Dujiang Canal: a successful example of the use of dredging and water diversion for flood control
The Dujiang Canal (Dujiangyan) is located in Sichuan Province on the middle reaches of the Minjiang River, a tributary of the Changjiang (Yangtze River) River. The Minjiang River discharges approximately 86.8 billion cubic meters of water per year, more than any other branch of the Changjiang River and approximately one and a half times as much as the Yellow River. It drops over 3000 meters between its source and its middle reaches, where it levels off to run through the Chengdu Plain. These physical characteristics cause the Minjiang River to flow rapidly in its upper reaches, picking up and carrying profuse quantities of silt and gravel downstream, only to slow abruptly when it reaches the Chengdu Plain. As it slows, it deposits its burden of sediment, silting up the watercourse and causing frequent flooding. The Chengdu Plain is the result of this accumulation of silt and gravel over millions of years.
Attempts to control flooding around the Minjiang River commenced early in the Warring States Period (476-221 BC). During the reign of King Zhao of Qin (306-251 BC), Li Bing (c.286-231 BC), the local governor, was given the responsibility for bringing the river under control. Following an intensive survey of the area's geological features and the distribution of its watercourses, Li Bing designed and built the great Dujiang Canal water conservancy project.
The Dujiang Canal consists of three main sections: Fish Mouth(Yu Zui), Precious Vase Mouth (Baoping Kou), and Flying Sand Weir(Feisha Yan). Fish Mouthi, like a big fish lying in the Minjiang River (as "A" in the sketch), is a watershed dividing the river into two parts: Inner River (D) and Outer River (E). Originally constructed of bamboo cages filled with cobblestones, this Fish Mouth was renovated in modern times using steel-reinforced concrete. The Outer River is relatively wide (with little higher riverbed) to keep more flux (60%) into the Outer River in rainy season and keep less flux (40%) into the Outer River in dry season, as well as more sands and stones into the Outer River. While the Inner River is narrow (with little deeper riverbed) to keep certain amount of flux into the Inner River even in dry season.After the two channels diverge at the Fish Mouth, the Outer River flowspast the bifurcating dam and downstream for several hundred kilometers, eventually feeding into the Changjiang River. The Inner River runs along the other side of the dam, which directs it into Precious Vase Mouth (I), continuesfor several hundred kilometers, branching off into numerous irrigation channels until it finally runs out. The Outer River is used primarily for flood control, as well as some irrigation and shipping, while the Inner River is used mainly for irrigation. A floodgate was built at modern times to adjust the flux of water between two channels more efficiently.
Precious Vase Mouth, like the neck of a bottle, is a canal cut through Yulei Mountain at the time of the Dujiang Canal's construction. Before the invention of explosives, the builders accomplished this by repeatedly heating the rock with wood fires and cooling the rock with the water from the river below to crack and weaken it. The strenuous work lasted seven years before the mountain finally yielded the opening. Flying Sand Weir (G), a lowered section of the bifurcating dam, is located shortly before Precious Vase Mouth and serves as a spillway in times of high water. Precious Vase Mouth and Flying Sand Weir work together as a water gate; they divide and control the flow again to keep water into irrigation ditches in optimal amount. In rainy season, the redundant water, restricted by the Precious Vase Mouth, will run over the Flying Sand Weir into the Outer River and thus prevent flooding. As its name shows, Flying Sand Weir, situated at the point where the Inner River makes its first turn after split from the Outer River at the Fish Mouth, also serves as a filter of sands and stones. Li Bing and his people cleverly used the fact that the waters flow at different speeds at the river turn, with the outer ring moving slowly and thus carrying most of the slit, which is then filtered at the weir and goes back into the Outer River.
By contrast with the water conservancy projects that breaking the flows with high dams, Dujiang Canal has no dam but only some dikes to keep the water flow naturally with different direction for different uses. Furthermore, historically, temporary dam was erected each year during the dry season to alternately block the Outer and Inner Rivers. This allows dredging of the profuse silt and gravel deposited along this section of the Minjiang River, in accordance with Li Bing's directive to "dredge channels deep, build dikes low."
In much the same way that acupuncture frees the flow of qi and blood through the meridians by removing blockages at their most accessible point, the Dujiang Canal takes full advantage of existing geographical features to direct the waters of the Minjiang River into the most beneficial channels. The construction of this great work of hydraulic engineering prevented flooding and effectively irrigated over 5000 square kilometers of bottomland in ancient China's Shu Prefecture, today's Sichuan Province. Ever since, the region has been known as the "Heavenly Land of Plenty."
Dujiang Canal, the world's oldest and only non-dam water project, is a prime example of the use of dredging and water diversion for flood control. It best embodies the ancient Chinese philosophy of "Integration of the Human and Heaven" --- that man should always stay in harmony with nature while taking its advantage rather than working against nature (such as damming a river to build a reservoir). The oldest operational water conservancy project in the world, the Dujiang Canal was added to UNESCO's list of World Heritage sites in 2000.
Illustration 1：都江堰（Overview of the Dujiang Canal）
Illustration 2：Sketch map of the Dujiang Canal
A: Fish Mouth (Yu Zui)
B: Outer River Dike (Jingang Di)
C: Inner River Dike (Jingang Di)
D: Inner River (Nei Jiang)
E: Outer River (Wai Jiang)
F: Minjiang River
G: Flying Sand Weir (Feishayan Weir)
H: Lambdoidal Dike (Renzi Di)
I: Precious Vase Mouth (Baoping Kou)
J: Lidui Park
K: One Hundred Zhang Dike (Baizhang Di)
L: Two Kings’ Temple (Erwang Temple )
