Orovill Dam

The maximum flow rate through the hydraulic units of the Hayat power plant is 480 m³ / s ^[4] . To pass excess water, the dam is equipped with the following culverts:

Main operational spillway

Located on the right bank, it is a water intake structure with gates, from which water flows into the Feter river channel along a concrete gutter with a length of 930 and a width of 54.5 meters with a springboard at the end. The maximum capacity of the main spillway is 4247 m³ / s .

Emergency Spillway

Located on the right bank of the water intake of the main spillway. Until 2017, it was never used. It is made in the form of a concrete overflow wall 530 meters long. The emergency spillway has no gates and begins to work when the water level in the reservoir exceeds the height of the overflow face. The designation of the emergency spillway is to allow catastrophic floods with an estimated recurrence rate of once every 450 years. Its maximum throughput is more than 13,000 m³ / s ^[7] .

After the accident of 2017the spillway structure is complemented by a 440 meter long cutoff wall of secant-pile wall in the soil 220 meters down the slope from the overflow wall and a concrete slope between them, made in the form of steps partially extinguishing the energy of the water flow ^[8] .

River Valve

Included in the system of underground tunnels on the left bank. Used to pass the waters of the Feter River during the construction of the dam. Since the 1980s, it has been used to regulate the temperature of the river in order to comply with environmental laws. It can be used to discharge water bypassing the hydraulic units of the Hayat power plant in an emergency ^[9] . The maximum throughput is 113 m³ / s ^[10] .

On July 22, 2009, waterworks employees tested the river valve. When the water flow through the valve reached 85% of the maximum, the vacuum created by it was collapsed and the partition wall was dragged into the flow. Three workers were knocked down by the air sucked into the tunnel, two of them were carried away over the edge of the work site. One of the workers was seriously injured by flying tools and parts of equipment while clinging to damaged metal structures. He received head injuries, broken arms and legs, cuts and bruises; and was hospitalized for four days. The Safety and Health Division filed six complaints with the Department of Water, five of which were rated “serious”. Two serious claims during the investigation were withdrawn, the total amount of the fine amounted to 76 125 dollars ^[11] .

In 2012, independent experts were recruited to investigate river valve safety. Due to the forecast of drought, an emergency valve repair was performed in 2014. In 2014-2015, it was used to maintain the temperature regime of the Feter River in accordance with the ^[12] .

The final repair of the valve and the installation of the deflector ring to dissipate the flow energy when using the valve at full power ^{[13] were} carried out in 2016–2017 ^[12] . On October 16, 2017, in connection with the emergency condition of the spillway structures of the waterworks, the Water Resources Department included the possibility of using a river valve in the plan to control the reservoir level during the winter flood ^[14] .

Emergency Prerequisites

The drawback of the emergency spillway design was the discharge of water directly onto an un-equipped hillside ^[15] . In 2005, this was noted by three environmental organizations, which proposed to make the condition of the next certification of the hydroelectric complex to strengthen the slope below the emergency spillway with concrete in order to avoid the development of erosion during its operation. The cost of the necessary work was estimated at $ 100 million . This proposal was rejected because the safety regulations for hydraulic structures allow serious landscape damage when rare catastrophic floods are missed ^[16] .

In 2009 and 2013, cracks appeared on the main spillway, requiring repair. Repair work was considered successful; in 2014–2016, the spillway was successfully inspected ^[17] .

In 2017, the main spillway has been used since January 13. In early February 2017, the reservoir level began to grow rapidly due to heavy rains, which by February 7 necessitated an increase in water flow through the main spillway to 1540 m³ / s ^[16] .

Spillway accident

On February 7, 2017, the destruction of the concrete trench of the main spillway and the erosion of sedimentary rocks around it began. Water passage through the main spillway was stopped for its inspection, which revealed a fracture site with a width of about 50 and a length of up to 100 meters ^[18] in that part of the trench where cracks were observed in 2013 ^[4] . The underlying rocks were washed several meters deep, erosion of the slope to the left of the spillway began ^{[Comm. 2]} . To test the possibility of further use of the damaged gutter, the discharge of water through it was resumed in a small volume. There was a likelihood of an emergency spillway being put into operation, for which purpose February 8 began clearing the slope below it and strengthening the foot of the overflow wall in bulk from boulders fastened with concrete.

A sharp increase in the inflow on February 9 forced to increase water flow through the damaged spillway to 1840 m³ / s , as a result of which the destruction of its gutter and erosion of the adjacent slope continued. Fragments of concrete and rocks formed a sandbank in the Feter River, which prevented the outflow of water from the Hayat Power Station. In the evening of the same day, the power plant had to be stopped ^[19] . Significantly increased turbidity caused damage to the hatchery located downstream of the river ^[20] .

On February 10, high inflow was maintained, as a result of which the commissioning of an emergency spillway was recognized as inevitable. Water consumption through the damaged main spillway was reduced to 1557 m³ / s in order to reduce its further destruction ^[19] .

Emergency development

On the morning of February 11, water went over the crest of the emergency spillway. Soil erosion on the slope was much stronger than expected and was accompanied by the formation of large holes, dangerously approaching the base of the overflow wall. The washing and collapse of the latter could lead to a catastrophic discharge of part of the reservoir. In these circumstances, the safety of the damaged main spillway was no longer taken into account, and the water flow through it was increased to 2831 m³ / s ^[21] .

Nevertheless, the operation of the reservoir below the ridge of the emergency spillway required time, during which the erosion of the slope under it continued. There was also the possibility that the destruction of the main spillway would spread up to the water intake structure. Therefore, on February 12, it was decided to evacuate the population downstream, which could suffer in the event of a catastrophic development of the situation. The evacuation affected about 200 thousand people ^[22] .

Crisis Resolution

Drainage through the emergency spillway stopped on the evening of February 12. The next day, work began on leveling and strengthening the slope under it, but further operation of the emergency spillway without serious repair and restoration work was impossible.

Discharge of the reservoir continued through the damaged main spillway. Fortunately, the spread of his damage up the slope did not occur. A large erosion pit and a deep ravine to the left of the former spillway location were formed below the site of the initial damage ^{[Comm. 2] [23]} . Due to stabilization of the situation, the compulsory evacuation of the population was canceled on February 14, but the warning of readiness for evacuation was upheld ^[24] . A decrease in the supply made it possible to gradually reduce the water consumption; on February 23, it was 1415 m³ / s ^[22] .

On February 27, a decrease in the reservoir level made it possible to stop the discharge of water through the damaged main spillway and to begin clearing the obstruction in the Feter riverbed. By this time, the spillway trench below the accident site was almost completely destroyed. The base of its upper part above the erosion pit was fixed with concrete ^[23] . Due to the cessation of discharge, there was a sharp drop in the water level in the river, which caused landslides on its banks and the formation of isolated reservoirs, from which a large number of fish juveniles of valuable species had to be saved. On March 3, hydraulic units of the Hyatt power plant were launched ^[25] .

From March 17 to March 27, the discharge of water through the damaged main spillway was resumed to reduce the level of the reservoir and to check the possibility of using the spillway during spring snowmelt. At a water flow rate of 1133 m³ / s, further structural failure did not occur ^{[26] [27]} . On March 22, the warning of readiness for evacuation was canceled ^[28] . The Department of Water Resources has begun developing a plan for the repair of spillway structures to ensure their safe operation by November 1, 2017 ^[29] .

Repair work

The restoration and improvement of the spillway structures of the Orovill Dam after the accident took more than two years. The initial $ 275.4 million repair contract was awarded to Kiewit Infrastructure West Co., a division of , one of the world's largest construction companies ^[30] . As of September 5, 2018, the total cost of repair and reconstruction of spillway structures, taking into account completed and planned work, was estimated at $ 1.1 billion ^[31] .

2017

On May 19, the use of the damaged main spillway for skipping the spring flood and creating a reserve of free reservoir capacity was completed. This allowed us to begin repair work on the main spillway ^[32] .

By November 1, 2017, the upper section of the gutter with a length of about 220 meters was repaired, and the next 265-meter section and the lower part of the gutter with a length of 105 meters were completely rebuilt. The middle part of the trench 320 meters long was restored in the form of a temporary construction of rolled concrete. Further work on the main spillway was postponed until next year due to the onset of the rainy season ^[33] .

At the same time, work was done to strengthen the slope below the emergency spillway to reduce erosion in the event of its subsequent use. 220 meters down the slope from the overflow wall, the construction of a cut-off wall made of bored piles in the ground began. The length of the cut-off wall is 440 meters, the depth is from 10 to 20 meters ^[34] .

2018

The erection of a cut-off wall in the ground below the emergency spillway was completed on March 7. On February 28, work began on covering the slope between the spillway crest and the cut-off wall with rolled concrete ^[34] . By the end of October, slope cover and the creation of a support under the emergency spillway wall made of rolled concrete were completed. Work continues on the reinforced concrete coating at the junction of the support with the overflow wall ^[35] .

Reconstruction of the main spillway continued on May 8, after the end of the spring flood period ^{[Comm. 3] [37]} . By October 31, the bulk of the work was completed. The Department of Water Resources said that the spillway, if necessary, will be ready for use from December 1 ^[35] . The new design in comparison with the old is significantly strengthened ^[38] .

2019

The completion of work on coating the junction of the support with the overflow wall of the emergency spillway is expected in early 2019 ^[35] .

Assessment of the accident and its causes

According to Ivan Vladimirovich Sliva, the chief expert of the RusHydro Russian company, the accident at the spillway structures of the Oroville waterworks is one of the most serious hydraulic accidents of the 21st century. Its immediate cause, he believes, is the formation of voids in the base of the main spillway tray by suffusion of soil with filtered water. Additional factors, in his opinion, were the lack of engineering and geological study of spillway damage during the repair of 2013 and the savings in the arrangement of an emergency spillway ^[39] .

• California Dam and Reservoir List

• Plum I.V. , Lapin G.G. Accident at the spillway structures of the Oroville waterworks // Hydrotechnical construction: journal. - 2017. - No. 11 . - S. 44-51 . - ISSN 0016-9714 .

• Construction of the Orovill Dam - Russian translation of the documentary
• Statistics of the Orovill Waterworks online at the US Department of Water's website.