[Objective] Dramatic decline in upstream sediment discharge, abnormal droughts during flood seasons, and recurrent extreme flood and storm surge disasters have triggered pronounced geomorphological adjustments within the Yangtze Estuary. This study aims to systematically investigate the long-term evolutionary characteristics and patterns of the North Branch and South Branch of the Yangtze River Estuary,and also seeks to analyze the driving mechanisms underlying their recent differential evolution and propose scientifically robust management strategies tailored to the distinct evolutionary traits of each branch. [Methods] The hydrological, sedimentological, and topographic datasets spanning from 1958 to 2022 were analyzed, the spatiotemporal variations in channel erosion and deposition were quantified, and the characteristic parameters of these processes were compred. Furthermore, the relationships between these morphological changes and their potential driving factors—including hydro-sediment dynamics, sediment source, and anthropogenic interventions were examined. [Results] The research results reveal significant differences in the evolutionary characteristics of the North and South Branches over the past decades.(1) North Branch: The recent evolution of the North Branch is predominantly characterized by accretion, with the channel progressively narrowing and shrinking annually, accompanied by notable alterations in tidal flat topography. The evolution of its channels and tidal flats is jointly influenced by tidal hydrodynamics and the impacts of reclamation projects. Owing to the combined effects of natural accretion and the reclamation of channels and tidal flats, the channel storage capacity of the North Branch has been continuously decreasing.(2) South Branch: In contrast, the evolution of the South Branch is characterized by alternating erosion and deposition with an overall trend toward stabilization. In recent years, the channel storage capacity of the South Branch has slightly increased, primarily attributed to reduced sediment load, enhanced runoff dynamics, and the effects of engineering measures following the operation of upstream reservoir clusters. Nevertheless, the erosion rate has slowed, and the overall planar morphology of the South Branch has remained generally stable. [Conclusion] This study confirms that the North and South Branches of the Yangtze Estuary are undergoing fundamentally different geomorphological adjustments: the North Branch is in a state of retreat and infilling, while the South Branch is experiencing erosional downcutting. This differential evolution is driven by the complex interplay of multiple factors, including anthropogenic activities (reclamation in the North Branch, regulation works in the South Branch) and changes in upstream boundary conditions (a drastic reduction in basin-derived sediment supply). Based on these findings, we propose differentiated management strategies. For the North Branch, the scale and pace of reclamation should be strictly regulated to avoid excessive land encroachment, and the water-sediment transfer regulation system between the North and South Branches should be optimized. For the South Branch, it is necessary to strengthen watershed ecological management to reduce soil erosion and optimize incoming sediment fluxes, thereby mitigating erosion risks. Additionally, enhanced monitoring, early warning, and emergency response systems are imperative: dynamic monitoring of channel morphology, tidal flat evolution, and flow-sediment regimes in both branches should be intensified to promptly detect changing trends in the riverbed. This study provides scientific and technical support for the integrated management, sustainable development, and disaster risk reduction of the Yangtze Estuary.

[Objective] More than 700 floodplains and polders are distributed along the mainstream of the middle and lower reaches of the Yangtze River, with a total flood storage capacity of about 16.41 billion m³. These areas serve as important spaces for flood discharge and storage of the Yangtze River and are also home to millions of people. Difficulties in operation during major floods, insufficient safety guarantees during ordinary floods, and a lack of management policies have become the most notable weak links in the Yangtze River flood control system. Existing studies lack in-depth investigation into the operation sequence and activation timing of floodplains and polders. They also do not thoroughly examine the differences in flood discharge and storage effects arising from various combinations of different types of floodplains (mid-channel bars and outer floodplains). This study constructs a two-dimensional unsteady flow mathematical model to quantitatively analyze the flood diversion effects under different activation water levels and operation modes, aiming to provide a scientific basis for the hierarchical optimal operation and flood control management of floodplains and polders. [Methods] The lower Jingjiang reach and the Hukou-Datong reach were selected as typical areas. A two-dimensional unsteady flow mathematical model was established based on MIKE 21, and the flood evolution processes of floodplains and polders under different operation modes were simulated under the 1954 flood condition regulated by the Three Gorges Reservoir and the upstream reservoir group. [Results] The operation of floodplains and polders effectively reduced short-term flood water levels. In the lower Jingjiang reach, the maximum water level reduction at Shishou and Diaoguan stations reached 0.44 m and 0.36 m, respectively, and the duration with reductions exceeding 5 cm lasted about 5.7 days. In the Hukou-Datong reach, the maximum water level reduction at Balijiang, Anqing, and other stations ranged from 0.07 to 0.16 m, and the duration with reductions exceeding 5 cm lasted 1.4 to 3.2 days. After the floodplains were fully filled, the peak-reduction effect was significantly weakened. The backflow of stored water during the flood recession period caused a slight rise in water level (about 1-4 cm). Comparison of different activation water levels showed that when the activation water level in the Hukou reach was increased from 20.5 m to 22.0 m, the maximum water level reduction along the reach increased by 0.07-0.13 m, but the duration with reductions exceeding 5 cm was shortened by 8-24 hours, indicating that activation at a higher water level could cope with more severe floods but resulted in a shorter duration of water level reduction. In addition, the peak-reduction effect was weaker when the floodplains were used only for flood storage than when flood discharge and storage were applied in combination. [Conclusion] Floodplains and polders are effective regulators for dealing with short-term excessive floods. In operation scheduling, it is necessary to balance the peak-reduction magnitude and the duration of action. Further studies should focus on the comprehensive effects of the combined operation of different types of floodplains and the adaptive management strategies.

[Objective] The Guanzhou Waterway, a typical goose-head-shaped braided channel in the lower reaches of the Yangtze River, is characterized by complex and dynamic shoals, braided channels, and flow-sediment diversion patterns. This study aims to: (1) analyze recent riverbed evolution characteristics (2003-2023) from multiple perspectives, including boundary conditions, diversion ratios, shoal dynamics, thalweg shifts, and erosion-deposition changes; (2) quantify the effects of revetment works, upstream reservoir impoundment, and downstream confluence processes; (3) predict future evolution trends; and (4) propose targeted measures for river regime stability. [Methods] Long-term hydro-morphological datasets were employed, including: (1) topographic surveys derived from 1∶10 000 scale maps from the years 1966, 1977, 1987, 1998, 2003, 2012, and 2023. These datasets were used to analyze changes in shoal areas (e.g., Qingjie Shoal, Fusheng Shoal), thalweg positions, and cross-sectional parameters (width, depth, width-depth ratio). (2) Hydrological data, including annual runoff and sediment load at Datong station from 1966 to 2022, were collected to characterize changes in the flow-sediment regime, particularly following the impoundment of the Three Gorges Reservoir in 2003. (3) Engineering records of historical revetment projects (e.g., Sanyiwei, Guanzhou Shoal) were compiled to assess their effects on channel boundary stability. Quantitative analyses included: (1) statistical comparisons of diversion ratios among branches (Dongjiang, Xinzhong Branch, Nanjiajiang); (2) calculations of erosion and deposition volumes (volume changes in the riverbed below +5 m elevation); and (3) trend analysis of key cross-sections (GZ2#, GZ7#, GZ19#) to identify dominant evolution patterns. [Results] (1) River regime stability under revetment works: continuous revetment works since the 1980s have stabilized the overall river regime. The width-depth ratio of key cross-section GZ7# decreased from 1.45 in 1998 to 1.28 in 2023, indicating channel stabilization. The Xinzhong Branch, previously active, became nearly inactive, with its dry-season diversion ratio dropping to approximately 1% in 2023 due to sedimentation at its entrance. (2) Effects of clear water discharge: after 2003, the annual sediment load at Datong station decreased by 68.5%, leading to net erosion in the study reach. From 1998 to 2023, the channel from the Qingjie Shoal inlet to Yangjiatao experienced net erosion of approximately 37.9 million m³, with the channel volume below the +5 m elevation increasing by about 6% from 2003 to 2023. Severe erosion was observed at the left bank of the confluence section and at the head of Qingjie Shoal. (3) Critical evolution trends: the diversion ratio of the Nanjiajiang Branch gradually increased from 15% in the 1980s to 24% in 2023 due to scouring along the left margin of Fusheng Shoal. However, its development was constrained by the nodal control of Huangshiji. The left-bank shoal in the confluence section was expected to continue eroding, threatening downstream stability. [Conclusion] This study highlights the critical role of revetment projects in stabilizing this historically unstable braided channel, while revealing new challenges posed by clear water discharge. Key findings include: (1) upstream changes, such as the shrinkage of the left branch in the Dongliu Waterway and the increased diversion to the right branch, cause a slight leftward shift of the main channel. After being deflected by Jiyangji, this shift leads to a minor rightward displacement of the diversion point between the left branch and Nanjiajiang. (2) Under the new flow-sediment regime, the reduced sediment load accelerates erosion in unprotected areas (e.g., the head of Qingjie Shoal and the left bank of the confluence section). Ongoing adjustments at the head of Qingjie Shoal may subtly alter the inflow conditions and diversion ratio of the Nanjiajiang branch. Targeted control measures are proposed for three critical zones: (1) the head of Qingjie Shoal, to manage scouring-induced changes in the inflow to the Nanjiajiang branch; (2) the left margin of Fusheng Shoal, to mitigate the enhanced deflection effects from Huangshiji; and (3) the left bank of the confluence section, to prevent downstream channel instability caused by persistent scouring. Enhanced monitoring and data collection in these areas are essential for ensuring future river regime stability.

[Objective] The navigation capacity of the Chenglingji-Wuhan section (Chengwu section) in the middle reach of the Yangtze River lags behind the economic development along the river. This study aims to analyze the improvement of channel dimensions to fully utilize the efficiency of this “golden waterway” and better support the socio-economic development along the river. [Methods] Based on the variation characteristics of the Chengwu section under new runoff and sediment conditions, this study utilized nearly a decade of runoff and sediment observation data and navigation charts collected after the discharge from the Three Gorges Project stabilized. A comprehensive verification was conducted on the channel conditions under different channel dimensions across the 22 waterways within the river section. The key challenges in dimension improvement were analyzed, and corresponding governance measures were proposed. [Results] The guarantee rates for the channel dimensions of 6.0 m×200 m and 6.0 m×150 m in the Chengwu section showed little difference. Among the 22 waterways, nine failed to meet the year-round verification criteria. The guarantee rate increased as the channel width decreased. Under the channel dimension of 6.0 m×200 m, the Wuqiao waterway had the lowest multi-year average guarantee rate of 77%, while the rates for the remaining waterways all exceeded 86%. Among the nine problematic waterways, four (Jiepai, Jiayu, Baishazhou, and Wuqiao) failed to meet the 6.0 m×200 m channel dimension during the dry seasons in most years, while the remaining waterways failed only in individual years. The total length of shoal areas accumulated to approximately 8.95 km, accounting for 3.92% of the total length of the river section. Overall, the natural conditions were favorable for increasing the channel depth to 6.0 m. The problematic waterways were mainly of the branching-channel type. Among them, the main issue in the Jiepai, Longkou, Jiayu, and Yanwo waterways was insufficient water depth, whereas for the others, it was primarily narrow channel width. The shoal areas were mainly located at the inlets of branching channels and local widening sections. [Conclusion] Guided by the governance thoughts of “integrating regulation and dredging for comprehensive management”, the improvement of channel dimensions can be achieved by implementing regulation projects to appropriately restrict the flow diversion into branching channels, thereby increasing the hydrodynamic force in the main channel. Additionally, the clear water released from upstream reservoirs should be utilized to scour the river channel. Combined with dredging methods and new intelligent navigation guidance technologies, these measures collectively facilitate the improvement of channel dimensions. Specifically, for the Jiepai waterway, channel regulation should be implemented in phases. Dredging should first be applied to the outlet of the left channel while ensuring the uninterrupted operation of the right channel as the main channel. After the left channel becomes navigable, flow-restricting structures can be constructed in the right channel. For the Jiayu waterway, regulating the central bar to increase the flow-split ratio towards the left branch, coupled with the strategy of narrowing the channel to concentrate flow for sediment scouring, can address local navigation obstructions in the left branch. Regarding the Wuqiao waterway, flow-guiding structures should be deployed at the tail of the Baishazhou bar upstream to direct the main flow from the Baishazhou waterway into the left branch around the submerged bar within the Wuqiao waterway. Additionally, training dikes should be built along the submerged bar. Integrating these with dredging and new navigation guidance technologies will improve the navigation conditions in the bridge area and enhance safety. This study is a preliminary exploration. Further in-depth studies may focus on strengthening the validation of engineering plans through model tests.

[Objective] To safeguard freshwater intake security in the Yangtze River Estuary against frequent saltwater intrusion, this study aims to identify an optimal freshwater replenishment scheme that balances saltwater suppression effectiveness with water resource utilization efficiency. [Methods] To address extreme saltwater intrusion resulting from the combined effects of low runoff and strong tidal dynamics, a two-dimensional coupled hydrodynamic and salinity transport model was developed. Numerical scenarios incorporating various replenishment discharges and durations were designed based on actual dispatch strategies. Key evaluation indicators included the spatiotemporal distribution of salinity isolines and the duration of the available water intake window. The effectiveness of saltwater suppression was comprehensively evaluated by quantifying the variations in environmental responses under different replenishment strategies. [Results] Simulation results indicated that, influenced by the complex topography of the Yangtze River Estuary, the spatial distribution of replenishment effects exhibited non-uniform characteristics. Under cascade replenishment schemes, the seaward retreat rate of salinity isohalines was approximately 1.52-1.59 km/(500 m³/s) in the North Channel and 1.75-1.92 km/(500 m³/s) in the South Channel. Salinity levels at the intakes of Chenhang Reservoir and Qingcaosha Reservoir showed an overall decreasing trend, with decay rates of approximately 0.03‰/(500 m³/s) and 0.05‰/(500 m³/s), respectively. Intake salinity exhibited an approximately linear relationship with replenishment discharge, with the Qingcaosha Reservoir demonstrating a relatively higher degree of responsiveness. In contrast, the relationship between the available water intake window and the replenishment duration was non-linear, exhibiting characteristics of marginal utility. The findings suggested that maintaining a replenishment discharge of approximately 9 900 m³/s might provide strong assurance for water intake, while a replenishment duration of around 13 days was likely to achieve optimal benefits. Furthermore, strong offshore winds tended to amplify frontal saltwater intrusion in the North Channel. [Conclusion] Increasing discharge from the Three Gorges Reservoir effectively suppresses saltwater intrusion and contributes positively to mitigating salinity hazards in the Yangtze River Estuary. However, replenishment performance is constrained by multiple factors, including the replenishment mechanisms and offshore wind fields. Optimization of replenishment strategies should consider both discharge magnitude and duration, as prolonged duration does not necessarily result in proportional benefits. Moreover, compared with the North Branch and South Channel, replenishment effects in the North Channel and at Qingcaosha Reservoir are more susceptible to wind field influences. These findings provide technical support for decision-making on water replenishment scheduling during extreme dry seasons.

[Objective] Although artificial intelligence-based water level forecasting methods have achieved promising results in predicting upstream water levels at various power stations, including the Three Gorges Reservoir, there remains room for improvement. To obtain more accurate water level predictions for the Three Gorges Reservoir, this study develops an ultra-short-term forecasting model with a 15-minute time scale based on deep learning techniques, providing enhanced technical support for real-time reservoir operation. [Methods] The dataset comprises four categories: (1) water level data from the Three Gorges Reservoir and downstream areas; (2) inflow and spillage flow rates of the Three Gorges; (3) total power output of the Three Gorges power plant; and (4) precipitation between Cuntan and the Three Gorges area. Four water level forecasting models, including a baseline model and three comparative models, were developed to predict water level changes over the next 24 hours. Each model was constructed using both LSTM and RNN neural networks. The primary distinctions among these models lie in the processing of input and output data as well as the temporal scales of the data. By comparing the performance of different models under varying conditions, we analyze how model configurations impact prediction accuracy. [Results and Conclusion] (1) Regardless of input conditions, water level forecasting models built with LSTM outperform those using RNN, achieving Mean Absolute Errors (MAE) of 3.58 to 4.40 cm and maximum absolute errors of 56.99 to 110.03 cm. (2) All four water level forecasting models constructed using LSTM exhibit good performance, with the best-performing model incorporating dynamic reservoir capacity and interval rainfall impacts, achieving an MAE of 3.58 cm and a maximum absolute error of 56.99 cm. (3) Differences in model input variables are the dominant factor affecting forecast accuracy across various conditions. Incorporating reservoir water level information allows the model to better account for dynamic reservoir capacity effects, while adding interval rainfall data provides more precise inflow estimates, significantly enhancing prediction accuracy. This approach reduces the MAE by 18.64% compared to the baseline model. This study demonstrates that integrating relevant hydrological and meteorological factors into LSTM-based models can substantially improve the precision of short-term water level forecasts, thereby supporting effective reservoir management.

[Objective] As a region characterized by extensive permafrost, the Qinghai-Xizang Plateau has undergone significant environmental changes under global climate change. Surface water dynamics serve as sensitive indicators of permafrost degradation. This study investigates surface water changes over the past 30 years in a typical permafrost degradation area of the eastern Qinghai-Xizang Plateau, distinguishes variations between permafrost and seasonally frozen ground zones, and analyzes their relationships with temperature and precipitation. [Methods] Landsat 5 and Landsat 8 satellite images from 1995 to 2024 (August data only) were processed using Google Earth Engine (GEE) to remove clouds and high-reflectance interference through median-pixel compositing. An empirical annual mean ground temperature model, corrected for slope and aspect, was applied to classify permafrost and seasonally frozen ground zones. Surface water bodies were extracted using the Normalized Difference Water Index (NDWI) with an Otsu global-local thresholding method. The results were further refined using slope and hillshade data derived from the ASTER Global Digital Elevation Model (GDEM). Surface water bodies were classified by area into four categories: ≤0.001,(0.001,0.01],(0.01,1],and (1,100] km². Monthly temperature and precipitation data from local meteorological stations were used to analyze correlations with water body metrics. [Results] Permafrost and seasonally frozen ground zones accounted for approximately 63% and 37% of the study area, respectively, with a classification accuracy of 88.1% as confirmed by field surveys. Between 1995 and 2024, the total number of water bodies increased by 40%, mainly driven by small water bodies (≤0.01 km²), while the total water surface area expanded by 29%, dominated by large water bodies ((1,100] km²). In permafrost zones, the number of water bodies increased by 85%, primarily due to small water bodies formed by thaw-induced subsidence, whereas the area increased by 28%. Seasonally frozen ground zones showed a moderate 16% increase in the total number of water bodies and a 28% increase in area, largely attributable to larger water bodies. Correlation analysis revealed significant positive relationships between temperature and water body metrics (r>0.75), with smaller water bodies exhibiting the highest temperature sensitivity. Conversely, precipitation generally had weak or negative correlations with water dynamics, particularly in permafrost zones, where heavy rainfall often promoted drainage and lake outflow. Seasonally frozen ground zones showed limited sensitivity to precipitation due to higher infiltration rates. [Conclusion] Rising temperatures primarily drive the expansion of surface water, exceeding the effects of precipitation. Permafrost zones are highly sensitive to warming, as indicated by rapid increases in small water bodies, whereas seasonally frozen ground zones maintain stable water body counts with area expansion driven by larger lakes. Precipitation plays a secondary or even negative role in water dynamics. The distinct responses of water bodies under different freeze-thaw conditions highlight the complexity of hydrological changes driven by climate warming, providing crucial insights for future environmental predictions and resource management on the Qinghai-Xizang Plateau.

[Objective] As the core water source area of the Middle Route of the South-to-North Water Diversion Project, runoff variations in the Danjiang River Basin directly influence water transfer capacity and source water security. Current research primarily relies on manually defined meteorological parameters, limiting scenario diversity and resulting in an insufficient scientific basis, yet studies on future runoff evolution remain scarce. This study aims to analyze the impact of future climate change on runoff in the Danjiang River Basin to provide a scientific basis for water resource management and the operation of the South-to-North Water Diversion Project. [Methods] This study first used the CN05.1 meteorological dataset and observed daily runoff data to construct and calibrate a SWAT hydrological model, simulated runoff variations during the historical period, and verified the model accuracy. On this basis, future meteorological data from six high-performing global climate models (GCMs) under three Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, and SSP5-8.5) of the Coupled Model Intercomparison Project Phase 6 (CMIP6) were bias-corrected using the Delta method to better represent the climatic characteristics of the Danjiang River Basin. Meanwhile, the PLUS model was applied to simulate two land use change scenarios to reflect potential future land use dynamics. The bias-corrected climate data were then combined with different land use scenarios and input into the SWAT model to simulate the spatiotemporal evolution of future runoff. [Results] (1) The SWAT model demonstrated excellent performance in simulating monthly streamflow at Jingziguan and Danfeng stations, with R² values exceeding 0.9 and NSE values above 0.8. The GCMs accurately captured the evolution patterns of temperature extremes and precipitation in the region, with correlation coefficients exceeding 0.85 for temperature and 0.7 for precipitation. (2) In the future, both temperature and precipitation in the Danjiang River Basin were projected to increase. Across all scenarios, temperature increases followed the pattern: late period>mid period>near period, with the most pronounced change under the high carbon scenario, where the late-period temperature rise reached 7.11 ℃, 2.72 times that under the low carbon scenario. Precipitation generally showed a continuous upward trend, with the largest increase (11.63%) under the low carbon scenario. The fastest increase occurred in summer, while winter precipitation under the low carbon scenario increased by 10.38%. (3) Runoff in the Danjiang River Basin exhibited significant spatiotemporal variability. The annual average runoff shifted from a decrease in the near period to an increase in the far period. Seasonal variations indicated significant increases in spring and winter, and decreases in summer and autumn, with the most pronounced increases in January and December and the most notable decreases in July and September. Spatially, downstream runoff increased markedly, with widespread growth in the mid to far period under the low carbon scenario, and prolonged decreases under the high carbon scenario. [Conclusion] (1) In the near term, the annual average runoff shows a downward trend with frequent fluctuations. Under the medium carbon scenario, the maximum annual runoff reduction reaches 25.32%. The high carbon scenario exhibits 16 abrupt changes during the study period without stable recovery, resulting in coexisting risks of extreme floods and droughts due to long-term runoff variability. (2) In the mid to long term, runoff generally recovers, although significant differences remain among scenarios. Under the low and medium carbon scenarios, runoff increases by up to 20.34%, which supports water supply security for the South-to-North Water Diversion Middle Route Project and meets the water demand of the basin’s ecosystems. In contrast, under the high carbon scenario, runoff increases by less than 1.00%, and supply risks persist. (3) The low carbon scenario is most favorable for the long-term development of the basin, while the high carbon scenario poses the greatest risks. Runoff recovery occurs earliest under the low carbon scenario, ensuring ecological water demand and water security. By contrast, under the high carbon scenario, recovery is slow, and fluctuations are frequent, increasing the probability of extreme droughts or floods.

[Objective] The changes and specific diffusive fluxes between carbon sources and sinks in the Three Gorges Reservoir have long been a focal issue in monitoring and analyzing greenhouse gas changes within the reservoir. Few studies have focused on the differences and potential relationships in greenhouse gas carbon fluxes of the water level fluctuation zone and the water body. To clarify the spatial interaction of greenhouse gas carbon fluxes between the water level fluctuation zone and the water body in the Hubei section of the Three Gorges Reservoir, this study selects the Shennong Stream, an important tributary in this section, as the research object. [Methods] The Picarro G2301 greenhouse gas online analyzer was employed to monitor carbon dioxide and methane fluxes in June, July, August, and September 2024. Based on their spatial positions, ArcGIS, SPSS, and other statistical analysis tools were applied to analyze the variation characteristics and interaction relationships. [Results] (1) Carbon source and carbon sink of carbon dioxide and methane carbon fluxes vary between the water level fluctuation zone and the water body in the study area. The carbon dioxide fluxes in the water level fluctuation zone were approximately 400 mg/(m²·h), with a standard deviation of 228.73 and a coefficient of variation of 0.55, indicating an emission state. The methane carbon fluxes ranged from -0.04 mg/(m²·h) to 0.01 mg/(m²·h), with an average value of -0.01 mg/(m²·h), including absorption state and emission state. The carbon dioxide fluxes in the water body were in an absorption state, and the methane fluxes of water body were similar to those in the water level fluctuation zone, exhibiting an absorption state in some months and an emission state in others. The corresponding standard deviation and coefficient of variation were 0.16 and 1.60, respectively. (2) The diffusive fluxes of greenhouse gases in the water level fluctuation zone and the water body varied in different months. The carbon dioxide fluxes in the water level fluctuation zone peaked in July, exceeding 650 mg/(m²·h), and the methane fluxes in the water body peaked in June, exceeding 0.30 mg/(m²·h). (3) The carbon dioxide and methane fluxes in the water level fluctuation zone and the water body of the study area differed in both correlation direction and magnitude. Methane and carbon dioxide fluxes in the water level fluctuation zone were negatively correlated, with a correlation coefficient of -0.44. The pattern was similar in the water body, and the correlation between methane and carbon dioxide fluxes in the water body was also negative, with a correlation coefficient of -0.89. The carbon dioxide fluxes in the water level fluctuation zone were positively correlated with those in the water body, with a correlation coefficient of 0.45, whereas the methane fluxes in the water level fluctuation zone and water body were negatively correlated, with a correlation coefficient of -0.78. (4) Monthly temperature during the monitoring period may affect greenhouse gas carbon fluxes in the water level fluctuation zone and the water body in the Hubei section of the Three Gorges Reservoir. The carbon dioxide fluxes in the water level fluctuation zone were negatively correlated with temperature, with a correlation coefficient of -0.53, while the methane fluxes were positively correlated, with a correlation coefficient of 0.13. The carbon dioxide fluxes in the water body were positively correlated with temperature, with a correlation coefficient of 0.51, while the methane fluxes were negatively correlated with temperature, with a correlation coefficient of -0.65. These patterns required further verification through additional monitoring data. [Conclusion] As the transition zone between aquatic and terrestrial ecosystems, the water level fluctuation zone of the Three Gorges Reservoir area experienced a large migration of material and energy and a high degree of interconnection,leading to a complex relationship between the carbon cycles of water, soil,and vegetation.Consequently,the influence of spatial changes should be considered when exploring the dynamics of greenhouse gas carbon fluxes.

[Objective] Hyperspectral remote sensing technology offers a new approach for non-contact, real-time sensing of water quality parameters. Existing research has shortcomings in developing dedicated sensing equipment and constructing multi-parameter collaborative inversion algorithms. This study aims to systematically explore the spectral response characteristics of five key water quality parameters—chemical oxygen demand (COD), total suspended solids (TSS), total phosphorus (TP), total nitrogen (TN), and ammonia nitrogen (NH₃-N)—based on a self-developed hyperspectral water sensing instrument, and to establish an adaptive hyperspectral sensing algorithm optimization framework for multiple water quality parameters. This framework aims to overcome the adaptability limitations of traditional single algorithms in complex water environments and provide technical support for achieving 24/7 continuous online water quality monitoring. [Methods] First, continuous spectral data and corresponding measured values of water quality parameters from a large number of water samples were collected using the self-developed hyperspectral water sensing instrument. Through spectral analysis, the absorption, reflectance characteristics, and differential patterns of COD, TSS, TP, TN, and NH₃-N within the visible to near-infrared spectral range were revealed, and the sensitive characteristic bands for each parameter were selected accordingly. On this basis, a multi-level, multi-type inversion model system was constructed. At the empirical model level, single-band regression, band ratio, and normalized difference index were employed to establish statistical relationships between spectral features and water quality parameters. At the machine learning level, BP neural network, random forest (RF), and XGBoost were introduced to fully exploit the nonlinear mapping relationships within the hyperspectral data. Furthermore, an adaptive algorithm optimization framework was proposed. By comparing the inversion accuracy and stability of various models across different water quality parameters and concentration ranges, this framework automatically matched the optimal inversion algorithm for each parameter, thereby achieving the optimal configuration for multi-parameter collaborative sensing. An independent dataset partitioning strategy was adopted for model training and validation, with the coefficient of determination (R²) and mean absolute percentage error (MAPE) as the core evaluation indicators to ensure the objectivity and reliability of the assessment results. [Results] The constructed adaptive algorithm optimization framework achieved excellent performance in the inversion of all five water quality parameters. In the training and validation datasets for the optimal algorithms, the R² values of the corresponding optimal models for each parameter ranged from 0.88 to 0.99, exhibiting extremely high fitting accuracy and generalization capability. From a practical accuracy perspective, 100% (COD), 91% (TSS), 90% (TP), 90% (TN), and 95% (NH₃-N) of total samples had a MAPE below 30%, fully meeting the practical requirements for water quality monitoring. Specifically, the inversion accuracy for the COD parameter was the highest, with all samples meeting the accuracy threshold, indicating that hyperspectral data possessed exceptional characterization capability for organic pollutant concentrations. NH₃-N followed closely, with 95% of samples meeting the accuracy requirement, reflecting significant spectral response characteristics of ammonia nitrogen in specific bands. The compliance rates for TSS, TP, and TN all exceeded 90%, verifying the universality and robustness of the framework in the inversion of different types of water quality parameters. Compared with traditional single-model methods, the adaptive optimization strategy significantly improved the overall accuracy and stability of multi-parameter collaborative inversion, effectively overcoming the adaptability deficiencies of single algorithms under complex water conditions. [Conclusion] This study proposes and validates an adaptive hyperspectral sensing algorithm optimization framework for multi-parameter water quality monitoring based on a self-developed hyperspectral water sensing instrument. The core innovations of this optimization framework lies in the following: first, it breaks through the conventional paradigm of “one algorithm fits all parameters” in traditional water quality hyperspectral inversion by implementing a multi-model competitive optimization mechanism, achieving automatic matching of the optimal algorithm for each parameter. Second, it integrates empirical models and machine learning models into a unified optimization system, balancing the model interpretability with nonlinear fitting capability. Third, the research findings demonstrate strong engineering application prospects and can directly support the construction of 24/7 continuous online water quality monitoring systems.

[Objective] This study investigates the synergistic and antagonistic effects of multiple factors on water self-purification capacity and examines self-purification efficiency of water under the combined influence of multiple factors, aiming to overcome the limitations of previous studies and provide new quantitative evidence for understanding water self-purification capacity in complex aquatic environments. [Methods] To investigate the interactions among multiple factors affecting water self-purification capacity, orthogonal experiments were conducted on water samples collected from the northwest side of a lake in Wuhan, which was affected by mixed pollution from domestic sewage and industrial wastewater and contained a microbial community. The orthogonal experiment results were analyzed using a combination of response surface methodology (RSM) and structural equation modeling (SEM) to reveal the nonlinear interaction mechanisms among typical influencing factors, including meteorological conditions, pollutant concentration background values, and biological activity. [Results] The results of orthogonal experiments showed that water self-purification efficiency (η), nitrification rate (r_N), and oxygen mass transfer efficiency (K_La) of water all varied nonlinearly across experimental groups, with maximum values observed at approximately temperature (T)=25 ℃, dissolved oxygen concentration (DO)=6 mg/L, flow rate (v)=0.1 m/s, microbial abundance (M)=5×10⁷ CFU/mL, and chemical oxygen demand (C)=100 mg/L. RSM analysis indicated that at the optimal parameter combination (T=24.8 ℃, DO=5.9 mg/L, v=0.27 m/s, C<100 mg/L, and M=4.7×10⁷ CFU/mL), η significantly increased to 83.24%, r_N exceeded 1 mg/(L·h), and K_La reached its maximum. In addition, in the binary factor interactions, the most significant interaction term was T×DO, with a synergistic contribution of 45.52%. SEM path analysis showed that v and T influenced the water self-purification process through both direct effect and indirect effect. Paths: v➝DO➝η; v➝DO➝Mi( microbial activity, including r_N and M)➝η; T➝DO➝η; T➝Mi➝η; T➝DO➝Mi➝η, where the direct effect of v on water self-purification process was 0.41, and the indirect effect was 0.25; and the direct effect of T on water self-purification process was 0.35, and the indirect effect was 0.25. The total effect of T and v on water self-purification process increased by 0.25 compared with the direct effect of each factor alone. [Conclusion] The results confirm that there is a synergistic amplification mechanism and significant threshold effects among different environmental influencing factors. Binary factor interactions not only show significant effects, but also factors originally negatively correlated with water self-purification capacity can significantly reduce their inhibitory influence on water self-purification efficiency after complex interactions with other factors. Additionally, factors such as flow rate and temperature affect the water self-purification process both directly and indirectly through their effects on other influencing factors. These findings provide a new quantitative basis for evaluating water self-purification ability and its controlling factors in complex aquatic environments.

[Objective] This study aims to quantitatively evaluate the effects of the emergency regulation of the Three Gorges Reservoir in response to the Tuanzhou polder breach emergency, thereby improving the understanding of flood control regulation of the Three Gorges Reservoir and laying a foundation for further optimization of its operation in the future. [Methods] A one-dimensional and two-dimensional coupled hydrodynamic model for the Jingjiang River reach and east Dongting Lake was established and validated with field-measured data. Hydrological conditions of the river-lake system under different regulation scenarios of the Three Gorges Reservoir were designed and simulated, and the effects of reservoir regulation on river-lake discharge and water levels were quantitatively analyzed. The development of emergencies in the Dongting Lake area (including Tuanzhou polder) before and after the breach, as well as the variation of water level at Qilishan, were analyzed. The effects of the Three Gorges Reservoir regulation on alleviating the flood-control pressure in the Dongting Lake area were discussed. [Results] Compared with the original regulation plan before the Tuanzhou polder emergency, the actual regulation of the Three Gorges Reservoir reduced the water level in east Dongting Lake by up to approximately 0.34 m, and advanced the time for the water level at Qilishan station to fall below the warning level by 18 hours. Flood-control emergencies in the Dongting Lake area mainly occurred during the high-water period after the water level exceeded the warning level and during the water recession period, and the regulation of the Three Gorges Reservoir reduced the probability of emergencies in the Dongting Lake area. [Conclusion] The emergency regulation of the Three Gorges Reservoir shortens the duration of the Qilishan water level exceeding the warning level and alleviates flood-control pressure in the Dongting Lake area. This study provides a scientific and quantitative evaluation of the regulation effects of the Three Gorges Reservoir, which provides a basis for further optimization of its operation.

[Objective] To evaluate the potential chain disaster effects such as debris flows under ultra-standard extreme hydrological conditions, this study aims to establish a comprehensive catastrophe evolution analysis system and propose optimized design solutions to enhance safety control capabilities from the source, transforming the safety control of tailing dams from passive response to proactive defense. [Methods] A combination of numerical simulation and engineering analysis was adopted. Focusing on a typical tailing dam, we constructed an integrated 3D catastrophe evolution analysis model covering the reservoir area and downstream regions. The finite element strength reduction method was used for numerical stability analysis of the tailing dam, accurately identifying potential sliding surfaces and instability failure zones through plastic strain cloud maps. Furthermore, multi-phase flow coupling simulation technology was introduced to combine the process of dam failure and subsequent debris flow progression. Within a digital elevation model, the entire evolution process of post-failure debris flow in downstream valleys was dynamically simulated, quantitatively acquiring critical disaster-causing parameters such as flow velocity, inundation extent, and depth. [Results] (1) Regarding dam stability, under the action of extreme flood levels, the maximum deformation zone identified by the finite element strength reduction method is not located at the dam body but correlates with the reservoir shape and topography. This area represents the most likely initial instability zone, highly susceptible to triggering local or overall landslides, thus inducing dam breaches.(2) Concerning the debris flow evolution process, simulations accurately depicted the descent paths and dynamic evolution characteristics of breached debris flow. Specifically, in terms of flow velocity, maximum flow velocities were observed in the immediate downstream area of the breach, indicating strong erosive capabilities; however, velocities gradually decreased with distance and widening terrain while still posing significant threats to key residential areas and infrastructure. In terms of inundation extent and depth, significant inundation areas formed in downstream valleys, with simulation results clearly delineating risk boundaries corresponding to different flood magnitudes. Maximum inundation depths reached several meters in downstream low-lying areas, directly threatening roads, buildings, and farmlands. Through coupled analysis, the full-chain disaster evolution characteristics from dam breach, debris flow formation to final deposition were identified. [Conclusion] This study proposes optimizing the existing drainage system of tailing dams with a stepped energy dissipation structure which significantly reduces the velocity and kinetic energy of descending floods, effectively controlling overflow erosion on the dam slope, fundamentally weakening the dynamic basis for overtopping destruction. It elevates the safety control system from traditional passive reinforcement and post-disaster rescue to a new phase of proactive intervention in water flow energy, preventing damage before it occurs, thereby markedly enhancing the ability of tailing dams to cope with sudden excessive floods. The research findings provide important theoretical support and technical references for risk assessment, emergency planning, and engineering renovation and expansion of similar tailing dams.

[Objective] This study aims to solve the problem of insufficient energy dissipation and flow disorder in stilling basins with low Froude number. The V-shaped pier is proposed, and the forward displacement rate of hydraulic jump position P is introduced as an evaluation indicator to quantify the effect of hydraulic jump regulation. The main purpose is to systematically examine the effects of Froude number Fr, V-shaped pier angle θ, row spacing γ, first row pier position Γ, pier height ratio ξ, and contraction ratio β on energy dissipation rate η and P, to reveal the corresponding energy dissipation mechanisms and provide optimal design parameters for practical engineering applications. [Methods] Physical model tests and numerical simulation were adopted to analyze flow field structure, turbulent kinetic energy dissipation rate, and turbulent scale. The physical model consists an upstream water tank, a test section, a triangular water weir, and a backwater system. The experimental design based on L₁₈ (3⁷) orthogonal table was used to evaluate the effects of six factors at three levels on η and P. At the same time, a three-dimensional numerical model using the RNG k-ε turbulence model was established to simulate both the optimal V-shaped pier condition and the non-pier condition. [Results] The orthogonal test results showed that Fr had the most significant effect on the energy dissipation rate η, and η increased notably with the increase of Fr (2.77-4.91). Under the optimal scheme, the energy dissipation rate η reached 95.58%. For the forward displacement rate P of the hydraulic jump position, the first row pier position Γ and the pier height ratio ξ demonstrated the greatest effects, and P reached 49.42% under the optimal scheme. The variance analysis verified the significance of each factor and determined the globally optimal parameter combination at Fr=4.91. Numerical simulations indicated that compared with the non-pier condition, the V-shaped pier layout reduced the maximum velocity of the pre-jump section by 39.4%, significantly shortening the flow field homogenization distance. The peak turbulent kinetic energy dissipation rate ε near the first row of piers was 14.52% higher than that in the non-pier case, indicating enhanced energy dissipation through strong shear and vortex action. The turbulence length scale L_T near the pier decreased to about 0.02 m, effectively suppressing the development of large-scale vortices. Downstream of the second row of piers, the L_T was homogenized to about 0.10 m, and the vortex scale at the apron was reduced by 81.25% to 0.03 m, reflecting a multi-stage vortex synergy energy dissipation mechanism. [Conclusion] The V-shaped pier proves to be an efficient auxiliary energy dissipator for stilling basins with low Froude numbers. The optimal parameter combination was determined as θ=90°, γ=0.2L, Γ=0.2L, ξ=0.66, β=0.26, which achieved an η of 95.58% and P of 49.42%. Overall, the V-shaped pier fundamentally optimizes the flow field, significantly improves the flow stability and energy dissipation efficiency, and reduces the downstream scour risk through mechanisms of diversion, impingement, shear, and multi-stage vortex dissipation. Future studies should focus on establishing the optimal parameter combination relationships in a wider range of Froude numbers.

[Objective] Under changing environmental factors such as sediment deposition, riverbed incision, and backwater effects induced by cascade reservoir impoundments, low-head hydraulic hubs in plain river basins often face the challenge that the design discharge curves of sluice structures do not match actual scheduling operation. To address the issues of low accuracy in the design discharge curves of low-head hydraulic hubs, this study, based on the analysis of the causes of discharge curve discrepancies, proposes a calibration method for discharge curves of sluice gates and demonstrates its performance through an engineering case study. [Methods] Taking a navigation and hydropower hub in the middle reaches of the Ganjiang River as an example, field measurements of discharge were conducted for sluice gates using a moving-vessel ADCP (Acoustic Doppler Current Profiler) measurement technique. The prototype observation results were used to calibrate empirical coefficients of hydraulic formulas and to validate the accuracy of a three-dimensional CFD (Computational Fluid Dynamics) numerical model. Under free-flow operation conditions of the sluice gates, the validated three-dimensional numerical model was employed to calculate discharge capacities of the sluice gates. The simulation results were further used to calibrate empirical coefficients in hydraulic formulas, enabling the calculation of sluice-gate discharge under different combinations of upstream and downstream water levels. [Results] The N-H-Q curve interpolation method provided high accuracy in calculating power-generation reference discharge and could be used to analyze the respective proportions of power-generation discharge and spilled discharge in the total outbound flow of the hub. When the sluice gate was partially opened to 0.5 m, the flow pattern downstream of the gate was submerged. When the gate was partially opened to 1.5 m, the flow transitioned to a free-flow pattern. Under the same conditions, the flow patterns through the sluice gate openings obtained from the three-dimensional CFD numerical simulation were consistent with field observations. Discharge values calculated using hydraulic formulas and three-dimensional CFD calculations agreed well with measured data, with a maximum deviation of less than 5% and an average deviation of approximately 1%. Considering variations in upstream and downstream water levels of the hub and the operational range of sluice gate openings, a family of calibrated discharge curves for partially opened gates was calculated. When the gate was fully open for free discharge, different submergence coefficient calculation methods were required to determine the discharge depending on the degree of downstream submergence. [Conclusion] By integrating hydraulic calculation, prototype observation,and three-dimensional numerical simulation,a family of discharge curves under both controlled and free-flow operations of sluice gates is developed,adaptable to changing field boundary conditions and unaffected by time-varying environmental factors such as inflow magnitude,downstream backwater effects,or unsteady water level-discharge relationships.The calibrated discharge curves deviate from the measured data by less than 5% and can be used for practical application in hub scheduling and operation.

[Objective] The ancient water storage and irrigation system in Sakya, Xizang autonomous region, listed in 2021 as a World Heritage Irrigation Structure, is a unique paradigm of high-altitude water resource management and an important testament to the diverse cultural heritage of the Chinese nation. Despite its profound value, existing research remains fragmented, and systematic studies from the perspective of water cultural heritage are particularly scarce. This study aims to fill this gap. [Methods] This study adopted a multidisciplinary approach integrating historical document analysis, field investigation, scientific principle verification, and value evaluation. First, classical Xizang texts, including Mkhas pa’i dga’ston and Rgyal rabs gsal ba’i me long, were examined to clarify the development stages and management traditions of the irrigation system. Second, field surveys were conducted to map the engineering structures (e.g., diversion weirs, reservoirs, and sluices) and to verify the current operation of the “Cuoben-Shuinv-Humin” management mechanism. Third, quantitative analysis was conducted to verify the scientific rationality of the system based on modern engineering theories such as the Bernoulli equation (potential energy conversion), Fourier’s law (thermal insulation and freeze protection), and hydraulic mechanics (pressure reduction principles). Finally, a comprehensive value evaluation framework was established, integrating theories from cultural heritage studies, ecological economics, and water resource management to systematically analyze the system’s tangible and intangible cultural heritage values. [Results] The development of the Sakya irrigation system experienced four key stages: 1) embryonic irrigation stage driven by nature worship during the Tubo period, 2) large-scale construction stage promoted by the integrated political and religious system in the Mongol-Yuan period, 3) stage of institutional innovation (e.g., a primitive “river chief system”) and ecological adaptation during the Ming-Qing dynasties, and 4) contemporary stage integrating traditional wisdom with modern technology. In terms of engineering structure, the system adopted a sophisticated “single-source, three-reservoir, sluice-regulated diversion” design, fully utilizing the local terrain for gravity-driven water conveyance without external energy input. The reservoirs were predominantly open earth-stone structures with capacities ranging from 30 000 m³ to 50 000 m³, reasonably arranged along the Chongqu River and composed of four core components: diversion weirs, channels or pipelines, reservoir bodies, and outlet pipe networks. Quantitative analysis verified three major scientific adaptations of the system to the plateau environment. (1) Efficient potential energy conversion: Utilizing a terrain height difference of 30-80 m, the system achieved an energy utilization efficiency exceeding 95%, and the head loss calculated using the Manning equation accounted for only 3.6% of the total head, which was sufficient to meet the irrigation demand of 100 000 mu (approximately 6 667 hectares) of highland barley fields. (2) Implicit application of pressure-reduction pool principles: The three-stage reservoir cascade reduced water pressure by 67% and channel scouring force by 96%, effectively preventing erosion of the earth-stone channels. (3) Thermal insulation and freeze protection design: Compared with pure stone structures, the composite structure of mortar-masonry stone and local clay reduced winter heat loss by 60% and lowered frost-heave risks by 80%, while reducing summer water evaporation by 60%, adapting to the plateau's extreme sub-frigid climate with winter temperatures as low as -25 ℃. The system demonstrated multidimensional values. (1) Political value: It provided the material foundation for Sakya to become the political and religious center of Xizang during the Mongol-Yuan period and promoted profound regional socio-economic transformation. (2) Economic value: More than 400 reservoirs currently in operation irrigate 40% of the highland barley cultivation area in Xizang, supporting Shigatse in becoming the “World Highland Barley Hometown”. (3) Cultural value: It integrates Xizang Buddhism, folk customs (e.g., rain-prayer rituals), and literary works (e.g., metaphors in Sa skya legs bshad), serving as a living carrier of water culture and a national water education base. (4) Ecological value: By using local materials and adapting to the terrain, the system minimized environmental disturbance while performing functions of flood regulation and groundwater recharge, embodying the ancient ecological wisdom of “harmony between human and nature”. [Conclusion] This study is the first systematic investigation of the Sakya irrigation system from the perspective of water cultural heritage, innovatively integrating traditional engineering practices with modern scientific principles and verifying the rationality of ancient high-altitude water resource management. The results demonstrate that the system is not only an engineering marvel, but also a living cultural-ecological system integrating physical structures, intangible management systems, and ecological ethics. Its multidimensional values highlight the diversity of Chinese water civilization and provide valuable reference for water resource management in high-altitude regions worldwide. To ensure the sustainable development of the system, targeted strategies are proposed: enhancing value interpretation through digital technologies, adopting holistic protection concepts such as “cultural routes”, implementing classified and phased protection mechanisms, and promoting the integrated development of culture and tourism. Through coordinated collaboration among government, communities, and academia, the Sakya irrigation system can be transformed from a static engineering heritage into a dynamic cultural-ecological entity, providing a model for the protection and development of global irrigation heritage and promoting the innovative development of plateau water culture.

[Objective] The integrated support structure of precast piles with dentiform curtains can effectively reduce the earth pressure acting on the support structure and enhance its overall performance. This study aims to quantitatively analyze the load-reduction characteristics of the dentiform walls in this structure, reveal the key mechanisms of load reduction, and provide a theoretical reference for the design and calculation of this novel composite support technology. [Methods] Based on the load-reduction mechanisms of the dentiform walls in an integrated support system of precast piles with curtains, a cycloidal slip surface conforming to the actual sliding failure mode of the soil behind the walls was introduced. A calculation method for the load-reduction effect of the dentiform walls under soil sliding conditions was developed using the horizontal thin-layer differential theory. Based on the validation through calculation examples, the impact of key parameters—including dentiform wall structural parameters (spacing, width, thickness), soil strength parameters (cohesion, internal friction angle), and soil-wall interface strength parameters (soil-wall cohesion, soil-wall external friction angle)—on the load-reduction effect of earth pressure was further analyzed. [Results] When the dentiform wall spacing decreased from 3 m to 1.2 m, the load-reduction ratio increased from 23.77% to 44.02%. When the dentiform wall width increased from 0.5 m to 3 m, the load-reduction ratio increased from 17.16% to 46.34%. When the dentiform wall thickness increased from 0.4 m to 1.0 m, the load-reduction ratio only increased from 32.66% to 36.66%. When soil cohesion (c) increased from 1 kPa to 16 kPa, the load-reduction ratio increased from 28.14% to 59.86%. When the soil internal friction angle increased from 16° to 31°, the load-reduction ratio increased from 20.43% to 42.68%. When the soil-wall interface cohesion (c₁) increased from 0.6 kPa to 9.6 kPa, the load-reduction ratio increased from 27.41% to 50.41%. When the soil-wall interface external friction angle increased from 9.6° to 18.6°, the load-reduction ratio increased from 30.33% to 36.21%, with the rate of increase gradually slowing down. [Conclusion] The developed calculation method effectively solves the problem that earth pressure is difficult to quantitatively analyze when the integrated support system of precast piles with curtains has dentiform walls. The dentiform walls exhibit a significant load-reduction effect, with the load-reduction ratio of earth pressure from the sliding soil behind the walls reaching over 50%. Decreasing the dentiform wall spacing and increasing its width can both significantly enhance the load-reduction effect. Since the walls have a certain width, the contact area between the back of the dentiform walls and the sliding soil mass is limited, and increasing the wall thickness does not lead to a notable improvement in the load-reduction effect. Increases in both the cohesion and internal friction angle of the soil behind the walls can significantly raise the load-reduction ratio of the dentiform walls. Therefore, on sites with better soil conditions, adding dentiform walls is more beneficial for improving the performance of integrated support of precast piles with curtains. The increase in the strength of the soil-wall interface also contributes to a higher load-reduction ratio of the dentiform walls, but the increase in soil-wall cohesion has a relatively more significant impact.

[Objective] The soil water characteristic curve (SWCC) can effectively reflect the relationship between soil water suction (matric potential) and water content, yet conventional testing methods pose challenges for the calibration of in situ soil hydraulic parameters. A more convenient, economical and reliable in situ testing approach is required for the rapid evaluation of in situ soil water status and hydraulic characteristics. [Methods] This study integrated soil dielectric theory with a soil water characteristic curve model and, by considering clay mineral composition, bound water, and free water, derived and established an evaluation model for the soil equivalent dielectric constant-matric suction characteristic curve (SEDCC). For model verification, remolded red clay samples with an average dry density of 1.2 g/cm³ were prepared from six regions of Yunnan Province. The prediction accuracy of the SEDCC curves for red clay across different regions was then systematically analyzed, and the feasibility of indirectly evaluating the SWCC of red clay using dielectric theory parameters of electromagnetic waves was further discussed. [Results] The results showed that the water retention characteristics of red clay were influenced by formation conditions, mineral composition, pore structure, and climatic environment, resulting in regional differences in the fitting accuracy of the Fredlund model. Volumetric water content was the key factor driving changes in matric suction and the equivalent dielectric constant. Using volumetric water content as a bridge, the equivalent dielectric constant gradually decreased as matric suction increased. By fitting the correlation between the equivalent dielectric constant and matric suction using the SEDCC model, the fitting accuracy of the SEDCC model in all regions was greater than 95%. An approximately symmetric relationship was observed between the matric suction inversion curve of red clay and the dielectric theory prediction curve. However, this relationship was still influenced by regional differences. By comparing the predicted values from the dielectric theory inversion curve with the measured values obtained by the filter paper method, it was found that the change trends of the soil water characteristic curves derived from the two were basically consistent, with an average relative error of 6.42%. [Conclusion] Therefore, the SEDCC curve proposed based on dielectric theory can fully reflect the coupling relationship between the soil equivalent dielectric constant and matric suction, indicating that the equivalent dielectric constant is a feasible indirect indicator for evaluating matric suction in red clay. Compared with traditional methods, this model is efficient, convenient, and nondestructive, and it shows potential for indirectly inverting the soil water characteristic curve using dielectric theory, thereby providing a convenient approach as well as theoretical model support and reference for the rapid and accurate detection of soil hydraulic parameters using electromagnetic wave techniques.

[Objective] To investigate the drag reduction effect of residual layers in periodic debris flows, we quantitatively revealed the amplification effect of the impact of subsequent waves due to the presence of residual layers. We propose evaluation indicators centered on the impact force ratio F^* (amplification of subsequent wave impact relative to the first wave) and the momentum ratio R (amplification of total momentum flux relative to the head momentum flux), aiming to elucidate the dynamic mechanisms responsible for the increased destructiveness of subsequent waves. [Methods] Using a mesoscale flume, we examined debris flows with three different solid contents (40%, 50%, and 60%). Sensors measured flow height, normal stress, shear stress, and pore water pressure. Considering that the movement of subsequent waves over residual layers can be approximated as quasi-steady hydraulic jumps, we calculated unit width impact forces from post-jump velocities and water depths based on momentum conservation. Dimensionless indicators F^* and R were constructed to quantify the amplification effects of multi-wave impacts and intra-wave momentum due to residual layers. [Results] (1) Higher solid content resulted in thicker residual layers, which tended toward a quasi-equilibrium state of erosion and deposition under multiple wave actions. (2) The presence of residual layers altered the flow regime of subsequent waves, with the initial wave typically exhibiting the highest Froude number (Fr). Subsequent waves showed an overall decrease in Fr, indicating a shift from inertia-dominated to gravity-dominated flow control. Increased solid content significantly reduced liquefaction and mobility, as indicated by decreased liquefaction degree λ with increasing solid content, suggesting enhanced effective stress and reduced fluidity. This change influenced the interaction intensity between subsequent waves and residual layers. (3) Residual layers significantly amplified the impact forces of subsequent waves, showing an “increase then stabilize” trend. Under all solid contents tested, impact forces generally exhibited a “rapid increase followed by stabilization”, consistent with the thickening and stabilization process of residual layers. Impact force ratios F^* were greater than 1 for subsequent waves, indicating amplified peak impact forces under identical release conditions. (4) Momentum analysis revealed that R stabilized in later waves, aligning with the trend of F^*. As R>1 indicated total momentum flux exceeding head momentum flux, the share of momentum carried by thicker residual layers drove stronger impacts of subsequent waves, especially pronounced under higher solid contents and thicker residual layers. [Conclusion] (1) In periodic debris flows, the formation and stabilization of residual layers constitute the primary processes leading to enhanced destructiveness of subsequent waves. Even if release conditions are identical for each wave, significant increases in impact loads can occur due to residual layer influences. (2) The indicator system F^* (for comparing external manifestations across waves) and R (characterizing internal momentum distribution within a single debris flow wave) provides a concise assessment framework with clear physical meanings: residual layers amplify impacts by contributing “hidden momentum”, thereby influencing destructiveness amplification. (3) Engineering practices focusing solely on the first wave’s impact for protective structure verification may underestimate the destructiveness amplification effects of multi-wave events. It is recommended to consider the influence of residual layers in designing check dam scales, dam heights, and safety factors.

[Objective] This study aims to propose a new grouting technology to effectively solve the anti-seepage problem of fully-strongly weathered granite strata by optimizing grouting technology and material mix proportion, and to investigate the anti-seepage effects of frequency-pressure grouting technology in such strata. [Methods] An anti-seepage project of fully-strongly weathered granite strata in the upper reservoir of a pumped-storage power station under construction was taken as the research object. An equilateral triangle hole layout was adopted, and five groups of grouting tests (A, B, C, D, E) were conducted sequentially in Ⅰ, Ⅱ, and Ⅲ holes. The influences of grouting technologies (constant-pressure grouting and frequency-pressure grouting), grouting hole spacing (60, 120, 180 cm), and grouting materials (pure cement slurry and cement-bentonite mixed slurry with five water-to-cement ratios of 5∶1, 3∶1, 2∶1, 1∶1, and 0.5∶1) on the anti-seepage effects of grouting in such strata were compared and analyzed. The grouting effects were further evaluated using permeability tests, single-hole ultrasonic tests, single-hole shear wave velocity tests, and borehole color television tests. [Results] Field grouting trials demonstrated that frequency-pressure grouting technology significantly optimized the grouting effect of fully-strongly weathered granite strata through dynamic pressure adaptation and precise flow control, and it was superior to traditional constant-pressure grouting in terms of permeability coefficient control, material cost efficiency, and fracture filling integrity. Cement-bentonite mixed slurry had better controllability than pure cement slurry, especially in addressing leakage during grouting, and it also greatly reduced the large grout consumption caused by grout leakage and the extended construction period caused by multiple waiting times for grout to set. For fully-strongly weathered granite strata with low strength and loose soil, water pressure tests could not be completed, and only water injection tests were conducted to assess permeability before and after grouting. Single-hole shear wave velocity tests could reflect the density of strata before and after grouting to some extent. Grouting spacing in the range of 60-180 cm had no significant effect on the grouting outcomes, whereas the use of frequency-pressure grouting technology effectively improved the compactness of strata and notably increased shear wave velocity. After frequency-pressure grouting treatment, the permeability coefficient of fully-strongly weathered granite strata decreased from the order of magnitude of 10^-3 to 10^-5, indicating a great improvement in anti-seepage capacity. This demonstrated the feasibility of applying frequency-pressure grouting technology for anti-seepage treatment of fully-strongly weathered granite strata, and it could partially or completely replace the traditional cut-off wall scheme, thereby simplifying anti-seepage treatment, reducing construction cost, and minimizing strata excavation. [Conclusion] Compared with traditional grouting methods, frequency-pressure grouting can smoothly adjust grouting pressure and inflow rate according to the characteristics of the grouted strata and real-time grouting feedback, which can avoid excessive fracturing of low-strength strata, uncontrolled grout diffusion, and excessive grout consumption. This technology can be applied to anti-seepage treatment of fully-strongly weathered granite strata. Combined with cement-bentonite mixed slurry, this technology can effectively solve problems such as grout leakage and excessive grout consumption encountered with traditional grouting methods.

[Objective] This study focuses on water content as the key controlling factor to clarify the time-dependent patterns of thixotropic strength recovery of Zhanjiang Formation structural clay under different initial water contents. The microscopic mechanism is interpreted through three pathways: pore structure evolution, particle reorganization, and water action. The findings are expected to provide experimental evidence and theoretical support for predicting strength recovery and evaluating the stability of thixotropic clay foundations. [Methods] Remolded Zhanjiang Formation structural clay specimens were prepared and subjected to a 150-day thixotropy test. Specimens at different thixotropic durations were investigated using macroscopic and microscopic tests. For macromechanical testing, unconfined compressive strength (UCS) tests were conducted on cylindrical specimens. Direct shear tests were conducted on ring-knife specimens to obtain UCS, cohesion (c), and internal friction angle (φ), which were used to evaluate thixotropic evolution. A thixotropic strength ratio was defined as A_t = m_t/m₀, and two indicators—A_t(q) (based on UCS) and A_t(τ) (based on cohesion)—were used to compare recovery characteristics among different strength parameters. For microstructure, fabric evolution was observed using an SEM. Pore parameters, including porosity (M) and abundance (C), were extracted to quantitatively analyze pore structure evolution. Particle parameters, namely probability entropy (H) and distribution fractal dimension (D), were used to quantitatively characterize particle orientation/orderliness and aggregation degree, respectively. [Results] (1) Stage-dependent recovery: Both UCS and cohesion (c) increased with thixotropic duration and showed two stages: a rapid and significant recovery phase during 0-30 d, followed by a slower, stable phase during 30-150 d. The increment during 100-150 d was small, indicating near-stabilization, after which the test was terminated. (2) Dual effect of water content: At the same thixotropic duration, UCS generally decreased with increasing water content, reflecting weakened particle contacts and bonding and thus reduced instantaneous strength. However, higher water content resulted in a faster strength recovery rate, especially at early stage, indicating that water promoted the kinetics of self-adaptive structural adjustment during thixotropic process. (3) Indicator-dependent differences: Cohesion exhibited a higher thixotropic strength ratio and faster recovery within 1 d, suggesting that shearing promoted directional particle alignment and optimized the friction-bonding interface, making c more sensitive to structural rebuilding than UCS. (4) Coordinated micro-parameter evolution: As thixotropic duration increased, M and c decreased continuously. Pores shifted from “large and numerous inter-aggregate pores” to “small and fewer intra-aggregate pores”, while the overall pore shapes remained mainly quasi-equant but became denser. Additionally, H and D decreased synchronously, indicating enhanced particle orientation/orderliness and increased aggregation. These changes were most significant within the first 30 d, consistent with the rapid macroscopic recovery stage. SEM observations revealed a transition from an “open flocculated-dispersed” fabric to a “closed flocculated-aggregated” fabric. Pores between and within aggregates decreased, while particle contacts and continuity of force-transfer paths improved, thereby supporting strength recovery. [Conclusion] The thixotropic strength recovery of Zhanjiang Formation structural clay exhibits distinct time-stage characteristics and strong sensitivity to water content. Recovery generally progresses through a rapid phase (0-30 d) and a stable phase (30-150 d). Higher water content reduces the strength level but significantly accelerates the strength recovery rate. Cohesion exhibits a higher thixotropic strength ratio than UCS because shear-induced particle orientation facilitates more effective structural reconstruction. Microscopically, synchronous decreases in M/C and H/D indicate pore reduction, particle ordering, and aggregation densification. Water enhances particle activity by altering relative particle positions and expanding migration pathways, thereby accelerating self-adaptive adjustment and strength recovery during thixotropic process. Innovations included: (1) parallel comparison of UCS and rapid direct shear parameters within a single thixotropic framework, revealing the cohesion recovery advantage caused by shear-induced particle orientation; (2) linking the macroscopic two-stage recovery pattern with the coordinated evolution of M, c, H, and D, forming an evidence chain of “structural rearrangement—aggregation densification—strength recovery”; and (3) demonstrating that higher water content, while reducing instantaneous strength, accelerates recovery by enhancing particle mobility/activity.

[Objective] The accurate prediction of pipe roof deformation is critical for ensuring construction safety in ultra-shallow buried tunnels. Existing analytical methods frequently oversimplify the complex interaction mechanisms between the pipe roof and surrounding soil, particularly neglecting the load transfer mechanism, stress release effects during excavation, disturbance-induced soil weakening, and the time-dependent behavior of initial support systems. This study aims to develop a comprehensive theoretical framework that integrates these multiple effects into a unified model. The primary objectives include: establishing a vertical load equation that incorporates both the soil arching effect at the tunnel crown and the circumferential micro-arching effect between pipes; utilizing the Pasternak elastic foundation beam theory to simulate soil-structure interaction more accurately; introducing variable subgrade coefficients and a load release coefficient to represent stress redistribution and excavation disturbances; and ultimately formulating a reliable method for predicting pipe roof deformation under realistic construction conditions. The proposed model seeks to provide a practical and theoretically sound tool for design optimization and risk mitigation in pipe-roofed tunnel projects. [Methods] The research methodology combined theoretical derivation, numerical discretization, and empirical validation. First, the vertical load acting on the pipe roof was calculated by considering dual arching effects: the soil arching above the tunnel crown, modeled based on Terzaghi’s trap-door theory with inclined slip surfaces, and the micro-arching between adjacent pipes, with the load distribution derived assuming a parabolic arch axis between pipe contact points. The pipe roof was then modeled as an Euler-Bernoulli beam resting on a Pasternak elastic foundation, accounting for shear interaction between adjacent soil springs, offering a significant improvement over traditional Winkler-based models. To capture the construction-phase effects, the longitudinal span of the pipe roof was divided into five distinct zones: a fully enclosed support zone, an unenclosed support zone, an unsupported zone, a plastically disturbed zone, and an elastically disturbed zone, each with specific definitions of subgrade modulus and stress release rate. The governing differential equation was discretized using the finite difference method, with virtual nodes introduced to handle boundary conditions, and solved programmatically using MATLAB. The model was calibrated and validated against field monitoring data from a real-world ultra-shallow tunnel project, with additional comparative analysis against existing analytical models to demonstrate its superior performance. A detailed parametric study was conducted to evaluate the influence of the subgrade coefficient in front of the face, the excavation advance length, and the length of the unenclosed support segment. [Results] Validation against field data showed excellent agreement, with the predicted maximum deflection of 22.1 mm differing by only 5% from the measured value of 23.2 mm, confirming the model’s accuracy. The deformation curve generated by the proposed method was wider and smoother than those from existing theories, more accurately reflecting the continuous beam behavior of the pipe roof and aligning closely with monitoring results. Parametric analysis revealed that increasing the subgrade coefficient (k₀) of the soil in front of the excavation face from 10 MPa/m to 90 MPa/m significantly reduced the maximum deformation from 33 mm to 17 mm, although the marginal benefit diminished beyond 90 MPa/m. In contrast, the excavation advance length (s) had an exponential impact on deformation. Increasing s from 1.0 m to 3.0 m caused the maximum deflection to approach 160 mm, far exceeding the typical control limit of 20 mm and severely threatening face stability. The length of the unenclosed support segment (b) was found to have a negligible effect on deformation. Furthermore, the load transfer capacity of the pipe roof was observed to be highly sensitive to all three parameters under high overburden ratios (H/B). Excessive increases in k₀, s, or b under these conditions led to a significant transfer of load onto the soil in front of the face, increasing the risk of face instability. [Conclusion] This study successfully develops a multi-effect coupled analytical method for predicting pipe roof deformation in ultra-shallow buried tunnels. The integration of the soil arching effect, micro-arching effect, stress release, excavation disturbance, and support delay into a single model provides a more realistic and accurate representation of mechanical behavior than previously available methods. It emphasizes the effectiveness of improving the soil modulus in front of the tunnel face and strictly controlling the excavation advance length to manage deformation, while indicating that minimizing the unenclosed support length has limited benefits. However, the current study does not consider shear forces between differential elements, soil stiffness hardening, or small-strain behavior. Future research should incorporate these aspects to further enhance the model’s comprehensiveness and accuracy for a wider range of geotechnical conditions.

[Objective] To investigate the meso-scale fracture mechanisms and crack evolution of hydraulic concrete under tensile loading, and to address the inadequate representation of initial defects and the interfacial transition zone (ITZ) in conventional meso-models, a four-phase 3D meso-scale model consisting of aggregates, mortar, an ITZ, and initial defects is established. The effects of initial defect ratio, ITZ thickness, and aggregate content on tensile strength, failure modes, and energy dissipation are quantified. The innovation of this study lies in explicitly introducing the initial defects with controllable volume fractions within a 3D meso-scale framework and coupling them with ITZ and aggregate factors to enhance the reproducible representation of experimental responses and failure patterns. [Methods] Uniaxial tensile tests on cylindrical specimens of ordinary-strength hydraulic concrete (C40) were conducted to obtain stress-strain curves and tensile strength for model validation. Numerically, a 3D meso-scale geometry was established based on randomly generated aggregates. Aggregates were modeled using a linear-elastic constitutive law, while mortar and ITZ were modeled with a damage-plasticity constitutive law. Spherical initial defects were randomly embedded in the mortar phase to represent pores and micro-cracks. After validating the model against experimental strength, curve morphology, and failure modes, a parametric study was performed to investigate the effects of initial defect ratio (0-7%), ITZ thickness (including a control without ITZ), and aggregate content (10%-40%) on tensile fracture behavior. [Results] (1) The four-phase 3D model well reproduced the mechanical response and crack propagation pattern throughout the uniaxial tensile process. Simulated tensile strength was generally overestimated when initial defects were not considered. (2) Simulated and experimental tensile strengths and stress-strain curves agreed well with a initial defect ratio of 1%-2%. As the defect ratio increased, tensile strength decreased significantly. Cracks tended to concentrate in the mid-region of the specimen and propagate rapidly, exhibiting more pronounced strength weakening, earlier instability failure, and concurrent deterioration in energy dissipation capacity during fracture. (3) Within the small range examined, variations in ITZ thickness had a relatively limited influence on peak tensile strength. However, omitting the ITZ led to systematic deviations in strength and failure mode, making it difficult to correctly capture crack paths and local damage evolution. (4) An increase in aggregate content further reduced the tensile strength and significantly altered crack propagation paths and patterns. This reflected an important modulating role of changes in the proportion of aggregate/ITZ weakened regions on crack evolution. [Conclusion] The proposed four-phase 3D meso-scale modeling method incorporating initial defects provides balanced accuracy in reproducing both mechanical responses and failure patterns. The study demonstrates that for ordinary-strength hydraulic concrete, properly characterizing an initial defect ratio of 1%-2% is crucial for improving the accuracy of tensile fracture simulation. An increase in the initial defect ratio significantly reduces tensile strength and promotes concentrated crack coalescence in the specimen’s mid-region. The ITZ has a non-negligible controlling effect on crack paths and macroscopic responses, and ignoring it will lead to significant errors. Aggregate content further affects strength and crack patterns by altering the proportion of weakened interfaces. The findings can provide references for the determination of meso-scale parameters for hydraulic concrete, risk assessment of dam cracking, and enhancement of structural safety.

[Objective] This paper aims to investigate the influence of slag content on the resistance to water penetration and pore structure of recycled aggregate concrete(RAC), to reveal the relationship between resistance to water penetration and pore structure, and to provide a reference for its durability research and engineering application. [Methods] The slag content was designed to replace cement at proportions of 0%, 10%, 15%, 20%, 25%, and 30%, respectively. Six groups of C30 RAC with slag were prepared according to these proportions. Tests on water absorption and water penetration height with different slag contents were conducted, as well as microscopic pore structure tests using low-field nuclear magnetic resonance. Based on the results of the water penetration tests and microscopic tests on RAC, the relative permeability coefficient was taken as the reference sequence, and the proportions of different pore types as the comparison sequence, Then,the grey correlation analysis method was used to analyze the relationship between the resistance to water penetration and the pore structure of RAC. [Results] The results showed that with increasing slag content, the water penetration height and relative permeability coefficient of RAC both decreased. Compared with the test group without slag, the water penetration height and relative permeability coefficient of RAC test group with 30% slag content decreased by 26.01% and 61.12%, respectively, indicating that the resistance to water penetration of RAC was significantly enhanced. Based on the experimental data, a relationship model between the relative permeability coefficient of RAC and the slag content was constructed, and the fitting degree of the model reached 98.97%. Furthermore, the results of microscopic pore structure tests using low-field nuclear magnetic resonance showed that the porosity of RAC also decreased with increasing slag content. This indicated that slag could refine and reduce the internal pores of RAC, increase its compactness, and thereby enhance its resistance to water penetration. Grey relational analysis showed that incorporating slag led to significant changes in the pore structure of RAC. Specifically, slag reduced the proportion of intermediate pores and large pores in RAC, leading to a decrease in overall porosity and a more compact microstructure, which in turn enhanced its resistance to water penetration. [Conclusion] Incorporating an appropriate amount of slag can improve the resistance to water penetration of RAC, providing a reference for its engineering application.

[Objective] Natural volcanic ash has environmental advantages in reducing cement consumption and lowering carbon emissions. However, due to its low reactivity, the use of volcanic ash alone often leads to reduced early-age performance of cement-based materials. This study aims to develop a volcanic ash-silica fume composite cementitious system based on low-reactivity volcanic ash from the Sichuan-Xizang region, and to systematically investigate its hydration behavior, microstructural evolution, and mechanical properties, thereby providing theoretical and technical support for the engineering application of volcanic ash. [Methods] Multiple characterization methods were used to systematically analyze the hydration characteristics and microstructure of the composite cementitious system. First, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to qualitatively and quantitatively analyze hydration products at different curing ages to investigate the formation of C-S-H gel and its microstructural characteristics in the volcanic ash-silica fume composite system. Second, thermogravimetric and derivative thermogravimetric analysis (TG-DTG) was conducted to examine the thermal decomposition behavior of the materials, thereby revealing the composite effect of volcanic ash and silica fume from a thermodynamic perspective. To further evaluate the influence of pore structure on strength, mercury intrusion porosimetry (MIP) was used to analyze the pore size distribution of samples at different curing ages. Meanwhile, mechanical performance tests, such as compressive strength tests, were conducted to evaluate the mechanical properties of systems with different mix proportions. [Results] The single volcanic ash-blended system exhibited relatively sluggish early hydration, resulting in a lower strength activity index and mechanical strength. In contrast, the incorporation of silica fume significantly promoted the formation of C-S-H gel, refined the pore size distribution, enhanced the densification of cement-based materials, and consequently improved both the early- and late-age strength of the composite system. At 28 days, the composite system containing 27% volcanic ash and 3% silica fume showed a significant advantage, with its compressive strength approximately 28% higher than that of the single volcanic ash-blended system, and with a markedly increased strength activity index. Microstructural analysis further indicated that the C-S-H gel in the composite system was dense and uniformly distributed, and the pore structure was effectively optimized. [Conclusion] The combined use of low-reactivity volcanic ash and silica fume significantly enhances hydration activity and mechanical performance, particularly strength. The novelty of this study lies in optimizing the combined dosage of volcanic ash and silica fume, which not only improves mechanical performance but also optimizes the microstructure, resulting in a denser structural system. Therefore, the volcanic ash-silica fume composite cementitious system shows strong engineering applicability and provides an important reference for the development of low-carbon and environmentally friendly cement replacement materials.

[Objective] Grouting has been recognized as an effective solution for water inrush treatment in underground engineering. Organic grouting materials such as polyurethane (PU) are strongly adhesive but expensive,whereas inorganic grouting materials such as waterglass (WG) are cheap but have low elasticity. To improve the performance of polyurethane grouting materials,we investigated the influence of waterglass content on the macroscopic and microscopic properties of polyurethane. [Methods] Polyphenyl polymethylene polyisocyanate (PAPI),waterglass,and polyether polyol were used as the basic raw materials,and the inorganic/organic hybrid method was adopted to prepare polyurethane foam grouting material modified by waterglass. The influence of waterglass content by weight (0,20%,40%,60%,80%) on the foaming rate,density,and compressive strength of the modified polyurethane was investigated. A fitting relationship between polyurethane density and elastic modulus was established. Scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) were used to measure the influence of waterglass content on the cross-sectional morphology and thermal stability of materials. [Results] (1) Macroscopic property results showed that,with increasing waterglass content,both the foaming rate and compressive strength first increased and then decreased. When the waterglass content was 20%, the foaming rate and compressive strength reached maximum. Under this condition, the foaming rate was 2 684%, and the compressive strength at 7 d was 38.9 MPa. In addition, the heat released by the reaction increased the instability of the cell structure and even caused cell collapse, resulting in decreases in foaming rate and compressive strength. Notably, when the density changed from 1.102 g/cm³ to 0.959 g/cm³, the compressive strength of materials decreased by approximately 70%. Based on the experimental results, the fitting relationship between the density and elastic modulus of the modified polyurethane was obtained as E∝ρ^1.8.(2) The results of scanning electron microscopy showed that, with increasing waterglass content, the smooth polyurethane surface was gradually covered by an amorphous inorganic phase surrounding the spherical cells. The structure of the materials eventually became loose and porous.(3) TGA results showed that the thermal stability of the modified polyurethane was better than that of pure polyurethane. The TG curve showed that when T<80 ℃, the thermal mass loss of the materials was only about 5%, indicating that the initial decomposition temperature of the materials was about 80 ℃. When T >600 ℃, the curve became stable, meaning that the modified polyurethane with waterglass content of 0, 20%, 40%, 60%, and 80% lost approximately 84%, 77%, 72%, 65%, and 53% of their mass, respectively. The curves then tended to stabilize, indicating that the incorporation of waterglass could enhance the thermal stability of polyurethane.(4) DTG (Differential Thermogravimetry) curve showed that the mass loss rate of polyurethane was significantly higher than that of the modified polyurethane. This was attributed to the fact that the inorganic components precipitated in the reaction system were encapsulated on the surface of polyurethane matrix. The presence of Si-O bonds increased the intermolecular forces, resulting in the need for more energy to cause thermal decomposition of the materials. Therefore, the addition of waterglass effectively reduced the thermal decomposition rate of polyurethane.[Conclusion] Polyurethane/waterglass (PU/WG) is an organic-inorganic hybrid material that has the advantages of both polyurethane and waterglass. Analysis of the relationship between waterglass content and the macroscopic and microscopic properties of the modified polyurethane shows that the addition of waterglass helps increase the foaming rate, compressive strength, and thermal stability of polyurethane. These findings offer data support for the performance improvement and system optimization of the modified oil-soluble polyurethane grouting materials.

[Objective] Severe reservoir sedimentation reduces storage capacity and increases global desilting costs. Traditional disposal of dredged sediments (SD), such as landfilling, occupy land resources and pose ecological risks. This study aims to prepare high-strength geopolymers for high-value utilization of solid wastes. [Methods] Sediment from the Zhongxian section of the Three Gorges Reservoir (D₅₀≈67 μm) as the primary raw material. A ternary system is constructed by incorporating ground granulated blast furnace slag (GGBFS) and Class C Grade II fly ash (FA) to overcome alkali activation constraints, including high SiO₂/Al₂O₃ molar ratio of the sediments and the low reactivity of clay minerals. Specimens were cured under standard conditions for unconfined compressive strength (UCS) measurement. XRD, SEM-EDS, and FTIR characterized the mineral composition, microstructure, and surface functional groups of the ternary geopolymers. Additionally, the leaching concentrations of heavy metals (Cr, As, Cd, Co) from the ternary geopolymers were analyzed using the TCLP method with ICP-MS. [Results] In the SD-GGBFS-FA ternary system, the 28-day UCS ranged 51.9-82.9 MPa. The 1-day UCS increased with GGBFS content, indicated diminishing marginal efficiency of GGBFS reinforcement. For GGBFS and SD fixed at 80% and FA at 20%, increasing GGBFS from 0% to 80% produced 1-day UCS increments of 45.8,26.9,22.9,6.4 MPa, respectively, indicating higher alkali activation efficiency when the GGBFS content was below 40%. XRD patterns revealed a typical amorphous characteristic peak in this specimen in the 28°-30° range. SEM of the B2F8S0 specimen revealed the formation of a continuous and dense C-A-S-H gel, indicating that the co-alkali-activated product of GGBFS and SD was C-A-S-H gel, providing primary mechanical support for the early strength development of the material. FA, rich in components such as hematite and mullite, were hardly susceptible to alkali erosion at early ages. Increasing FA content improved stability of strength at later stages (28 d). At FA content of 40% or more, the UCS from 7 d to 28 d remained stable or even increased slightly, contrasting sharply with the significant strength attenuation of the GGBFS-SD system. XRD patterns showed that for B2S0F8 specimen (20% GGBFS + 80% FA) cured for 28 d, the crystalline peak of limestone (CaO) in FA disappeared, and characteristic diffraction peaks of zeolite-type C-A-S-H minerals emerged. FTIR revealed that, after 28 d of curing, the intensities of Si-O-Si stretching and Si-O bending vibration peaks in the B2S0F8 specimen remained stable, indicating greater geopolymer stability in this system than that in the B2S8F0 system. SEM confirmed that the tacharanite generated in the B2S0F8 system filled the pores, improving the compactness of the matrix and maintaining the long-term strength development. TCLP leaching tests showed that the leaching concentrations of Cd and Co from geopolymers were significantly lower than those from raw materials, indicating that Cd and Co could be stabilized by the geopolymer system. However, Cr and As mainly existed as anionic species, and the leaching concentrations of Cr and As from some samples increased after geopolymerization. [Conclusion] High-calcium GGBFS promotes the dissolution of low-activity aluminosilicate components in reservoir sediment clay minerals and forms dense C-A-S-H structures that enhance structural stability. In high-calcium environments, FA undergoes alkali-activated secondary reactions to generate tacharanite, which fills geopolymer gel pores and maintains long-term strength stability of the GGBFS-FA system. B2S6F2 achieves 72.4 MPa at 28 d, and its heavy metal leaching meets Class Ⅰ criteria. The ternary GGBFS-SD-FA system enables the preparation of environmentally safe, high-strength geopolymers, providing a reference for high-value sediment utilization.

[Objective] This study aims to evaluate the long-term effects of the impoundment and operation of the Three Gorges Reservoir on the shoreline ecological environment of the middle and lower reaches of the Yangtze River, thereby providing scientific evidence and technical support for subsequent policy formulation and the implementation of the Yangtze River protection strategy. [Methods] The shoreline of the Zhicheng-Hukou section in the middle reaches of the Yangtze River was selected as the study area. Satellite images for four representative years (2004, 2010, 2016, and 2023) at the same water levels combined with UAV real-scene imagery from specific sampling sites during 2021-2023 and water level data from hydrological monitoring stations were utilized to systematically analyze the spatiotemporal dynamic evolution characteristics of the shoreline space after the impoundment of the Three Gorges Reservoir. Remote sensing images were used to calculate large-scale changes in shoreline water surface area for each section in the four representative years, and UAV data were employed to analyze shoreline slope and inundation at selected sampling sites. [Results] (1) After 20 years of impoundment and operation of the Three Gorges Reservoir, the water surface area and shoreline area in the Zhicheng-Hukou section of the middle reaches of the Yangtze River remained generally stable, with the standard deviation of the large-scale water surface area proportion controlled within 2.4%. Additionally, sedimentation occurred in local areas, particularly at river bends, requiring enhanced monitoring of these areas in the future. (2) Analysis of shoreline inundation and slope changes showed that from 2021 to 2023, slope variations along parts of the middle reaches of the Yangtze River shoreline were not significant. At the Zhijiang and Shishou sampling sites, the standard deviation of shoreline slope distribution remained within 2.0%, indicating overall stability. [Conclusion] The impoundment and operation of the Three Gorges Reservoir have a limited impact on the shoreline spatial patterns of the middle reaches of the Yangtze River, and overall changes in shoreline space in the middle and lower reaches of the Yangtze River remain minor. Overall water surface area and shoreline slope remain stable. However, shoreline area has changed in certain local sections, indicating that monitoring and management need to be continuously strengthened in the future. The innovations of this study lies in 1) integration of multi-source monitoring data to achieve high-precision monitoring of spatial dynamic changes in the shoreline of the middle reaches of the Yangtze River; 2) exploration of the influence mechanisms of the Three Gorges Reservoir impoundment on shoreline spatial patterns from two dimensions: large-scale water surface area changes and local shoreline morphological evolution.