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Journal of Hydro-environment Research 4 (2010) 3e13 www.elsevier.com/locate/jher

Dujiangyan Irrigation System e a world cultural heritage corresponding to concepts of modern hydraulic science

Shuyou Cao, Xingnian Liu, Huang Er*

State Key Laboratory of Hydraulics and Mountain River Engineering, University, , Sichuan 610065, Received 28 June 2009; accepted 28 September 2009

Abstract

The ancient Dujiangyan Irrigation System (DIS) was listed as a World Culture Heritage Site by the World Heritage Center, UNESCO in 2000. DIS still plays a crucial role in flood control, irrigation and water supply for Chengdu Plain in Sichuan Province. The immense advances in science and technology achieved in ancient China are graphically illustrated by the DIS. The system is appropriately arranged in accordance with the terrain and topography of the river and Chengdu plain, thus successfully solving the problem of sand discharge, flood control, and water distribution. Consequently the task of gravity diversion could be fulfilled over a long period and across the whole irrigation district. Since 1940s, series of prototype observations, hydraulic physical model experiments, and numerical modeling, have been conducted to explore the design philosophy, new planning scheme and key techniques for modern reconstruction. At the same time, science mechanism and river dynamics of the wonder being based upon are discovered. A selective literature review of the DIS is offered in this paper to introduce record history of the original construction and sustainable development, engineering and science values, and regularly restoration experiences. Based on analysis of accurate literatures, a new understanding to the original headwork structure of Dujiangyan is provided. Ó 2010 International Association for Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rights reserved.

Keywords: Dujiangyan; Hydraulic history; Fluid mechanics; Hydraulic model experiment; Literature review

1. Introduction another is water distribution system in 7 cities and 37 counties (Fig. 3). For over two thousand years the whole system has The ancient Dujiangyan irrigation system (DIS) was listed functioned perfectly, serving in flood control, irrigation, as a World Culture Heritage Site by the World Heritage navigation and wood drifting. It has contributed greatly to the Center, UNESCO in 2000. This system still controls the waters richness of Chengdu Plain and helped it earn its reputation as of the Minjiang River and distributes it to the fertile farmland ‘‘The Land of Abundance’’ (Ministry of Culture, 2003). of the Chengdu plain. The system is a major landmark in the The system is appropriately arranged in accordance with development of water management and technology. The the terrain and topography of the river and Chengdu plain, thus immense advances in science and technology achieved in successfully solving the problem of sand discharge, flood ancient China are graphically illustrated by the DIS (World control, and water distribution. Consequently the task of Heritage Center, UNESCO, 2009). As shown in Fig. 1, DIS is gravity diversion could be fulfilled over a long period and located in central Sichuan Province, which covers two main whole irrigation district. The irrigation district covers 3 components. One is the headwork the key control project for watersheds with a total area of 23.2 Â 103 km2 today, namely water division and intake at Dujiangyan City (Fig. 2), and Mingjiang River, Tuojiang River, and Fujiang River. DIS is planned to a total irrigation area of 1.01 million ha based on * Corresponding author. actual irrigation area of 0.68 million ha at present (Sun, 2004). E-mail addresses: [email protected] (S. Cao), [email protected] (X. According to the traditional understanding, the ancient Liu), [email protected] (H. Er). headwork of Dujiangyan was a non-dam intake project. It is

1570-6443/$ - see front matter Ó 2010 International Association for Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jher.2009.09.003 4 S. Cao et al. / Journal of Hydro-environment Research 4 (2010) 3e13

Fig. 1. Location of Dujiangyan Irrigation System (DIS) in Sichuan Province. located on the top of an alluvial fan where the Minjiang River just exits from the Longmen Mountains. The layout of the main hydraulic headwork is shown in Fig. 4 (Du, 2008). The hydraulic headwork of ancient Dujiangyan were mainly composed of three main components: 1) Fish Mouth, a diver- Fig. 3. Irrigated district of DIS. sion embankment diverting flow to the intake stem channel; 2) Flying Sand Sluice, a flood and sediment sluice; and 3) Bao- channel bends, and the annual maintenance. (Cao et al., 1988a, pingkou (Top of Precious Vase), a water intake with flood 2000a; Chen, 1983; CCE and TDME, 1978; Du, 1980, 2008; control function. A series of data analysis, mathematical Jin, 1988; Xu, 1981). modeling, prototype observations and hydraulic physical model experiments have been conducted since 1940s. The 2. History of the original construction and sustainable purposes of those research are: 1) to investigate the scientific development principles corresponding to modern fluid mechanics, including effects of head hydraulic structures to flood control, sediment The construction and sustainable development of DIS is an exclusion, and management schemes; 2) to study the scientific outstanding example of keeping abreast with times and bases for modern sustainable development of the system, promoting harmony between mankind and nature. For including the construction of a reinforced concrete checkgate example, the irrigated area enlarged rapidly since 1940s as on Outer River and the water regulation project, Zipingpu Table 1 (WROSC, 2004; DAB, 2004). The processes of the reservoir, and others. In spite of the headwork of the DIS was original establishment and long period development of DIS modified time to time since the original construction, modern can be divided into three main periods: the original research reveals that the effective sediment exclusion is construction times, the development times, and the modern attributed to the general layout of the three main head times. The original construction of DIS was firstly recorded in hydraulic structures, the utilization of secondary currents in the famous book titled as ‘‘the Historical Records’’ by Sima (91 B.C.) in the Han Dynasty more than 2100 years ago. After Sima, numerous historical records of Dujiangyan can be found in the history (Tan, 2004, 2009). Based on evaluating historical records, Guo (1994) suggested that to understand of Chinese hydraulic civilization one should start from Dujiangyan. Recently, Feng (2006) further suggested for the establishment of Dujiangyan Study in order to save this civilized heritage based on Dujiangyan’s great historical value and modern function.

2.1. Original construction (256 B.C.e206 B.C.)

During the Warring States period of ancient China, Fig. 2. Overview of the headwork of Dujiangyan (Li and You, 2007). Kingdom Qin conquered Kingdom Shu in 316 B.C. On the S. Cao et al. / Journal of Hydro-environment Research 4 (2010) 3e13 5

Fig. 4. General layout of Dujiangyan reach of Mingjiang River in 1973. 1. Fish Mouth (FM), 2. Inner River 3. Outer River 4. Flying Sand Sluice (FSS) (Du, 2008). basis of the previous water conservancy project, Li Bing, the destroyed and washed downstream for 1 km by extreme flash governor of the Shu Shire under the Qin State, built a diversion flood on June 26, 910; 2) During the war of Mongolia embankment called Yuzui (front-end likes a fish mouth) in the conquered China (1235e1267); 3) During the Sichuan war of middle part of the Minjiang River at the upper end of the Qing Dynasty conquered Ming Dynasty (1644e1681); 4) by Chengdu Plain where the river just runs out from the moun- flash flood on October 9, 1933, which was induced by burst tainous region in 256 B.C. The division embankment was of block-lakes formed by Diexi earthquake 150 km upstream constructed to stabilize the course of the main flow (the Outer Dujiangyan on August 25, 1933. In spite of being destroyed River). Meanwhile, a new channel, the Inner River, was dug a few times, the DIS was recovered quickly. The name out by the side of the hill to intake water. Moreover, Bao- ‘‘Dujiangyan’’ was first appeared in historical record in Yuan pingkou (throat of the intake like the tap of a precious vase) Dynasty, and the title of the permanent water intake, ‘‘Bao- was excavated through the hard conglomerate rock mass in pingkou’’ was firstly appeared in Ming Dynasty (Tan, 2009; order to intake water and also to control flood. Then, at the WROSC, 2004; DAB, 2004). upstream of Baopingkou on the side next to the Outer River, Feishayan (a sediment and flow spillway) was built by woven bamboo baskets filled with local pebbles and boulders to 2.3. Modern times (since 1940s) discharge the surplus flood and sediment into the Outer River, assure the irrigation demand and domestic supply, and even- The DIS started its modern times since 1940s. The symbol tually, prevent droughts and floods in the Chengdu Plain. Since of the beginning of the modern times was the first modern then, the project is honored as the ‘Treasure of Sichuan’, planning of the permanent solutions (WRBSC, 1943) and the which played a crucial role in flood control, irrigation and first modern physical scale model experiments for the DIS in water supply for Chengdu plain. On the other hand, as a part of 1941 (Tan, 2004, 2009; Zhou, 2006). The Chengdu Hydraulic the Dujiangyan water distribution system, the channel system Laboratory was established in Guanxian County (Dujiangyan of Chengdu plain was improved, especially twins river of City at the present) in 1941, and moved to Chengdu on the Chengdu City (Chen, 2005). As a key river training project, invitation of in 1945. During this period Mingjiang River, Fu Jiang River and Tuojiang River were of time, three modern experiments on DIS were conducted at connected (Tan, 2004, 2009; Xu, 1981; Zhang and Hu, 2006). the laboratory, which are: 1) Scour experiment on Yuzhui Historical records about Li Bing and earliest Dujiangyan (Fish Mouth) and Inner Channel of DIS, Sichuan; 2) Study were mainly focused on ‘‘the Historical Records’’ (Sima, on backwater of Inner Channel of DIS, Sichuan; and 3) 91B.C. reprint in 2002) by Sima, the State Records of Study on hydropower of upstream, Mingjiang (Zhou, 2006). Huayang (Huayang Guozhi, Chang, 349), and the Commen- A British historian of Chinese science, Joseph Needham, tary on the Waterways (Shui Jing Zhu, Li, 472e527). visited Dujiangyan in 1943. He visited the hydraulic model of Dujiangyan and Li Bing’s temple. He met the head of the experiments, Dr. Chang Y. L., who got his Ph.D. degree at 2.2. Development times (206 B.C.e1930s) the University of Manchester. Dr. Chang was a professor of Sichuan University at that time. Joseph Needham was excited The DIS experienced a period of ceaseless development by those modern experiments during the Second Would War, from Han Dynast to early 20th century, over 2100 year. The and suggested Dr. Chang providing research papers about head control project was improved gradually, and the irri- modern research of Dujiangyan for publication in UK gation area increased step by step. Irrigation cannels (Needham, 1945, 1948; Tan, 2004, 2009). Needham further extended to Chengdu plain quickly to establish a mult-benefit introduced the Dujiangyan irrigation system to the western as channel network, which ensured irrigation, flood control and a typical example of the rice-based irrigation systems of navigation in Chengdu Plain. As a result, Chengdu became South China in both its history and its size. From the point of an economic center of South-western China. Unfortunately, view of management, he noted that a government-managed the DIS was destroyed seriously at least four times owing to irrigation system was constructed around 2200 years ago historical wars or nature hazards: 1) Headwork was (Needham, 1971). 6 S. Cao et al. / Journal of Hydro-environment Research 4 (2010) 3e13

Table 1 management science. The system science concept in planning, Statistics of irrigated area of the DIS. design, construction, and management of the Dujiangyan is Date 1940s 1960 1980s 1990s perfect judged by modern system technique (Xiong and Guo, Irrigated area (Million ha) 0.188 0.452 0.572 0.668 1989; Wei, 1991).

Large reconstruction of the DIS includes mainly the 3.1. Hydrology and water resources conditions checkgate at Outer River finished in 1974 to replace temperate water adjust work, wood-tripods (macha) with bamboo-cages The Dujiangyan irrigated area located in subtropical moist (Zhulong), into permanent reinforced concrete checkgate, monsoon climate district with abundant rainfall, crisscross which makes a new development of the DIS. As reported by river networks and rich ground water. The main water supply Nickum, in the first 40 years following the establishment of of Dujiangyan is inflow from upper reach of the Minjiang the People’s Republic of China, the total irrigated area River, which is a mountain river with small annual flow increased 2.2 times, and the total grain production increased variation in Sichuan Province, with a stem stretching 735 km, 3.7 times. The process of irrigation and agricultural develop- and a catchment area over 135,000 km2. Upper Mingjiang ment, however, has not been widely charted in the literature River above Dujiangyan headwork is 340 km in length and (Nickum, 1982; Yasuyuki, 1997). 23,037 km2 in watershed area, with a vast expanse of flow Coming to the new millennium, Dujiangyan irrigated 0.668 field, thick forests and wide grassland. Outflow thawed snow million hectares farmland in 7 cities and 37 counties in Sichuan all the year round together with water pooled from precipita- province in 2004. Zipingpu reservoir, the second generation tion ensure the requirement of Dujiangyan. The hydrological project of Dujiangyan was finished in 2006. Professor Graf data of upper Mingjiang River are listed in Table 2 (Sun, 2004; (1984) introduced Dujiangyan in his book, Hydraulics of Sedi- Liu et al., 2003, 2008). Under typical hydrology and water ment Transport. He described that an outstanding mark of resources conditions, water supply of DIS can be ensured. hydraulic engineering is the most complicated net of an irrigation system designed about 240 B.C. Some of the irrigation canals from that century and rules for their operation are said to be still in 3.2. Unique topography conditions use (Graf, 1984). Twenty years later, Professor Graf was invited to visit Dujiangyan and participated in the International Symposium Chengdu basin can be divided into three geomorphic units: on 2260 Anniversary of Dujiangyan. After understanding more the western mountain, the central plain and the eastern hill. The about Dujiangyan, Professor Graf suggested that we should let the highest elevation of 4364 m above sea level is at Miaojiling peak world understand Dujiangyan (Graf, 2004). Professor Gopa- in Dayi County, while the lowest point of 385 m is at Yunhe lakrishnan, the secretary general of the International Commission village in Jintang County. Chengdu plain is an alluvial fan by on Irrigation and Drainage (ICID), praised that Dujiangyan Mingjiang River and Tuo River, with an average elevation of e project is a great contribution of China to humankind (Gopa- 500 m above sea level, and slopes of 0.003 0.005, which is lakrishnan, 2004). More experts published positive opinions for suitable for gravity irrigation, and avoid water logging (Tan, Dujiangyan in recent years (Peng and Xiao, 2004; Xu, 2004; 2009). Mingjiang River flows out of the mountains, the river bed Wang, 2004). A series of books, Development and Culture of expands abruptly while its water flow slows down. The Dujiangyang, were published in 2004, which are recognized as Dujiangyan head-works is on the top of the alluvial fan with an the milestones for Dujiangyang studies in the new millennium elevation of 739 m, which is commanding point of entire (DAB, 2004; Peng and Xiao, 2004; RSMD, 2004; Tan, 2004; Dujiangyan irrigated district. Corresponding the topography, WROSC, 2004; DAB, 2004). water distribution channel is constructed in accordance with the land configuration, thereby controlling the Dujiangyan irrigated 3. Concepts of modern science in Dujiangyan area with gravity flow (Sun, 2004).

What is the reason behind the sustainable development of 3.3. Scientific layout of headwork Dujiangyan for more than 2200 years? To answer this question and to further improve the system, a series of field surveys, Dujiangyan has been continuously explored the rules of experiments, and numerical modeling were conducted in harmony between mankind and water over 2000 years. With Sichuan University and jointly with other institutions for the socioeconomic development and improvement of productivity, reconstruction of DIS, mainly for the constructions of modern people have kept abreast with their times by continuously Outer checkgate and a large water regulation project, the Zipingpu reservoir, since1970s (Cao et al., 1988a,b, 2000a,b; Table 2 CCE and TDME, 1978; Chen, 1983; Chen and Hu, 1985; Hydrological data of Dujiangyan. Chen and Kuang, 1983; CCE and TDME, 1978; Du, 1980, Average annual flow discharge 467 m3/s 2008; Fang et al., 1988; Hua and Chen, 1981; Liu et al., 2003, Lowest flow discharge 100 m3/s 2008; Sun, 2004; Sun et al., 2006a,b). Modern research shows Average annual surface inflow 14.73 billion m3 that the DIS satisfies modern principles in system science, Average annual ground inflow 15.27 billion m3 3 hydraulics, river mechanics, hydrology, geomorphology, and Total water resources 29 billion m S. Cao et al. / Journal of Hydro-environment Research 4 (2010) 3e13 7 innovating headwork engineering techniques, adopting shows that in spite of a great shift of river course has occurred in advanced project technologies and building materials, the reach downstream of the junction of Baisha River. However, reformed the management system and operation regime, and the river course, from Majiatuo to the diversion work, appears to satisfying people’s increasing demand for water within the be stable without change for 35 years. Erosion and deposition of irrigated areas, under the condition of the sustained develop- river bed are slight (in general, the variation of bed elevation is ment of Dujiangyan (Cao et al., 1988a,b; CCE and TDME, about 0.1e0.4 m for different years). This is one of reasons that 1978; Du, 1980, 2008; Tan, 2004, 2009). Dujiangyan headwork is appropriately designed for all flow The positions of the diversion embankment (Jingang Di) conditions (Du, 1980, 2008). and its front-end (Yuzhui) changed in according to the changes of upper inflow, sedimentation and river processes. Historical 3.4. Earthquake resistance ability records show that the earliest diversion work was located near the conjunction of the Baisha River and the Mingjiang River, In the history of DIS, two earthquakes with magnitude 700 m above the current position. Later, owing to huge flood, greater than 7.5 Ms (Richter scale) were recorded upstream of scouring and destruction of the river bed resulting from the Dujiangyan headwork. The Diexi earthquake with multiple earthquakes at the upper reach of the Mingjiang a magnitude of 7.5 Ms, occurred in Diexi Town, Mao County River, the shifts of the meandering channel, the changes of on August 25, 1933. The straight distance between the water flow paths and the bamboo basket composition of the epicenter and the Dujiangyan headwork is 118 km. The project which led to poor impact immunity and stability and earthquake destroyed the town of Diexi and surrounding frequent destroyed by flood with huge stones. As a result, villages, and caused landslides that resulted in more than ten people had to shift the position of division work frequently. quake lakes. After 45 days, Dujiangyan head work was totally The greatest shift took place in 910 A.D. when flood flashed destroyed by a huge flash flood owing to burst of block-lakes, the entire bamboo basket water division (Fish Mouth) to about and about 2500 people were killed by the flood on October 9, 1 km downstream, just in front of the water intake (Bao- 1933 (Zhou et al., 1992). However, the straight distance pingqou). In fact, the position of Yuzui is the control point of between the epicenter and the headwork is just 12 km in headwork, which decides the locations of other projects of the Wenchuan earthquake in 2008, but the headwork was only headwork, and affects the distribution proportion of water and mildly damaged, comparing with the magnitude of 8.0 Ms. sediment. For example, during reconstruction in early Qing Most buildings in protected area of the Dujiangyan World Dynasty, the Yuzhui was located at about 300e400 m Heritage Site were severely affected, and six building upstream of Baopinqou, at the location of present Feishayan complexes collapsed, including the Er-wang Temple during (FSS). With the improvement of annual maintenance scheme, the 5.12 Wenchuan earthquake in 2008. Part of the main body Yuzhui was moved gradually up stream to the present position of the Dujiangyan irrigation system is fractured and most of before 1827 (Tan, 2009). the infrastructure was damaged by a landslide after the For modern times, a comparison of the river reach of earthquake as well (World Heritage Center, UNESCO, 2009). Dujiangyan headwork in 1973 (Fig. 4) and in 1938 (Fig. 5) The modern Outer River Checkgate crippled owing to the axis

Fig. 5. General layout of Dujiangyan reach of Mingjiang River in 1938. 1. Fish Mouth (FM); 2. Inner River; 3. Outer River; 4. Flying Sand Sluice (FSS); 5. Long masonry revetment (Du, 2008). 8 S. Cao et al. / Journal of Hydro-environment Research 4 (2010) 3e13 of hoisting device was shifted downstream more than 10 cm. However, it is a wonder that Dujiangyan irrigation system was never stopping to deliver water to Chengdu City anytime, even on May 12, 2008. The first-aid repair was carried out day and night, and the gate was reopened at 3 am, May 13, 2008, the next day of Wenchuan earthquake. The headwork of Dujiangyan is just 21 km away from the epicenter and its seismic intensity during the earthquake was about VIII-IX, while its original seismic fortification intensity was VII. As a result, buildings were seriously destroyed by the earthquake (Zhang and Jin, 2008). However, the headwork of Dujiangyan was just cracked in several lines (Fig. 6). The earthquake resistance ability of Dujiangyan is strong enough, comparing with the damage induced by flash flood (Fig. 7).

Fig. 7. Fish Mouth after a flash flood, Nov. 23, 2002. 3.5. Discussion on the headwork structures of ancient Dujiangyan widely adopted in river closure, regulate water and flood control in Dujiangyan before 1974 as shown in Fig. 8. Therefore The headwork structure of ancient Dujiangyan is recognized a conclusion is proposed that the headwork system is always traditionally as a non-dam intake project. However accurate a checkgate dam system. The difference between ancient and historic records show that the division and intake structure modern Dujiangyan is only construction materials. The check- actually formed a checkgate dam division system. Sima gate dam was constructed by bamboo-cages with wood-tripods (91B.C.) did not mention the division work. He recorded only in ancient project, and by the reinforced concrete and steel gate cutting Lidui at headwork as a intake work, and excavated twins today (Fig. 9). channels to Chengdu for navigation as the first task, and irri- gation next. Chang (349) recorded firstly that Bing blocked up Mingjiang river and built an embankment. Furthermore, Li 4. Hydraulics researches on Dujiangyan since 1970s (526) recorded that Bing blocked up the river and built a diver- sion embankment with two openings at right and at left Besides physical scale model studies and reconstruction respectively. In fact, those records show that a temporary low planning study for Dujiangyan in 1940s, intensive researches dam and embankment by local nature material were constructed were conducted since 1970s. Prototype survey, physical in ancient times, and used continually in maintenance practice hydraulics modeling, and numerical modeling were conducted each year until 1974. Construction materials were wood-tripods for the construction and evaluation for the checkgate dam in with bamboo-cages. Bamboo-cage is a long sausage-shaped Outer River (finished in 1973), and for the construction of basket of woven bamboo filled with stones, to be used as Zipingpu reservoir (finished in 2006). Results of those a temporary dam under the support of wood-tripods. Structure of researches provide not only scientific support for design of bamboo-cage with wood-tripods is simple and cheap but those modern projects, but also interpretation of why the effective and has been used from over 2000 years. It is still ancient Dujiangyan so successful.

Fig. 8. The traditional checkgate dam built by bamboo-cages with wood- Fig. 6. Fish Mouth after 5.12 wenchuan earthquake (after DAB, May 17, 2008). tripods (source: DMB). S. Cao et al. / Journal of Hydro-environment Research 4 (2010) 3e13 9

Table 3 Water Level difference (between concave and convex banks) in representative section of fish mouth. Discharges (m3/s) 376 668 2600 Differences (m) 1.37 1.80 0.80

curvature, with a radius of 1090e2080 m at PGS causes the secondary currents bring the clear water of upper layers less than 50% of total discharge into intake located on the concave side by FM. With bottom currents, about 60% of suspended load and 70% of gravel-cobble bed load are conducted into Outer River located on the convex side. Based on the proto- type data, the percentage of gravel-cobble entering the intake is only about 26% of that of the Minjiang River. And that is Fig. 9. Modern Outer checkgate dam built by reinforced concrete and steel 29% based on the results of the distorted movable model. The gate. secondary currents can be indicated by the differences of water levels between the concave bank and convex bank. The differences of water levels at cross section between FM and 4.1. Model-Prototype correlation study for headwork PGS were observed on prototype shown in Table 3. Water levels on the left (concave) bank are much higher than that on Model-Prototype correlation Study on headwork of the right (convex) bank (Cao et al., 1988a). Dujiangyan was conducted at Sichuan University by a joint On the undistorted fix-bed model, the values and directions of team from Beijing and Sichuan Province in 1970se1980s local flow velocities of four cross sections were observed. The (Cao et al., 1988a,b; Chen, 1982, 1983; CCE and TDME, distances of four sections above FM are 38 m, 100 m, 158 m, and 1978; Chen and Hu, 1985; Du, 1980, 2008; Fang et al., 1988; 218 m respectively. Three vertical profiles of velocities for each Hua and Chen, 1981; Jin, 1988). The basic hydrologic and section were observed: right, middle, and left. The data show that topographic data used in the model experiments were provided the differences of direction of flow between surface and bottom by prototype observations. Two hydraulic physical models are 2e7, and the largest difference was 30. The above data served different purposes. The styles of sediment control indicated that the strong secondary currents exist around FM. structures, flow structures around water control structures and Flying Sand Sluice (FSS) conducts the surplus flow and wood drifting were studied on the undistorted fixed-bed model gravel-cobble bed load automatically into Outer River once the with a scale of 1:60. Because of the wide range size distri- intake flow exceeds 350 m3/s. When the discharge diversion bution of gravel-cobble bed load particles and the complex ratios at FSS are 20%w40%, the gravel-cobble ejected by FSS characteristics of the transport rate, a distorted movable bed are 60%w70%. In flood season the intensive secondary currents model was built for bed load study. The distorted movable bed caused by the river bend ahead of FSS exclude almost all bed load model has a horizontal scale of 1:240 and a vertical scale of up to a maximum size of 700 mm into Outer River by FSS when 1:120. The distorted movable model was calibrated by field the discharge of Minjiang River is over 2000 m3/s. In 1977, the data as well as the experiment results of undistorted fixed-bed flood peak discharge of Minjiang River was 4720 m3/s, and that model. The scales of physical parameters of distorted movable of Inner River was 2350 m3/s. The water depth on FSS was 2.0 m. model were decided by the principles of dynamic similarity Thousands cubic meters of gravel-cobble were observed along and modified by field data. the FSS after the flood. The largest size of bed load carried from Prototype observations include: 1) four cross sections upstream by secondary currents can be observed as large as around Fish Mouth for bed load measurement, 2) twenty-seven 900 mm  800 mm  500 mm. sections for channel bed topography measurement along 4 km Baopingkou, only with a narrow width of 20 m and 1/4 of river reach, and 3) five hydrographic stations for the obser- Inner River width, functions as a flood-sediment control vations of water stage, discharge, and transport rate of sus- intake. The coordination between Baopingkou and FSS, whose pended load. More than one thousand field data of bed load elevation of crest is 2.2 m above the Inner River bed, transport rate had been collected.

Table 4 4.1.1. Hydraulic structures and sediment exclusion Comparison of flow conveyed to downstream irrigation channel (Qi) before Both model experiments and prototype observations show and after construction of Outer River Checkgate; Qm = Mingjiang discharge. that the adequate general layout of three hydraulic head Before Qm (m3/s) 421.0 444.0 451.0 452.0 456.0 structures function well to exclude sediment. As shown in Qi (m3/s) 247.4 305.1 312.9 339.0 309.3 Fig. 4, Fish Mouth, the division embankment, is located just Qi/Qm (%) 58.76 68.72 69.38 75.00 67.83 downstream of a pebble gravel shoal (PGS) in the middle of After Qm (m3/s) 428.0 442.0 444.0 445.0 453.0 3 the river. PGS is submerged in flood season and the main flood Qi (m /s) 338.2 424.7 429.9 408.9 445.5 current diverts directly toward the Outer River. The local Qi/Qm (%) 79.02 96.09 96.82 91.89 98.34 10 S. Cao et al. / Journal of Hydro-environment Research 4 (2010) 3e13

Table 5 Equilibrium transport rate of gravel-cobble bed load obtained from physical models. Q(m3/s) 200 400 600 800 900 1000 1140 1300 1700 2000 2600 Qs (kg/s) 0 5 20 45 92 158 260 410 675 920 1300 Annual Yield (1963) 1.50 million tons guarantees the inflow at Baopingkou at a satisfactory value of 4.1.3. Transport rate of bed load 350e740 m3/s. Less than eight percent of the total sediment of The statistics of 675 sets prototype data shows that the the Minjiang River can be carried into the Baopingkou. transport rate of cobble-gravel bed load can vary in spite of Similar to the hydraulic head structures, a series of small FM remaining the same flow condition. Table 5 shows the equi- are built, which separate the water intake channels from the librium transport rate Qs in the model. Based on the central flood-sediment sluice channels in the whole irrigated area. limit theorem of probability theory and verified by Kolmo- Therefore, the largest amount of sediment entering the irri- gorov-Smirnov test, the transport rate of cobble-gravel bed gated area can be excluded from irrigation channel automati- load can be described by lognormal probability distribution at cally by branch flood-sediment sluices. Finally, the average a 95% level of confidence (Cao et al., 1988a). Evaluation annual sediment volume deposited in the irrigation channel is studies conducted continuously in the new millennium also only about 20,000 m3, which is dredged away annually at the supported the construction of Outer River checkgate (Sun end of the winter before 1974. et al., 2006a,b; Wang et al., 2006; Zheng et al., 2006).

4.1.2. Modern outer river checkgate dam 4.2. Effect and protection to the Dujiangyan project by In the non-flood season, the discharge of Minjiang River is the Zipingpu dam less than 600 m3/s, and the minimum discharge is even less 3 than 400 m /s. At that time, the water to the Inner River Zipingpu reservoir, the main water regulation project for cannot meet the demand of irrigation by natural water intake. DIS, was completed in 2006. The sketch map between The old-fashioned wooden cofferdam was built across Outer Zipingpu dam and Dujiangyan as Fig. 10. The main functions River every spring so as to adjust diversion water at FM. of the project are irrigation and water supply, with compre- Tripod-wooden cofferdam would often be damaged, and was hensive benefits in power generation, flood control, environ- difficult to be adjusted. Based on model experiments, an eight ment protection and tourism. The construction of the Zipingpu opening checkgate with a total width of 104.2 m was built reservoir will raise the flood prevention standard, in the across Outer River in 1974. The width of each opening is downstream Chengdu plain, from once every 10 years to once 12 m, and the total net width of checkgate is 96 m which is every 100 years. The water supply guarantee rate will be equal to the local average width of Outer River. The elevation improved by 10 percentage points in Dujiangyan areas. In of the gate step is 729.0 m which is at the same level as local general, the reservoir plays an important role in sustainable average river bed elevation. The height of gates is 4 m, and the development of society and economy in Sichuan Province. maximum water stage at the checkgate is 732.5 m. The The Zipingpu reservoir is located on the Minjiang River, 9 km experiment of Outer River Checkgate was done on undistorted upstream from the Dujiangyan City and 60 km from Chengdu river model. City, the capital of Sichuan Province. The check and design The new building of the checkgate not only keeps the flood levels are 883.10 and 871.20 respectively, and the stability of the river regime of hydraulic head structure reach normal and minimum pool levels are 877.00 and 817.00 m but also improves the environment of water diversion and respectively. The total capacity of the reservoir is sediment exclusion of the project. When discharge is smaller 3 3 1.112 Â 109 m . The dam has a maximum height of 156 m. than 500 m /s almost all of the water in Minjiang River can The dam crest is 634.8 m in length and 884 m in elevation. be conducted into irrigated area after building the checkgate, The total storage volume is 11.83 Â 106 m3. Based on 60 years as compared to only about 70% before building of the natural hydrological data (1937e1999) at the site of Zipingpu structure. Eight hundred million m3 of more water is taken annually. The comparison of conducted water between before and after the building of Outer River Checkgate in prototype shown in Table 4, where Qm is the discharge of Minjiang River, and Qi is the discharge of intake water. Based on the model experiments, 30% of sediment carried in Minjiang River was conducted to the Inner River before reconstruction. The prototype observations show that the sediment into Inner River decreases by about 27% after the building of the checkgate. The model and prototype measurements of bed topography verify that the deposition around the hydraulic head structures decreased after the operations of Outer river checkgate dam. Fig. 10. Sketch map between Zipingpu dam and Dujiangyan. S. Cao et al. / Journal of Hydro-environment Research 4 (2010) 3e13 11

Table 6 Discharges of inflow and outflow, Zipingpu reservoir. Frequency of flood Flood peak m3/s Highest water level m Maximum outflow m3/s Decreased flood peak m3/s Once 100 a 6030 861.6 2390 3640 Once 1000 a 8300 871.2 2677 5623 Maximum 12,700 883.1 4856 7844 dam, 10-years frequency flood is 3760 m3/s; 50-years be scoured when discharge over 3000 m3/s. The maximum frequency flood is 5340 m3/s; 100-years frequency flood is scouring depth reaches 9 m when discharge reaches 3640 m3/s. 6030 m3/s; and the record maximum flood is 5840 m3/s Slight clean water scouring can be found at Feishayan, (1964). After Zipingpu reservoir is completed, floods can be Baopinkou, Inner River and Outer River, but can be main- adjusted by the flood control volume of the reservoir as Table tained by feed natural pebbles during experiment to form an 6. For example, a 100-year frequency flood (6030 m3/s) from armoring layer. Finally, the safety of Dujiangyan headwork upstream of the reservoir can be reduced by the reservoir to can be guaranteed. 2390 m3/s, less than a 10-year frequency flood (3760 m3/s). As a result, the great Dujiangyan is protected by the Zipingpu 5. Conclusions dam (Liu et al., 2003, 2008). Hydraulics experiments were conducted at the Sichuan A selective literature review of the Dujiangyan Irrigation University in 2000s. The tasks were 1) fluvial processes of System is offered in this paper, to introduce the historical Dujiangyan reach under the condition of scour by clean water record of the original construction and sustainable develop- released from the Zipingpu; 2) Optimize engineering measures ment, engineering and science values, and regularly resto- to scour protection; 3) Evaluate the protection function of the ration experiences. The processes of original establishment Zipingpu reservoir to the Dujiangyan. A distorted model was and long period development of Dujiangyan can be divided built for bed load study and general erosion downstream of into three main periods as: original construction times, Zipingpu, with a horizontal scale of 1:160 and a vertical scale development times, and modern times. The system is of 1:80. The local clean water scour, flow structures around developing quickly in modern times since 1940s. The irri- water control structures were studied on the undistorted model gated area enlarged rapidly from 0.188 ha in 1940s to 0.668 with a scale of 1:80 (Fig. 11). in 1990s. Numerical and experimental results show that 80% of A series of data analysis, prototype observations, math- suspended load will deposit in the reservoir in 30 years. ematical modeling and hydraulic physical model experi- Suspended load will be equilibrium between erosion and ments have been conducted since 1940s, to explore the deposition in 70 years. All of bed load will deposit in the design philosophy, new planning scheme and key tech- reservoir, and no bed load transport to Dujiangyan in100 niques for modern reconstruction. Concepts of modern years. Therefore, Dujiangyan reach of Mingjiang River will be science in Dujiangyan are summarized as hydrology scoured by clean water sluiced from the Zipingpu reservoir. conditions for guarantee of water supply, unique topography Scouring depths at different locations are observed in the conditions, scientific layout of headwork, earthquake undistorted model. The most serious scour takes place in front resistance ability. of the division embankment (Yuzhui): the scouring depths Based on analysis of the well-established literatures, the increase with the increase of discharges. The foundation will headwork system of Dujiangyan has always been a checkgate dam water diversion system. The difference between ancient and modern Dujiangyan is just construction materials and structures. The checkgate dam was constructed by bamboo- cages with wood-tripods in ancient project as a cofferdam, and by the reinforced concrete with steel gate today. The prototype-model correlation study on Dujiangyan Project shows that the rational general layout of three main hydraulic head structures function well for flood control and sediment exclusion. The strong secondary currents caused by meandering channel can exclude intensive sediment to main- tain the water intake project effectively. Based on modern hydraulics study, the building of the Outer River checkgate dam improves the environment of water conduction and sediment exclusion since 1974. As the second generation of Dujiangyan, the Zipingpu reservoir functions mainly in irrigation and water supply, with comprehensive benefits in power generation, flood control, Fig. 11. Undistorted model of DIS. environment protection and tourism. The water supply 12 S. Cao et al. / Journal of Hydro-environment Research 4 (2010) 3e13 guarantee rate will be improved by 10 percentage points in Du, Guohan, et al., 1980. A study on the reconstruction of the Dujiangyan Dujiangyan areas. The Zipingpu reservoir will raise the flood headwork and the pebble- gravel bed load transport. recover issue. Journal e control standard of the downstream Dujiangyan and the whole of Sediment Research, 12 22 (in Chinese). Du, Guohan, 2008. Sedimentation Research and Practice of Hydraulic Engi- Chengdu plain, from once every 10 years to once every 100 neering. Waterpower Press, Beijing, China, ISBN 978-7-5084-5584-6 (in 3 years. A 100-year frequency flood (6030 m /s) from upstream Chinese), pp. 126e212. of the reservoir can be reduced by the reservoir to 2390 m3/s, Fang, Duo, Cao, Shuyou, Liu, Xingnian, Cheng, Jiayang, 1988. Computer less than a 10-year frequency flood (3760 m3/s). As a result, model of the bed changes caused by pebble in Dujiangyan. 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