01 July 2020, Volume 37 Issue 7

    

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SPECIAL CONTRIBUTION
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  Water Problems and Their Countermeasures in the Protection and Developing of the Yangtze River

  XU Ji-jun, WANG Yong-qiang

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 1-6. https://doi.org/10.11988/ckyyb.20191387

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  At present, the systems of comprehensive utilization of water resources and comprehensive disaster prevention and reduction have been basically formed in the Yangtze River Basin, and the framework of water environment improvement and water ecological protection and governance has been preliminarily established. Nevertheless, the Great Protection of Yangtze River and the high-quality development of the Yangtze River Economic Belt in the new era pose new requirements while shortcomings still exist. In the present paper, we probed into current existing problems, including extensive water usage, severe waste water discharge, declining aquatic biodiversity, and frequent local floods and droughts. According to the integrity, systematicness and inherent law of the Yangtze River water system, we put forward strategies and coordinated governance approaches for four water problems: water environment deterioration, water ecology damage, water resources shortage, and flood and drought disasters.
RIVER-LA KE SEDIM ENTATION AND REGULATION
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  Channel's Morphological Change and Riverbed's Erosion-Deposition Process of Xiongjiazhou Reach in Lower Jingjiang River

  LIU Yi-lun, LI Zhi-wei, CHEN Yi-min, YANG Han-yuan, WANG Zan-cheng

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 7-14. https://doi.org/10.11988/ckyyb.20190508

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  Xiongjiazhou reach is located at the tail of the lower Jingjiang River in the midstream of Yangtze River. The morphology of Xiongjiazhou reach transited from straight to slightly sinuous and then gradually evolved into S-shape. It is currently the most sinuous segment in the Jingjiang reach. Subjected to incoming water and sediment load, and affected by riverbank boundary conditions as well as natural and artificial cutoffs, Xiongjiazhou reach has undergone frequent local adjustment in terms of river regime. Since the chute cutoff in the convex bank of Xiongjiazhou reach, a new anabranching channel was formed, leading to the coexistence of the main channel and the anabranching channel, which changed the flow conditions at the outlet of the Xiongjiazhou reach and further affected the bank collapse rate and riverbed erosion-deposition process in Qigongling bend. According to remote sensing imagery data and representative sections in dry season, the entire Xiongjiazhou reach has been creeping towards the downstream and the channel has been widened. The newly formed anabranching channel in the Xiongjiazhou bend is experiencing expanded scouring, with an average width of 70 m in seven years. The deep trough along the outflow section develops from the left bank to the right bank, and the thalweg of downstream Xiongjiazhou is deflected to the right bank. After the impoundment of the Three Gorges Reservoir, the thalweg has been moving close to the concave bank of Qigongling bend, whereas the original deep trough was deposited into two bars emerging above water surface. After reaching the apex of Qigongling bend, the thalweg gradually shifted to the concave bank. The results of this research provide reference for the implementation of the river regime control project in this segment, and offer ideas for the next step in the study of river regime control.
WATER RESOURCES AND ENVIRONMENT
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  Factors Influencing Sustainability of Public-Private Partnership Projects in Water Environmental Governance Based on Structural Equation Modeling

  WANG Lun-yan, XIA Qing, LI Hui-min, MA Ying

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 15-21. https://doi.org/10.11988/ckyyb.20190321

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  Current researches on sustainability mostly focus on economic, social and environmental fields, among which the public-private partnership (PPP) project in water environment governance receive inadequate attention. In the present study, structural equation modeling (SEM) is employed to study the factors affecting the sustainability of PPP project in water environment governance from aspects of economy, society, resources and environment, engineering and project management by constructing a complete set of sustainability evaluation index system. In addition, a method for calculating the weight of each indicator is defined, and the indicators that have the greatest impact on project sustainability in the aforementioned five aspects are identified as follows: sustainable cash flow, public satisfaction, standard-reaching rate of river sectional water quality, renewal capacity of project facilities, and organization structure. The research findings offer reliable reference for the public and private sector to take appropriate measures to improve the sustainability level of PPP project of water environment governance.
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  Variation Characteristics of Graded Rainy Days and Rainfall Amount in Poyang Lake Area

  ZHANG Fan-ping, XU Xin-fa, CHENG Jing-qing, WEN Tian-fu, LIU Zhang-jun

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 22-28. https://doi.org/10.11988/ckyyb.20190284

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  In pursuance of investigating the variation characteristics of rainfall amount in Poyang Lake area, the daily rainfall dataset (including rainy days and rainfall amount) of Duchang hydrometric station, located at middle Poyang Lake, with a 60-year common record length of the daily rainfall from 1953 to 2012, is classified into different levels according to rainfall classification standard. The variation characteristics and distribution features of rainy days and rainfall amount are examined with methods of trend and catastrophe test, Hurst index, precipitation concentration degree (PCD) and precipitation concentration period (PCP). Results reveal that: (1) The number of average annual rainy days is 134, of which light rainy days, moderate rainy days, heavy rainy days and torrential rainy days account for 70.9%, 18.66%, 8.2% and 2.98%, respectively; during the past decades, the total annual rainy days, light rainy days and torrential rainy days exhibit a significant decreasing trend and is likely to continue. Total annual rainy days changed abruptly (reduced by 14 days) in 1978. In spring and summer, heavy and torrential rainy days occupy a high portion, whereas in autumn and winter, light and moderate rainy days are dominant. (2) The average annual rainfall amount is 1 335.66 mm, of which light rain, moderate rain, heavy rain and torrential rain account for 20.74%, 29.50%, 27.35% and 22.41%, respectively; the annual rainfall amount exhibits a significant and lasting decreasing trend and changed abruptly (declined by 301.26 mm) in 1976. (3) The precipitation concentration degree (PCD), which varies greatly due to distinguishable distribution feature of rainy days and precipitation in different years, has an average of 0.4; the precipitation concentration period (PCP) is mainly from mid-March to mid-June, of which from mid-April to mid-May accounts for 71.43%. The decrease of rainy days is mainly caused by declining light rainy days, but annual rainfall amount is affected by rainfall of all levels. PCD, closely related to the ratio of heavy rain and torrential rain to annual precipitation, is significantly positively correlated with the uniformity coefficient of precipitation.
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  Variation Characteristics of Water Exchange Between Mekong River and Tonle Sap Lake

  LI Chang-wen, YOU Zhong-qiong, XU Zhao-ming, HUANG Li-yao

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 29-34. https://doi.org/10.11988/ckyyb.20190248

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  Tonle Sap Lake is the largest river-connected lake of Mekong River. The water exchange between Tonle Sap Lake and Mekong River has always been a major focus in the international community. Long series of hydrological data of four river-lake controlling stations were employed to analyze the time and magnitude variation characteristics of the water exchange between Mekong River and Tonle Sap Lake. Results show that the water exchange between Mekong River and Tonle Sap Lake is highly intensive. Tonle Sap Lake has obvious peak clipping and dry replenishing effect on the runoff of Mekong River. During flood season (May to September), backward flow from Mekong River to Tonle Sap Lake lasts 122 days on average every year, with a total volume of 37.7 billion m³, accounting for 14.4% of the incoming flow from upper Mekong River in the same period; in particular, the backwater volume from July to September occupies 88.6% of the total annual backwater volume. The average flood peak of backwater is 8 402 m³/s, accounting for 20% of the inflow from Mekong River in the same period. After the flood season, from October to next April, the stored flood of Tonle Sap Lake replenishes to Mekong River, lasting for 244 days on average every year, with a total volume of 71.1 billion m³, which is 1.96 times of the backwater volume from Mekong River and accounts for 29.9% of flood volume of lower Mekong River. Particularly, the replenishing flow from October to next January occupies 83.1% of the total supplementing volume. The interannual variations of duration, volume, and peak discharge of backflow as well as duration, volume, and peak discharge of replenishing flow are stable. The amplitudes reach 76 days, 28.3 billion m³, 6 095 m³/s, and 76 days, 47.4 billion m³ and 4 677 m³/s respectively, with the variation coefficient at 0.07-0.24. The volume of backwater and supplementing water is not only related to the volume of inflow from Mekong River and Tonle Sap Lake, but also affected by the difference of water level between river and lake. The research findings offer scientific evidence for the governance of the Mekong Delta and Tonle Sap Lake Area.
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  Calculating Ecological Instream Flow Demand in Data-scarce Plain River: A Case Study of Tongshun River in Wuhan Reach

  ZHANG Yun-chao, ZENG Kai, WANG Jia-sheng, CAI Jin-song

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 35-40. https://doi.org/10.11988/ckyyb.20190121

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  Conventional methods for calculating ecological instream flow demand could not be applied directly to data-scarce rivers as they require observational hydrological or biological data in long sequence. Tongshun River in Wuhan reach is a plain river with data scarcity as it has obvious swales without data of long sequence. We arranged ten representative sections in Tongshun River in Wuhan reach, and replaced long-sequence measured flow with artificial trial flow to obtain the relationship between artificial trial flow and hydraulic parameters such as river width, water depth, average velocity and wetted perimeter using MIKE11. On this basis, we calculated the ecological instream flow demand in the study area using the wetted perimeter method and the eco-hydraulics method which are based on the correlation between hydraulic parameters and discharge. Results demonstrated that the ecological instream flow demand on the prerequisite of maintaining the basic morphology and habitat of Tongshun River should be at 26 m³/s. The calculation scheme proposed in this paper provides a feasible means to calculating the ecological instream flow demand in data-scarce plain rivers.
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  Impact Analysis of Precipitation and Runoff Variation on Simulation Accuracy and Parameters of Monthly Water Balance Model

  WU Di, GUO Jia-li, XIANG Xiao-li, YU Zhong-bo

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 41-46. https://doi.org/10.11988/ckyyb.20190311

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  The purpose of this research is to assess the reliability of hydrological models, two-parameter monthly water balance model for example, in changing environments. First of all the consistency of precipitation and runoff data in 104 typical catchments in the United States is analyzed by Mann-Kendall method; in line with variable fuzzy theory, the impact of the consistency of precipitation and runoff data on the simulation accuracy of the two-parameter monthly water balance model is examined; furthermore the influences of the consistency of precipitation and runoff data and the climatic characteristics of catchment on the model parameters are also investigated. Results show that the consistency of precipitation or runoff data is damaged in 92.31% of the study catchment. Evaluation on the simulation effect of the model with variable fuzzy sets implies that the variation of precipitation and runoff trend weakens the simulation ability of hydrological model. The deterioration of precipitation consistency is the main reason for the weakening of hydrological model simulation ability. Moreover, the variable fuzzy method could accurately identify the secondary factors affecting the simulation ability. In addition, model parameters C and SC both increase with the augment of the annual average runoff coefficient. Parameter C represents the wetting degree of the basin, and parameter SC represents the storage capacity of the basin. The research findings offer technical support for flood control, draught relief, water resources planning and management.
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  A Method of River Flow Prediction Based on VMD-BP Model

  ZHAO Li-xue, HUANG Jie-jun, CHENG Xue-jun, SHEN Shao-hong, YUAN Yan-bin

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 47-52. https://doi.org/10.11988/ckyyb.20190326

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  River flow is an important ecological indicator for monitoring hydrological issues and managing water resources. Flow forecast is also significant for providing guidance and reference for water conservancy construction, navigation planning, and water resources dispatching. In the present research, a VMD-BP (variational mode decomposition and back propagation) model of forecasting river flow is proposed and constructed by combining the advantages of VMD in dealing with non-stationary sequences and the ability of BP neural network in tackling nonlinear fitting problems. The model is verified by using daily water level and flow data in 1998 and 1999 at Yichang Hydrological Station of the Yangtze River. Results indicate the VMD-BP model solves the multi-value relations between water level and flow to some extent and mitigates the volatility of data. The predicted result is better than those of linear fitting regression model and BPNN model, and the prediction error is merely 1.61%. Therefore, the VMD-BP model can be considered as an effective method for river flow prediction.
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  Future Precipitation Change in the Belt and Road Region under Representative Concentration Pathway Scenarios

  HUANG Xiao-hui, YUE Qun, ZHANG Min

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 53-60. https://doi.org/10.11988/ckyyb.20190325

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  The future precipitation change in the Belt and Road region in 2020—2099 is estimated under different representative concentration pathway (RCP) scenarios. The monthly precipitation under different RCP scenarios of global climate model (HadGEM2-ES) is first of all corrected using four transfer functions of the quantile mapping method for a more reliable estimation result. On such basis, the change trend and regional differences are examined. The root mean square error (RMSE) and precipitation deviation (diff) are analyzed to compare the correction efficacy of quartile mapping method. Results demonstrate that the correction effect of linear function PTF1 reaches the optimum. The uncorrected HadGEM2-ES model overestimates the trend of precipitation increase in Central Asia in the RCP2.6 scenario; the RCP4.5 scenario overestimates the increase in precipitation in Central Asia, East Asia and Russia; the other two scenarios RCP6.0 and RCP8.0 overestimate the increase in precipitation in East Asia and Russia. In the four scenarios after correction, the precipitation increases in East Asia, Southeast Asia and Russia, while mostly decreases in North Africa and West Asia. In future, the average annual precipitation is generally increasing, with the average value climbing in East Asia and Russia, whereas declining in West Asia and North Africa. The abrupt change in precipitation differs with scenarios: RCP2.6 scenario has the least abrupt change, while the other three scenarios perform diversely in different regions.The analysis results provide an effective reference for future policy formulation or project construction.
WATERSOIL CONSERVATION AND ECOCONSTRUCTION
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  Quantitative Analysis of the Effects of Vegetation on Soil Erosion in the Three-River Headwaters Region Using Logarithmic Mean Divisia Index Model

  HE Qian, DAI Xiao-ai

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 61-67. https://doi.org/10.11988/ckyyb.20190279

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  In the purpose of quantitatively evaluating the influence of vegetation change on soil erosion modulus, we conducted pixel statistical calculation and adopted the RUSLE model to explore the changes of vegetation coverage and soil erosion modulus in three periods from 2000 to 2015. Moreover, we employed the logarithmic mean Divisia index (LMDI) decomposition model was used to quantitatively analyze the contribution of vegetation to the changes of soil erosion modulus from the pixel scale. Results showed that the vegetation coverage in the Three-River Headwaters Region displayed an overall increasing trend, yet still with some differences in different periods. In general, the soil erosion modulus had been decreasing, and the spatial distribution characteristics between the changes of erosion modulus and the changes of vegetation coverage in each period were not identical, mainly because soil erosion change was caused by multiple factors. Vegetation had a positive effect on changes of soil erosion, and the contribution values ranged between -100 t·hm^-2·a^-1 and 100 t·hm^-2·a^-1. The spatial distribution characteristics of the contribution values were consistent with the changes of vegetation coverage. The LMDI model is a good method to quantitatively analyze the effects of vegetation on changes of soil erosion modulus from the pixel scale and provides new ideas for soil erosion management. The ecological conservation and restoration project in the Three-River Headwaters Nature Reserve has achieved benefits, and environmental projects should be strengthened.
FLOOD PREVENTION AND DISA STER REDUCTION
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  Characteristics of Underlying Surface in Small-scale Mountain Flood Disaster Area: Case Study on Guanshan River Basin

  HAN Pei, REN Hong-yu, WANG Si-teng, DONG Lin-yao, ZOU Ming-yang, WANG Zhi-gang, ZHANG Ping-cang

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 68-74. https://doi.org/10.11988/ckyyb.20190541

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  The purpose of this study is to offer basic data support for the construction of small-scale mountain flood disaster prevention and early warning in response to national requirements. The Guanshan River drainage basin is taken as a case study. By using multiple approaches including geoscience statistics, vegetation index calculation, object-oriented and human-computer interaction interpretation, spatial superposition analysis, field survey, and etc., the underlying surface characteristics, such as the topography, slope, vegetation coverage, and land use pattern are obtained, and the potentially affected villages and populations are comprehensively analyzed. The Guanshan River Basin is characterized by low middle part and high edges with the lowest point as the basin's outlet. The average historical disaster elevation and slope are 415 m and 21°,respectively. The average vegetation coverage of the Guanshan River Basin is 71%. Most of the basin is covered by forest and grass, and historical disasters mainly happened in less-vegetated area along rivers, ditches and roads. Residential houses as the major disaster-bearing body for mountain flood disasters, account for 1% of the total area; bare land and sloping land account for 2% of the total area. Potential disaster-stricken houses mainly concentrate in the lower positions of Guanshan River, Yuanjia River, Lyujia River and Xihe River, with a total of 8,106 people and 2,023 households affected. The bend, narrow river segment, and upstream and downstream bayonet area at the exit of Guanshan River Basin are not conducive to rapid flood discharge and easily lead to mountain flood disasters. Under heavy rainfall, bare land and steep slope in low-vegetation area are prone to breed mountain flood disaster, while less-vegetated along rivers and roads are subjected to mountain flood disasters. Early warning and prevention work should be strengthened as twelve villages in the Guanshan River Basin are threatened by potential hazards, among which Wulongzhuang, Dahewan, Zhaojiaping, Lyujiahe, Horseshoe Mountain, Xihe and Guanting Village are key points of mountain flood disaster prevention and construction.
ENGINEERING SAFETY AND DISASTER PREVENTION
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  Cusp Catastrophe Model of the Evolution Mechanism of High-risk Operations in Water Conservancy Projects

  JIANG Xin, LUO Dong-li, LI Wei, CHEN Yao, WU Jing-han, JIN Liang-hai

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 75-81. https://doi.org/10.11988/ckyyb.20191535

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  The aim of this research is to mitigate emergencies in high-risk operation of water conservancy projects and to control the impact of emergency events.First of all,the diagram of evolution mechanism of emergency is obtained via analyzing the influencing factors of safety risks in high-risk operations.The cusp catastrophe model for high-risk operation emergency of water conservancy projects is then established based on the catastrophe theory with internal motivation and external incentives as control variables and construction safety dynamics as state variable. The formation process of emergency event is examined using the model, and the role of splitting factors and regular factors before and after emergencies and the trajectory of event evolution are expounded.Moreover,six safe statuses,inclusive of safe operations(two types), external hazard-hidden operations, internal hazard-hidden operations, and dangerous operations(two types),are summarized;corresponding preventive measures, control measures,and emergency measures are put forward.The research results suggest that enhancing the quality of workers is a key factor in avoiding emergencies, and the behavior of managers is extremely influential before and after the events. The present model is a further theoretical attempt for the safety guarantee of high-risk operations in hydraulic engineering.
ROCKSOIL ENGINEERING
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  Numerical Simulation of Zonal Disintegration of Surrounding Rock Based on Strain Rate Damage Constitutive Model

  TANG Li-zhong, CHEN Ying-yi, LIU Chang, SHEN Fan

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 82-87. https://doi.org/10.11988/ckyyb.20190153

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  The generation of zonal disintegration of surrounding rock is analyzed by using the numerical simulation software ANSYS/LS-DYNA. Piecewise linear elastoplastic model is used to fit the rock's total stress-strain curve. The excavation of roadways is regarded as a dynamic process by using Cowper-symonds constitutive model in combination with GISSMO model, which take the effects of strain rate and damage on mechanical properties of rock into consideration in mesoscopic scale. Meanwhile, the mechanical properties of rock under various stress conditions are depicted more accurately based on stress triaxiality. The element failure criteria are developed based on maximum strain criterion and maximum tensile stress criterion. The complete stress-strain curve obtained by numerical simulation of uniaxial compression of rock shows good fitting results. According to previous work and engineering practice, the model is modified, and the tunneling and excavation method is adopted in the simulation. The tunneling excavation rate has an impact on the zonal disintegration. Comparison between simulation results and actual observation data demonstrates that the two are consistent.
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  Model Test on Shallow Failure of Fully and Strongly Weathered Rock Slope Induced by Rainfall

  LIU Guang-ning, LI Cong, LU Bo, ZHU Jie-bing, WANG Xiao-wei, FENG Shi-guo

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 88-95. https://doi.org/10.11988/ckyyb.20190217

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  Taking the typical rainfall-induced landslides in the pilot test area of the Pearl River-Xijiang Economic Belt as the research object, we carried out three sets of physical model test on the whole process of rainfall-induced landslides by introducing the hydrodynamic similarity principle and taking the Weber number criterion as the rainfall similarity criterion. By analyzing the response law of slope to rainfall infiltration, the influence of infiltration process on slope deformation, the slope failure process and failure modes, we preliminarily obtained the formation mechanism of rainfall-induced landslide and the characteristics of disastrous rainfall in South China. (1) The high porosity caused by strong weathering of shallow rock and soil is the material basis of rainfall-induced landslides in the test area. The dissipation of matric suction of unsaturated rock and soil is the essential cause of rainfall-induced landslides in the test area. The specific matric potential (water content) distribution after rainfall infiltration lays the condition for rainfall-induced landslides. (2) The cumulative rainfall needed for totally weathered granite landslide disaster induced by rainfall is smaller than that of strongly weathered clastic rock landslide, but the sliding influence range of strongly weathered clastic rock landslide is larger than that of fully weathered granite landslide. (3) The cumulative rainfall required for landslide disaster induced by heavy rainfall is smaller than that induced by intermittent rainfall. Heavy rainfall is more likely to induce landslide, and the induced landslides are larger in scale.
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  Strata Deformation Law of Water-rich Loess Tunnel in Different Reinforcement Schemes

  HUO Run-ke, QIU Tian, LI Shu-guang, CAO Xin-xiang, QIAN Mei-ting

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 96-104. https://doi.org/10.11988/ckyyb.20190290

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  With the tunnel segment of Xi'an Metro Line 5 as engineering background, we examined and compared the ground settlement, soil deformation around the tunnel, and mechanics effect of water-rich loess tunnel reinforced by different methods (dewatering reinforcement and grouting reinforcement) via a seepage-stress coupling numerical model. We further validated the numerical result according to site monitoring data. Results demonstrated that the maximum ground settlement of the tunnel strengthened by dewatering reinforcement was 13.7 times that by grouting reinforcement. The laws of soil deformation around the tunnel under these two reinforcement schemes were consistent, and the deformation value of surrounding rock in ten days of excavation reached about 70%-80% of the stable value. Soil around the tunnel suffered from compressive stress under grouting reinforcement, while tensile stress appeared in the soil layer of the middle wall and the middle plate during the excavation under dewatering reinforcement. The stress of the lining under grouting reinforcement was larger than that under dewatering reinforcement. The extreme value of plastic zone under dewatering reinforcement was 11.3 times that of grouting reinforcement, mainly distributing at the shoulders, waists and feet of the arch on both sides. The simulated values of ground settlement and soil deformation around the tunnel under the two reinforcement schemes were similar and basically consistent with the change law of the monitored value.
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  Experimental Study on Physical Characteristics and Microstructure of Acid-contaminated Lishi Loess

  WANG Nian-qin, LU Xing-sheng, MA Xiao

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 105-108. https://doi.org/10.11988/ckyyb.20190293

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  The purpose of this research is to investigate the influence of acid pollution on the physical properties of loess. We prepared acid-contaminated loess specimens by immersing Lishi formation loess into hydrochloric acid solutions (concentrations of 0.5, 1, 2, 3, 4, 6, 8, 10 mol/L), and also prepared distilled water-immersed specimens as the control group. Through laboratory tests on the variations of specific gravity, void ratio and plasticity index of acid-contaminated Lishi loess as well as micro-structure analysis, we revealed the variation mechanism of physical properties of Lishi loess. Results demonstrated that: 1) the specific gravity of loess declined from 2.73 to 2.69, presenting a linear progressive reduction; 2) the increase of acid concentration led to a gradual increase in the void ratio of specimens from 0.77 to 1.33, showing a single exponential growth; 3) with the increase of acid concentration, the plasticity index of specimens augmented from 11.89 to 13.73 and then reduced to 7.34; 4) after acid action, the microstructure of Lishi loess changed remarkably. With the increase of acid concentration, the surface of soil particles was rougher; the skeleton particle looser; fine particle content smaller and pore area larger.
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  A Method of Initializing FLAC^3D Saturated-unsaturated Seepage Field

  FENG Wen-kai, WU Zhuo-lin, LIN Bu-lei, ZHU Quan-wei, ZHOU Yu-long

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 109-114. https://doi.org/10.11988/ckyyb.20190234

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  The establishment of initial seepage field close to the real state is an important prerequisite for the correct analysis of slope rainfall infiltration. In view that methods of initializing saturated-unsaturated seepage field in FLAC^3D are rarely reported, in this paper we put forward and introduce in detail the setup method of initializing saturated-unsaturated seepage field. According to the five distribution forms of matric suction, two initialization methods are proposed corresponding to the linear and non-linear distribution laws of matric suction. For the linear distribution of matric suction, MIDAS modeling and FLAC^3D built-in FISH language are employed to initialize the seepage field. For non-linear distribution, since it is difficult to represent the complex geometry of groundwater level surface by using function, the groundwater level is simplified into a plane for the initial setting of seepage field by using FISH programming language. Comparison with conventional initialization method demonstrated that the initial seepage field has a great influence on the final calculation result of slope rainfall infiltration. When the initial seepage field is closer to the real state, the calculation accuracy is higher.
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  A Relative Permeability Model for Unsaturated Flow in Consideration of Hysteresis Effect

  PENG Zi-mao, HUANG Zhen

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 115-119. https://doi.org/10.11988/ckyyb.20190424

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  Relative permeability coefficient is an important content in the study of seepage characteristics of unsaturated soils. In consideration of the hysteresis effect of unsaturated soil seepage caused by the non-uniformity of soil pores, a soil-water characteristic curve (SWCC) model reflecting the hysteresis effect of unsaturated flow is constructed by using the fractal theory and the Buckingham-Darcy law. On such basis, a model of relative permeability coefficient of unsaturated flow is derived. The proposed model is simple in form and contains only one parameter. The rationality of the model proposed in this paper is verified by five representative sets of test data of relative permeability coefficient. Compared with the Assouline model, the proposed model has superior fitting results.
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  Experimental Study on Effect of Matric Suction on Strength of Slip Soil in Limestone Zone

  ZHENG Wen-xiao, GAN Jian-jun, LI Jian-hua

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 120-124. https://doi.org/10.11988/ckyyb.20190278

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  As an important mechanical parameter of the water-soil interaction in unsaturated soil, matric suction is of great significance for researching the shear strength of unsaturated soil and the stability of landslides induced by changes in rainfall or reservoir water level. In this research, the effect of rainfall-induced change in matric suction on the slip soil in limestone zone was examined via GDS triaxial test system with standard stress path. Matric suction was set different at four values and net confining pressure three levels. The curves of matric suction versus maximum shear stress, strain versus shear stress, and net confining pressure versus shear stress were obtained and further analyzed using the variable double stress principle of unsaturated soil strength theory. Results evinced that under the same matric suction condition, shear strength increased rapidly with the climbing of confining pressure at first and when shear strain reached 10%, such increase attenuated. Under the same confining pressure and unsaturated conditions, shear stress rose with the increase of matric suction. Total cohesive force is in a linear relation with matric suction. The slope of this linear relation, which also equals the related angle of matric suction, is 15.5°. When matric suction is 0 kPa, 30 kPa, 60 kPa and 90 kPa, respectively, the total cohesive force is 22.7 kPa, 33.4 kPa, 43.8 kPa, and 46.9 kPa, respectively, and the corresponding internal friction angle is 21.8°, 23.6°, 26.2°, and 24.5°. According to Fredlund and Ralaadjo's double stress strength theory of unsaturated soil, a modified formula for the shear strength of the slip soil of the studied landslide is proposed for reference.
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  Experimental Study on Matric Suction of Tailings Sand by Filter Paper Method

  XIN Bao-quan, DUAN Zheng-xiao, WAN Lu

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 125-129. https://doi.org/10.11988/ckyyb.20190289

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  The purpose of this study is to obtain the rule of tailings sand's matric suction varying with different water content. On the basis of particle gradation test of tailings sand, the matric suction in moisture absorption stage and dehumidification stage of tailings sand were tested respectively using contact filter paper method. Twelve groups of matric suction with a water content ranging from 9% to 40% were acquired. The soil-water characteristic curve (SWCC) of tailings sand in moisture absorption and dehumidification stages is established, and the variation characteristic of SWCC is analyzed. Results indicate that the tailings sand belongs to silty clay. The SWCC of tailings sand in a large suction range obtained by filter paper method approximates an inverted “S” shape. The SWCC of tailings sand can be divided into four stages. With the increase of water content, matric suction decreases in stages. The matric suction of tailings dam measured by filter paper method varies in a large range from 2 711.095 kPa to 22.459 kPa in the process of moisture absorption and from 28.205 kPa to 3 607.825 kPa in the process of dehumidification. The variation law of matric suction in the process of moisture absorption and dehumidification is basically the same. However, the change paths are different and obviously hysteretic. The matric suction in dehumidification process is higher than that in moisture absorption process.
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  Dynamic Modulus and Damping Ratio of Sandy Gravel Material Based on Large-scale Dynamic Triaxial Test

  TAN Fan, ZHANG Ting, XU Han

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 130-134. https://doi.org/10.11988/ckyyb.20190439

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  The dynamic modulus and damping ratio of sandy gravel material of a sandy gravel dam with asphalt concrete core wall are tested by large dynamic triaxial tester. The dynamic stress-strain characteristics of sandy gravel dam material are studied. The influence of confining pressure on the maximum dynamic modulus, dynamic shear modulus ratio and damping ratio is analyzed. The characteristic parameters such as the dynamic modulus and damping ratio of the dam shell material and transition material are determined using the modified equivalent linear viscoelastic model. Test results show that with the climbing of confining pressure, the dynamic shear modulus of sandy gravel dam material increases while damping ratio decreases. With the growth of dynamic strain, the dynamic shear modulus declines while damping ratio increases. By incorporating normalized dynamic strain into the modified equivalent linear model, the influence of confining pressure on dynamic modulus attenuation and damping ratio are better eliminated, and moreover the dynamic deformation characteristics of sandy gravel dam material are more accurately reflected.
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  Experimental Study on Uniaxial Compression of Recycled Aggregate Concrete

  HAN Yu-zhou, AN Xu-wen, XIAO Bei, HUANG Tian-xiang, GUO Zi-qing

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 135-139. https://doi.org/10.11988/ckyyb.20191584

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  The constitutive relation of recycled aggregate concrete was constructed based upon the stress-strain relation under uniaxial compression with varying replacement ratio of recycled aggregate and water-cement ratio. With road waste concrete as recycled aggregate, 45 prismatic specimens with dimensions of 150 mm×150 mm×300 mm were prepared for test. Results unveiled that with the increase of the replacement ratio of recycled aggregate, the peak stress f_c of recycled aggregate concrete declined at first, and then climbed slightly but decreased afterwards; peak strain ε₀ augmented firstly and then decreased. With the increase of water-cement ratio, peak stress f_c presented different variation trends—yet in general a decline trend—due to varied replacement ratio; peak strain ε₀ increased firstly and then remained unchanged or decreased slightly. In general, the peak stress of recycled aggregate concrete is slightly smaller than that of natural concrete, and strain slightly greater than that of natural concrete. The research results offer technical support and reference for the popularization and application of recycled aggregate concrete.
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  Experimental Study on Thermal Conductivity of Red Clay in Drying Process

  CHEN Bo, YAN Rong-tao, LIANG Wei-yun, TIAN Hui-hui, WEI Chang-fu

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 140-146. https://doi.org/10.11988/ckyyb.20190328

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  The dynamic change in the three-phase composition of rock and soil in natural environment poses difficulty to the accurate test of thermal conductivity of soil. The heat transfer property of soil can be indirectly obtained through the thermal conductivity dryout curve (TCDC). In this research, the change rule of thermal conductivity of Guilin red clay in drying process was examined by using pressure plate and KD2 Pro soil thermal properties analyzer. Before the matric suction reached the air entry value, the saturation of soil remained unchanged, and the thermal conductivity of soil specimens with varied dry density increased slightly; in this stage, thermal conductivity was mainly related with the change of soil density caused by the change of suction. When matric suction exceeded the air entry value, the thermal conductivity gradually decreased with the reduction of saturation degree; at this stage, moisture content was the main factor affecting thermal conductivity. Based upon the experimental results, Lu's model and William's model were employed to fit the thermal conductivity curve of red clay in the drying process. Both models have good fitting results, hence can be used to simulate the change of thermal conductivity in red clay in drying process.
HYDRAULIC STRUCTURE AND MATERIAL
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  Influence of Raw Materials on Performance of Hydraulic Roller Compacted Concrete

  GUO Wen-kang, ZHANG Xiao-bo, WANG Shu-yin, GUI Wei

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 147-152. https://doi.org/10.11988/ckyyb.20190526

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  To scientifically utilize local raw materials for the roller compacted concrete (RCC) of a large hydropower dam project, we conducted performance test on RCC prepared with local raw materials (ordinary Portland cement, artificial granite aggregate, and grade II fly ash), and compared the mixture properties, physical and mechanical performances, deformation, durability, and adiabatic temperature rise between the RCCs of the present project and Longtan hydropower project. On such basis, we further investigated the influences of raw materials on RCC performance. Results unveil that local raw materials can be used to prepare RCC that meets the requirements of design specification. The mixture property of RCC is mainly affected by water consumption for normal consistency of cement, particle gradation and water absorption of aggregate. The physical and mechanical properties, deformation, and durability of RCC are controlled by comprehensive factors inclusive of cement type, fly ash type and dosage, CaO content and loss on ignition in fly ash, as well as aggregate strength. The adiabatic temperature rise of RCC mainly depends on the type of cement, hydration heat, and cement consumption. The lower the heat value of cement hydration and the less the amount of cement consumption, the smaller the adiabatic temperature rise of RCC. According to the research results, we put forward suggestions for the rational selection of local materials for RCC dam.
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  Experimental Study on Impact-abrasion Resistance of Recycled Aggregate Pervious Concrete

  CHEN Shou-kai, ZHANG Zheng-nan, ZHENG Yong-jie, GUO Lei, WANG Lun-yan

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 153-159. https://doi.org/10.11988/ckyyb.20190514

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  Impact-abrasion resistance is the key factor that affects the service life and applicability of recycled aggregate pervious concrete. At present, systematic research is lacking in China and abroad. The influences of water-cement ratio, chopped basalt fiber, fly ash, replacement rate of recycled aggregates, and amount of double-grained aggregate (5-10 mm, 10-20 mm) on the wear resistance of recycled aggregate pervious concrete were examined in this research. Test results indicate that when water-cement ratio exceeds 0.27, the mass loss from the recycled aggregate pervious concrete increases as the water-cement ratio rises, which means that the impact-abrasion resistance is ideal at a water-cement ratio of 0.27. The incorporation of 10% fly ash could improve the impact-abrasion resistance of recycled aggregate pervious concrete. As the replacement rate rises, the impact-abrasion resistance ameliorates at first and then reduces; the impact-abrasion resistance is optimal at the replacement rate of 50%. The double-grained grade is conducive to the improvement of impact-abrasion resistance of recycled aggregate pervious concrete. In addition, strength, density, and porosity are the main factors affecting the impact-abrasion resistance of recycled aggregate pervious concrete. In general, the greater the strength and density or the lower the porosity, the better the impact-abrasion resistance tends to be.
NUMERICAL MANIFOLD METHOD BASED ON INDEPENDENT COVERS
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  Automatic Two-dimensional Computation Based on Arbitrary Mesh Division

  SU Hai-dong, FU Zhi, XIE Zhi-qiang

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 160-166. https://doi.org/10.11988/ckyyb.20190661

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  Finite Element Method (FEM) is inconvenient in mesh division and subdivision, difficult in precise modeling of exact geometry, and costs large amount of labor operations. In view of this, we propose an approach of arbitrary mesh subdivision in the 2D solving domain based on convex decomposition idea using Manifold Method based on independent covers presented previously, in which cover meshes are of arbitrary shape, arbitrary connection and arbitrary subdivision. On this basis, with the help of error estimation and h-p version self-adaptive technology in previous studies, we attempt to implement the automatic static analysis of 2D linear-elastic structure, including the automatic subdivision of the solving domain, and the automatic elevation of polynomial orders. Two numerical examples, one of which is a gravity dam and the other is a plate with a small circular hole, are given to illustrate the validity of the present method. Especially in the latter, the whole procedure is exhibited, involving the input of geometry information and computational parameters in CAD, automatic CAE modeling with exact geometry, automatic self-adaptive analysis, as well as the automatic output of computational results. Hence, the automatic CAE computation and CAD/CAE integration are realized preliminarily.
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  Numerical Computations Based on Cover Meshes with Arbitrary Shapes and on Exact Geometric Boundaries

  SU Hai-dong, FU Zhi, XIE Zhi-qiang

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 167-174. https://doi.org/10.11988/ckyyb.20181071

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  Finite element meshes should keep regular shape as much as possible, and ensure correct connections through nodes. These requirements pose a great burden to the pre-processing procedure of numerical computations for solving domains with complex shapes. On the other hand, curve boundaries in practical situations are usually discretized into shapes which finite element meshes can describe, resulting in an imprecise simulation of exact geometry defined in CAD. In view of this, cover meshes with arbitrary shapes and arbitrary connections are implemented using Manifold Method based on independent covers. Exact geometric boundaries of CAD models and boundary conditions are simulated in CAE analyses. The solving domain is divided into block meshes with arbitrary shapes which can contain curve boundaries. And two approaches, including analytical integration method with simplexes and numerical integration method, can be used for the block integration. The thin strips for cover overlapping are considered only in the integration process, but are not necessarily involved in the generation of computation models. Essential boundary conditions are strictly applied through boundary strips, including the boundary conditions on curves. Moreover, two numerical examples are given to illustrate the validity of the method. Cover meshes with arbitrary shapes bring about a new path for numerical computations based on exact geometric models and automatic pre-processing procedures.
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  Numerical Method for Solving One-dimensional Convection-Diffusion Equation Based on Independent Covers

  LIU Ya-jun, GONG Ya-qi, SU Hai-dong

  Journal of Changjiang River Scientific Research Institute. 2020, 37(7): 175-182. https://doi.org/10.11988/ckyyb.20190907

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  In solving one-dimensional convection-diffusion equations, present numerical methods are prone to suffer from stability and accuracy problems caused by numerical oscillation and pseudo-diffusion. In view of this, an idea of applying Numerical Manifold Method (NMM) based on independent covers (the approximation using polynomial series piecewise-defined) to the numerical solution is proposed. The solution formula of the one-dimensional convection-diffusion equation is derived based on the standard Galerkin method. The posterior error estimation method about the continuity of the first-order derivative of field variable in the narrow overlapping area between independent covers is used for the automatic solving by h-p hybrid self-adaptive analysis with mesh refinement and ascending series order. The results of the steady-state and unsteady-state analysis examples show that the numerical solution of the piecewise-defined series steadily approximates and finally well fits the exact solution. For the convection-dominated problem, the adaptive solution effectively avoids numerical oscillation. In addition, the error index of the residual by substituting the numerical result back to the differential equation is successfully attempted. If the differential equation is solved point by point, the method would the most stringent error judgment for the numerical solution so far.