapse. A failure of the concrete top of the spillway would allow up to 30 feet (9 m) vertical of Lake Oroville through the gap in an uncontrolled deluge.[18] The flow over the main spillway was increased to 100,000 cu ft/s (2,800 m3/s) to try to slow the erosion of the auxiliary spillway.[19]

By 8 p.m. on the evening of February 12, the increased flow had successfully lowered the water level to below the auxiliary spillway top, causing the auxiliary spillway to stop overflowing. However, evacuation orders were not rescinded. The stop in water flow allowed the erosion there to be hastily inspected and stabilized with boulders.[18] Engineers worried that the damage would be transferred to the main spillway, not only making future repairs more expensive, but also that the damage to the main spillway could grow uphill to the point that it endangered the main spillway gates, leaving no safe way to release water. The extent of such damage was unknown, hidden by water spray and darkness; it was expected to be assessed on the morning of the 13th.[18]

On February 13 helicopters began to drop sandbags and large rocks in the area of the now dry auxiliary spillway, in order to protect the base from erosion.[20]

By February 13, 188,000 people in the vicinity were reported evacuated.[21] About 23,000 National Guardsmen were ordered to be ready for "immediate deployment if the dam spillway should fail" to help with evacuation and relief efforts.[20]

References

  1. ^ "Hyatt Powerplant". California Department of Water Resources. Retrieved 2017-02-13.
  2. ^ a b c "Oroville Facilities (FERC Project No. 2100)" (PDF). California Department of Water Resources. Jan 2005. Retrieved 2017-02-13.
  3. ^ "2016 Annual Review of the construction and operation of the State Water Project" (PDF). California Water Commission. Retrieved 2017-02-13.
  4. ^ Plumer, Brad (February 13, 2017). "The Crisis at Oroville Dam, Explained". Vox.
  5. ^ Kasler, Dale; Sabalow, Ryan; Reese, Phillip. "Crater in Oroville Dam spillway will continue to grow, officials warn, as reservoir levels climb". Sacramento Bee. Retrieved February 9, 2017.
  6. ^ Graff, Amy (February 10, 2017). "Gaping hole in Oroville Dam spillway is growing, officials warn". SFGate. Retrieved February 10, 2017.
  7. ^ "Fish evacuated from Feather River Hatchery". KRCR. February 11, 2017. Retrieved February 13, 2017.
  8. ^ "The Latest: California lake damage may near $100 million". NewsOK.com. February 10, 2017. Retrieved February 11, 2017.
  9. ^ "'Amazing engineering feat' saves millions of fish at Feather River Fish Hatchery". ChicoER. Retrieved February 13, 2017.
  10. ^ "Sacrificing California spillway may avoid emergency releases". Retrieved February 11, 2017.
  11. ^ "Use of untested emergency spillway yet again a possibility at crippled Oroville Dam". Sacramento Bee. Retrieved February 12, 2017.
  12. ^ CA DWR (February 10, 2017). "Lake Oroville Releases Slowed to Avoid Erosion" (PDF). Retrieved February 12, 2017.
  13. ^ "In Historic First, Lake Oroville Flows Over Dam's Emergency Spillway". KQED News. February 11, 2017. Retrieved February 11, 2017.
  14. ^ Associated Press (February 10, 2017). "The Latest: Emergency Spillway Use Likely at Oroville Dam". ABC News. Retrieved February 10, 2017.
  15. ^ "Water flowing out of Lake Oroville emergency spillway". SF Gate. February 12, 2017. Retrieved February 12, 2017.
  16. ^ @CA_DWR (February 12, 2017). "Flows down the auxiliary spillway peaked last night at 1 a.m. at 12,600 cfs. Lake elevation levels are trending down.@CALFIRE_ButteCo" (Tweet) – via Twitter.
  17. ^ Staff, KCRA (February 13, 2017). "Thousands from Yuba, Butte, Sutter counties evacuated". KCRA. Retrieved February 13, 2017.
  18. ^ a b c Reese, Phillip; Sabalow, Ryan (12 February 2017). "Experts: State left with few options while trying to avert disaster at Oroville Dam". Sacramento Bee.
  19. ^ "BREAKING: Fearing collapse of emergency spillway at Oroville Dam, Oroville evacuated". Sacramento Bee. February 12, 2017. Retrieved February 12, 2017.
  20. ^ a b "Tons Of Rocks Bolster Failing Oroville Dam Spillway". CBS Bay Area. 2017-02-13. Retrieved 2017-02-13.
  21. ^ Schmidt, Samantha and Hawkins, Dave (February 13, 2017). "'NOT a drill': 188,000 evacuated, emergency declared, as Calif's massive Oroville Dam threatens floods". Washington Post. Retrieved 13 February 2017.{{cite news}}: CS1 maint: multiple names: authors list (link)
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