Three Gorges Reservoir sees largest flood peak since its construction

The Three Gorges Dam, constructed and operated by the China Three Gorges Corporation (CTG), saw the safe passage of peak of the fifth flood of the Yangtze River this year at 8 am on August 20 when the inflow reached 75,000 cubic meters per second, the largest flood since the dam was completed in 2003.

In accordance with the latest flood control operation instructions, the Three Gorges Dam has opened 11 spillway holes, generating an outflow of 49,200 cubic meters per second and achieving a peak reduction rate of 34.4%.

This round of flooding has broken multiple records in terms of flood peak and flood discharge. The Three Gorges Project had only seen two floods with an inflow rate of 70,000 cubic meters or more per second, in 2010 and 2012 when the peak was 70,000 cubic meters and 712,000 cubic meters respectively.

Heading into this year’s monsoon season, the Yangtze River has been hit by frequent, long-lasting heavy rainfall, resulting in the total cumulative precipitation in the basin exceeding that of 1998 and ranking second in the hydrological records. Due to two successive heavy rainfalls in the Jialing River and the basin of Min River and the Tuo River in August, Min River was hit by its highest flood peak in history. At the same time, the Jialing and Tuo Rivers have both experienced their highest flood peak flow rates in history. Floods from the upper reaches of the Yangtze River have simultaneously entered into the Three Gorges Reservoir, with just a five-day gap between the fourth and fifth floods. The two floods came around the same time of day and had similar high peaks and flow rates.

In order to effectively combat the huge floods, CTG has carried out multiple measures strictly in line with flood control operation instructions from the Ministry of Water Resources and the Yangtze Water Resources Committee. After carefully monitoring the extreme rainfall situation in early August, CTG accurately forecasted the two floods would have a flow rate of over 60,000 cubic meters per second, the peak time and the peak flow rates a week, three days and 24 hours in advance respectively. Based on these predictions, CTG was able to prepare by lowering the water level by over 10 meters and setting aside 80% of the flood control capacity in advance of the major floods.

Meanwhile, CTG has utilized reservoirs in the upper reaches of the Yangtze River to help the Three Gorges Project retain floodwater. As Wudongde reservoir elevated its water level by 1.5 meters per day and Xiluodu and Xiangjiaba reservoirs jointly held back the floodwater, the flood flow into the lower reaches of the Yangtze River is expected to reduce by about 2 billion cubic meters, greatly easing the pressure on the Three Gorges Project. CTG has also issued flood warnings and responded in a timely manner to the flood, strengthening its joint consultation with other government agencies, releasing information more frequently, and increasing on-site personnel to ensure it has adequate emergency response forces.

Compared with the major floods of 1998 and 2012, the Yangtze River basin has now established a joint flood control operation system in which the reservoirs coordinate to discharge floodwater from the upper and lower reaches of the Yangtze River in an orderly manner. During this round of flooding, the inflow of the Three Gorges Reservoir has been reduced by 9,000 cubic meters per second on average. This is equivalent to the water level detected at Chongqing Cuntan Port Station, the main flood control station in the lower reaches of the Jinsha River, lowering to about two meters while the Three Gorges Reservoir maintains its current water level. Joint flood control has effectively relieved the flood control pressure of the Sichuan and Chongqing sections of the Yangtze River and underscored the mitigation capacity of the cascade reservoirs in the river.

Since the flood season arrived in 2020, the Three Gorges Project has successfully mitigated nine floods with a flow rate of more than 30,000 cubic meters per second, which is the second most since its construction. The cumulative amount of retained floodwater when Yangtze River’s fifth flood is over is expected to exceed 28 billion cubic meters, the highest in history and equal to about 2,000 times the size of West Lake, the renowned freshwater lake in Hangzhou.

By Huang Yukai