The Beijing News interviewed Wang Hao, academician of the Chinese Academy of Engineering, and Zhang Boting, deputy secretary-general of the Chinese Society of Hydropower Engineering, on the impact of floods on the Three Gorges Dam
Water conservancy experts say that the incoming flood does not affect the safety of the Three Gorges Dam
Affected by heavy rainfall in the Jialing River, Minjiang River, Tuojiang River, Wujiang River, Fujiang River and the Three Gorges section in the upper reaches of the Yangtze River, at 14:00 on July 26, the inflow of the Three Gorges Reservoir reached 50,000 cubic meters per second, ushering in No. 3 of the Yangtze River in 2020 flood. The peak flow at 14:00 on the 27th reached 60,000 cubic meters per second.
According to the latest dispatch order of the Yangtze River Water Conservancy Commission of the Ministry of Water Resources, the Three Gorges Project will control the discharge of 38,000 cubic meters per second, block the flood and cut the peak by 36.7%, and reduce the pressure on flood control in the middle and lower reaches of the Yangtze River. At 8 o'clock on the 18th (the No. 2 flood of the Yangtze River) before this, the inflow of the Three Gorges reservoir rose to 61,000 cubic meters per second, and the outflow flow was 33,000 cubic meters per second, with a flood interception rate of 45.9%.
What is the impact of the floods on the Three Gorges Dam? Will there be displacements as widespread outsiders? How is the safety of the dam monitored? A reporter from Beijing News interviewed Wang Hao, standing director of the Chinese Society of Dam Engineering and academician of the Chinese Academy of Engineering, and Zhang Boting, deputy secretary-general of the Chinese Society of Hydropower Engineering.
The Three Gorges Dam played a role in peak shaving
The Beijing News: Every time a flood peak hits, the Three Gorges Dam will play a role in cutting the peak. For example, the flood interception rate on July 18 was 45.9%. What does this mean?
Zhang Boting: The flood interception rate is the ratio of the intercepted flow to the incoming flow of the intercepted flood. For example, this year's peak No. 2 (the storage volume at 8 o'clock on the 18th is 61,000 cubic meters per second) entered the Three Gorges Reservoir, and the flood retention rate was 45.9%, which means that 45.9% of the floods were blocked in the Three Gorges Reservoir, so the flood was discharged. The amount is 33,000 cubic meters per second.
Beijing News: How is the flood blocking rate determined?
Zhang Boting: How much flood the Three Gorges Dam needs to intercept is a systemic issue. It requires comprehensive consideration of the upstream and downstream conditions and the storage capacity. The flood interception rate is determined by different levels. For example, if the flood volume is less than 30,000 cubic meters per second, it will be dispatched by the Three Gorges Group itself; if the flood volume is higher than 30,000 cubic meters per second, the Yangtze River must be appointed, exceeding 56700 cubic meters. The meter per second needs to be decided by the National Flood Control and Drought Relief Headquarters.
Beijing News: Why can't the Three Gorges Dam stop the flood all at once?
Zhang Boting: Actually, the Three Gorges Dam often stops flooding all at once. For example, in years when the precipitation is not very large, the Three Gorges usually does not discharge floods throughout the year, which means that all the flood peaks are 100% blocked. Then, by increasing the flow of power generation, the flood was released.
However, the Three Gorges Dam cannot always stop all floods, especially in years with abundant rainfall. Because the total reservoir capacity of the Three Gorges reservoir area is 39.3 billion cubic meters, the flood control reservoir capacity is 22.15 billion cubic meters, and the annual inflow of the upstream of the Three Gorges Dam is about 450 billion cubic meters. 5% of the amount. Therefore, in a flood year, the Three Gorges Dam can usually only achieve peak-shaving for each peak during the flood season, but cannot stop all floods.
Beijing News: In comparison, what level is the flood interception rate of the Three Gorges Dam?
Zhang Boting: If you have to compare, it can be compared with the Hoover Dam in the United States, but the flood interception rate of the Hoover Dam is basically equivalent to 100%. The storage capacity of the Hoover Dam is equivalent to more than twice the annual runoff of the Colorado River, so the Hoover Dam is fully capable of stopping all floods at any time. Water is released when water is needed downstream, and it is not released when it is not needed.
The Three Gorges Dam is safer in the flood season than in the dry season
Beijing News: For example, the flood volume of No. 2 is relatively large. Does this have any impact on the safety of the foundation of the Three Gorges Dam?
Wang Hao: Flood No. 2 is the largest flood that has reached the Three Gorges since it entered the flood season this year. It has not reached the once-in-a-millennium standard and falls within the normal operating range of the Three Gorges. According to the historical flood data for 2500 years, the largest flood on the Yangtze River occurred in 1870, with a peak discharge of 105,000 cubic meters per second. The design flood standard of the Three Gorges Dam is once in a thousand years, and the corresponding flood flow is 98,800 cubic meters per second. When the design standard flood is encountered, the Three Gorges Dam operates normally. In the event of a flood of "10% once in a 10,000-year event", the peak flow rate reached 124,300 cubic meters per second, the water level in front of the dam was 180.4 meters, and the distance from the top of the dam was 4.6 meters. The dam can still be discharged safely and normally. flood.
Zhang Boting: Now it is said on the Internet that the flood will affect the safety of the foundation of the Three Gorges Dam, but this is not the case. When the flood came, the dam was safe. Why do you say that? Because when the flood comes, the Three Gorges Dam must free up its storage capacity to deal with the upstream water, and realize the peak shaving function for each flood peak. The water level in the Three Gorges reservoir area is relatively low, generally not higher than 160 meters. However, in winter and spring, when the Yangtze River is in dry season, the Three Gorges Dam needs to store water to resist drought, and the water level in the reservoir area is high, generally reaching 175 meters. At this time, the displacement of the top section of the dam is larger than that during the flood period, and the structural force should be correspondingly larger. Therefore, the safety of the Three Gorges Dam during the dry season is no problem, and there is no problem during the flood season.
The Three Gorges Dam is a very safe concrete gravity dam
Beijing News: Why can the Three Gorges Dam be safe?
Wang Hao: Because the Three Gorges Dam uses a concrete gravity dam, it is the dam type with the strongest overload capacity. It has high safety and is not afraid of flooding. In the history of dam construction in the world, there has not been a concrete gravity dam that failed after flooding. precedent. In extreme cases, floods exceed 124,300 cubic meters per second, and even if the dam overflows, there will be no dam collapse accident. In addition, the Three Gorges Dam has 23 deep spillway tunnels and 22 surface spillway tunnels, which can effectively release floodwaters, thereby maintaining the safety of the dam.
Beijing News: Does a concrete gravity dam not break even if there is a hole in it?
Zhang Boting: Yes, the characteristic of a concrete gravity dam is that even if a hole is opened, water will leak from that hole. It is equivalent to failing to close the gate and will not cause the dam to collapse. Concrete gravity dam is a very safe type of dam. It relies on the friction generated by its own weight and the foundation to resist the pressure of upstream water. Each dam section can be established independently. Therefore, floods basically pose no threat to dams such as the Three Gorges. Of course, the concrete gravity dam has very high requirements for foundation conditions, and the location of the Three Gorges Dam meets the conditions for building a concrete gravity dam. Therefore, as long as the conditions are met, the construction of concrete gravity dams on important rivers is generally given priority.
Beijing News: The concrete strength of the concrete gravity dam is also very high?
Wang Hao: The concrete used in the Three Gorges Dam has the characteristics of high durability, high crack resistance, and excellent construction performance. Since the water storage operation for 17 years, the cement and fly ash in the internal concrete are still continuously hydrated, and the concrete strength is still With continuous growth, the design strength of concrete is 25 MPa, which has increased to 43 MPa so far. From an international perspective, the Hoover Dam in the United States built in 1936 has a design strength of 25 MPa. In 1994, the average strength of core samples tested by the Bureau of Reclamation was 50 MPa. After more than 80 years of operation, the concrete strength is still increasing. The Three Gorges Dam has a wide dam bottom and a large amount of concrete. The hydration reaction will continue to occur for a long time, and the concrete strength of the dam will continue to increase.
The displacement indexes of the Three Gorges dam are all within the design allowable range
Beijing News: Is the displacement of the Three Gorges Dam normal?
Wang Hao: Any object will deform under the action of gravity and external load. The settlement and horizontal displacement of the Three Gorges Dam under the action of its own weight, water thrust and temperature load are in line with the law of gravity dams, and the actual deformations are also within the reasonable controllable range allowed by the design. In fact, this deformation is difficult to observe with the naked eye, and it is difficult to find without monitoring data.
Beijing News: What are the main factors affecting the displacement of the dam?
Zhang Boting: Mainly affected by the upstream water pressure. Displacement occurs mainly at the top of the dam, so the farther to the top of the dam, the greater the magnitude of the displacement, and the more difficult it is to move toward the dam foundation. And generally, the more the flood period, the smaller the displacement of the dam.
Beijing News: Why is this happening?
Zhang Boting: Because the water level difference between the upstream and downstream of the Three Gorges Dam during the current flood period is small, the force is small. In winter and spring, during the dry season of the Yangtze River, the water level difference between the upstream and downstream sides of the Three Gorges Dam is relatively large, so the displacement will be larger. In other words, judging from the deformation and displacement of the dam, the Three Gorges Dam is safer in the flood season of the Yangtze River than in the dry season.
Beijing News: There is a view that dam displacement is also affected by thermal expansion and contraction.
Zhang Boting: Thermal expansion and contraction also have an impact, but the impact is small. Because there are expansion joints between the dam sections, they are made of waterproof material. If there is no seam, the dam will have to be squeezed open after the heat expands.
Beijing News: Judging from the current situation, how large is the displacement of the Three Gorges Dam?
Wang Hao: According to the monitoring data, the settlement of the Three Gorges Dam has stabilized. The maximum vertical displacement of the dam foundation is 26.69 mm, and the maximum horizontal displacement in the upstream and downstream directions is 4.63 mm. The horizontal displacement in the upstream and downstream directions of the dam crest changes periodically due to the influence of water level and temperature, and the maximum displacement 28.70 mm. The conclusion of the quality inspection expert group of the Three Gorges Project is that "From the monitoring data, the displacement range of the Three Gorges dam conforms to the law and all indicators are within the design allowable range." Since the Three Gorges Dam was impounded to a high water level of 175 meters in 2010, it has been operating for 10 years.
Comprehensive monitoring to ensure the safety of the Three Gorges Dam
Beijing News: What are the safety assessment indicators for the Three Gorges Dam?
Zhang Boting: There are actually many safety assessment indicators. The main concern of the public is dam failure, but for a hydropower station, there are many safety assessment indicators, such as seepage flow. Small seepage is allowed, but if it reaches a certain level, it will be unqualified.
Beijing News: How do we observe and know these changes?
Wang Hao: During the construction of the Three Gorges Dam project, we simultaneously buried a total of 12,087 instrument monitoring points for 14 projects in three categories. These instruments have maintained a 99.8% intact rate so far. The Three Gorges Dam is second to none in terms of the categories and coverage of safety monitoring measures, the integrity rate of internal observation instruments after more than ten years of operation, and the automation level of data processing.
In addition, the Three Gorges Project has also built a large-scale and fully-functional monitoring system for water and rain conditions in the river basin. The number of flood reporting stations has reached 1,401, and the area of the upper Yangtze River basin is controlled by approximately 580,000 square kilometers. This has achieved rapid monitoring of water and rain conditions and reservoir information in the basin. Collection, storage and processing; a complete set of meteorological and hydrological forecasting systems has been established. The forecast period of hydrological and meteorological forecasts in the basin is up to 7 days, and the accuracy of 24-hour flow forecasting exceeds 98% on average. Through forecasting and pre-discharge, it can ensure that floods are avoided. dam.
Zhang Boting: Not only the Three Gorges Dam, but all important dams in our country are equipped with a lot of automatic observation and monitoring equipment. Moreover, the monitoring data is not only controlled by the dam affiliated company, but also by the National Energy Administration's Dam Safety Supervision Center and other relevant agencies in real time. Our country has a relatively high degree of automation in this area, and in the future, real-time monitoring of important hydropower stations in the country will gradually be realized.
Beijing News: In addition to automatic monitoring, we also conduct regular inspections on dams like the Three Gorges Dam, right?
Wang Hao: The party and the state attach great importance to the quality and safety of the Three Gorges Project. Based on the supervision of the whole process of construction, the State Council has established a special quality inspection expert group for the Three Gorges Project. From the beginning of the construction period to the present, at least twice a year to the project. Conduct a comprehensive inspection on site and submit a quality inspection report directly to the State Council.
Zhang Boting: Our country has regular safety inspection requirements for all important dams. The state attaches great importance to the safety of the Three Gorges Project. The State Council’s Three Gorges Project Quality Inspection Expert Group inspects the Three Gorges Project every year. The number of this expert group is 15 this year, all of whom are the most authoritative experts in various fields. For example, the first team leader was Academician Pan Jiazheng (Editor's Note: He presided over the design and construction of dozens of dams in China, and was the director of the Expert Committee for the Three Gorges Project Demonstration and Construction), and now Academician Chen Houqun (Editor's Note: He has presided over the completion Research on the seismic structure of the Three Gorges, Liujiaxia, Xiaolangdi and Xiluodu large-scale projects has provided a scientific basis for solving major engineering problems). Check separately according to the content of the separate demonstration of the Three Gorges Dam.
The Beijing News: What is my country's level of dam monitoring capabilities?
Zhang Boting: It should be said that not only there is no gap with foreign countries, but we may still be ahead. Because most of the dams in our country were built later, the level is relatively high, and the level of monitoring automation is relatively high.
Beijing News reporter Xiao Longping and intern Qiu Xing