In February 2017, the main spillways and Oroville Dam emergency were damaged, prompting the evacuation of over 180,000 people living downstream along the Feather River and relocating fish hatcheries.
Heavy rains during California's 2017 floods damaged the main channel on Feb. 7, so California's Water Resources Department closed the dam to assess the damage and contemplate the next steps. The rain finally raises the lake level to flow over the emergency spillway, even after the damaged main spillway is reopened. As water flows over an emergency spill channel, erosion toward the head threatens to damage and tear down the concrete weir, which can send a 10 meter high water wall to the Feather River below and flood the community downstream. No collapsing happened, but water further damaged the main channel and eroded the slope of the emergency.
Video Oroville Dam crisis
​​ Background
Oroville Dam, an important part of the California Water State Project, is an embankment embankment of land on the River Feather, east of the city of Oroville in Northern California. This dam is used for flood control, water storage, hydroelectricity, and water quality improvement in the Sacramento River Delta-San Joaquin. Completed in 1968, it was the highest ground dam in the United States, at 770 feet (230 m). This makes it difficult for Lake Oroville, the second largest manmade lake in the state of California, capable of storing more than 3.5 million acre-feet (4.4 km 3 ). The adjacent Edward Hyatt Powerplant has six power plants with a total installed capacity of 819 megawatts (MW) of electricity.
For the purpose of flood control, some space in the Oroville Reservoir must remain dry to catch floods, a practice that has caused controversy in another California dam over the amount of wasted water. Dam operators are required to dispose of water based on the chart contained in the Oroville Dam Reservoir Regulation Manual , a flood control guide developed by the US Army Engineer Corps. At the time of the incident, the Oroville Dam Reservoir Regulation Manual was last updated in 1970 and the discharge chart was based on climatological data and runoff projections that did not take into account significant climate change or floods in 1986. and 1997. It is unclear whether the manual the outdated is a significant factor in the crisis of February 2017.
The dam complex is designed with four routes for water crossing from Lake Oroville:
- Through a hydro-electric generator, which has a combined maximum flow rate of 16,950 cuÃ, ft/s (480 m 3 /s).
- Through the river stream (also known as the bypass valve) which has a capacity of 5400 cuÃ, ft/s (150 m 3 /s).
- Through the main spill (service) located at the northwest part of the dam, which is used to control reservoir heights by releasing large amounts of excess water rapidly to a 3,050 ft (930 m) long channel extending into the river below. The water flow to the main spillway is controlled by eight radial (Tainter) gates that have a combined maximum discharge capacity of 270,000 cuÃ, ft/s (7,600 m 3 /s) when the lake elevation is 905 feet. Since the leakage threshold elevation is 813.6 feet (248.0 m), it can only be used at lake heights above that level.
- Above an emergency dike, consisting of a high concrete dam along the 1,730 feet (530 m), 30 feet (9.1 m) high beside the main channel and a slope of land adjacent to the abutment. After the lake reaches a height of 901Ã, ft (275 m) above sea level on average - 21 feet (6 m) below the height of the main dam structure - water flow, uncontrolled, above the weir and down the slope until it reaches the river below.
In 2005, the dam was re-licensed by the Federal Energy Regulatory Commission, in which three environmental advocacy groups filed a motion with FERC raising concerns about the use of emergency spillways. Environmental advocacy groups cited a 2002 technical memorandum by the Yuba County Water Agency noting that significant erosion of abutments would occur if emergency spillways were used, which could cause significant downstream damage and disrupt the operation of Edward Hyatt Powerplant. The groups are urging the FERC to renew the Oroville Dam extension permit on the slope of the concrete embankment with concrete and to update the Oroville Dam Reservoir Regulation Manual .
According to the FERC engineering guidelines: "Emergency spillways can be used to obtain high hydrological safety levels with minimal additional costs because their rare use is acceptable to maintain significant damage when used and they may be designed with lower structural standards than those used for additional spillways. "The FERC specifies that emergency deployment structures conform to their engineering standards and that the concerns of erosion posed by environmental advocacy groups are exaggerated. The FERC has determined that emergency spillways can handle 300,000 cuÃ, ft/s (8,500 m 3 /s) of debits and renew the dam licenses without requiring concrete layers of emergency spillways, estimated to cost more than $ 100 million.
Prior to February 2017, the main spillway was reviewed last in August 2016. During the inspection, the spillway was examined visually without the examiner entering the launch channel. Supervisors enter the launcher channel for inspection in 2014 and 2015 without finding any worries. Cracks in spillway concrete were repaired in 2009 and 2013.
Maps Oroville Dam crisis
Timeline of the crisis
Damage to the main spillway
In early February 2017, high inflows into Lake Oroville caused dam operators to start using the main concrete spillway to control the lake level. Between 6-10 February 2017, 12.8 in (330 mm) rain falls on the Feather River Basin. The entry of water into Lake Oroville rose from 30,000 cuÃ, ft/s (850 m 3 /s) on February 6 to more than 130,000 cuÃ, ft/s (3,700 m 3 /s) at noon on February 7, before easing around 75,000 cuÃ, ft/s (2,100 m 3 /s) the next day. The dam operators began to increase the flow to the main channel to 54,500 cuÃ, ft/s (1,540 m 3 /s) on February 7 but immediately noticed an unusual flow pattern. The water flow beneath the spillway is suspended for investigation, revealing a crater about halfway through which the concrete layer is eroded and water out of the concrete channel. DWR consults with FERC and other dam security agencies on this issue and prepares for the possibility of using emergency spillways.
During the two test streams down the spillway on February 8-9, the crater length increased from 250 feet (76 m) to 300 feet (91 m). Meanwhile, the Lake Oroville level is rising as inflows into Lake Oroville skyrocket from about 130,000 cuÃ, ft/s (3,700 m 3 /s) on Feb. 7 to peak of 190,435 cuÃ, ft/s (5,395 m 3 /s) on February 9th. The DWR is then faced with two options: use the main spillway, know it will likely be more damaged, or let Lake Oroville rise until it overtopped the emergency spillway.
On February 9, the main spillway reopened. DWR hopes that using spillways that are damaged with limited flow can drain enough lakes to avoid the use of emergency spillways, which potentially damage the electrical grid serving hydroelectric installations. The next day, the discharge in the main spillway was reduced from 65,000 cuÃ, ft/s (to 1,800m 3 /s) to 55,000 cuÃ, ft/s (1,600 m 3 /s) , but this flow is not enough to prevent the lake going up. On 11 February, the lake level reached 901 feet (275 m) above average sea level and water began to flow - as designed - above the concrete dam along the top of the emergency sewer, flowed into the emergency spillway for the first time in dam history.
Emergency spillway failure
As the lake level rises, steps are taken to prepare an emergency channel for use. On February 10, 2017, workers began cutting down trees on the hillside under an emergency dike.
Shortly after 8:00 am on February 11, 2017, emergency spillways began carrying water for the first time since the dam construction in 1968. Since spillways are separate structures of dams, officials state that no major dangers are being breached, and evacuations Oroville was not considered at that time, as officials stated that there was no threat to public safety. The uncontrolled flow above the weir reaches 12,600 cuÃ, ft/s (360 m 3 /s).
Erosion at the base of the weir - expected - grows much faster than anticipated. Future erosion of the emergency spillway threatens to weaken and tear down the concrete weir, which in this case the 30 foot-high water wall will be sent to the Feather River below and the downstream flood community. Afraid of destruction, the Butte County Sheriff's Office issued an evacuation order from the Oroville area. To reduce the pressure on the emergency spillway and prevent the possibility of collapse of the weir, DWR almost doubled the volume of the main spillway discharge from 55,000 cuÃ, ft/s (1,600 m 3 /s) to 100,000 cuÃ, ft/s ( 2,800 m 3 /s). Hours later, the water stopped flowing over the weir into the emergency spillway and officials began assessing the state of the emergency spillway.
Evaluation of the Feather Valley Valley
On February 12, 2017, evacuations were ordered for those in the lowlands along the Feather River Valley in Butte, Yuba and Sutter districts, due to the failure to anticipate emergency spillways. In particular, erosion on the hillside grows up toward the concrete lips of the emergency spill channel, which causes fear to collapse. Failure of the top of the concrete of the spillway will allow up to 30 feet (9 m) vertical of Lake Oroville through a gap in uncontrolled bursts. The flow above the main spillway was increased to 100,000 cuÃ, ft/s (2,800 m 3 /s) to try to slow down the erosion of the emergency spillway.
At 9 pm on the evening of February 12, an increase in flow has managed to lower the water level until it is under an emergency levee, causing an emergency leak to stop overflowing. A stop in the water flow allows the erosion there to be hasty and stable with the rocks. The engineers fear that the damage will be diverted to the main channel, not only making the repair more expensive in the future, but also damage to the main spillway can grow up to the point of endangering the overflow main gate, leaving no safe way to release it. water. The extent of such damage is unknown; it is expected to be assessed on the morning of 13 February. On February 13 helicopters began dropping sandbags and boulders in the now dry emergency escape area, to protect the base from erosion. On February 13, 188,000 people around it were reportedly evacuated. Around 23,000 National Guards are ordered to be ready for "immediate deployment if the spill dam must fail" to assist with evacuation and relief efforts. On February 14 at 2:45 pm, the above evacuation orders were reduced to evacuation hours by emergency management officials and the California Water Resources Department, and people were allowed to return to the evacuated area.
Downstream effects
Direct damage from damage is limited to the downstream area of ​​the offense, eroding the hillside to form a canyon. However, the main danger is that the spill channel can erode back toward the gate as it is carried by the water flowing into the crater. Finally, this will threaten the overflow gate, close to the actual dam.
Approximately 150,000 cubic meters (110,000 m 3 ) of erosion debris clog the Feather River channels under a dam, preventing the release of water from a hydroelectric plant, thereby reducing the overall capacity of the dam to release water. Debris is brought downstream and causes damage to the Hawar Fish Fish Breeding due to high turbidity. State workers began to evacuate fish and eggs from hatcheries in an effort to reduce the damage and evacuate more than nine million downstream fish to a satellite hatch adjacent to Thermalito Afterbay.
On February 27, the flow to the spillway is temporarily turned off, allowing the crew to begin removing debris in the river in preparation for the resumption of the power plant. After the spillway is turned off, it appears that the erosion toward the head along the spillway channel has been largely stable, reducing the immediate threat of gate failure and uncontrolled release of water. The bedrock under the upper half of the spillway looks much stronger than the rock drifting beneath the center of the spillway. In addition, the strength of the falling water has dug the original crater into the deep plunge pool that acts as a hydraulic pad, throwing away most of the energy that may have caused additional damage to the spillway.
Investigation
The Federal Energy Regulatory Commission (FERC) ordered the California Department of Water Resources (DWR) to assemble a team of five independent experts to assess dams and issue recommendations. On February 17, 2017, DWR commissioned an independent Consulting Board (BOC) to investigate the causes, reviews and comments on improvements to Bend Oroville. A memoranda (report) prepared by the Board of Commissioners is posted on the DWR website. DWR gathers another Independent Forensic Team (IFT) to determine the cause of spillways incidents, including the effects of operations, management, structural design and geological conditions. DWR also requested full-time FERC staff to provide direct supervision, review and approve an improvement plan that will make the dam safe for the 2017-18 flood season.
Department of Water Resources 2017-18 Operation Plan
DWR has prepared the Lake Oroville Flood Control Operation Plan 2017/2018.
Public hearings
On May 11, 2017, the trial took place.
References
External links
- Media related to Oroville Dam crisis on Wikimedia Commons
- Incident Oroville Spillways, California Water Resources Department
- Photogallery about the incident from the California Department of Water Resources
- Satellie Drama Shows Oroville Dam from Drought to Excess
Source of the article : Wikipedia
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