01 August 2021, Volume 38 Issue 8

    

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SPECIAL CONTRIBUTION
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  Evolution Trend of Basic Channel Scour and Morphological Adjustment in Yangtze River Estuary from 2002 to 2018

  YU Wen-chou, ZHANG Zhi-lin

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 1-8. https://doi.org/10.11988/ckyyb.20201057

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  The effect of clear water discharge on the river bed evolution of the Yangtze River estuary after the Three Gorges water storage is a common concern of sediment researchers. On the basis of recent changes of sediment and boundary conditions, we analyze the characteristics, morphological evolution and trend of erosion and siltation of the basic channel riverbed in the Yangtze River Estuary. Our findings reveal a comprehensive strong scour of the basic channel of the Yangtze River Estuary. The river bed shape develops toward a smaller width-depth ratio. The erosion distribution of the upper, middle and lower riverbeds in each section reflects that the upstream section of the Yangtze River Estuary is largely affected by runoff, while the downstream is largely affected by tidal energy. The overall river regime of the Yangtze River Estuary will remain relatively stable steadily under the actions of bank protection and recent encirclement projects and shoreline renovation projects. In the presence of a sharp decline of sediment after water storage in the Three Gorges, the basic channel will be continuously scoured, and river bed shape will still develop to be narrower and deeper.
RIVER-LAKE PROTECTION AND REGULATION
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  Post-assessment of the 2018 Maintenance Dredging Project of Diaoyuzui Shoal

  YAN Xia, YAO Shi-ming, XIA Wei, ZHOU Yin-jun, LI Zhi-jing

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 9-13. https://doi.org/10.11988/ckyyb.20200616

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  Diaoyuzui Shoal, located in a large curved passageway in the fluctuating backwater area of the Three Gorges Reservoir, was featured with curved and dangerous waterway and rapid current. A maintenance dredging project was first carried out in 2018. In this paper we analyzed the rationality, adaptability and sustainability of the dredging work based on summarizing the characteristics of shoals obstructing navigation, the dredging plan and the implementation process, etc., and then evaluated the effect of the dredging work and summarized its problems and shortcomings. Results demonstrated that the dredging design was well adaptive to riverbed evolution and annual water-sediment transport rules; the dredging was conducted when the water level of the Three Gorges storage reach rose, which not only alleviated the contradiction between construction and navigation, but also doubled the effect by taking the advantage of post-flood scouring. Nevertheless, the stability and sustainability of the channel are difficult to be maintained because the dredging project does not change the water and sediment conditions and boundary conditions that cause the riverbed evolution. Therefore, we suggested to strengthen the underwater terrain observation of the reach and carry out waterway regulation works in time according to the evolution trend of the shoal.
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  Propagation of Surge Wave in Three Gorges Reservoir Area Reach from Zhutuo to Dam Site

  LU Cheng-wei, CHEN Mo-fei, ZHANG Yu-long, ZHOU Jian-zhong

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 14-18. https://doi.org/10.11988/ckyyb.20200540

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  The construction and operation of reservoir groups have changed the natural flood propagation characteristics, which are mainly reflected on the change of flood propagation time. By establishing hydrodynamical model of the reach from Zhutuo to the dam site of Three Gorges Reservoir, we simulated and analyzed the flood wave propagation characteristics under the conditions of different reservoir water levels, different surge wave flows and different maximum flow durations, hence revealing the influences of such various factors on the surge wave propagation in the reach. Results suggest that the factors affecting the propagation of surge wave in the river channel are complex. Surge wave flow, maximum flow duration and reservoir water level all exert influence on the propagation, among which the maximum flow duration has the smallest influence on the propagation time of the flood, followed by surge wave flow and reservoir water level. The research findings offer technical support for the precise forecast of incoming flood and the optimized scheduling of Three Gorges Reservoir.
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  Ecological Base Flow of Three Major Outlets Rivers of the Middle Yangtze River into the Dongting Lake Area

  LI Kai-xuan, LI Zhi-wei, HU Xu-yue, CHEN Bang, WANG Zan-cheng

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 19-24. https://doi.org/10.11988/ckyyb.20200549

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  In recent years, the ecological flow in the three outlets rivers of the middle Yangtze River into Dongting Lake area is in severe shortage due to the construction and operation of large hydropower projects, the continuous reduction of diversion discharge of the three major outlets rivers, and the unreasonable utilization of water resources. We propose a method to determine the ecological basic flow by comprehensively considering the calculated results of different hydrological methods based on daily discharge data of main hydrological stations from 2003 to 2018 and monthly average discharge from 1973 to 2002 in the three major outlets rivers. We further obtain the recommended values of annual and monthly ecological basic discharge and analyze the guarantee rate and influence factors. Results demonstrate that the annual ecological base flow at Xinjiangkou, Shadaoguan, Mituosi, Kangjiagang and Guanjiapu hydrological stations is 208.18 m³/s, 49.44 m³/s, 75.39 m³/s, 2.52 m³/s and 93.94 m³/s, respectively, and their corrsponding guarantee rate of ecological base flow is 84.83%, 37.52%, 51.88%, 21.84% and 42.60%, respectively. The highest monthly ecological base flow of all hydrological station appears in July, and decreases in August, September, and June in turn. The ecological base flow and its guarantee rate in flood seasons are significantly higher than those in non-flood seasons. The proposed method and calculated results could be taken as reference for aquatic ecological restoration in the Dongting Lake area.
WATER RESOURCES
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  Flood Characteristics and Hydrological Simulations in Monsoon Area Based on Experimental Watersheds

  LIU Yue, ZHANG Jian-yun, CHEN Xin, WANG Jie, BAO Zhen-xin, GUAN Xiao-xiang, LIU Cui-shan, WANG Guo-qing

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 25-32. https://doi.org/10.11988/ckyyb.20200659

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  Study on flood characteristics and hydrological simulations are of great significance to flood control and disaster mitigation. According to the measured data of five experimental watersheds in Anhui Province, we analyze the characteristics of the floods in each watershed, and then simulate the floods in each watershed by using TOPMODEL to examine its applicability in flood forecasting in the monsoon region of east China. Our results reveal that the rainfall-runoff scatter points of floods in each experimental watershed all present a good positive correlation. The rainfall amount increases with the rising of elevation, and the runoff coefficients in mountainous area are greater than those in the plain and hilly area. The Wawuliu watershed which has the smallest area is the most sensitive to storms; the Suncun watershed features the smallest difference in the modulus of flood peak and the longest flood rising time; while the Huangshan watershed has the longest flood duration. The average value of watershed's topographic index gradually increases with the rising of altitude, while the value corresponding to the largest proportion drops. And the larger the standard deviation of the topographic index is, the more diversified land use types the watershed has. The TOPMODEL can generally reproduce the flood processes of each experimental watershed, and the captured results of large flows are better than those of small ones. The simulation of natural watersheds is better than that of human-disturbed watersheds.
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  Spatial and Temporal Variations of Precipitation in Recent Six Decades in the Water Receiving Area of Central Yunnan Water Diversion Project

  REN Xiao-hua, HE Shi-hua, MA Xiao-qing, LAI Yi-zhou, CHEN Shi-hang, DU Juan

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 33-40. https://doi.org/10.11988/ckyyb.20200652

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  Studying the spatial and temporal distribution of precipitation in the water receiving area of the water diversion project in central Yunnan Province is conducive to the scientific regulation and rational allocation of regional water resources. Based on the six-decade precipitation data of eight rainfall stations in the water receiving area, we investigated into the spatial and temporal distribution characteristics of precipitation in the area by using cloud model, Mann-Kendall test, wavelet analysis and collaborative Kriging interpolation method. The period from each June to August saw concentrated precipitation within the year, while from December to next February, precipitation was much less. The precipitation was unevenly distributed in summer and had the worst stability in August.The precipitation was even and small in winter. In addition, the annual precipitation decreased with a rate of 9.92 mm/10a which had abrupt changes and periodic fluctuations. The abrupt change points were concentrated between 1982 and 1992. And the precipitation experienced three cycles of wet-dry changes in the study period. The precipitation was also unevenly distributed in space. In spring, summer and autumn, precipitation in the south and the west was greater than those in the north and in the east, whereas in winter, the spatial distribution of precipitation was less regular. In summary, attentions should be paid to the flood prevention and drought relief in the water receiving area, the regional debris flows monitoring, and the prevention of summer drought.
WATER ENVIRONMENT AND WATER ECOLOGY
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  Carbon, Nitrogen, and Phosphorus in Sediments of Honghu Lake: Spatial Distribution and Pollution Evaluation

  PAN Xiong, GU Wen-jun, LI Huan, HU Yuan, ZHANG Wei, LIN Li

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 41-46. https://doi.org/10.11988/ckyyb.20200640

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  Honghu Lake, the seventh largest freshwater lake in China, has been undergoing serious eutrophication in recent years. Investigating into the content and distribution characteristics of nitrogen, phosphorus and organic matters in the sediment of Honghu lake is of great significance for acquiring its eutrophication status and ecological risks of nitrogen and phosphorus pollution. In the present research, we deployed eight sampling points and collected 50 cm height columnar sediments in October 2019. By measuring the content of total nitrogen (TN), total phosphorus (TP) and organic matters (OM) of sediment specimens from different depths, we examined the spatial distribution patterns of TN, TP and OM and their correlations. Furthermore, we evaluated the corresponding pollution status by using comprehensive pollution index. Results manifested that the TN content of Honghu sediments ranged from 467.8-8 454.5 mg/kg, averaging 2 167 mg/kg, indicating heavy pollution. Nearly half of surface sediments presented a TN content over 5 000 mg/kg. TP content ranged from 502.7 to 1 252.4 mg/kg, with an average value of 693.8 mg/kg. Except for Yangchai lake which suffered heavy pollution, most of the areas were subjected to medium TP pollution. The content of organic matters ranged from 5.0% to 24.9%, with an average value of 9.6%, also implying heavy pollution. In vertical scale, TN, TP and OM accumulated significantly in the surface layer of the sediments from 0 to 20 cm, and then declined rapidly with the increase of depth.
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  Spatio-temporal Variation of Water Quality in Middle-lower Hanjiang River

  ZHANG Sheng, LIN Li, WANG Zhen, PAN Xiong, LIU Min, DONG Lei, TAO Jing-xiang

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 47-53. https://doi.org/10.11988/ckyyb.20200610

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  In the aim of exploring the characteristics of the spatio-temporal changes of water quality in middle-lower Hanjiang River in wet season and dry season, a comprehensive survey was carried out on the middle-lower Hanjiang River (Danjiangkou-Wuhan reach) during the wet season in June 2019 and the dry season in January 2020. Eighteen sampling sections in the main stream and two sampling sections in the tributaries were arranged. The water quality was evaluated by using the overall pollution index and the overall trophic level index. The results indicated that the overall pollution index of water in the mainstream of the middle-lower Hanjiang River had no significant difference between wet season and dry season; in sectional scale, yet, the pollution of most sampling sections in wet season was more severe than that in dry season. Spatially, the change trends along the river during wet season and dry season were similar. Sections in Xiangyang, Hanchuan, and Wuhan were subjected to severe pollution, mainly total nitrogen (TN), total phosphorus (TP), and ammonia nitrogen (NH₃-N), among which TN concentration exceeded the limit of class II standard (GB 3838-2002) in all sections in both wet season and dry season, apparently larger in wet season, and TP concentration of most sections in wet season was greater than that in dry season, with 27.8% sections exceeding the limit of class II standard, and NH₃-N concentration of all sections were below the limit of class II standard in dry season, while exceeding the limit only in a few sections in wet season. Moreover, the overall trophic level in middle-lower Hanjiang River presented an increasing trend along the river, ranging from moderate nutrition to mild eutrophication. Except for larger fluctuations in dry season, the overall trophic level index saw no significant difference between wet season and dry season. The water quality in the middle-lower Hanjiang River was mainly affected by point source pollution and non-point source pollution, the confluence of tributaries, and water conservancy projects. The research findings offer scientific basis for water resources protection and sustainable development in the middle-lower Hanjiang River.
WATER-SOIL CONSERVATION AND ECOLOGICAL RESTORATION
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  Assessment of Eco-environmental Quality in Xiushan County Using Modified Remote Sensing Ecological Distance Index Model

  WANG Zhi-chao, HE Xin-hua

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 54-59. https://doi.org/10.11988/ckyyb.20200609

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  A rapid and accurate assessment of regional eco-environmental quality and its change is of important reference to ecological monitoring in the karst rocky desertification region. With Xiushan County in Chongqing as research area and the Landsat 5/8 imagines as main data resources, the eco-environmental quality and its change in the Xiushan County were assessed by establishing modified remote sensing ecological distance index (MRSEDI), which was calculated by rocky desertification index (RI), normalized difference built-up and soil index (NDSI), wetness index (WI), and normalized difference vegetation index (NDVI). Results unveiled that: 1) The average values of MRSEDI in 1990, 2002 and 2016 were 0.617, 0.665 and 0.748, respectively, presenting a rising trend. 2) The area of excellent and good eco-environmental quality in the research area increased over the 26 years, from 55.94% in 1990 to 82.67% in 2016. 3) The eco-environmental quality in most of the study area changed for the better or remained unchanged over the 26 years. The improved area accounted for 54.01%, while the unchanged area occupied 34.49%. In conclusion, Xiushan county boasts a good and ever-improving eco-environmental quality, implying that the eco-environmental protection measures over the 26 years are effective.
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  Performance of Electrokinetic Removal of Cadmium from Contaminated Soil Treated with CaCl₂-Humic Acid

  HU Yan-ping, WANG Zhen-hua, LI Qing-yun, TANG Xian-qiang, ZHANG Di-meng

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 60-65. https://doi.org/10.11988/ckyyb.20200498

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  In an attempt to enhance the removal of Cadmium(Cd) from paddy field soil and meanwhile alleviate the influence on soil's pH value, we conducted electrokinetic removal test with CaCl₂-humic acid solution activating the mobility of Cd in soil. We observed the changes in the content of available Cd and the pH value of soil by changing the concentration of CaCl₂-humic acid and the time of activation. In addition, we examined the changes in the removed content of Cd, the total concentration of Cd , the concentration of available-state Cd, and the pH value of soil after activation and electrokinetic drainage. Results manifest that the solution with 0.5% CaCl₂ and 1% humic acid has an optimum activation effect on the Cd in soil, by raising the available-state Cd from 0.3 mg/kg to 0.44 mg/kg and cutting the pH value by only 0.4. After the drainage of the overlying water, Cd can be further removed by electrokinetic drainage. The electrokinetic removal content of Cd was about three times that in the overlying water. After both activation and electrokinetic drainage, the total content of Cd dropped from 1.72 mg/kg to about 1.20 mg/kg, the content of available-state Cd declined to 0.27-0.31 mg/kg, while the pH value of each section of the soil was relatively uniform and barely changed, ranging between 5.45 and 6.30. The present research method could maximize the removal of Cd in soil while reducing the impact on pH value, which ensures the safe production of crops.
ENGINEERING SAFETY AND DISASTER PREVENTION
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  Dam Deformation Prediction Model Based on FCM-XGBoost

  YANG Chen-lei, BAO Teng-fei

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 66-71. https://doi.org/10.11988/ckyyb.20200508

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  Deformation is a crucial indicator to evaluate the safety of dam. With the increase in the number of deformation measuring points, however, prediction often lags because analyzing all the measuring points is time costly. Moreover, despite that traditional machine learning algorithms have ameliorated prediction accuracy, unreasonable selection of parameters has a great impact on prediction results and the process of establishing model is extremely complicated. In view of this, we introduced the fuzzy C-means clustering (FCM) and eXtreme Gradient Boosting algorithm (XGBoost) to partition the deformation measuring points according to the similarity of the change rules, and then established XGBoost prediction model for each partition. Taking the perpendicular line deformation monitoring data of the arch dam as an example, we verified the reliability of the clustering results and compared the XGBoost result with that of random forest prediction model. Result suggest superiority of the XGBoost prediction model in data pretreatment, modeling time, and prediction accuracy.
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  Application of Time Series InSAR to Deformation Monitoring in Central Line Project of South-to-North Water Transfer

  ZHANG Yong-guang, TIAN Fan, LI Ying-chun, LIU Hao-jie

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 72-77. https://doi.org/10.11988/ckyyb.20191501

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  The Central Line Project of South-to-North Water Transfer is a large-scale linear water conveyance building in China. To guarantee the safe operation of main canals is an important measure to promote people's livelihood and economic development in water-receiving areas. In this paper, the improved SBAS InSAR technology was used to process the 90-view Sentinel-1A data covering the main canal of Huixian segment of the Central Line of South-to-North Water Transfer from April 11, 2015 to March 09, 2019. The deformation information of the Huixian segment and its surroundings in the aforementioned time period was obtained. The settlement of Huixian segment mainly distributed in the southwestern area of the city center of Huixian and the northern area of Luohe Town, while the main canals were basically stable with no large-scale settlement funnel and only a few small settlement areas along the line. The expansive soil segment of Huixian County canal was further investigated comprehensively in consideration of expansive soil characteristics, rainfall, and soil moisture. Result demonstrated a periodic settlement trend, and the settlement was highly correlated with rainfall and soil moisture.
HYDRAULICS
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  Fluctuation Intensity Characteristics of Concentric Annular Slot Spiral Flow around Rear Piped Carriage's Body with Different Spacing-Diameter Ratios

  WU Jian, LIU Ya-kun, SUN Xi-huan, LI Yong-ye, ZHANG Di, YANG Jiao

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 78-83. https://doi.org/10.11988/ckyyb.20200559

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  Pipeline hydraulic conveyance of tube-contained raw-material is a new type of material transportation method which appears in modern logistics industry and has special advantages. In this paper, the distribution characteristics of fluctuation intensity of axial, vertical and lateral velocities of concentric annular slot spiral flow formed by double-piped carriages with different spacing-diameter ratios (S_D) are studied by means of model test. Results reveal that with the increase of S_D, the average value and range of fluctuation intensity of axial, vertical and lateral velocities remain basically stable, while the standard deviation and variation coefficient decrease. The characteristics of fluctuation intensity of vertical and lateral velocities are similar. Regardless of the S_D, the fluctuation intensities of vertical and lateral velocities are smaller than that of axial velocity by one order of magnitude, but the variation coefficient is about three times that of axial velocity. The reasonable spacing-diameter ratio of the double-piped carriages is 5-7. The research result is of referential value for the engineering application of tube-contained raw-material pipeline hydraulic conveyance.
ROCK-SOIL ENGINEERING
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  Geostress Measurement Accuracy and Its Extensive Quantitative Characterization

  HAN Xiao-yu, WU Ai-qing, XU Chun-min

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 84-90. https://doi.org/10.11988/ckyyb.20200461

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  In this paper we studied the definition and extensive characterization of geostress measurement accuracy in view of the present situation of the research field. First of all, we reviewed the geostress measurement methods, commonly used techniques, and the problems in measurement accuracy as well as their causes. From the perspective of closed-loop measurement, we gave our definitions of geostress measurement and geostress measurement accuracy. On the basis of classifying and summarizing historic researches, we presented the composition and structure of a characterization system for geostress measurement accuracy, expounded the significance of accuracy research, the difference between measurement method and measurement technology, as well as the accuracy problems. Furthermore, we presented the idea and connotations of quantitatively characterizing all factors of geostress measurement accuracy and gave the 3D physical model test device for measurement accuracy and its coordinate setting and characterization program. We also introduced the characterization formulae of all factors which include indexes of average-mode and maximum-mode in 2D and 3D measurement. In addition, we discussed the method of calculating angle error and explained the relationship between characterization modes and technical requirements. Taking the example of characterizing the measurement accuracy of the physical model test results of USBM borehole deformation gauge and CSIRO hollow inclusion strain gauge, we demonstrated the process of our extensive quantitative characterization. By comparing the characterization indexes with the original ones, we found that the overall measurement error of the single index of the maximum-mode of USBM and CSIRO is 10% and 27%, respectively. By using the present extensive characterizing method in consideration of all factors, we found that the measurement accuracy of USBM is superior to that of CSIRO.
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  Early Warning and Prediction of Side Displacement and Deformation of Soft Soil Foundation Pit

  WANG Juan, WANG Xing-ke

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 91-96. https://doi.org/10.11988/ckyyb.20200619

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  Early-warning model and prediction model for the side displacement of foundation pit were built in the aim of accurately grasping the deformation characteristics of foundation pit's side displacement in soft soil area. In the early-warning model, the early-warning discrimination indices were constructed using the multifractal detrended fluctuation analysis method, and then the change trends of the discrimination indices were determined by the Spearman rank test, hence the early-warning classification was completed. In the prediction model that is based on ridgelet neural network, the model parameters were optimized by rough set theory and trial and error method. Case study demonstrated that the early warning of the case in this paper was at level two, which indicated that the side displacement of the foundation pit tended to develop toward an unfavorable direction. Monitoring should be strengthened to improve the early warning for construction safety. In addition, the prediction accuracy and robustness of the ridgelet neural network can be effectively enhanced by parameter optimization, with the average relative error of the prediction results not exceeding 2%. The prediction results were consistent with the early warning results, which proved the accuracy of the analysis results.
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  Sensitivity Analysis for Parameters of Buried Steel Pipes Based on Orthogonal Test Method

  WU He-gao, YU Jin-hong, SHI Chang-zheng, SHI Ya-zhu

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 97-103. https://doi.org/10.11988/ckyyb.20200671

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  In the design of buried steel pipes, the influences of surrounding soil and trench on the steel pipes need to be taken into consideration, which are researched inadequately. The sensitivities of such parameters inclusive of the deformation modulus E₁ of backfill, the deformation modulus E₂ of sand bedding, the bedding angle θ, the trench bottom width B, and the inclination angle α of trench sidewalls to test indices (maximum vertical deformation of pipe, circumferential bending stresses of pipe top, springline, and bottom) were examined via an orthogonal test method. Results suggest that E₁, E₂, and α are highly sensitive to the test indices, while B bears low sensitivity to the test indices, and θ has the lowest sensitivity. The high soil deformation modulus and narrow excavation trench are beneficial to the structure of buried steel pipes, while the inclination angle of trench sidewalls has a complex influence on pipes and needs to be analyzed in line with specific situation. The research findings provide references for the parameter design of buried steel pipes.
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  Development of Freezing Temperature Field of Extra-long Connecting Aisle and Its Impact on Tunnel Deformation

  CHEN Jun-hao, CHEN Bi-jian, ZHUANG Yan, WANG Le-xiao

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 104-111. https://doi.org/10.11988/ckyyb.20200499

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  To probe into the influence of super long connecting passage on shield tunnel and surrounding strata in active freezing process, we examined the temperature field and displacement field of the 65.8 meters long cross passage, the shield tunnel and surrounding soil layer by using ANSYS with the frozen connecting channel between Ziyang station and Wuliting station of Fuzhou Metro Line 2 as engineering background. Results suggest that in the first 21 days of the active freezing, the average rate of frozen wall diffusion at the cross section of freezing pipe amounted to 57 mm/d, while in the last 29 days the average rate was only 27.6 mm/d; in the active freezing period, we observed a deformation concentration area of 10 meters in width and 2.5 meters in height near the bell mouth, which had a greatest impact on ground deformation. Freezing had a significant impact on the horizontal displacement of the arch shoulder and the vertical deformation of the arch foot. In the end of the active freezing period, the horizontal displacement of the arch shoulder was 85% larger than that of the right wall of the shield tunnel, and the vertical displacement of the arch foot was 45% larger than that of the arch crown. In addition, the structural deformation value and deformation growth rate both increased with the elongation of freezing time. Therefore, we suggested controlling the active freezing time and method reasonably to ensure the safety of buildings (structures).
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  Nonlinear Analysis on Bearing Characteristics of Flexible Piled Raft Foundations in Non-homogeneous Cohesive Soil under Excavation

  JIU Yong-zhi, ZHANG Zhen, LI Jian, SHEN Huan, SONG Zhen-hong

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 112-119. https://doi.org/10.11988/ckyyb.20200630

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  A simplified nonlinear approach for analyzing flexible piled raft foundation in non-homogeneous cohesive soil under excavation is proposed to investigate the influence of excavation on the bearing capacity of the foundation. The flexible piled raft is modeled as a thin plate through the finite element discretization. With the impact of soil excavation around the pile into consideration, the flexibility coefficient of pile top in non-homogeneous cohesive soil under excavation is obtained by employing the load transfer method; based on solutions for the stress and displacement in layered elastic half space, the interactions of the pile group-soil system are taken into account to determine the flexibility matrix of the pile group-soil system in non-homogeneous cohesive soil under excavation. By integrating finite element method and finite difference method, the simplified nonlinear analysis method is obtained. Comparison between calculated results and test and theoretical results verified the validity of the present method. With the influence of soil excavation ignored, the load sharing ratio of soil in piled raft foundation will be underestimated, and the non-uniformity of load distribution at the top of foundation pile at the bottom of raft will be larger in the analysis results. The research results offer a basis for balance design for variable-rigidity of group piled foundation and piled raft foundation under excavation.
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  Experimental Study on Shear Mechanical Properties of Composite Improved Loess under Wet and Dry Cycles

  JI Hui, ZHANG Tao, LIU Bao-jian

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 120-126. https://doi.org/10.11988/ckyyb.20200504

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  Fibers effectively improve the mechanical properties of soil. In this paper, the influences of glass fiber content, fiber length, curing age, and drying-wetting cycles on the shear mechanical properties of fiber-straw ash-lime improved composite loess were examined through unconsolidated and undrained triaxial shear tests. Results demonstrated that with the addition of fiber, the stress-strain curve of the improved soil gradually displayed strain hardening features. With the rise of fiber content and fiber length, the ultimate deviatoric stress, cohesion and internal friction angle first increased but then decreased. The optimal ratio is that the fiber content is 0.4% and the fiber length is 9 mm. Under drying-wetting cycles, the ultimate deviatoric stress, cohesion and internal friction angle of the improved soil decayed gradually; the rate of such decay was large in the early stage and tended to be stable in the later stage. The decay amplitude was obviously affected by fiber content and fiber length. Regardless of the number of drying-wetting cycles, the strength and stability of the composite improved soil enhanced with the expansion of curing age
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  Influence of Freeze-Thaw Cycles on Mechanical Properties and Fracture Evolution of Cement-Soil

  SUN Tao

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 127-132. https://doi.org/10.11988/ckyyb.20200485

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  In the aim of understanding the changes of cement-reinforced silty soft soil under freeze-thaw cycles, we conducted triaxial compression tests, CT scanning, and SEM scanning on cement-soil specimens undergone 0-10 times of cyclic freezing and thawing to examine the mechanical properties and microstructural damage of soil specimens. Results revealed that the stress-strain curves of cement-soil displayed strain hardening features. With the increase of freeze-thaw cycles, the cohesion and internal friction angle shrank in exponential and linear trends, respectively. Binary results of CT scanning images manifested that the crack rate rose exponentially with the increase of freeze-thaw cycles. During cyclic freezing and thawing, the change of fracture rate synchronized with those of strength indicators. Moreover, from SEM scanning we observed losses of cementitious agents in the cement-soil and decay of compactness among particles, which led to the expansion of cracks and degradation of mechanical properties.
HYDRAULIC STRUCTURE AND MATERIAL
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  Time-varying Damage of Macro-and-micro Properties of Fly Ash Concrete in Corrosive Media

  WANG Zong-xi, YAO Zhan-quan, HE Liang, WU Han-han, LIU Zi-mei

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 133-138. https://doi.org/10.11988/ckyyb.20200519

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  The corrosion resistance of fly ash concrete fully soaked in 2 mol/L NH₄Cl solution as corrosive medium was studied in this paper. Before dissolution, the stomatal structure of fly ash concrete was tested and analyzed. The loss of compressive strength together with the change amplitude and change law of dissolution depth after the dissolution was investigated. Moreover, the structure and pore distribution of concrete after dissolution with different amounts of fly ash (0%, 15%, 30%, 45%) were examined via nuclear magnetic resonance and field emission scanning electron microscopy. Research results manifested that fly ash refined the internal pores of concrete, reduced pore connectivity and improved pore size distribution, which effectively enhanced the structural safety and corrosion resistance of concrete in water environment. Compared with ordinary concrete dissolution area, fly ash concrete dissolution area had ettringite crystals generated inside, indicating that the formation of ettringite can reduce corrosion damage.
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  Measures Improving Quality of Concrete Aggregate Production for Wudongde Ultra-high Arch Dam

  HU Jin-wu, LI Guo, DENG Le-qing, PENG Hao-yang

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 139-145. https://doi.org/10.11988/ckyyb.20200657

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  Quality of artificial aggregate is the foundation of concrete quality, which is of great significance to the safety and stability of concrete dams during long-term operation. In line with the characteristics of artificial aggregate production system for the dam concrete of Wudongde Project, we summarized the whole process of aggregate production from raw material mining to aggregate processing, and put forward measures of optimizing the production techniques of coarse and fine aggregates. Results manifested that the particle size of coarse aggregate is effectively controlled, and the output and quality of fine aggregate (fineness modulus, stone powder content and particle content) are improved, hence enhancing the quality of dam concrete production for Wudongde Project. The quality and safety of the dam's entire life cycle have also been guaranteed.
INFORMATION TECHNOLOGY APPLICATION
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  Control Regulation Calculation for Basin Flood: Configuration Coupling Technique and Application

  TANG Hai-hua, LI Qi, HUANG Li-yao, ZHOU Chao, LUO Bin

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 146-150. https://doi.org/10.11988/ckyyb.20200511

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  In the aim of quickly responding to the uncertain demand of calculating basin flood control and regulation, object modeling is carried out for basin flood control system. The complex flood control tasks are decomposed into relatively independent model algorithm components. The dynamic adaptation between flood control objects and model components under different flood control scenarios is studied. A configuration coupling method for calculating multi-type flood control is proposed. In view of the flood control system of the upper Yangtze River, the method is applied to the comprehensive analysis of reservoir storage and river flood discharge through dynamic coupling calculation integrating flood dispatching, backwater simulation and flood evolution in channel. The operational response speed is greatly improved. This method is flexible in task configuration, strong in adaptability and boasts high degree of reuse, hence would be playing an important supporting role in the intelligent control regulation analysis and consultation decision judgment of basin flood.
INSTRUMENTATION DEVELOPMENT AND TESTING TECHNIQUES
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  Monitoring Technology and Its Error Analysis for Freeze-Thaw of Dam Core Soil in Cold Regions

  ZHANG Zhen-yu, YU Qi-hao, FANG De-yang, YUE Pan, WANG Xin-bin, CHEN Kun

  Journal of Changjiang River Scientific Research Institute. 2021, 38(8): 151-158. https://doi.org/10.11988/ckyyb.20200542

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  During the construction of core rockfill dam in cold regions in winter, it is an important precondition to quickly judge the freeze-thaw state of filled soil in the core to ensure the project's quality and the rapid construction. According to field tests, the infrared thermometric technique can be used to quickly detect the surface temperature and the freeze-thaw status of the core at long distance with a large area (more than 10 000 m²). However, under different ambient conditions, the error of surface radiometric temperature measured by infrared thermometric technique was 2-8 °C compared with contact temperature. In view of this, we conclude that environmental radiation and the surface emissivity are major factors that affect the error of infrared temperature measurement, of which the environmental radiation is mainly controlled by air temperature, while surface emissivity is affected by soil surface shapes caused by compaction patterns and geotextile covering. In association with field test data, we established the correction relationship between surface radiometric temperature and contact surface temperature. The research achievement provides important guidance for freeze-thaw detection technique of soil in engineering construction in cold regions.