sustainability Article Sustainability of the Dujiangyan Irrigation System for over 2000 Years–A Numerical Investigation of the Water and Sediment Dynamic Diversions Xiaogang Zheng 1,2 , Ehsan Kazemi 2 , Eslam Gabreil 3, Xingnian Liu 1 and Ridong Chen 1,* 1 State Key Laboratory of Hydraulics and Mountain River Engineering, College of Water Resource and Hydropower, Sichuan University, Chengdu 610065, China; [email protected] (X.Z.); [email protected] (X.L.) 2 Department of Civil and Structural Engineering, University of Sheffield, Sheffield S1 3JD, UK; e.kazemi@sheffield.ac.uk 3 Department of Civil Engineering, University of Gharyan, Gharyan 52GG + FM, Libya; [email protected] * Correspondence: [email protected] Received: 30 January 2020; Accepted: 10 March 2020; Published: 20 March 2020 Abstract: The Dujiangyan Irrigation System (DIS), located in the western portion of the Chengdu Plain at the transitional junction between the Qinghai-Tibet Plateau and Sichuan Basin, has been in operation for about 2300 years. The system automatically uses natural topographical and hydrological features and provides automatic water diversion, sediment drainage and intake flow discharge control, thus preventing disastrous events in the region in a ‘natural’ way. Using a numerical modeling approach, this study aims to investigate the reasons behind this natural behavior of the system and provide a better understanding of the complex mechanisms which have caused the sustainability of the DIS for over two millennia. For this purpose, a two-phase flow model based on the Shallow Water Equations (SWEs) is developed to simulate the fluid and sediment motions in the DIS. A coupled explicit-implicit technique based on the Finite Element Method is applied for the fluid flow and a Sediment Mass (SM) model in the framework of the Lagrangian particle method is proposed to simulate the sediment motion under different flow discharge conditions. The results show how different components of the DIS make full use of the hydrodynamic and topographical characteristics of the river to effectively discharge sediment and excess flood to the downstream and create an environmentally sustainable irrigation system. Keywords: Dujiangyan; sediment mass model; hydraulic properties; sediment diversion; sustainable development 1. Introduction The ancient water conservancy projects, such as the control project of Nel-Hammurabi on the Euphrates River built by the Babylon Kingdom, the manmade canals constructed by the ancient Roman Empire [1], and the agricultural irrigation systems dating back to the 1st-6th centuries in Israel [2] have long fallen into disuse. However, there are some old irrigation structures that are still in operation, such as the irrigation in sediment-laden rivers developed in ancient Peru [3]. Moreover, the qanat system, a subterranean aqueduct used to convey water for irrigation and domestic consumption, originated in Iran as early as the 7th century BCE [4], was brought to Spain in the 8th century, and then to the New World in the 16th century [5,6]. This system still irrigates farmlands in Iran at the present day. The Dujiangyan Irrigation System (DIS) is one of the ancient irrigation systems which are still in operation. This system began in the 3rd century BCE, is characterized by the non-dam intake project Sustainability 2020, 12, 2431; doi:10.3390/su12062431 www.mdpi.com/journal/sustainability Sustainability 2020, 12, x FOR PEER REVIEW 2 of 15 43 The Dujiangyan Irrigation System (DIS) is one of the ancient irrigation systems which are still in 44 operation. This system began in the 3rd century BCE, is characterized by the non-dam intake project 45 and is still discharging its functions perfectly [1]. It was listed as a World Cultural Heritage Site in 46 2000 and a World Heritage Irrigation Structure in 2018. The main headworks of the DIS are located 47 in the transitional region, where the river flows from a gorge to an alluvial plain. The location has a 48 50 km distance and 273 m elevation difference from Chengdu City, China (Figure 1). This is on the Sustainability 2020, 12, 2431 2 of 15 49 commanding point of the alluvial plain so as to prevent floodwater flowing directly to the Chengdu 50 Plain [7]. Natural topographical and hydrological features are used to exclude sediment and divert 51 andwater is still for dischargingirrigation without its functions the use of perfectly dams. The [1]. sy Itstem was has listed produced as a World benefits Cultural in flood Heritage prevention, Site in 52 2000agricultural and a World irrigation, Heritage and Irrigationwater consumption Structure for in many 2018. years The main [8]. headworks of the DIS are located 53 in the transitionalRecently, a dam region, was where constructed the river upstream flows fromof the a DIS gorge (about to an 4 alluvialkm from plain. the Fish The Mouth). location The has a 54 50reservoir km distance of the and dam 273 can m be elevation seen in Figure difference 1. The from dam Chengduconstruction City, was China completed (Figure in 12006,). This and is since on the 55 then, it has provided a more stable water supply to the Dujiangyan areas during the dry season, and commanding point of the alluvial plain so as to prevent floodwater flowing directly to the Chengdu 56 also reduced the peak flow discharge of the river during the flood season. Since this dam is a Plain [7]. Natural topographical and hydrological features are used to exclude sediment and divert 57 relatively new structure and the focus of the present study is on the historical performance of the DIS water for irrigation without the use of dams. The system has produced benefits in flood prevention, 58 based on its natural and topographical characteristics during the past 2000 years, the effect of the dam agricultural irrigation, and water consumption for many years [8]. 59 is not taken into consideration in this study. 60 61 FigureFigure 1. 1.Sketch Sketch map map ofof thethe study area. The The inset inset map map (top (top right) right) shows shows the the location location of the of the Dujiangyan Dujiangyan 62 IrrigationIrrigation System System (DIS) (DIS)in in China.China. The Puyang, Baitia Baitiao,o, Zouma, Zouma, and and Jiang’an Jiang’an Ri Riversvers all allflow flow from from the the 63 BaopingkouBaopingkou (the (the bottle-neck bottle-neck channelchannel of the DIS). 64 Recently,The headworks a dam wasof the constructed DIS are composed upstream of thre of thee primary DIS (about components: 4 km from (1) The the Fish Fish Mouth Mouth). (the The 65 reservoirfront end of looks the dam like a can fish be mouth), seen in a Figurediversion1. The emba damnkment construction dividing the was Minjiang completed River in into 2006, the and Inner since 66 then,River, it hasprimarily provided for airrigation, more stable and water the Outer supply River, to the mainly Dujiangyan for flood areas and during sediment the discharge; dry season, (2) and 67 alsoFeishayan, reduced a the low peak spillway flow dischargedam removing of the sediment river during and the excess flood water season. from Since the Inner this dam River is ainto relatively the 68 newOuter structure River; andand the(3) focusBaopingkou of the present(bottle-neck study channel), is on the a historical water intake performance automatically of the controlling DIS based on 69 itsintake natural discharge and topographical from the Inner characteristics River (Figure during2). the past 2000 years, the effect of the dam is not taken into consideration in this study. The headworks of the DIS are composed of three primary components: (1) The Fish Mouth (the front end looks like a fish mouth), a diversion embankment dividing the Minjiang River into the Inner River, primarily for irrigation, and the Outer River, mainly for flood and sediment discharge; (2) Feishayan, a low spillway dam removing sediment and excess water from the Inner River into the Outer River; and (3) Baopingkou (bottle-neck channel), a water intake automatically controlling intake discharge from the Inner River (Figure2). The need to understand how the DIS has preserved its functionality for more than 2000 years and the need for protecting it have urged researchers and engineers to investigate the hydrodynamics of the system, such as flow motion, sediment transport, and riverbed evolution. They have been conducting research in the last few decades on the mechanisms of water and sediment diversion under different discharge conditions in order to improve the methods of protecting the system. Since the first physical model experiment of the DIS conducted in 1941 [9], scholars have studied the characteristics of the water and sediment transport by using the prototype survey [10,11], physical hydraulics model [12], and numerical model [13–15]. As for the dynamic water diversion process, based on the ancients’ experience, more than 60% of the Minjiang River typically goes into the Inner River for the use of the Chengdu Plain during the low discharge period, while in the flood period, this ratio can reduce to less than 40% [16]. The analysis of the prototype data carried out by Sun et al. suggested that after the construction of the check gate across the Outer River in 1974, the water distribution into the Inner Sustainability 2020, 12, 2431 3 of 15 River was close to 75% in spring time and only 37.5% in the flood season [10]. Later, as a complement to the laboratory experiments conducted in Sichuan University [12], a two-dimensional (2D) numerical model has been used to study the performance of the system under different inflow conditions [13]. Sustainability 2020, 12, x FOR PEER REVIEW 3 of 15 70 71 FigureFigure 2. 2.General General layout layout of of the the DIS. DIS. 72 ManyThe need irrigation to understand projects in historyhow the were DIS abandoned has preserved due toits sedimentation.