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Development of check-dam systems in gullies on the Loess Plateau, China
Environmental Science & Policy 7 (2004) 79–86
Development of check-dam systems in gullies on the Loess Plateau, China Xu Xiang-zhou∗ , Zhang Hong-wu, Zhang Ouyang Sediment Laboratory, Department of Hydraulics and Hydropower Engineering, Tsinghua University, Beijing 100084, China
Abstract The Loess Mesa Ravine Region and the Loess Hill Ravine Region, cover 200,000 km2 of the Loess Plateau in China and have serious problems of soil and water erosion. Two primary ways to control the sediment pouring into the Yellow River from this area are planting and engineering measures. The former is not suitable for the Loess Plateau due to the arid climate and the barren soil, while some of the latter means, such as terrace farmlands, are vulnerable to floods. As a widespread engineering measure, the check-dam system in gullies is one of the most effective ways to conserve soil and water in the Loess Plateau. At present, the amount of sediment retained by check-dam systems is the largest of all methods and the potential is promising. The dam farmlands so created have become important high-yield croplands or orchards with enriched fertile soil and ample water. This paper reviews the history and principles of check-dams and discusses future theoretical and experimental studies which are needed for the further implementation of this system. © 2004 Elsevier Ltd. All rights reserved. Keywords: Loess Plateau; Check-dams; Soil and water conservation; Strategy to control the Yellow River; Yellow River; Soil erosion
1. Introduction Check-dams are the most widespread structures for conserving soil and water in the Loess Plateau. However, they have not been described extensively in literature outside China. This paper presents a review of check-dam systems in gullies on the Loess Plateau. By making use of the local geography and climate, the people of the Loess Plateau of China skillfully invented the check-dam system in gullies several centuries ago, to retain sediments and to form farmland. Check-dams at the Kanghe Gou watershed of the Fen-xi County, built in the Ming Dynasty 400 years ago, are still in good condition. As one of the primary measures to conserve water and soil, the check-dam project has been given great emphasis ever since the founding of the People’s Republic of China. Fig. 1 shows a check-dam in the Loess Plateau. By 2002, about 113,500 check-dams had been built, creating 3200 km2 of farmlands with high productivity, and intercepting a total of 700 million m3 of sediments that pour into the Yellow River. Thus, the check-dam system is the most important and well-known project in China to conserve soil and water. Generally, a check-dam is composed of three parts: the embankment, the spillway, and the outlet. Sometimes some simple check-dams are constructed without spillways or outlets. Fig. 2 is a Planform of a typical check-dam. In a ∗ Corresponding author. Tel.: +86-10-62788585; fax: +86-10-62776772. E-mail address: [email protected] (X. Xiang-zhou).
1462-9011/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.envsci.2003.12.002
small watershed, various dams can be built, such as productive dams for forming farmland, flood-control dams for preventing floodwater and intercepting sediments, and water-storage dams for irrigation. A group of such dams constitutes a check-dam system.
2. Characteristics of this area The Loess Plateau is located in the upper and middle reaches of the Yellow River, encircled by the Ela Mountains to the west, the Taihang to the east, the Yanshan Mountains to the north and the Qingling and Funiu Mountains to the south. The plateau covers a total area of 624,000 km2 , and over 60% of the land is subjected to soil and water losses. The most crucial areas are the Loess Mesa Ravine Region and the Loess Hill Ravine Region, which together cover 30% of the total area. As an area with the most severe area of soil and water losses in the world, average erosion reaches 5000–10,000 (km2 per year), sometimes even up to 20,000–30,000 tonnes km−2 per year) (Meng, 1996). 2.1. Climate Climate is an essential factor controlling alluvial sedimentation and erosion (Tilman Rost, 2000). Most areas of the Loess Plateau are arid or semi-arid with dry air and little cloud, but short of moisture. The average annual precipitation on the Plateau is only 350–550 mm, which gradually
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Fig. 1. A check-dam in ZiFangGou, Shannxi Province, China (photo: Zhang Ouyang).
decreases from the southeast to the northwest. Moreover, precipitation is mainly concentrated in the rainy season from June to September, which accounts for 60–70% of the total, most of which is in the form of high intensity rainstorms. For these reasons, soil erosion predominately occurs in this period. 2.2. Landforms The main geomorphic landforms on the Loess Plateau are plateau, ridge, mound and various gullies. Because of the low gradient, little erosion takes place on top of the plateau, except for intense erosion in the gullies at the edge of the plateau. Undulating terrain in the Loess Hill Ravine Region is typified by crisscross gullies dissecting thick loess overburden on top of ancient landforms, to form ridges and mounds: mounds are domed in shape, round or elliptical. Dam Farmland Flood-Control Dam Slope
Slope
Flood-Control Channel Silo
Embankment Spillway
Seepage
Lower Reach of the Dam
Fig. 2. Planform for a check-dam.
The strip-formed “ridges” typically measure several tens of kilometers in length and scores or even hundreds of meters wide at some localities. Fig. 3 shows the typical landform of the Loess Plateau. Since soil erosion occurs intensely in gullies, ridges and mounds, unique geomorphologic features with numerous gullies and fragmented landforms are formed gradually. As a rule, an area with a high gully density often bears severe soil erosion. 2.3. Geology The Loess Plateau is capped with the deepest loess in the world. The Quaternary loess is widely distributed, covering more than 70% of the total area. Loess is mostly silty sandy loam of loose structure comprising 60–70% of grains 0.02–0.05 mm in size and 10–15% of readily soluble carbonates, besides being of high porosity reaching 40–50% and having vertical joints, rendering the soil very liable to be eroded under action of water, gravity or wind (Yellow River Conservancy Commission, 1988). Tectonic movements accelerate erosion. The Loess Plateau is one of the most active areas of tectonic movement in China. In the Quaternary, the crust has been uplifted locally by 150–200 m. Soil erosion in the valleys is aggravated by the increased energy of the water. Also, earthquakes bring about gravitational erosion and destroy landform structure, and lead to the collapse and sliding of large areas on the Loess Plateau (Hui and Mingan, 2000). 3. Strategy to control erosion on the Loess Plateau Two primary ways to control the sediment pouring into the Yellow River in this area are planting and engineering measures. The former means constructing systems to control
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Fig. 3. A sight of loess ridges and mounds in the Loess Plateau, China (photo: Xu Xiangzhou).
wind erosion and water erosion, and engineering measures mean setting up civil engineering structures such as reservoirs, check-dams, or walls to store soil and water. Some means, such as land-use change, combine planting and engineering measures. Which is the most effective, is still in dispute. 3.1. Planting It is said that the Loess Plateau in ancient China was an area with luscious grasslands and dense forests (Shi, 1981; Chen et al., 2001), which were destroyed and attacked by severe soil and water erosion due to undue deforestation by humans (Fu and Gulinck, 1994). A typical viewpoint (Meng, 1996) is that forest cover on the Loess Plateau had been over 50% greater in the past, and this implies that soil and water conservation could be achieved by planting trees and grasses on the slopes. In the middle reaches of the Yellow River, especially in the areas that presently suffer soil and water losses most severely, the same severe environment has lasted for a long time: accelerated erosion started 250,000–300,000 years ago, and was further accelerated in the last 50,000 years, which was before mankind started to impact on the environment (Comprehensive Survey Team on Loess Plateau of Chinese Academy of Sciences, 1992a, 1992b). This suggests that the ultimate determinant in causing severe soil and water erosion on the Loess Plateau is not human activities, but the arid climate and the unfavourable geology and topography. However the suggestion that the forest cover was up to 50% or more greater in the past still remains in doubt. It might be expected that forests and grasslands that had ex-
isted in history should still be in existence, except for a change in scale or type (Meng, 1996). During the Quaternary, due to uplift of 3500 m of the Qinghai-Tibet Plateau, the monsoon system of Southeast Asia was formed and the climatic difference between the humid southeast coast and the dry northwest inland was enhanced. Furthermore, local landforms, for example, the Qinglin Mountains, with an altitude of 3000 m, influenced the monsoon and affected the climate of the Loess Plateau. Distinct floral zones and forests with grasslands were created on the Loess Plateau. As a result, the forest cover has never been above about 53% of the total plateau area. Pollen studies, combined with stable carbon-isotope analysis of organic matter, show that changes in the paleovegetation are profound in response to climate forcing. Persistent steppe vegetation and fossil elephant fauna also suggest long-lasting dry and warm climate conditions in the last 3.0–2.7 million years (Han et al., 1997). Control of soil and water erosion on the Loess Plateau by planting measures alone has been proved to be unsuccessful during recent decades. Firstly, trees are difficult to keep alive owing to the arid climate and barren soil, and even if they survive, most of them do not grow strong enough to control soil and water losses. An investigation in 1998 showed that the survival rate of trees in this year was 53% and that of grasses was 24.2% in the Hekou-Longmen region in an area of 110,000 km2 (Xu and Wang, 2000). Next, even where approriate vegetative engineering measures are taken, it is often impracticable to carry out the entire plan. In the early 1980s, the emphasis of soil and water conservation on the Loess Plateau was on planting, nevertheless, sediment continued to pour into the Yellow River.
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After a reconsideration of the policy, the check-dam system was reconsidered in the mid-1980s, and a new type of check-dam system comprising strong-structured high dams named key projects was undertaken (Feng, 2000). A key project dam is generally made of concrete and is designed to resist one-in-a-100-year floods, and can therefore prevent other check-dams in the system from being destroying by quite major floods. 3.2. Engineering measures There are two kinds of engineering measures in the Loess Mesa Ravine Region and Loess Hill Ravine Region. Some structures, such as farmland terracing, emphasize regulating slopes whereas others focus on controlling gullies. Many researchers have regarded the regulating of slopes as more important, and their main idea is to intercept rainfall on the slope. Soil and water losses can be controlled as long as the total rainfall is filtered out in situ. However, some critical deficiencies exist in this approach. (a) Gully erosion accounts for more than 60–90% of the soil erosion in the watersheds (Zhang, 1993). Even though sediments from the slopes are fully captured, leaking sediments from the small watersheds could not be reduced very much. (b) It is difficult to intercept sediment on steep slopes in situ. (c) Structural stabilization of constructions on the slope is not very reliable. (d). The constructions for controlling slopes such as farmland terracing are inefficient in confronting a drought, and water-storing dams for irrigation in the gully have to be built as a complement. Some other scholars insist on regulating gullies and slopes simultaneously. However, conflicts will occur in the initial stage of building check-dams in gullies, as constructions for controlling slopes for retarding sediments will slow down the speed of the formation of farmland, and more time will be needed to reach relative stabilization for the whole check-dam system. Recently, an engineering project for controlling gullies was proposed for the Yellow River (Feng, 1994; Zhang et al., 1999; Xu et al., 2002). It was suggested that in order to retard total sediment and floodwater in the small watersheds of the Loess Plateau, check-dams should be built step by step along the valleys to form layers of flat land with various elevations. It was anticipated that when the mounds and ridges of the plateau were cut and the valleys were filled to form layers of flat land, the geographical environment would be greatly improved, and soil and water losses would be controlled as well. Moreover, in order to make the dam system relatively stable more quickly, advanced modern engineering measures, such as directional blasting, hydraulic fill and transport by bulldozers are introduced to cut the mounds and fill the valleys. However when this approach is adopted, in order to avoid reducing the transport of soil from the hill slope and the velocity of formation of farmland in the early age of forming dam-land, neither planting nor engineering measures could be adopted on the hill slope.
3.3. Benefits of the check-dam system in gullies The check-dam system, especially when including key project dams for controlling gullies, is the largest engineering project to harness soil erosion effectively, and should act as the last line of defence to impound water and conserve soil. In addition, the fertile farmland so formed provides an excellent base for agriculture. The check-dam system in gullies is the most effective measure to control sediment pouring into the Yellow River. Due to serious silting, levees along the lower reaches have to be heightened ever year, thus an “Above Ground River” is formed with a riverbed 10 m higher than the surrounding ground. In fact, most of sediment in the lower Yellow River comes from the Loess Plateau. Hence, reducing sediment pouring into the Yellow River from the Loess Plateau is the most effective solution, although many other methods such as dredging and dike heightening have also been tried. In the lower reaches of the Yellow River, the amount of retained sediment is between 16,680 and 72,840 tonnes per hectare. The more serious soil and water erosion is, the more valuable are check-dams. Enormous amounts of sediment retained from small watersheds by check-dams not only form large-scale fertile farmland but also safeguard the Yellow River against overflow. Thus, the social benefit is remarkable. The mean decrease in sediments pouring into the Yellow River from the three main tributaries between Hekou and Guanbao city in recent decades is shown in Table 1. Table 2 gives the percentage of sediment retained by check-dams in the Yellow River watershed—7127 mm3 are held up by check-dam systems, which is 66.9% of the total amount. Meanwhile, 3812 km2 of farmlands were formed as a result of the measures taken. From Tables 1 and 2, it can be concluded that check-dam systems have made a great contribution to preventing sediment from pouring into the Yellow River. A consequence of this success is the creation of a new type of agriculture, which emerges as more and more food supplies are produced from the dam farmland. Farmland
Table 1 The mean decrease in sediment pouring into the Yellow River from the three main tributaries—Wuding River, Fenhe River, Huangpucuan River (Xu and Wang, 2000) Items
Wuding River (106 tonnes)
Fenhe River (106 tonnes)
Mean decrement of sediment in every decade 1960s 43 50 1970s 125 52 1980s 96 53 1990s 99 54
Huangpucuan River (106 tonnes) 1 3 9 16
Percentage to the total sediment yielded from the small watershed 1960s 16.04 59.00 1.20 1970s 51.83 72.90 4.44 1980s 64.40 92.20 16.57 1990s 50.51 92.40 36.62
Xu Xiang-zhou et al. / Environmental Science & Policy 7 (2004) 79–86 Table 2 Amount of retained sediment in the Yellow River watershed (Zeng et al., 1999) Period
Sediment retained by check-dams (106 m3 )
1952–1962 1963–1969 1970–1979 1980–1989 1990–1995 Total
600.72 1004.42 2151.37 1823.77 1547.04
Total retained sediment (106 m3 ) 647.42 1,135.26 2694.69 3144.77 3032.65
Percentage of the sediment retained by check-dams 92.8 88.5 79.8 58.0 51.0
7127.32
10,654.79
66.9
formed by dams is highly suitable for planting due to the fertile soil and abundant water. In contrast with the farmland on slopes, dam farmland is enriched in nitrogen, phosphorus and organic substances: their contents in the dam farmlands are 7–12%, 8–10% and 120–140%, respectively, higher than that in the slope farmland. Moreover, the water content in the dam farmlands is two to three times higher than that in the slope farmlands (Li, 1995). According to field investigations on check-dam systems in the Shanxi Province (Fang, 1999), including Kanghe Gou in Fenxi County, Dong Gou in Lingshi County and Liu Gou in Ji County, 750–1500 kg corn is harvested per hectare of the dam farmland, which is 8–10 times higher than that on the slope farmland. At present, the dam farmland only occupies 9% of the whole area of farmland in the Loess Hill Ravine Region, but their grain yield is 23.5% or more higher than elsewhere (Xu and Wang, 2000). Thus, there is now a popular proverb: “One would rather own a hectare of dam farmland than ten hectares of slope farmland.” It is impractical to convert all slope farmland to forestland or grassland and then import food supplies from other provinces, as the agricultural population in the Loess Plateau amounts to 40 million (Liu, 2001), and is scattered widely in remote villages with unfavourable transportation conditions. Nevertheless, if the people take full advantage of all potential dam farmland, and continue to preserve the present terrace farmland, the food supplies could support the regional population and all slope farmland could be abandoned eventually. 4. Working principles of the check-dam system in gullies Over 100,000 check-dams have been built on the Loess Plateau in the last 50 years, and the agriculture of the check-dam systems has become a marvelous landscape on the vast plateau. Thus, it is essential to study the working principles and to improve the design theory for the check-dam system in gullies. 4.1. Relative stability of the check-dam In the design and construction of check-dam systems, elements such as the layout, the height of the dams and the
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controlled area of the small watersheds should all meet the standard for relative stabilization. In the 1960s, the concept of “relative stability of the check-dam system” emerged, which stemmed from the recognition of the key factors that determined the natural balance of check-dams. Inspired by the natural Juqiu, a kind of dam land resulting from a land-slip which never overflows when impounding the floodwater and soil for centuries, people came to recognize the important role of dam height and the ratio of dam land area to that of the controlled watershed. It was observed that if the above parameters reached a certain value, soil and water in the small watershed could be internally absorbed without raising the height of the dam. Check-dam system design has to fulfill several purposes (Fang, 1995): 1. To prevent flooding i.e. to ensure the check-dam system can withstand rainstorms. 2. To guarantee harvest from the dam farmland i.e. to reduce the loss of crops due to rainstorms. 3. To conserve floodwater and sediment by impounding. 4. To ensure that increase in height and repair of the dams after prolonged use are unnecessary. To attain relative stability of the check-dam system to meet these purposes, many elements have to be considered: hydrological, geographical and geological conditions of the controlled small watersheds, area of the dam farmland, varieties of the crops, etc.. Empirically, the check-dam becomes relatively stable when the ratio of the dam farmland area to that of the controlled watershed is between 1:25 and 1:15. When the impounded water depth is less than 0.8 m and the storage time is shorter than 3–7 days, a dam designed for enduring the biggest rainstorm in a century is found to be relatively stable (Zeng et al., 1995). The coefficient for relative stability of the dam system I is defined as the ratio of the area of dam farmland to that of the watershed corresponding to flood frequency p. When the flood frequency p is 2% and the depth of impounded water in the dam farmland is equal to the critical value, the coefficient I is called the critical coefficient for relative stability of the dam system Ic . If I > Ic , the dam system is stable, and if I > Ic , the dam system is not stable. Ic =
Wp δF
(1)
Where, Wp is the amount of impounded floodwater (m3 ) when the flood frequency p is 2%, δ is the design depth of water for the dam farmland (m); and F is the area of controlled watershed by the check-dam (km2 ) (Zeng et al., 1999).Many natural check-dams, such as the Balihe Gully and Sanshilihe Gully in the Shannxi Province, the Laobatou and Qianqiuzi in the Gansu Province have achieved relative stability. The Huangtuwa in the Zizhou County of the Shannxi Province, another famous natural check-dam covering 40 hectares (40,000 m2 ) of lowlands, has favourably run for
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more than 400 years. Some check-dam systems built in the 1960s, such as the Kanghe Gou dam system in the Shanxi Province and the Wangmao Gou dam system in the Shannxi province, are also typical projects attaining relative stability, and have retained enormous amounts of sediments and harvested plentiful crops. Relative stability of the check-dam system is the ultimate goal. 4.2. Mechanism of check-dams in retaining sediments and decreasing erosion The mechanism of check-dams to retain sediments and to decrease erosion can be explained as follows. The riverbeds in gullies will rise by virtue of siltation behind the check-dams. As a result, cutting down of the gullies by scouring, and collapse of gully sides are reduced. At the initial stages of the check-dam, sediments are retained and floodwater is impounded within the check-dam. At this stage, therefore, the check-dam also has the function of reducing the erosion in the lower reaches by reducing the flood peak. In the later stages of the check-dam, the flow velocity will be reduced due to the wider and gentler gradient of the newly formed farmland. Consequently, sediment transport capacity of the flood decreases and sediment is deposited along the valley. Meanwhile, as flourishing crops grow on the fertile dam farmland, floodwater will be filtered and sediment will remain. Obviously, check-dams have not much influence on erosion on the slopes. However, the check-dams can maintain relative stability even when they retain soil lost from the slopes. The amount of loss on slopes is much smaller than that in the gullies. In the Loess Hill Ravine Region, sediments yield from the gullies is 60–70% of the total amount. In the Loess Mesa Ravine Region, the sediment yield is 90% or more of the total amount, and is about nine times of that from the slopes (Fang et al., 1998). Slope engineering, such as the construction of farmland terraces and the planting of trees, can be adopted in the later operational period of the check-dam when the dam farmland has been formed. At this stage all of the slope farmlands can potentially be converted to woodlands or grasslands by virtue of the development of the check-dam agriculture. Moreover, if we cut the mounds and ridges erected on the Loess Plateau and fill the valleys to form layers of flat land, soil erosion on the slopes can be reduced. Thus, as the ultimate goal for soil and water conservation in the small watersheds, relative stability of the check-dam system can be attained.
5. History of the check-dam system in the last 50 years Although check-dams had been built in China for centuries, it is since the founding of the People’s Republic of China that they have been constructed on a large scale as a matter of policy (Feng, 2000).
In the autumn of 1949, more than 30 demonstration check-dams were built at the Kanghe Gou, Ma Gou and Yaopu River in Fenxi County, Shanxi Province, and attained a remarkable accomplishment in blocking of sediments and increasing foodstuff output. From then on, check-dams have spread extensively in soil erosion areas of the Loess Plateau. By the end of 1950s, many rudimentary check-dam systems, with flood-control dams, had been formed. With the number of check-dams increasing, problems of preventing floods became serious due to lack of design theory and poor construction quality. In the 1960s, an integral check-dam system which functioned for controlling floods, retaining sediments and producing grain had emerged in the Fenxi county, Shanxi Province. Thereafter, the Yellow River Conservancy Commission put forward a scheme for constructing 946 check-dams to prevent sediment from pouring into the Sanmenxia Reservoir, known as the Thousand Dam Scheme. With advantages such as drought-resistance, fertile and good yields on dam farmland, great enthusiasm was inspired for building check-dams in the late 1960s and the 1970s. It was estimated that most of the 100,000 check-dams in China were built at this period. However, due to poor construction quality, in 1977 and 1978 more than 80% of the check-dams in the Shanbei region were destroyed in fierce rainstorms after a long period of drought. These failures diminished the confidence in developing check-dams. Some damaged dams or dangerous dams were repaired or reinforced, but in general the emphasis was shifted to slope engineering such as planting and constructing terrace farmland in this period. As time went on, it became clear that slope engineering was unsatisfactory and incapable of conserving soil and water in the Loess Plateau due to its inability to withstand large rainstorms. Hence in the 1980s there was renewed interest in check-dam systems in gullies.In 1983, the key project of Conservation of Soil and Water in Gullies was initiated by the State Commission of Planning, and experiments were carried for three years. Since the relevant departments have prepared detailed plans, technical criteria and regulations, the project progresses well. However progress is slow: compared to the 100,000 check-dams in the 1960s and 1970s, only 1118 check-dams had been set up from 1986 to 1999 in the Loess Plateau (Feng, 2000). In the Loess Plateau, it is estimated that 6758 gullies are suitable for building check-dam systems, where 21,000 key projects, including 242,000 check-dams, could be built in the future. Table 3 lists the detailed layout. At this speed it would take more than 100 years to realize the scheme as above to control the main soil and water loss area in the Loess Plateau. A series of articles on check-dam systems in gullies on the Loess Plateau were published in the beginning of 1990s (Kang, 1993) and aroused passionate discussion on the strategy for controlling the Yellow River. Consequently, importance was also attached to researching theories for constructing check-dams, and funds have been granted for
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Table 3 Scheme for the check-dam systems in the Loess Plateau (Huang, 2000) Region Loess Hill Ravine area I, II Loess Hill Ravine region III, IV, V Loess Mesa Ravine region Wind Sand region Soil-stone mountain region Terrace region Total a
Area (km2 )
Number of key projectsa
Number of check-damsa
84,980 93,413 25,017 36,238 51,264 8341
13,447 4788 2153 242 340 30
47,486 77,866 21,859 31,961 53,218 9610
299,253
2100
242,000
The key projects and check-dams include the existing 1289 projects and 112,000 dams.
work on relative stability and optimum design of check-dam systems. As the technology is progressing and the theories for constructing check-dams are more mature, a common viewpoint has emerged that the most effective way to control sediment pouring into the Yellow River is to carry out modern engineering projects whose main element is the check-dam system in gullies.
6. Problems and suggestions It is necessary to focus national attention in China on the control of the Yellow River by check-dam systems. As more investment from the government is needed, policies encouraging individuals and local governments to develop check-dams ought to be established too. At the same time, theories on the development of check-dam systems should be extended and improved. Physical model experiments should be carried out in the context of typical small watersheds with unfavourable topography in the Two-River (Kuye River and Tuwei River) and Two-Chuan (Huangpu Chuan River and Gushan Chuan River) Zone of the Loess Plateau. Problems worthy of study are proposed as follows: (1) To construct and test hydraulic models of small watersheds in the Loess Plateau. It is well known that scale model experiments are effective in demonstrating processes and studying mechanisms of soil and water loss on a large scale or at high speed (Jiang et al., 1994; Yuan et al., 2000; Wischmeier, 1976). However, since the loess is so fine and erosive, and other erosive factors and phenomena such as rain, overland flow, gravity erosion are so complicated, an integrated scaling law for hydraulic models of small watersheds in the Loess Plateau has still not been produced. In spite of many experimental plots in situ to measure runoffs from slopes, few indoor model experiments with strict enlarging (or shrinking) scales to simulate the original hydraulic phenomena small watershed have been completed in recent decades. (2) To study the relative stability and optimum design of the check-dams. Based on ample data and investigations on the spot, hydraulic models should be made for the soil and water erosion area in the Loess Plateau.
Then, studies on the condition and standard of the relative stability for check-dam systems in gullies should be carried out, and scientific methods to investigate flood frequency should also be developed. After that, rules on designing the check-dam systems in various gullies through computers, and theories on check-dam stability, should be established. In this way, it is hoped that floodwater and sediments can be controlled and the dams will become relatively stable at last. We have completed a series of indoor model experiments to study the mechanism to intercept sediment and the relative stability of check dams. Results in detail will be published in other papers. 7. Conclusion In conclusion, the most effective way to conserve soil and water in the Loess Plateau is to construct check-dam systems in gullies. If layers of flat land were formed as the mounds and ridges were cut to fill the valleys, the geographical condition of the Loess Plateau would be greatly improved, and soil and water losses would be thoroughly controlled as well. The policy to spread check-dams is feasible and great benefits are attached for the local people by virtue of developing agriculture of check-dam system. Some problems such as destruction due to floodwater can be resolved as the design standards and the layout theory are improved. Acknowledgements The project was supported by the Yellow River Conservancy Commission Fund for Soil and Water Conservation (Contract No. 2000.06) and National Nature Science Fund (No. 40201008). The authors wish to acknowledge Professor Liang Dongbai and Doctor Cong Yu for amending this paper on English writing. References Chen, L., Wang, J., Fu, B., Qiu, Y., 2001. Land-use change in a small catchment of Northern Loess Plateau, China, agriculture. Ecosyst. Environ. 86, 163–172.
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Comprehensive Survey Team on Loess Plateau of Chinese Academy of Sciences. 1992a, Resource, Environment and Social Economy Data of Loess Plateau Zone, Economics Press of China, 14 (in Chinese). Comprehensive Survey Team on Loess Plateau of Chinese Academy of Sciences. 1992b, Brief report on comprehensive control and development on the Loess Plateau, Economics Press of China, pp. 445–464 (in Chinese). Fang, R., 1999. Outlook of the check-dam system agriculture in Shanxi Province. Soil Water Conserv. China 169, 4–7 (in Chinese). Fang, X., 1995. Criterion and condition for the relative stability of check-dam system, Soil Water Conserv. China 169, 29–33 (in Chinese). Fang, X., Wang, Z., Kuang, S., 1998. Mechanism and effect of check-dam to intersect sediment in the middle reach of Yellow River, J. Hydraul. Eng. 262, 49–53 (in Chinese). Feng, G., 1994. Check system agriculture is worth studying, Sci. Technol. Rev. 4, 18–21 (in Chinese). Feng, G., 2000. The key to bring the Yellow River under control is to speed up construction of check-dams in the Sandy and Grit Areas. Sci. Technol. Rev. 145, 53–57 (in Chinese). Fu, B., Gulinck, H., 1994. Land evaluation in an area of severe erosion: the Loess Plateau of China. Land Degrad. Rehabil. 5, 33–40. Han, J., Fyfe, W.S., Longstaffe, F.J., Palmer, H.C., Yan, F.H., Mai, X.S., 1997. Pliocene-pleistocene climatic change recorded in fluviolocustrine sediments in Central China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 135, 27–39. Huang, Z., 2000. Practice and assumption for construction of the dam system eco-engineering for gullies in Loess Plateau. Soil Water Conserv. China 22, 1–4 (in Chinese). Hui, S., Mingan, S., 2000. Soil and water loess from the Loess Plateau in China. J. Arid Environ. 45, 9–20. Jiang, D., Zhou, Q., Fan, X., Zhao, H., 1994. Simulated experiment on normal intergral model of water regulating and sediment controlling for small watershed. J. Soil Water Conserv. 8, 26–30. Kang, X., 1993. Check-dam agriculture, the radical method to control Yellow River. Sci. Technol. Rev. 62 (8), 16 (in Chinese). Li, Z., 1995. Construction and outlook analysis for check-dams in the Loess Plateau. J. Soil Water Conserv. 4, 44 (in Chinese). Liu, W., 2001. Discussion on control strategy for soil and water loss in the Loess Plateau. Soil Water Conserv. China 208, 21–22 (in Chinese). Meng, Q. (Ed.), 1996. Soil and Water Conservation in the Loess Plateau, vol. 75, Water Resource Press of Yellow River, pp. 316–317 (in Chinese). Shi, N., 1981. Forestry in the Middle Reach of Yellow River During History, Shanlian Book Store, pp. 1–5 (in Chinese). Tilman Rost, K., 2000. Pleistocene paleo-environmental changes in the high mountain ranges of Central China and adjacent regions. Quat. Int. 65-66, 147–160. Wischmeier, W.H., 1976. Foreword, In: The Proceeding of a National Conference on Soil Erosion, Soil Erosion: Prediction and Control, Soil Conservation Society of America, pp. viii–ix.
Xu, M., Wang, G., 2000. To accelerate the construction of check-dams in the Loess Plateau. Yellow River 22, 26 (in Chinese). Xu, X., Zhang, H., et al., 2002. Check-dam system in gullies–the most effective measure to conserve soil and water in Chinese Loess Plateau, In: Proceedings of the 12th International Soil Conservation Organization Conference, vol III, Tsinghua University Press, pp. 503–509. Yellow River Conservancy Commission, 1988. Soil and water conservation in the Yellow river valley, Shanghai Educational Publishing House, 7 and 40. Yuan, J., Lei, T., Jiang, D., Zhou, Q., 2000. Simulated experimental study on normalized integrated model for different degrees of erosion control for small watersheds. Trans. Chin. Soc. Agric. Eng. 16, 22–25 (in Chinese). Zeng, M., Fang, X., Kang, L., Wang, E., 1995. It is doubtless for the check system in Gullies to become relativly stable. Yellow River 17, 18–21 (in Chinese). Zeng, M., Zhu, X., Kang, L., Zuo, Z., 1999. Effect to intercept sediment and decrease erosion and development prospect of check-dams in water and soil erosion areas. Res. Soil Water Conserv. 6, 127–132 (in Chinese). Zhang, T., 1993. Discussion Outline on Loess Plateau, Environment Press of China, p. 109 (in Chinese). Zhang, H., Zhang, J., Yao, W., 1999. Strategy to control the Yellow River. J. Sedim. Res. 2, 1–4 (in Chinese).
Xu Xiang-zhou received his BSc and MS in Civil Engineering from Dalian University of Technology, and is at present a PhD student in Hydraulics and Hydropower Engineering of Tsinghua University. His doctoral thesis aims at understanding the similarity of scale models on soil and water erosion. He is the first-inventor of three Chinese patents. He is currently engaged in the series of programs financed by the Yellow River Conservancy Commission Fund for Soil and Water Conservation and National Nature Science Fund. Zhang Hong-wu received his PhD from Hydraulics and Hydropower Engineering of Tsinghua University. He is Professor and Director of the Yellow River Research Center of Tsinghua University, technological committeeman of the Yellow River Conservancy Commission, vice-director for the Sedimentation Committee of Chinese Hydraulic Engineering Society, etc. His current research aims at physical scale models for controlling the Yellow River and minimising soil and water erosion on the Loess Plateau. He is the author of about 100 papers and 12 monographs. Zhang Ouyang is a post-doctoral researcher in Hydraulics and Hydropower Engineering of Tsinghua University studying soil and water erosion on the Loess Plateau.
Development of check-dam systems in gullies on the Loess Plateau, China Xu Xiang-zhou∗ , Zhang Hong-wu, Zhang Ouyang Sediment Laboratory, Department of Hydraulics and Hydropower Engineering, Tsinghua University, Beijing 100084, China
Abstract The Loess Mesa Ravine Region and the Loess Hill Ravine Region, cover 200,000 km2 of the Loess Plateau in China and have serious problems of soil and water erosion. Two primary ways to control the sediment pouring into the Yellow River from this area are planting and engineering measures. The former is not suitable for the Loess Plateau due to the arid climate and the barren soil, while some of the latter means, such as terrace farmlands, are vulnerable to floods. As a widespread engineering measure, the check-dam system in gullies is one of the most effective ways to conserve soil and water in the Loess Plateau. At present, the amount of sediment retained by check-dam systems is the largest of all methods and the potential is promising. The dam farmlands so created have become important high-yield croplands or orchards with enriched fertile soil and ample water. This paper reviews the history and principles of check-dams and discusses future theoretical and experimental studies which are needed for the further implementation of this system. © 2004 Elsevier Ltd. All rights reserved. Keywords: Loess Plateau; Check-dams; Soil and water conservation; Strategy to control the Yellow River; Yellow River; Soil erosion
1. Introduction Check-dams are the most widespread structures for conserving soil and water in the Loess Plateau. However, they have not been described extensively in literature outside China. This paper presents a review of check-dam systems in gullies on the Loess Plateau. By making use of the local geography and climate, the people of the Loess Plateau of China skillfully invented the check-dam system in gullies several centuries ago, to retain sediments and to form farmland. Check-dams at the Kanghe Gou watershed of the Fen-xi County, built in the Ming Dynasty 400 years ago, are still in good condition. As one of the primary measures to conserve water and soil, the check-dam project has been given great emphasis ever since the founding of the People’s Republic of China. Fig. 1 shows a check-dam in the Loess Plateau. By 2002, about 113,500 check-dams had been built, creating 3200 km2 of farmlands with high productivity, and intercepting a total of 700 million m3 of sediments that pour into the Yellow River. Thus, the check-dam system is the most important and well-known project in China to conserve soil and water. Generally, a check-dam is composed of three parts: the embankment, the spillway, and the outlet. Sometimes some simple check-dams are constructed without spillways or outlets. Fig. 2 is a Planform of a typical check-dam. In a ∗ Corresponding author. Tel.: +86-10-62788585; fax: +86-10-62776772. E-mail address: [email protected] (X. Xiang-zhou).
1462-9011/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.envsci.2003.12.002
small watershed, various dams can be built, such as productive dams for forming farmland, flood-control dams for preventing floodwater and intercepting sediments, and water-storage dams for irrigation. A group of such dams constitutes a check-dam system.
2. Characteristics of this area The Loess Plateau is located in the upper and middle reaches of the Yellow River, encircled by the Ela Mountains to the west, the Taihang to the east, the Yanshan Mountains to the north and the Qingling and Funiu Mountains to the south. The plateau covers a total area of 624,000 km2 , and over 60% of the land is subjected to soil and water losses. The most crucial areas are the Loess Mesa Ravine Region and the Loess Hill Ravine Region, which together cover 30% of the total area. As an area with the most severe area of soil and water losses in the world, average erosion reaches 5000–10,000 (km2 per year), sometimes even up to 20,000–30,000 tonnes km−2 per year) (Meng, 1996). 2.1. Climate Climate is an essential factor controlling alluvial sedimentation and erosion (Tilman Rost, 2000). Most areas of the Loess Plateau are arid or semi-arid with dry air and little cloud, but short of moisture. The average annual precipitation on the Plateau is only 350–550 mm, which gradually
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Fig. 1. A check-dam in ZiFangGou, Shannxi Province, China (photo: Zhang Ouyang).
decreases from the southeast to the northwest. Moreover, precipitation is mainly concentrated in the rainy season from June to September, which accounts for 60–70% of the total, most of which is in the form of high intensity rainstorms. For these reasons, soil erosion predominately occurs in this period. 2.2. Landforms The main geomorphic landforms on the Loess Plateau are plateau, ridge, mound and various gullies. Because of the low gradient, little erosion takes place on top of the plateau, except for intense erosion in the gullies at the edge of the plateau. Undulating terrain in the Loess Hill Ravine Region is typified by crisscross gullies dissecting thick loess overburden on top of ancient landforms, to form ridges and mounds: mounds are domed in shape, round or elliptical. Dam Farmland Flood-Control Dam Slope
Slope
Flood-Control Channel Silo
Embankment Spillway
Seepage
Lower Reach of the Dam
Fig. 2. Planform for a check-dam.
The strip-formed “ridges” typically measure several tens of kilometers in length and scores or even hundreds of meters wide at some localities. Fig. 3 shows the typical landform of the Loess Plateau. Since soil erosion occurs intensely in gullies, ridges and mounds, unique geomorphologic features with numerous gullies and fragmented landforms are formed gradually. As a rule, an area with a high gully density often bears severe soil erosion. 2.3. Geology The Loess Plateau is capped with the deepest loess in the world. The Quaternary loess is widely distributed, covering more than 70% of the total area. Loess is mostly silty sandy loam of loose structure comprising 60–70% of grains 0.02–0.05 mm in size and 10–15% of readily soluble carbonates, besides being of high porosity reaching 40–50% and having vertical joints, rendering the soil very liable to be eroded under action of water, gravity or wind (Yellow River Conservancy Commission, 1988). Tectonic movements accelerate erosion. The Loess Plateau is one of the most active areas of tectonic movement in China. In the Quaternary, the crust has been uplifted locally by 150–200 m. Soil erosion in the valleys is aggravated by the increased energy of the water. Also, earthquakes bring about gravitational erosion and destroy landform structure, and lead to the collapse and sliding of large areas on the Loess Plateau (Hui and Mingan, 2000). 3. Strategy to control erosion on the Loess Plateau Two primary ways to control the sediment pouring into the Yellow River in this area are planting and engineering measures. The former means constructing systems to control
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Fig. 3. A sight of loess ridges and mounds in the Loess Plateau, China (photo: Xu Xiangzhou).
wind erosion and water erosion, and engineering measures mean setting up civil engineering structures such as reservoirs, check-dams, or walls to store soil and water. Some means, such as land-use change, combine planting and engineering measures. Which is the most effective, is still in dispute. 3.1. Planting It is said that the Loess Plateau in ancient China was an area with luscious grasslands and dense forests (Shi, 1981; Chen et al., 2001), which were destroyed and attacked by severe soil and water erosion due to undue deforestation by humans (Fu and Gulinck, 1994). A typical viewpoint (Meng, 1996) is that forest cover on the Loess Plateau had been over 50% greater in the past, and this implies that soil and water conservation could be achieved by planting trees and grasses on the slopes. In the middle reaches of the Yellow River, especially in the areas that presently suffer soil and water losses most severely, the same severe environment has lasted for a long time: accelerated erosion started 250,000–300,000 years ago, and was further accelerated in the last 50,000 years, which was before mankind started to impact on the environment (Comprehensive Survey Team on Loess Plateau of Chinese Academy of Sciences, 1992a, 1992b). This suggests that the ultimate determinant in causing severe soil and water erosion on the Loess Plateau is not human activities, but the arid climate and the unfavourable geology and topography. However the suggestion that the forest cover was up to 50% or more greater in the past still remains in doubt. It might be expected that forests and grasslands that had ex-
isted in history should still be in existence, except for a change in scale or type (Meng, 1996). During the Quaternary, due to uplift of 3500 m of the Qinghai-Tibet Plateau, the monsoon system of Southeast Asia was formed and the climatic difference between the humid southeast coast and the dry northwest inland was enhanced. Furthermore, local landforms, for example, the Qinglin Mountains, with an altitude of 3000 m, influenced the monsoon and affected the climate of the Loess Plateau. Distinct floral zones and forests with grasslands were created on the Loess Plateau. As a result, the forest cover has never been above about 53% of the total plateau area. Pollen studies, combined with stable carbon-isotope analysis of organic matter, show that changes in the paleovegetation are profound in response to climate forcing. Persistent steppe vegetation and fossil elephant fauna also suggest long-lasting dry and warm climate conditions in the last 3.0–2.7 million years (Han et al., 1997). Control of soil and water erosion on the Loess Plateau by planting measures alone has been proved to be unsuccessful during recent decades. Firstly, trees are difficult to keep alive owing to the arid climate and barren soil, and even if they survive, most of them do not grow strong enough to control soil and water losses. An investigation in 1998 showed that the survival rate of trees in this year was 53% and that of grasses was 24.2% in the Hekou-Longmen region in an area of 110,000 km2 (Xu and Wang, 2000). Next, even where approriate vegetative engineering measures are taken, it is often impracticable to carry out the entire plan. In the early 1980s, the emphasis of soil and water conservation on the Loess Plateau was on planting, nevertheless, sediment continued to pour into the Yellow River.
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After a reconsideration of the policy, the check-dam system was reconsidered in the mid-1980s, and a new type of check-dam system comprising strong-structured high dams named key projects was undertaken (Feng, 2000). A key project dam is generally made of concrete and is designed to resist one-in-a-100-year floods, and can therefore prevent other check-dams in the system from being destroying by quite major floods. 3.2. Engineering measures There are two kinds of engineering measures in the Loess Mesa Ravine Region and Loess Hill Ravine Region. Some structures, such as farmland terracing, emphasize regulating slopes whereas others focus on controlling gullies. Many researchers have regarded the regulating of slopes as more important, and their main idea is to intercept rainfall on the slope. Soil and water losses can be controlled as long as the total rainfall is filtered out in situ. However, some critical deficiencies exist in this approach. (a) Gully erosion accounts for more than 60–90% of the soil erosion in the watersheds (Zhang, 1993). Even though sediments from the slopes are fully captured, leaking sediments from the small watersheds could not be reduced very much. (b) It is difficult to intercept sediment on steep slopes in situ. (c) Structural stabilization of constructions on the slope is not very reliable. (d). The constructions for controlling slopes such as farmland terracing are inefficient in confronting a drought, and water-storing dams for irrigation in the gully have to be built as a complement. Some other scholars insist on regulating gullies and slopes simultaneously. However, conflicts will occur in the initial stage of building check-dams in gullies, as constructions for controlling slopes for retarding sediments will slow down the speed of the formation of farmland, and more time will be needed to reach relative stabilization for the whole check-dam system. Recently, an engineering project for controlling gullies was proposed for the Yellow River (Feng, 1994; Zhang et al., 1999; Xu et al., 2002). It was suggested that in order to retard total sediment and floodwater in the small watersheds of the Loess Plateau, check-dams should be built step by step along the valleys to form layers of flat land with various elevations. It was anticipated that when the mounds and ridges of the plateau were cut and the valleys were filled to form layers of flat land, the geographical environment would be greatly improved, and soil and water losses would be controlled as well. Moreover, in order to make the dam system relatively stable more quickly, advanced modern engineering measures, such as directional blasting, hydraulic fill and transport by bulldozers are introduced to cut the mounds and fill the valleys. However when this approach is adopted, in order to avoid reducing the transport of soil from the hill slope and the velocity of formation of farmland in the early age of forming dam-land, neither planting nor engineering measures could be adopted on the hill slope.
3.3. Benefits of the check-dam system in gullies The check-dam system, especially when including key project dams for controlling gullies, is the largest engineering project to harness soil erosion effectively, and should act as the last line of defence to impound water and conserve soil. In addition, the fertile farmland so formed provides an excellent base for agriculture. The check-dam system in gullies is the most effective measure to control sediment pouring into the Yellow River. Due to serious silting, levees along the lower reaches have to be heightened ever year, thus an “Above Ground River” is formed with a riverbed 10 m higher than the surrounding ground. In fact, most of sediment in the lower Yellow River comes from the Loess Plateau. Hence, reducing sediment pouring into the Yellow River from the Loess Plateau is the most effective solution, although many other methods such as dredging and dike heightening have also been tried. In the lower reaches of the Yellow River, the amount of retained sediment is between 16,680 and 72,840 tonnes per hectare. The more serious soil and water erosion is, the more valuable are check-dams. Enormous amounts of sediment retained from small watersheds by check-dams not only form large-scale fertile farmland but also safeguard the Yellow River against overflow. Thus, the social benefit is remarkable. The mean decrease in sediments pouring into the Yellow River from the three main tributaries between Hekou and Guanbao city in recent decades is shown in Table 1. Table 2 gives the percentage of sediment retained by check-dams in the Yellow River watershed—7127 mm3 are held up by check-dam systems, which is 66.9% of the total amount. Meanwhile, 3812 km2 of farmlands were formed as a result of the measures taken. From Tables 1 and 2, it can be concluded that check-dam systems have made a great contribution to preventing sediment from pouring into the Yellow River. A consequence of this success is the creation of a new type of agriculture, which emerges as more and more food supplies are produced from the dam farmland. Farmland
Table 1 The mean decrease in sediment pouring into the Yellow River from the three main tributaries—Wuding River, Fenhe River, Huangpucuan River (Xu and Wang, 2000) Items
Wuding River (106 tonnes)
Fenhe River (106 tonnes)
Mean decrement of sediment in every decade 1960s 43 50 1970s 125 52 1980s 96 53 1990s 99 54
Huangpucuan River (106 tonnes) 1 3 9 16
Percentage to the total sediment yielded from the small watershed 1960s 16.04 59.00 1.20 1970s 51.83 72.90 4.44 1980s 64.40 92.20 16.57 1990s 50.51 92.40 36.62
Xu Xiang-zhou et al. / Environmental Science & Policy 7 (2004) 79–86 Table 2 Amount of retained sediment in the Yellow River watershed (Zeng et al., 1999) Period
Sediment retained by check-dams (106 m3 )
1952–1962 1963–1969 1970–1979 1980–1989 1990–1995 Total
600.72 1004.42 2151.37 1823.77 1547.04
Total retained sediment (106 m3 ) 647.42 1,135.26 2694.69 3144.77 3032.65
Percentage of the sediment retained by check-dams 92.8 88.5 79.8 58.0 51.0
7127.32
10,654.79
66.9
formed by dams is highly suitable for planting due to the fertile soil and abundant water. In contrast with the farmland on slopes, dam farmland is enriched in nitrogen, phosphorus and organic substances: their contents in the dam farmlands are 7–12%, 8–10% and 120–140%, respectively, higher than that in the slope farmland. Moreover, the water content in the dam farmlands is two to three times higher than that in the slope farmlands (Li, 1995). According to field investigations on check-dam systems in the Shanxi Province (Fang, 1999), including Kanghe Gou in Fenxi County, Dong Gou in Lingshi County and Liu Gou in Ji County, 750–1500 kg corn is harvested per hectare of the dam farmland, which is 8–10 times higher than that on the slope farmland. At present, the dam farmland only occupies 9% of the whole area of farmland in the Loess Hill Ravine Region, but their grain yield is 23.5% or more higher than elsewhere (Xu and Wang, 2000). Thus, there is now a popular proverb: “One would rather own a hectare of dam farmland than ten hectares of slope farmland.” It is impractical to convert all slope farmland to forestland or grassland and then import food supplies from other provinces, as the agricultural population in the Loess Plateau amounts to 40 million (Liu, 2001), and is scattered widely in remote villages with unfavourable transportation conditions. Nevertheless, if the people take full advantage of all potential dam farmland, and continue to preserve the present terrace farmland, the food supplies could support the regional population and all slope farmland could be abandoned eventually. 4. Working principles of the check-dam system in gullies Over 100,000 check-dams have been built on the Loess Plateau in the last 50 years, and the agriculture of the check-dam systems has become a marvelous landscape on the vast plateau. Thus, it is essential to study the working principles and to improve the design theory for the check-dam system in gullies. 4.1. Relative stability of the check-dam In the design and construction of check-dam systems, elements such as the layout, the height of the dams and the
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controlled area of the small watersheds should all meet the standard for relative stabilization. In the 1960s, the concept of “relative stability of the check-dam system” emerged, which stemmed from the recognition of the key factors that determined the natural balance of check-dams. Inspired by the natural Juqiu, a kind of dam land resulting from a land-slip which never overflows when impounding the floodwater and soil for centuries, people came to recognize the important role of dam height and the ratio of dam land area to that of the controlled watershed. It was observed that if the above parameters reached a certain value, soil and water in the small watershed could be internally absorbed without raising the height of the dam. Check-dam system design has to fulfill several purposes (Fang, 1995): 1. To prevent flooding i.e. to ensure the check-dam system can withstand rainstorms. 2. To guarantee harvest from the dam farmland i.e. to reduce the loss of crops due to rainstorms. 3. To conserve floodwater and sediment by impounding. 4. To ensure that increase in height and repair of the dams after prolonged use are unnecessary. To attain relative stability of the check-dam system to meet these purposes, many elements have to be considered: hydrological, geographical and geological conditions of the controlled small watersheds, area of the dam farmland, varieties of the crops, etc.. Empirically, the check-dam becomes relatively stable when the ratio of the dam farmland area to that of the controlled watershed is between 1:25 and 1:15. When the impounded water depth is less than 0.8 m and the storage time is shorter than 3–7 days, a dam designed for enduring the biggest rainstorm in a century is found to be relatively stable (Zeng et al., 1995). The coefficient for relative stability of the dam system I is defined as the ratio of the area of dam farmland to that of the watershed corresponding to flood frequency p. When the flood frequency p is 2% and the depth of impounded water in the dam farmland is equal to the critical value, the coefficient I is called the critical coefficient for relative stability of the dam system Ic . If I > Ic , the dam system is stable, and if I > Ic , the dam system is not stable. Ic =
Wp δF
(1)
Where, Wp is the amount of impounded floodwater (m3 ) when the flood frequency p is 2%, δ is the design depth of water for the dam farmland (m); and F is the area of controlled watershed by the check-dam (km2 ) (Zeng et al., 1999).Many natural check-dams, such as the Balihe Gully and Sanshilihe Gully in the Shannxi Province, the Laobatou and Qianqiuzi in the Gansu Province have achieved relative stability. The Huangtuwa in the Zizhou County of the Shannxi Province, another famous natural check-dam covering 40 hectares (40,000 m2 ) of lowlands, has favourably run for
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more than 400 years. Some check-dam systems built in the 1960s, such as the Kanghe Gou dam system in the Shanxi Province and the Wangmao Gou dam system in the Shannxi province, are also typical projects attaining relative stability, and have retained enormous amounts of sediments and harvested plentiful crops. Relative stability of the check-dam system is the ultimate goal. 4.2. Mechanism of check-dams in retaining sediments and decreasing erosion The mechanism of check-dams to retain sediments and to decrease erosion can be explained as follows. The riverbeds in gullies will rise by virtue of siltation behind the check-dams. As a result, cutting down of the gullies by scouring, and collapse of gully sides are reduced. At the initial stages of the check-dam, sediments are retained and floodwater is impounded within the check-dam. At this stage, therefore, the check-dam also has the function of reducing the erosion in the lower reaches by reducing the flood peak. In the later stages of the check-dam, the flow velocity will be reduced due to the wider and gentler gradient of the newly formed farmland. Consequently, sediment transport capacity of the flood decreases and sediment is deposited along the valley. Meanwhile, as flourishing crops grow on the fertile dam farmland, floodwater will be filtered and sediment will remain. Obviously, check-dams have not much influence on erosion on the slopes. However, the check-dams can maintain relative stability even when they retain soil lost from the slopes. The amount of loss on slopes is much smaller than that in the gullies. In the Loess Hill Ravine Region, sediments yield from the gullies is 60–70% of the total amount. In the Loess Mesa Ravine Region, the sediment yield is 90% or more of the total amount, and is about nine times of that from the slopes (Fang et al., 1998). Slope engineering, such as the construction of farmland terraces and the planting of trees, can be adopted in the later operational period of the check-dam when the dam farmland has been formed. At this stage all of the slope farmlands can potentially be converted to woodlands or grasslands by virtue of the development of the check-dam agriculture. Moreover, if we cut the mounds and ridges erected on the Loess Plateau and fill the valleys to form layers of flat land, soil erosion on the slopes can be reduced. Thus, as the ultimate goal for soil and water conservation in the small watersheds, relative stability of the check-dam system can be attained.
5. History of the check-dam system in the last 50 years Although check-dams had been built in China for centuries, it is since the founding of the People’s Republic of China that they have been constructed on a large scale as a matter of policy (Feng, 2000).
In the autumn of 1949, more than 30 demonstration check-dams were built at the Kanghe Gou, Ma Gou and Yaopu River in Fenxi County, Shanxi Province, and attained a remarkable accomplishment in blocking of sediments and increasing foodstuff output. From then on, check-dams have spread extensively in soil erosion areas of the Loess Plateau. By the end of 1950s, many rudimentary check-dam systems, with flood-control dams, had been formed. With the number of check-dams increasing, problems of preventing floods became serious due to lack of design theory and poor construction quality. In the 1960s, an integral check-dam system which functioned for controlling floods, retaining sediments and producing grain had emerged in the Fenxi county, Shanxi Province. Thereafter, the Yellow River Conservancy Commission put forward a scheme for constructing 946 check-dams to prevent sediment from pouring into the Sanmenxia Reservoir, known as the Thousand Dam Scheme. With advantages such as drought-resistance, fertile and good yields on dam farmland, great enthusiasm was inspired for building check-dams in the late 1960s and the 1970s. It was estimated that most of the 100,000 check-dams in China were built at this period. However, due to poor construction quality, in 1977 and 1978 more than 80% of the check-dams in the Shanbei region were destroyed in fierce rainstorms after a long period of drought. These failures diminished the confidence in developing check-dams. Some damaged dams or dangerous dams were repaired or reinforced, but in general the emphasis was shifted to slope engineering such as planting and constructing terrace farmland in this period. As time went on, it became clear that slope engineering was unsatisfactory and incapable of conserving soil and water in the Loess Plateau due to its inability to withstand large rainstorms. Hence in the 1980s there was renewed interest in check-dam systems in gullies.In 1983, the key project of Conservation of Soil and Water in Gullies was initiated by the State Commission of Planning, and experiments were carried for three years. Since the relevant departments have prepared detailed plans, technical criteria and regulations, the project progresses well. However progress is slow: compared to the 100,000 check-dams in the 1960s and 1970s, only 1118 check-dams had been set up from 1986 to 1999 in the Loess Plateau (Feng, 2000). In the Loess Plateau, it is estimated that 6758 gullies are suitable for building check-dam systems, where 21,000 key projects, including 242,000 check-dams, could be built in the future. Table 3 lists the detailed layout. At this speed it would take more than 100 years to realize the scheme as above to control the main soil and water loss area in the Loess Plateau. A series of articles on check-dam systems in gullies on the Loess Plateau were published in the beginning of 1990s (Kang, 1993) and aroused passionate discussion on the strategy for controlling the Yellow River. Consequently, importance was also attached to researching theories for constructing check-dams, and funds have been granted for
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Table 3 Scheme for the check-dam systems in the Loess Plateau (Huang, 2000) Region Loess Hill Ravine area I, II Loess Hill Ravine region III, IV, V Loess Mesa Ravine region Wind Sand region Soil-stone mountain region Terrace region Total a
Area (km2 )
Number of key projectsa
Number of check-damsa
84,980 93,413 25,017 36,238 51,264 8341
13,447 4788 2153 242 340 30
47,486 77,866 21,859 31,961 53,218 9610
299,253
2100
242,000
The key projects and check-dams include the existing 1289 projects and 112,000 dams.
work on relative stability and optimum design of check-dam systems. As the technology is progressing and the theories for constructing check-dams are more mature, a common viewpoint has emerged that the most effective way to control sediment pouring into the Yellow River is to carry out modern engineering projects whose main element is the check-dam system in gullies.
6. Problems and suggestions It is necessary to focus national attention in China on the control of the Yellow River by check-dam systems. As more investment from the government is needed, policies encouraging individuals and local governments to develop check-dams ought to be established too. At the same time, theories on the development of check-dam systems should be extended and improved. Physical model experiments should be carried out in the context of typical small watersheds with unfavourable topography in the Two-River (Kuye River and Tuwei River) and Two-Chuan (Huangpu Chuan River and Gushan Chuan River) Zone of the Loess Plateau. Problems worthy of study are proposed as follows: (1) To construct and test hydraulic models of small watersheds in the Loess Plateau. It is well known that scale model experiments are effective in demonstrating processes and studying mechanisms of soil and water loss on a large scale or at high speed (Jiang et al., 1994; Yuan et al., 2000; Wischmeier, 1976). However, since the loess is so fine and erosive, and other erosive factors and phenomena such as rain, overland flow, gravity erosion are so complicated, an integrated scaling law for hydraulic models of small watersheds in the Loess Plateau has still not been produced. In spite of many experimental plots in situ to measure runoffs from slopes, few indoor model experiments with strict enlarging (or shrinking) scales to simulate the original hydraulic phenomena small watershed have been completed in recent decades. (2) To study the relative stability and optimum design of the check-dams. Based on ample data and investigations on the spot, hydraulic models should be made for the soil and water erosion area in the Loess Plateau.
Then, studies on the condition and standard of the relative stability for check-dam systems in gullies should be carried out, and scientific methods to investigate flood frequency should also be developed. After that, rules on designing the check-dam systems in various gullies through computers, and theories on check-dam stability, should be established. In this way, it is hoped that floodwater and sediments can be controlled and the dams will become relatively stable at last. We have completed a series of indoor model experiments to study the mechanism to intercept sediment and the relative stability of check dams. Results in detail will be published in other papers. 7. Conclusion In conclusion, the most effective way to conserve soil and water in the Loess Plateau is to construct check-dam systems in gullies. If layers of flat land were formed as the mounds and ridges were cut to fill the valleys, the geographical condition of the Loess Plateau would be greatly improved, and soil and water losses would be thoroughly controlled as well. The policy to spread check-dams is feasible and great benefits are attached for the local people by virtue of developing agriculture of check-dam system. Some problems such as destruction due to floodwater can be resolved as the design standards and the layout theory are improved. Acknowledgements The project was supported by the Yellow River Conservancy Commission Fund for Soil and Water Conservation (Contract No. 2000.06) and National Nature Science Fund (No. 40201008). The authors wish to acknowledge Professor Liang Dongbai and Doctor Cong Yu for amending this paper on English writing. References Chen, L., Wang, J., Fu, B., Qiu, Y., 2001. Land-use change in a small catchment of Northern Loess Plateau, China, agriculture. Ecosyst. Environ. 86, 163–172.
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Xu Xiang-zhou received his BSc and MS in Civil Engineering from Dalian University of Technology, and is at present a PhD student in Hydraulics and Hydropower Engineering of Tsinghua University. His doctoral thesis aims at understanding the similarity of scale models on soil and water erosion. He is the first-inventor of three Chinese patents. He is currently engaged in the series of programs financed by the Yellow River Conservancy Commission Fund for Soil and Water Conservation and National Nature Science Fund. Zhang Hong-wu received his PhD from Hydraulics and Hydropower Engineering of Tsinghua University. He is Professor and Director of the Yellow River Research Center of Tsinghua University, technological committeeman of the Yellow River Conservancy Commission, vice-director for the Sedimentation Committee of Chinese Hydraulic Engineering Society, etc. His current research aims at physical scale models for controlling the Yellow River and minimising soil and water erosion on the Loess Plateau. He is the author of about 100 papers and 12 monographs. Zhang Ouyang is a post-doctoral researcher in Hydraulics and Hydropower Engineering of Tsinghua University studying soil and water erosion on the Loess Plateau.