Tongjiyan Weir, also known as Yuanjiyan Weir, is located on the Meizhou-Pengshan Plain in the Sichuan Basin of China. Sitting at the confluence of the Minjiang River, Nanhe River and the Xihe River, it irrigates about 35,000 ha of farmland in Xinjin, Pengshan, Dongpo and Qingshen counties under Chengdu and Meishan cities.

Tongjiyan Weir was first built in 141 BC. Wen Weng, the then governor of Sichuan, ordered the construction of a giant weir on the Minjiang River in Wuyang County, or the east of Pengshan today. As the headwork of the Tongjiyan Irrigation Scheme, this weir featured six water gates to regulate water flow according to the irrigation demand. As Li Jifu of the Tang Dynasty writes in his Yuanhe Chorography of Counties and Prefectures, Tongjiyan Weir, 12.5 km to the southwest of Pengshan County, was a giant structure on the Minjiang River with six water gates for the purpose of irrigation.

During the period of 713-741 AD, Zhangchou Jianqiong, the then governor of Sichuan, rebuilt the Tongjiyan Weir, expanding the irrigated area to more than 10,000 ha. In 1145 of the Song Dynasty, Julong Tingshi, the mayor of Meizhou, presided over another rehabilitation of the weir. At that time, the weir was 924 m long, a rare record for an ancient weir, and the irrigated area grew to exceed 22,000 ha.

Tongjiyan Weir was built with bamboo cages filled with stones. Different types of cages called Shunlou and Pinlou were used for the construction of levees and the weir, respectively. Pinlou has a trapezoidal cross section. Every year, before the spring irrigation, craftsmen would make cages with local bamboos, fill them with stones, and put them into the river to block the water for diversion. These building materials, enjoying both strength and flexibility, were readily available, affordable and easy to use. Lu You, a famous poet of the Song Dynasty, once wrote a poem to depict the rehabilitation of Tongjiyan Weir: “lying on a rainbow, the barrage is three hundred meters long; …bamboos from the West Mountain are woven into ten thousand cages.” Wang Menggeng, a local magistrate in the Qing Dynasty, also recorded such a spectacular scene in his poem. Song Hao, another local magistrate in the Qing Dynasty, drafted the Statute of Tongjiyan Weir, setting out special regulations on the construction techniques and annual repair method of the headwork.

In ancient times, the dam-less water diversion technique was popular in the Sichuan basin. Tongjiyan Irrigation Scheme is one of the few projects of the Minjiang River basin which relies on barrages for water diversion. Tongjiyan Weir intersects diagonally with the Nanhe River with an angle of 137.5°. As the largest and longest-standing movable weir in Chinese history, Tongjiyan Weir represents major progress made in the field of hydraulic engineering. The bamboo cages filled with stones are easy to make and use materials which are locally available. A weir built with such materials could both withstand flooding and meet the irrigation demand downstream. As a result, the bamboo cages and pebbles had not been replaced by a concrete dam with sluices until recently.

Tongjiyan Irrigation Scheme has three main canals, 65 branch canals, and thousands of lateral canals and field ditches.

The rivers to the west of the Minjiang River, including the Maohe River and Liquanhe River, intersect with the canals of the irrigation scheme. Meanwhile, different levels of canals are distributed in a braided pattern. The main stream of the Minjiang River is replenished with melted snow, and the seasonal changes in its runoff correspond to the shifts between the rainy and dry seasons. The impact of rainstorms covering large areas could spread through flood peaks. The braided distribution of the canal system, combined with its flood discharge gates, could widen the river bed when flood peaks arrive and help discharge flood at different sections so as to quickly reduce the water level.

The distribution of the irrigation canals displays a complex and interlocking nature. Despite being man-made, the canals share similar characteristics with natural watercourses.

Tongjiyan Irrigation Scheme has played an irreplaceable role in flood control and water ecosystem improvement in the small basins on the west bank of the Minjiang River, fundamentally bettering the urban and rural ecosystems of the region.

The structure was ahead of its times in terms of project formulation, engineering design, construction techniques, dimensions of the structure itself, quantum of water diverted, and size of the command;

The water intake of Tongjiyan Weir is located at the confluence of Minjiang River, Xihe River and Nanhe River, which has guaranteed its water supply. On its right bank stands the Xiujue Mountain which is hard enough to resist the scour, solving the problem of convex bank erosion. By adapting to and even utilizing the natural conditions, the Weir, with its smart siting, has successfully transformed water from a damaging force into a source of prosperity for the Chengdu plain.

The structure made outstanding contribution to enhancing food production, livelihood opportunities, rural prosperity, and poverty alleviation in a region;

Sichuan used to be a major source of tax income in ancient China (the Tang and Song Dynasties), and the region surrounding Dujiangyan Weir and Tongjiyan Weir was the main contributor in Sichuan. In the Song Dynasty (960-1279), the irrigated area of Tongjiyan Weir exceeded 22,000 ha. Located 250 m upstream of the confluence of the Minjiang River and the Nanhe River, the Weir enjoys sufficient area for rainwater collection and thus abundant water for irrigation at its downstream. Standing at the starting point of the Meizhou-Pengshan alluvial plain, the Weir is able to cover vast farmland, increase grain yield and bolster regional agricultural development and rural prosperity.

The structure contributed to the evolution of efficient and contemporary engineering theories and practices;

Tongjiyan Weir was built with bamboo cages filled with stones. Different types of cages called Shunlou and Pinlou were used for the construction of levees and the weir, respectively. Pinlou has a trapezoidal cross section. In this way, the water resource could be fully tapped, and the impact on natural watercourse minimized. By utilizing the local terrain and how the Minjiang River flows around the curve, Tongjiyan Weir could perfectly meet the demand for water diversion, flood discharge, and sediment flushing and serve the local people without causing damage to the natural resources.

The structure is an example of attention to environmental aspects in its design and construction;

Its headwork and different levels of canals, with the functions of irrigation, flood control, water transport, and urban water supply, have formed a canal network similar to that of natural watercourses. Its building materials such as stones, timbers and bamboo are locally available. Tailored to the local terrain and resource conditions, the Scheme fits well into the water ecosystem and is an embodiment of ancient people’s perception of nature and the philosophy of man learning from and being an integral part of nature.

The structure was an example of engineering marvel or excellence at the time of its construction;

In ancient times, the dam-less water diversion technique was popular in the Sichuan basin. Tongjiyan Irrigation Scheme is one of the few projects of the Minjiang River basin which relies on barrages for water diversion. Tongjiyan Weir intersects diagonally with the Nanhe River with an angle of 137.5°. As the largest and longest-standing movable weir in Chinese history, Tongjiyan Weir represents major progress made in the field of hydraulic engineering.

The structure was unique in some positive and constructive way;

The rivers to the west of the Minjiang River, including the Maohe River and Liquanhe River, intersect with the canals of the irrigation scheme. Meanwhile, different levels of canals are distributed in a braided pattern. The main stream of the Minjiang River is replenished with melted snow, and the seasonal changes in its runoff correspond to the shifts between the rainy and dry seasons. The impact of rainstorms covering large areas could spread through flood peaks. The braided distribution of the canal system, combined with its flood discharge gates, could widen the river bed when flood peaks arrive and help discharge flood at different sections so as to quickly reduce the water level and prevent losses. The overflow surface is usually stepped to facilitate energy dissipation.

The structure bears the stamp of a cultural tradition or a civilization of past;

The history of the Tongjiyan Irrigation Scheme exceeds 100 years (dating back to 141 BC). The Scheme is composed of water diversion structures such as weirs and barrages and canal systems. Its engineering form remains basically unchanged till today. As the theme of a large number of ancient poems and essays, the Scheme is deeply rooted in Chinese history and culture and has itself cradled multiple local sub-cultures of poetry, longevity, bamboo, and loyalty and piety.

Engineering utility vis-à-vis designed utility

Tongjiyan Weir was first built in 141 BC. Wen Weng, the then governor of Sichuan, ordered the construction of a giant weir on the Minjiang River in Wuyang County, or the east of Pengshan today. As the headwork of the Tongjiyan Irrigation Scheme, this weir featured six water gates to regulate water flow according to the irrigation demand. Later, he ordered the construction of a new canal to facilitate the transport of bamboo and wood from the mountains to the west of the Weir. With these building materials, thousands of water storage facilities were built.

In 740, Zhangchou Jianqiong, the then governor of Sichuan, rebuilt the Tongjiyan Weir, expanding the irrigated area to more than 10,000 ha, which has laid a solid foundation for the sustainability of this water-diverting system. In 907, Zhang Lin, the mayor of Meizhou, launched another round of rehabilitation, further enlarging the irrigated area to 15,000 ha and benefiting the locals greatly; as a result, the water supply to the downstream farmland was improved, further boosting local agricultural development.

In 1145 of the Song Dynasty, Julong Tingshi, the then mayor of Meizhou, presided over the rehabilitation of the Weir, after which the Weir was 924 m long with 119 tube weirs. During the Xinglong Period (1163-1164) of the Southern Song Dynasty, Fan Chengda, the then governor of Sichuan and a water expert, initiated another round of restoration. He built embankments with stones, installed 49 water gates, set up water gauges, and ensured orderly irrigation; the local people benefited greatly. The maximum irrigated area of the Tongjiyan Irrigation Scheme was 22,667 ha in the Song Dynasty (960-1279).

In 1432 of the Ming Dynasty, the Weir was again rehabilitated. Water was diverted to Pengshan through 16 canals, nourishing 1667 ha of farmland. This was the first rehabilitation in the Ming Dynasty, though the benefit it brought was much smaller than that of the Tang and Song Dynasties. In 1442, a relatively large-scale restoration was carried out; it is recorded that both civilians and soldiers were called up to work, and local officials were ordered by the Emperor to control the cost.

In 1733 during the Qing Dynasty, Huang Tinggui presided over another round of rehabilitation. Learning from the Dujiangyan Weir, he replaced giant rocks with bamboo cages filled with pebbles as building materials to reduce cost. The new materials, enjoying both flexibility and solidness, were cheaper and easier to use, more resistant to the impact of the turbulent water, and more adapted to the deformation of the weir foundation. However, they lacked durability, which means annual repairs were needed. In 1753, the local officials initiated a project of repair and expansion. By 1755, the canal system in Meishan had been restored. In addition, new embankments were built at the headwork, the old weir grew longer, and more than 40 km of old ditches were dredged. In Pengshan, 28 abandoned canals were restored, irrigating 2615 ha; in Meizhou, 14 abandoned canals were restored, watering 1952 ha, and if the irrigated area of Xinjin is included, the total irrigated area reached 4874 ha.

In 1802, the officials of Meizhou, Xinjin and Pengshan bought land from a local resident called Lv Chao and opened a 500-meter-long canal upstream of the headwork of Tongjiyan Weir. The canal, called Baijihe, diverted water from the Yangmahe River to the Weir. In addition, they blocked the ditch connecting the Xihe River with the Minjiang River. In this way, the water supply was increased, and irrigation water was guaranteed.

In 1915, Xu Yuanlie, the director of the water conservancy bureau of Pengshan, reported to the provincial government and required rehabilitation of Tongjiyan Weir. He planned to rebuild the weir body with stone slabs and cement them with tabia and only use bamboo cages filled with pebbles to block the temporary discharge gates. He also suggested using Macha, a temporary damming tool used at Dujiangyan Weir.

With agricultural and industrial development in the region, the old headwork was unable to meet the newly added water demand as a result of economic growth. Therefore, a permanent dam was built in 2005, ending the history of low-dam water diversion. The new dam is 417 m long: the overflow dam is 172 m long, and 245 m are the flood discharge gates (14) and flushing sluices (3).

Present State of Conservation

The management agency of the Scheme has adopted effective protective measures to minimize the risks of encroachment and water pollution and to increase its resilience to flooding, earthquake and pressure brought by the development of tourism.

As part of the publicity campaign, an exhibition hall has been built to display the cultural relics and basic information about the project; relevant activities have been organized to raise public awareness of water saving, water pollution, and heritage conservation.

Management

Many unique water measuring facilities and standards have been introduced. To monitor and regulate the water level, stones with six grooves were used as water gauges; and the standards of water level regulation set by Zhangqiu Jianqiong remained in use till the Qing Dynasty; iron walls with embedded tubes, invented by the locals in practices are tools of water measurement and allocation rarely seen in China. At the water intakes of the branch canals of the Tongjiyan Irrigation Scheme, water was diverted by weirs or the above-mentioned tubes, the size of which is proportional to the size of the irrigated farmland. Made of stone or iron, these tubes are very durable. In 1764, one local official set up steles in the Erwang Temple of Xinjin with inscriptions of the standards and specifications of the tubes and the bans on changes without permission. From the water governance practice over the past 2000 years, the managers of Tongjiyan Irrigation Scheme have summed up the principles of weir operation, formulated annual repair guidelines, mastered the technique of draining out excess water and sediment at proper sites, and set up a management system tailored to the Scheme by establishing a professional management agency and the weir chief system.