Effects of check-dams on sediment storage-release in Chabagou Watershed

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1 第 26 卷第 2 期农业工程学报 Vol.26 No 年 2 月 Transactions of the CSAE Feb Effects of check-dams on sediment storage-release in Chabagou Watershed Zhang Luan 1,2, Shi Changxing 1, Zhang Hao 3 (1. Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing , China; 2. Graduate university of Chinese Academy of Sciences, Beijing , China; 3. College of Information and Business, North University of China, Taiyuan , China) Abstract: For clarifying the response of sediment yield to the strengthening of human interventions in the middle Yellow River, Chabagou Watershed was selected as the study area to explore the relationship between development of check-dams and dynamic of sediment storage-release both in time and space. Construction of check-dams in the Chabagou Watershed started in the late 1950s, speeded up swiftly in the 1960s and 1970s, and nearly stopped in the 1980s and 1990s. The results showed that most of eroded sediment was stored in the Chabagou Watershed in the 1980s, with a sediment delivery ratio of The low sediment delivery ratio can be attributed to the check-dams, which can not only trap sediment, but also reduce the frequency and intensity of hyper-concentrated flow to a greater extent. However, it is worth noting that the storage capacity of check-dams in the Chabagou Watershed had been decreased significantly due to filling up of early built dams as well as slowdown of dam construction since the 1980s. Moreover, the potentiality of sediment release from destroyed dams is increasing as a result of low design standards and poor maintenance of existing dams. Therefore, the role of check-dams as soil and water conservation measures should be paid adequate attention in the future. It is necessary to strengthen construction and management of check-dams for maintaining and even promoting their sediment trapping capacity. Key words: erosion, soil conservation, water conservation, sediment transport, sediment traps doi: /j.issn CLC number: TV5 Document code: A Article ID: (2010) Zhang Luan, Shi Changxing, Zhang Hao. Effects of check-dams on sediment storage-release in Chabagou Watershed[J]. Transactions of the CSAE, 2010, 26(2): (in Chinese with English abstract) 0 Introduction Sediment problem is the key issue of the Yellow River. Though the opinion that the multi-year average sediment delivery ratio is close to 1 on the Loess Plateau has been the basic understanding to the academic community for a long time [1-2], soil erosion and sediment transport in the middle Yellow River had become much more complicated due to the strengthening of human interventions, and the regional variations in the response of sediment storage-release on different space and temporal scales [3-5]. In the long history of struggle against soil erosion, people in Loess Plateau in China have created check-dam, which is one of the most effective engineering measures in trapping sediment, resolving hanging river problem, increasing basic farmlands of high quality, solving food problems, promoting returning farmland to forest, and improving eco-environment, as well as stimulating industrial Received date: Revised date: Foundation item: National Natural Science Foundation of China(Grant No ) Biography:Zhang Luan(1982-),Ph D, Research direction: Soil erosion and conservation, Graduate School of the Chinese Academy of Sciences, Beijing , China. zhangluan03@yahoo.cn Corresponding author:shi Changxing(1963-), Ph D, Professor, Research direction: Fluvial geomorphology, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing , China. shicx@igsnrr.ac.cn restructuring and rational water utilization, and improving people's living and production conditions. The past 50 years can be divided into four phases of check-dam construction, i.e. demonstration ( ), widely promotion ( ), accelerated construction ( ) and consolidation and reparation (after 1981). Now, a great number of check-dams that retard sediment transport to downstream in Loess Plateau region in northern Shaanxi Province have emerged. The dam system in the Chabagou Watershed is a typical example. It is recorded that soil and water conservation measures were implemented in the Chabagou Watershed first in the 1950s [6]. By the end of 1989, both the quantity and quality of soil and water conservation project had been greatly increased. Many studies have been done to elucidate the importance of check-dams in trapping sediment and reducing erosion [7-9]. Based on hydrological data, Xu and Sun [10] indicated that sediment storage-release was affected strongly by sediment trapping and releasing of check-dams, which could act as artificial sinks when they were empty and intact, and in contrast, as sediment sources when they collapsed or were destroyed. However, the relationship between development of check-dams and dynamic of sediment storage-release both in time and space has not been explained in detail. Therefore, with more available detailed data of sediment load and check-dams in the Chabagou Watershed, and by using statistical methods, Googleearth, Arcgis and other technology on raw data mining, authors intend to further

2 第 2 期张鸾等 : 岔巴沟流域淤地坝对泥沙存贮 - 释放的影响 65 investigate the relationships between temporal-spatial changes of sediment storage-release and the development and construction of check-dams. 1 Study area The Chabagou Watershed is located at the longitude of ' and latitude of 37 31' and covers an area of 205 km 2, 24.1 km in length and 7.22 km in width on average. There are one main stream named Chabagou and 11 main branches in the watershed (Fig.1). The outlet station is set up at Caoping, with a drainage area of 187 km 2. Fig.1 Schematic diagram of Chabagou Watershed As a result of the continental dry climate, rainfall distribution in the Chabagou Watershed is uneven, 70% concentrating in the three months from July to September and falling mainly in intensive and short rainstorms. With broken terrain, destroyed natural vegetation and agricultural activities, the highly erodible loess in the Loess Plateau tends to generate hyper-concentrated flows in rainstorms, leading to serious soil erosion. The extensive occurrence of hyper-concentrated flows on the Loess Plateau can be regarded as a zonal phenomenon [11]. According to the observations at Zizhou Hydrological Station from 1954 to 1958, the average and largest annual erosion modulus was t/km 2 and t/km 2, respectively. Located in the transitional zone with sandy loess and loess, the study area has been subjected to more intensive soil erosion as a result of silt-sized particle of the loess there besides other causes. To control the serious soil erosion, soil and water conservation measures, such as terraces, forests and grass, check-dams, have been put in practice on the Loess Plateau and developed to a large scale since the 1950s. 2 Data sources and methods The study of the relationship between the development and sediment-trapping effects of check-dams was based on data of reservoirs and check-dams in the Chabagou Watershed, which was provided by local water authorities and include construction time, location, height, storage capacity and area, as well as the reserved area of soil and water conservation measures. In addition, sediment reduction mechanism of check-dams was also explored by analyzing ten-year average sediment delivery ratio in the watershed in the 1980s. For calculating sediment delivery ratio, authors used data of precipitation and sediment yield recorded at Caoping Station from 1980 to 1989 [12], Chabagou Digital Elevation Model (DEM) with a 25 m resolution, Land-use data and soil properties database (from Data Center of Resources and Environmental Sciences, Chinese Academy of Sciences). In order to quantify the effects of check-dams on sediment storage and release in the 1980s, the sediment delivery ratio was calculated. It is defined as the ratio of the suspended sediment load measured at the outlet to total soil erosion of a watershed: SDR=Y/T. In this study, the factor Y is the suspended sediment load measured at Caoping Station, and the factor T is calculated by RUSLE (Revised Universal Soil Loss Equation) equation with the channel erosion being taken into account. Details of the method for calculating soil erosion can be found in another paper of the same author (Zhang et al. [13], 2009). 3 Results and discussions 3.1 Development of check-dams Construction time and number of check-dams Of 484 registered reservoirs and check-dams in the Chabagou Watershed, 83.68% is recorded with specific construction time. The statistics of the amount and construction time of these dams give the following results: Construction of check-dams in the Chabagou Watershed began at Only 25 check-dams were built in the 1950s, mostly in In the 1960s, the number of check-dams began to increase, and it reached 124 by the end of In the 1970s, it was up to 248. In the 1980s and 1990s, there were only 13 newly built check-dams, and some existing check-dams were repaired or reinforced. Figure 2 gives the tendency of the development of check-dams in the Chabagou Watershed. The trend of histogram graph also reflects the development process of check-dams in the Loess Plateau in northern Shaanxi region. Fig.2 Changes in number of check-dams in 40 years Before 1980, 372 dams were built. Among them 180 check-dams were more than 10 meters in height and more

3 66 农业工程学报 2010 年 than m 2 in area, and 63 of the 180 check-dams were more than 15 meters in height. The percentage of these two categories of dams is shown in Fig.3. It can been seen that the percentage of dams of more than 10 meters in height showed a decreasing trend from the 1950s to the 1960s and an increasing trend after the 1960s, while that of the dams of more than 15 meters in height showed an increasing trend from the 1950s to the 1970s. The percentage of dams of more than 10 meters in height was lower than 50% in its highest and was much higher than that of dams of more than 15 meters in height, which was still lower than 37% in the 1970s. This indicated that, even being improved in the 1970s, most of the check-dams had a lower design standard in the Chabagou Watershed. This would greatly increase the risk of serious collapse of dams in future. Figure 4 shows the spatial distribution of dams in different periods. It can be clearly seen that the dams built in the 1950s were more concentrated, only in some branches of Liqugou, Yandonggou, Gaojiagou and Liujiagou. The number of dams in the 1960s was more than that in the 1950s, but the dams concentrated in the central valley. In the period of much more dams were built and they were widely distributed in the whole watershed and some appeared in the uppermost small gullies. Fig.3 Changes in percentage of check-dams with different scales Fig.4 Location of check-dams in Chabagou Watershed in different periods The above results showed that a lot of check-dams were constructed in the 1960s and 1970s. They had played a very important role in trapping sediment in the Chabagou Watershed which had only an area of 205 km 2. However, the fact is that heavy and concentrated rainstorms, although short, usually cause a large volume of floods carrying a large sediment load. These floods are mostly beyond the capacity of small-scale check-dams, resulting in the damage and collapse of dams. According to Wang s [14] (1996) report, in the rainstorms in August 1973 and 1975, 43.6% and 30.5% of dams were damaged in Yanchuan County and Yanchang County in Shaanxi Province, respectively. High-intensity rainstorms in rainfall data are common in Shaanxi Province. For this reason, it is necessary to analyze the status of check-dams The status of check-dams of different period Based on the records of reservoirs and check-dams in the Chabagou Watershed in three periods, which include capacity, silted area, sediment volume, erosion amount and remaining capacity, as well as height, thickness and notes, statistics of 451 check-dams is made by dividing the status of dams into four types: unfilled, filled, destroyed and collapsed. Results are as follows: In the booming period, sediment trapping of the dams was effective. However, the dams were filled gradually with time. Up to 1978, 206 dams had been totally filled up, and 143 dams were partly filled. In 1993, the number of partly filled dams decreased to 26. In 2001, only two of the 451 check-dams had remaining capacity. Although some dams were heightened and repaired before the flood season in 2001, 106 dams were still totally filled-up (Fig.5). Some dams were heightened for several times every three to five years for sustaining their capacity of silt detainment and promoting their capacity of flood prevention. However, they were still threatened by extraordinary floods. Statistics showed that there were 47 collapsed dams and 55 totally destroyed dams within the whole watershed after the flood season of The numbers increased to 204 and 70 before the flood season of 1993, and 223 and 120 in 2001, respectively. The collapsed and totally destroyed dams together account for 76.05% of the total (Fig.5). Spatial distributions of collapsed dams, totally filled-up dams and partly filled dams differ among the three periods mentioned above (Fig.6). Reasons for this phenomenon are

4 第 2 期张鸾等 : 岔巴沟流域淤地坝对泥沙存贮 - 释放的影响 67 quite complicated, and may result from the diversities of repairing time, height, capacity, quality, etc. Fig.5 Changes of status of check-dams in different periods Fig.6 Status of check-dams in Chabagou Watershed in 2001 It can be seen from the above analysis that dams in the Chabagou Watershed have been developed greatly since the 1950s. However, later in the 1970s, rainstorms destroyed many dams and put many in danger. With fewer new dams being built in the 1980s and 1990s and the decrease of silting capacity of the poorly maintained dams, 92.8% dams in the Chabagou Watershed had collapsed, been destroyed or filled-up before the rain season of Given by Ye [6], the preserved silting area behind dams built upstream of Caoping Station from 1956 to 2000 validates the above facts (Fig.7). Fig.7 Changes of silting area of check-dams from 1956 to Sediment reducing mechanism and benefit of check-dams The role of check-dams in sediment trapping has been confirmed by many studies [8,15]. According to a survey of check-dams in Yulin and Yan an carried out by Shaanxi Provincial Water Resources Bureau recently, there was total silted area of hm 2 behind dams by the end of Report of Jiao [9] showed that the sediment trapping effects of single dams in the loess hilly-gully area in five watersheds, including the Huangfuchuan River, the Kuye River, the Jialu River and the Tuwei River, were 30.8%, 32.5%, 52.9%, 23.3% and 41.5%, respectively, indicating positive roles of dams in trapping sediment and reducing erosion. Our data support this finding, too. It could be calculated that the volume of silts behind dams in the Chabagou Watershed was m 3 and m 3 by the ends of 1978 and 1993, respectively. Assuming a bulk density of 1.35 t/m 3, the mass of silts would be tons and tons, respectively. Therefore, in the 14 years from 1978 to 1993, about tons of sediment was trapped by dams. It would be a total of about tons in the 1980s if the yearly mean rate of sediment accumulation was the same. However, the mass of silts behind dams is much less than the difference between the calculated volume of soil erosion and sediment load measured at the outlet of the watershed. Our result showed that, the sediment delivery ratio in 1980s was 0.325, or the percentage of sediment storage was 67.5% in the watershed, while only 12.1% of eroded soil had been checked behind dams. This means sediment in Chabagou Watershed was unbalanced. It seems to be a mistake, but it is reasonable. Soil erosion in the Chabagou Watershed was serious in natural condition, even in the areas with bedrock exposed. The frequent hyper-concentration flows carried a great deal of sediment out of the watershed. However, the implementation of soil and water conservation measures in the 1970s and 1980s, such as terraces, planting grass and check-dams, changed this situation by altering local micro-topography and increasing precipitation infiltration. The decrease of runoff and sediment erosion was effective in reducing hyper-concentrated flow [16]. Xu [16] analyzed the relationship between 5-year moving average value of hyper-concentrated flows and the area with soil and water conservation measures. His results suggested that the frequency of sediment-laden flows had been noticeably lowered due to the effects of soil and water conservation measures, especially check-dams, on erosion reduction. The comparison between sediment delivery ratio and the volume of silt behind dams in the Chabagou Watershed in the 1980s also showed that, in addition to sediment trapping, check-dams effectively reduced the frequency and intensity of hyper-concentration flows and sediment yield. Besides, check-dams also reduced erosion by raising erosion base level, which controlled the down-cutting of valleys and the extension of gully heads.

5 68 农业工程学报 2010 年 4 Conclusions Check-dams in the Chabagou Watershed have played a very important role in reducing sediment yield by trapping sediment and lowering the frequency and intensity of sediment-laden flows. However, this study also reveals some problems of check-dams in the Chabagou Watershed, such as the lower design standards, lacking of large dams, poor management. Conclusions and recommendations of this study are as follows: 1) Constructing check-dams in the Chabagou Watershed started in the late 1950s, boomed in the 1960s and 1970s, and stagnated in the 1980s and 1990s. Most of the early built dams had a low designing standard and a low capacity. In the booming period, sediment trapping of the dams was effective. With the filling up of more and more dams in the Chabagou Watershed, damage or collapse of dams in storms has increased with time. 2) In addition to sediment trapping, check-dams depressed the sediment yield by reducing the frequency and intensity of hyper-concentration flows and by reducing soil erosion in gullies through raising their erosion base level. 3) The role of check-dams as soil and water conservation measures should be paid adequate attention in the future. It is necessary to strengthen construction and management of check-dams for maintaining and even promoting their sediment trapping capacity and reducing the frequency of high sediment-laden flows. [References] [1] Chen Yongzong. Time variation of origin of sediment of the Yellow River and yield of sediment as a result of erosion[j]. Soil and Water Conservation in China, 1988, (1): (in Chinese with English abstract) [2] Yao Wenyi, Wang Weidong. Review of researches on the source of the Yellow River sediment[j]. Yellow River, 1997, (6): (in Chinese with English abstract) [3] Xu Jiongxin, Yan Yunxia. Scale effects on specific sediment yield in the Yellow River basin and geomorphological explanations[j]. Journal of Hydrology, 2005, 307(1/2/3/4): [4] Fang Haiyan, Chen Hao, Cai Qiangguo. Effect of spatial scale on suspended sediment concentration in flood season in hilly loess region on the Loess Plateau in China[J]. Environment Geology, 2007, 54(6): [5] Zheng Mingguo, Cai Qiangguo, Chen Hao. Effect of vegetation on runoff-sediment yield relationship at different spatial scales in hilly areas of the Loess Plateau[J]. North China, 2007, 27(9): [6] Ye Aizhong. Study on Simulation of Catchment Water Cycle in Changing Environment[D]. Wuhan: Wuhan Unversity, (in Chinese with English abstract) [7] Li Jing, Zheng Xinmin. Analysis on erosion reduction mechanism and sediment reduction function of check-dams[j]. Bulletin of Soil and Water Conservation, 1995, 15(2): (in Chinese with English abstract) [8] Fang Xuemin, Wan Zhaohui, Kuang Shangfu. Mechanism and effect of silt-arrest dams for sediment reduction in the middle Yellow River basin[j]. Journal of Hydraulic Engineering, 1998, (10): (in Chinese with English abstract) [9] Jiao Juying, Wang Wanzhong, Li Jing, et al. Silting land and sediment blocking benefit of check-dams in hilly and gully region on the Loess Plateau[J]. Transactions of the Chinese Society of Agricultural Engineering, 2003, 19(6): (in Chinese with English abstract) [10] Xu Jiongxin, Sun Ji. Study of temporal variation of check-dam construction in the Wudinghe River basin and some suggestions for some countermeasures[j]. Journal of Soil and Water Conservation, 2006, 20(2): (in Chinese with English abstract) [11] Xu Jiongxin. Erosion caused by hyper-concentrated flow on the Loess Plateau of China[J]. Catena, 1999, 36(1/2): [12] Yellow River Conservancy Commission of Ministry of Water Resources of the PRC. Hydrology Yearbook of People's Republic of China Hydrological Data of the Yellow River Basin[Z]. Zhengzhou[s.n.]: [13] Zhang Luan, Shi Changxing, Du Jun, et al. Research on sediment storage-release of a small watershed in loess hilly-gully area[j]. Research of Soil and Water Conservation, 2009, 16(4): (in Chinese with English abstract) [14] Wang Wanzhong, Jiao Juying. Sedimentation caused by rainfall erosion and runoff in Loess Plateau and sediment transport by Yellow River[M]. Beijing: Science Publishing House, (in Chinese) [15] Ran Dachuan, Luo Quanhua, Liu Bin, et al. Effect of soil-retaining dams on flood and sediment reduction in the middle reaches of Yellow River[J]. Journal of Hydraulic Engineering, 2004, 5(2): (in Chinese with English abstract) [16] Xu Jiongxin, Yao Wenyi, Han Peng, et al. Research on Erosion Processes in the Middle Reaches of Yellow River by Coupling Climate-Landform-Vegetation[M]. Beijing: Science Press, 2009: (in Chinese)

6 第 2 期张鸾等 : 岔巴沟流域淤地坝对泥沙存贮 - 释放的影响 69 岔巴沟流域淤地坝对泥沙存贮 - 释放的影响 张鸾 1,2, 师长兴 1, 张灏 (1. 中国科学院地理科学与资源研究所陆地水循环及地表过程重点实验室, 北京 ; 2. 中国科学院研究生院, 北京 ; 3. 中北大学信息商务学院, 太原 ) 摘要 : 为分析黄河中游流域产输沙对人类干预增强的响应机制, 该文选取岔巴沟为坝系小流域研究区, 探讨淤地坝的建设和发展对流域内泥沙存贮 - 释放时空变化的影响 研究表明,20 世纪 80 年代岔巴沟处于泥沙存贮状态,10 a 平均泥沙输移比为 0.325, 经分析, 始于 20 世纪 50 年代, 并于 年代得到发展,80 90 年代趋于稳定的岔巴沟淤地坝拦截了大量泥沙是造成泥沙存贮的主要原因 和拦沙作用相比, 其减少高含沙水流发生频率, 降低高含沙水流发育程度的作用更大 值得注意的是, 淤地坝在几十年的发展过程中可淤库容显著下降, 多数原有坝地淤满, 加上 20 世纪 80 年代以后坝体建设速度减缓, 以及设计标准低, 后期管理和维护不利, 垮坝和毁坝造成泥沙再释放的潜在性在增加 因此, 今后在对淤地坝作为一项重要的水保措施引起足够重视的同时, 维护和巩固现有淤地坝的拦泥库容也非常必要 关键词 : 侵蚀, 土壤保持, 水资源保护, 泥沙输移, 淤地坝 3