Dam Knowledge | Xiaolangdi: Turbulent waves appear in a calm lake, and there is no flooded beach for thousands of miles

When talking about water and sediment regulation on the Yellow River, we have to mention the Xiaolangdi Reservoir; and to understand the Xiaolangdi Reservoir, we must first understand the Yellow River.

The Mother River with a Changeable Temperament

The Yellow River is the second largest river in China after the Yangtze River, with a total length of 5,464 kilometers and a drainage area of ​​more than 750,000 square kilometers. It originates from the Yugur Basin at an altitude of 4,500 meters at the northern foot of the Bayankala Mountains on the Qinghai-Tibet Plateau, flows through nine provinces and autonomous regions including Qinghai, Sichuan, Gansu, Ningxia, Inner Mongolia, Shanxi, Shaanxi, Henan, and Shandong, and finally flows into the Bohai Sea. Along the way, the Yellow River has nurtured a long and splendid regional ethnic culture and is the birthplace of the Chinese nation. However, the Yellow River has a changeable character. While nurturing the children of China, it often changes its course and breaks its banks at will, swallowing up villages and flooding crops.

Distribution of major water conservancy hubs on the mainstream of the Yellow River (Information provided by Lin Shengrui and illustration by Li Yuxiao)

The middle reaches of the Yellow River are loess tableland areas. Due to long-term drought, the surface vegetation is sparse and has poor resistance to rain erosion. Summer rainstorms and floods wash away a large amount of exposed surface loess into the river. In the lower reaches, the Yellow River is wide and shallow, and the flow rate slows down, which leads to a decrease in the ability of the water to carry sand. Some coarse sand is precipitated, causing the river to silt up year by year, forming a "hanging river" with the riverbed far higher than the ground outside the embankment. At the same time, because the riverbed and even most of the embankments are composed of soft sand, it is difficult to restrain the fierce and roaring floods, resulting in frequent breaches and diversions in the lower reaches of the Yellow River in history. Each breach will cause a great disaster of "water flooding Jinshan and barren land for thousands of miles."

In ancient times, Dayu controlled floods by splitting three gates, dredging the river, and diverting water eastward into the sea. Dayu was recorded in history for this. Legend has it that Xiaolangdi Village was also given the honor of Dayu's name because of "losing Danyang at night", and was praised as a model of sacrificing one's own family for the sake of the whole. But thousands of years have passed, and the Yellow River is still flooded frequently. Famous water control experts of all dynasties have been working hard, but with little success. Since the founding of New China, in order to cure the "chronic disease" of frequent breaches in the lower reaches of the Yellow River in history, not only the Yellow River embankment has been raised and widened, but also large reservoirs such as Longyangxia, Liujiaxia, Wanjiazhai, and Sanmenxia have been built one after another. The Xiaolangdi Reservoir, built in 2001, has raised the flood control standard of the lower reaches of the Yellow River to once in a thousand years.

Xiaolangdi Water Conservancy Project discharging floodwater (Photo source: Tuchong Creative)

The past and present of Xiaolangdi Project

The idea of ​​Xiaolangdi Reservoir was proposed as early as the 1930s. At that time, water conservancy experts proposed to build a dam on the Yellow River to put a chain on the Yellow River dragon. The three dam sites were Sanmenxia, ​​Dalongmen and Xiaolangdi. However, due to frequent wars and turbulent times, this idea could only be a fantasy. After the founding of New China, at the second session of the First National People's Congress held in July 1955, Deng Zihui, then Vice Premier of the State Council, made a "Report on the Comprehensive Plan for the Radical Treatment of Yellow River Floods and the Development of Yellow River Water Conservancy", in which the Sanmenxia Water Conservancy Project and Xiaolangdi Water Conservancy Project were included in the package of projects for the control of water and sand in the Yellow River.

The Sanmenxia Project, the "first dam on the Yellow River" (Photo source: Tuchong Creative)

On April 13, 1957, the construction of the Sanmenxia Water Conservancy Project began. It was the first large-scale water conservancy project built on the mainstream of the Yellow River with flood control as the main purpose and comprehensive utilization. In September 1960, the project was basically completed. However, the flood control problem of the Yellow River tributaries such as the Yiluo River and the Qin River remained unsolved. The construction of the Xiaolangdi Water Conservancy Project was imminent.

In the autumn of 1954, more than 500 people from the Yellow River Conservancy Commission's geological exploration team moved into Xiaolangdi and began to conduct long-term and detailed exploration and drilling work in the Xiaolangdi area of ​​the Yellow River. Subsequently, the Yellow River Conservancy Commission's geological team, mountain team, geophysical team, and surveying team moved in one after another. The Baoshan Hydrological Station of the Yellow River Conservancy Commission also moved to Xiaolangdi Village to obtain first-hand hydrological data of the Xiaolangdi area of ​​the Yellow River. On April 9, 1991, the Xiaolangdi Water Conservancy Project was officially included in the national plan and was determined to start construction during the "Eighth Five-Year Plan". On September 12, 1994, Li Peng, then Premier of the State Council, solemnly announced: "The construction of the Xiaolangdi Water Conservancy Project on the Yellow River has started!" The Xiaolangdi project entered a construction period of 11 years. Subsequently, on October 28, 1997, the Yellow River was successfully diverted; on January 9, 2000, the first unit generated electricity; in December 2001, the project was completed. On April 7, 2007, the Xiaolangdi Water Conservancy Project successfully passed the national comprehensive completion acceptance.

The Xiaolangdi Project is in line with international practices and has fully implemented the project legal person responsibility system, bidding system, construction supervision system and contract management system. Three international consortiums consisting of 14 companies and construction units from four countries won the bid to build three civil engineering projects: the dam project, the flood discharge system project and the water diversion and power generation system. Foreign funds, technology, talents and advanced management experience have been introduced, forming an international engineering management model with Chinese characteristics. The Xiaolangdi Project dam adopts an inclined core wall rockfill dam with a maximum designed dam height of 154 meters, a dam crest length of 1,667 meters, a dam crest width of 15 meters, and a maximum dam bottom width of 864 meters. The dam body has a filling volume of 518,500 cubic meters, and the foundation concrete anti-seepage wall is 1.2 meters thick and 80 meters deep. Its filling volume and concrete anti-seepage wall are the largest in China. The Xiaolangdi Water Conservancy Project has an important strategic position, a large project scale, complex geological conditions, special water and sand conditions, and strict application requirements. It is called one of the most complex water conservancy projects in the world by Chinese and foreign water conservancy experts.

Model of the inclined core rockfill dam of Xiaolangdi Reservoir (Photo by Wang Lili/Fotoe

The banks of the Xiaolangdi Reservoir are Xiaoshan, Shaoshan and Mangshan in the Qinling Mountains, and Wangwushan in the Zhongtiao and Taihang Mountains. Its completion can effectively control the Yellow River floods, and can raise the flood control standard of Huayuankou in the lower reaches of the Yellow River from once in 60 years to once in 1,000 years, basically eliminating the threat of ice floods in the lower reaches of the Yellow River and slowing down the siltation of the lower reaches of the river. The Xiaolangdi Reservoir can also effectively regulate the runoff in the non-flood season with long-term storage capacity, and increase the amount of water for urban and industrial water supply, irrigation and power generation. It is at the key position of connecting the upper and lower reaches to control the water and sand in the lower reaches, and controls 100% of the sediment transport of the Yellow River.

Practice of water and sediment regulation in Xiaolangdi Project

In order to ensure that Xiaolangdi Reservoir has more flood control storage capacity, the Yellow River Flood Control Headquarters will order the reservoir to discharge a large amount of previously stored water in advance before the flood season every year. In order to make these discharged water flows play a greater role, the Yellow River Conservancy Commission organized experts from the Yellow River Water Science Institute, Design Institute, River Affairs Bureau, Hydrological Bureau and other relevant departments to develop a complete set of practical plans for Yellow River control through experimental demonstration and research on the Yellow River model, called water and sand regulation. It aims to use limited water resources to transport the silt deposited in the Sanmenxia Reservoir to the Bohai Sea as much as possible. Since June 2002, sand has been flushed and disturbed upstream of the Sanmenxia Reservoir (intended to lower the "Tongguan elevation" - referring to the corresponding water level at Section 6 of the Yellow River Tongguan Hydrological Station when the flood flow is 1,000 cubic meters per second. Its changes reveal the siltation of the lower reaches of the Weihe River) and discharged through the Sanmenxia Reservoir's sand discharge holes. The Xiaolangdi Reservoir is discharged using the principle of "heterogeneous density flow" (water bodies of different specific gravities flow in layers in a basically constant and uniform fluid, starting from the "entry point", the reservoir water becomes clear on top and turbid on the bottom, with the turbid water flowing in the middle of the water body) of the Xiaolangdi Reservoir.

Schematic diagram of density flow (Photo source: Hydrology and Water Resources Bureau of Yellow River Conservancy Commission)

From 2018 to 2021, the decision-makers of the Yellow River adopted a bold attempt to empty the Xiaolangdi Reservoir before the flood season (the premise is to ensure that there is enough water after the flood season to provide the reservoir with drought resistance and prevent downstream flow interruption, which requires accurate medium- and long-term forecasts to support it), and used the joint dispatch of the Wanjiazhai and Sanmenxia reservoirs and the floods generated in the middle reaches to flush the silt deposited in the Sanmenxia and Xiaolangdi reservoirs in the early stage. During this period, the author was at the Xiaolangdi hydrological station and witnessed the maximum sediment content monitored by the hydrological station, which increased year by year from less than 2 kilograms per cubic meter in 2017 to 378 kilograms per cubic meter in 2021. The reservoir flushing effect is very obvious.

Xiaolangdi Reservoir regulates water and sediment, and the pattern formed by the trickle when the water level in the reservoir drops (Image source: Tuchong Creative)

In order to prevent the discharged silt from accumulating in the downstream river channel, the four reservoirs of Sanmenxia, ​​Xiaolangdi, Luhun and Guxian will be jointly dispatched in the later stage to output a large amount of clean water to flush the downstream river channel and ensure the flood-carrying capacity of the downstream river channel. On June 28, 2020, in order to verify the actual water-passing capacity of the main channel of the lower reaches of the Yellow River, the Yellow River Flood Control Headquarters adjusted the maximum discharge flow of the Xiaolangdi Reservoir to 5,680 cubic meters per second. On July 1, the flow of the Aishan checkpoint downstream reached 4,800 cubic meters per second, but there was no floodplain along the entire downstream. According to statistics, as of the pre-flood period of 2020, after nearly 20 years of water and sand regulation, the average scouring depth of the riverbed in the lower reaches of the Yellow River reached 1-2 meters, increasing the water-passing capacity of the main channel by about 200 cubic meters per second. Such achievements are gratifying.

Xiaolangdi Reservoir, fishing boats singing in the evening (Photo source: Tuchong Creative)

Nearly 20 years have passed. In addition to its functions of flood control, ice prevention, silt reduction, water supply, irrigation and power generation, the Xiaolangdi Hydropower Project has also been transformed into a national 4A-level tourist attraction with beautiful scenery and rich culture, one of the top ten tourist hotspots in Henan Province, and is also known as the "Little Thousand Island Lake in the North". Domestic and foreign tourists can enjoy the visual feast brought by the Xiaolangdi Hydropower Project.