Why has this water conservancy project, which was first built during the Warring States Period, benefited mankind for two thousand years?

After visiting Dujiangyan and Qingcheng Mountain, Mr. Yu Qiuyu left behind the famous saying "Worship the water at Dujiangyan and seek the truth at Qingcheng Mountain". Dujiangyan, as an important water conservancy project in the Chengdu Plain, has played an important role for two thousand years and brought endless benefits to future generations. However, hydrological management has been a major issue in my country since ancient times. The management of the Yellow River is a typical example. All dynasties have invested a lot of manpower and material resources, but it has not been completely solved so far. Factors such as climate change and river evolution have posed challenges to the subsequent maintenance of water conservancy projects and are difficult to sustain.

So, what is so special about Dujiangyan? Why did it make Chengdu the "Land of Abundance"? What was the process of its construction? Why has Dujiangyan been used until now? This is closely related to its scientific design, construction quality and ability to adapt to the environment. At the same time, Dujiangyan is also the crystallization of the wisdom of the ancient Chinese working people, embodying the concept of harmonious coexistence between man and nature, which is an important reason why it has been able to survive for thousands of years.

Dujiangyan

▏Flooding in the Minjiang River

Minjiang River, as an important tributary of the Yangtze River, has an average flow of about 2,800 cubic meters per second at its estuary, which makes its annual runoff more than twice that of the Yellow River. Its basin area is vast, reaching 135,882 square kilometers, of which 126,280 square kilometers are located in Sichuan. Minjiang River flows through many places, including Wenchuan, Chengdu, Meishan, Leshan, Shuangliu, etc. Its hydrological characteristics are characterized by Dujiangyan as the upstream, which is the main water source for the local area.

In ancient times, the Sichuan Basin and surrounding areas in Shu were described in poems and articles as having steep mountains. As Li Bai described in "The Difficult Road to Shu": "Above, there are high peaks where six dragons circle the sun, and below, there are winding rivers where the waves rush against the water." There are many rugged mountains in Sichuan, but after passing through them, you can see the plains formed by the impact of rivers. The economic development potential of the plains is huge. However, before the construction of Dujiangyan, once a flood occurred, the entire Chengdu Plain would become a vast ocean, and development would be almost impossible.

Sichuan is a basin with a subtropical monsoon climate. The main crop here is rice, especially in the Chengdu Plain. The rice grows and matures in summer and autumn, which are also the seasons when floods in the Minjiang River are frequent. Before Dujiangyan was renamed, it was called "Qian'an", which was the transition zone between the Qinghai-Tibet Plateau and the Chengdu Plain. The Minjiang River rushes directly to Dujiangyan from top to bottom. At that time, people were almost helpless in the face of the raging floods, facing the dilemma of "people may become fish and turtles".

However, the Minjiang River did not destroy the entire Chengdu Plain due to floods, but chose to bypass the Yulei Mountain and march southward. This resulted in frequent floods in the western part of the Chengdu Plain, while the eastern part was very dry. It brought both abundant water resources and abundant products, but also the challenges of natural disasters such as floods. However, it was these challenges that prompted people to seek solutions. The construction of the Dujiangyan Irrigation System enabled the Sichuan region to remain stable and prosperous in the face of natural disasters.

▏Construction of Dujiangyan

At the end of the Warring States Period, King Zhao of Qin realized the strategic importance of the Bashu region to the country, and he sent Li Bing, a man who was well versed in astronomy and geography, to serve as the governor of Shu County. In 256 BC, after Li Bing took office, his first task was to cure the floods in the Minjiang River, develop local agriculture, and lay a solid economic foundation for Qin to unify China.

Li Bing and his son worked with local water management experts to study the project. They drew on the experience of their predecessors and conducted in-depth field investigations on the topography and water flow in the Minjiang River area. Finally, they decided to select the Yulei Mountain where the Minjiang River crosses the mountains and plains. This is the apex of the Chengdu fan-shaped alluvial plain and an ideal location for water diversion. This will ensure diversion irrigation in the irrigation area.

The Dujiangyan Irrigation Project is mainly divided into three parts: "Fish Mouth", "Flying Sand Weir" and "Baopingkou". These three parts have clear division of labor and reasonable configuration, which enables Dujiangyan to play its role for a long time. Li Bing and his team carefully designed and cleverly used natural conditions to enable Dujiangyan to effectively prevent floods, divert water and divert water, providing strong support for local agriculture and people's lives.

▏The three main parts of Dujiangyan and their working principles

1. Fish mouth

Deep in the middle of the river, there stands a unique landscape. It is like the work of nature, cleverly dividing the Minjiang River into two. This is the Fish Mouth of Dujiangyan. As the name suggests, the Fish Mouth is shaped like a fish's mouth, dividing the rolling Minjiang River into two branches, the inner and outer branches. It is mainly responsible for flood discharge, ensuring that the river water can pass smoothly during the flood season. The Neijiang River is located on the east side. As the main canal for artificial water diversion, it is responsible for providing irrigation water for farmland. The water intake of the Neijiang River is 150 meters wide, while the water intake of the Waijiang River is slightly narrower, 130 meters wide.

By cleverly utilizing the advantages of the terrain, Yuzui regulates the flow of river water according to a specific water diversion ratio. In the spring when flowers are blooming, the water level of the Minjiang River is low, with about 40% of the river water flowing into the Yangtze River and 60% flowing into the Inner River to ensure the water demand for spring farming. When the rainy season arrives at the turn of spring and summer, the water level rises, and Yuzui plays its magical function, directing 60% of the water to the outer river and 40% to the inner river. Such a diversion ratio not only protects the irrigation area from flooding, but also meets the water demand of farmland.

杩様

The exquisite design of the fish mouth is also reflected in its harmonious coexistence with the surrounding environment. The inner and outer diamond dikes on the outer edge echo each other with the fish mouth, and together maintain the stability of the diversion ratio. The secret lies in the use of the movement law of the water line in the bend: it goes straight when there is a lot of water, and it goes curved when there is less water. At the critical point of the deflection of the water line in the flood and dry seasons, the water flow at different water levels will flow into the inner river or the outer river as needed. In order to better regulate the water flow, people also adopted an ancient regulation method-making a chariot. A triangular bracket is made of three mutually tied wooden stakes, and the chariots are placed and connected at the location where construction is required. At the same time, a pebble cage is placed in each chariot to ensure the stability of the chariot. This method is generally used in spring when there is less water flow to increase the water supply of the inner river.

Today, the design and construction of the Yuzui section is more modern. The traditional wooden sluice has been replaced by a permanent sluice gate, which pays more attention to flexibility and time security. The Yuzui is now built with solid stone masonry and concrete, and the embankment is continuously extended downstream. The King Kong Dike has also been completed.

Although the location of Yuzui has changed many times in history, it has not had much impact on the entire Dujiangyan water conservancy system. As a symbol of the upstream location, Baizhang Dike works together with Yuzui to help discharge floodwaters, drain sediments, and regulate water volume. This ancient cooperation mechanism has played an important role in the past and present, ensuring the normal operation of the entire Dujiangyan water conservancy system.

2. Feishayan

We came to Feishayan, which is the middle section of the Dujiangyan Irrigation Project and is mainly responsible for sand discharge. It was built during the Longshuo period of Emperor Gaozong of the Tang Dynasty, around 663 AD, and is the main channel for flood discharge and sand discharge in Neijiang. Feishayan was built from a gap in the lower section of the Jingang Dike, located on the opposite bank of Hutouyan, with the top of the weir 2 meters higher than the riverbed. It can discharge water that exceeds the water volume in the irrigation area to the outer river, thus ensuring that the plain will not be affected by floods.

The bend circulation is caused by the centrifugal force. The water level on the concave bank is high and the water depth pressure is high. Under this pressure, the water at the bottom of the concave bank flows to the convex bank, forming a bend circulation. The strong force formed by this circulation can "throw" a large amount of sand and gravel to the outer river, thus avoiding the blockage of the inner river. In addition, the "low weir" principle of Feishayan allows the inner river water that exceeds the upper limit of the Baopingkou flow to overflow the top of the weir and flow into the outer river. If a large-scale flood is encountered, the weir will collapse by itself, allowing the water to return to the main flow of the Minjiang River. All of this demonstrates the wisdom and talent of the ancients, allowing the Dujiangyan water conservancy project to last for thousands of years and benefit generations of people.

3. Baopingkou

In Dujiangyan, there is an amazing engineering miracle, that is, Baopingkou. It is the iconic part of the Dujiangyan water diversion project. It is an artificial gap dug from Yulei Mountain to Minjiang River. This gap presents a unique shape of wide at the top and narrow at the bottom. The bottom width is about 14 meters, and the top shrinks to about 28 meters. The height is about 18 meters. The function of Baopingkou is to regulate the water flow together with Feishayan.

In ancient times, this project relied entirely on people's wisdom and efforts, and was artificially excavated using methods such as burning stones and pouring cold water to expand and contract. When floods came, Baopingkou and Feishayan cooperated with each other to make part of the flood flow to the outer river, thereby slowing down the speed of the inner river flow and achieving the effect of flood control. After passing through Baopingkou, the water flow followed the changes in the terrain and formed a branch water diversion irrigation canal system, which moistened the vast farmland on the Chengdu Plain.

On the cliff on the left bank of Baopingkou, there are dozens of stone lines, called water scales, each about 30 cm apart. This is the earliest water level scale in my country, used to observe the rise and fall of water levels. The water level scale of Dujiangyan has been continuously expanded with the increase of irrigation area, and it has reached 24 strokes in the Qing Dynasty. Feishayan and Baopingkou cooperate with each other to regulate water flow and discharge sand. After the rapids pass through Baopingkou, they form a vortex outside the mouth, which is called "Fulongtan".

The water conservancy construction of Dujiangyan fully utilized the local natural conditions and had little impact on the environment. The distinct climate and stable rainfall in the rainy season make it not only have the functions of water diversion and irrigation and flood control, but also have the functions of water transportation and urban water supply. Today, Dujiangyan is still an important water conservancy project, providing solid support for my country's agricultural and urban development.

▏ Annual maintenance of Dujiangyan

Since its construction, Dujiangyan has benefited mankind for two thousand years. The greatest feature of this project is that it completely complies with the natural flow of the river and does not violate its laws at all. The reason why it has been able to stand for two thousand years is because of the convenience it brings to people, so people have never stopped repairing it every year for two thousand years.

During the reign of Emperor Ling of the Han Dynasty, "Du Shui Jian" and "Du Shui Chang" were set up to maintain water conservancy projects. In the Song Dynasty, an annual maintenance system was established that is still in effect today. Every year during the dry season in winter and spring and the slack season in farming, people would carry out annual maintenance, the main task of which was to repair the bamboo cage structure in the waterway and clear the accumulated silt. In order to measure the depth of the inner water, Li Bing placed three stone piles of human figures to determine the water level by "dry water does not flood the feet, and flood water does not exceed the shoulders". In the Ming Dynasty, the stone elephants were replaced by a ten-foot-long lying iron.

Today, the bamboo cage structure of the Yuzui diversion dike has been replaced by reinforced concrete to make it stronger. However, the traditional bamboo cages are still in use today. Today, the Dujiangyan Irrigation Project has become the world's oldest and only surviving damless water diversion project still in use.

▏Benefit future generations

Over the course of thousands of years of history, the water conservancy project at Dujiangyan has been like a solid bridge, dividing the Minjiang River into two parts, preventing it from flooding, and providing an endless source of water for farmland irrigation. It not only promoted the political, economic and cultural development of the early Sichuan region, but also served as a symbol of the benefits of agricultural irrigation.

With the gradual improvement of the irrigation canal system, the expansion of Baopingkou has led to the continuous expansion of the irrigation area, from Pi County in the Qin Dynasty to the hinterland of Chengdu, with an irrigation area of ​​more than 10,000 hectares. In modern times, the power of science and technology has strengthened and enhanced the water conservancy project of Dujiangyan, and Yuzui, Baopingkou, Lidui and all the dikes have been carefully protected.

Today, Dujiangyan is even better than before, and its charm is timeless. As an old saying goes, "Those who plant trees will enjoy the shade." The Dujiangyan Irrigation Project has been properly managed since its inception, making it an important pillar of Sichuan's economic development.

(Picture from the Internet)

Author | Kiwi

Graduated from Lincoln University, New Zealand, with a major in finance. Has a strong interest in popular science knowledge and has published popular science articles in many popular science journals. Focuses on facts and actively explores cutting-edge technology.