[Objective] Global warming has exacerbated extreme climate events and their compound hazards, causing increasingly severe impacts on both ecological environments and socioeconomic systems. [Methods] Based on daily meteorological observations from the Yangtze River Basin from 1961 to 2022, the percentile threshold method was used to identify extreme climate events, and four major types of compound extreme climate events were constructed from both temporal and spatial dimensions. The spatiotemporal distribution and evolution characteristics of compound events in the Yangtze River Basin were systematically analyzed. [Results] (1) The spatial distribution of the frequency and annual average duration of compound heatwave-drought events in the Yangtze River Basin generally exhibited a pattern of “low in the upstream source area and high in most parts of the middle-lower reaches”. The annual average frequency of regional heatwave-drought events was 1.8-1.9 times, the average duration per single event was 9.6-9.9 days, and the longest duration was 53-59 days. Both the frequency and duration of these regional heatwave-drought events showed an increasing trend over the past 62 years, with a more significant increase in the upstream. (2) The spatial distribution of the frequency of drought-flood abrupt alternation events in the Yangtze River Basin was characterized by “low in the upper reaches and high in the middle-lower reaches”. The frequency was approximately three events per decade in the upper reaches and six events per decade in the middle-lower reaches. Over the past 62 years, the occurrence frequency has exhibited a slight increasing trend, with the growth rate in the middle-lower reaches being twice that of the upper reaches. (3) The spatial distribution of the frequency of compound flood-heatwave events in the Yangtze River Basin exhibited a pattern of “low in the upstream source area and high in most parts of the middle-lower reaches”. The frequency was approximately five events per decade in the upper reaches and seven events per decade in the middle-lower reaches. Over the past 62 years, it has shown a significant increasing trend. (4) Compound flood events involving both the upper and middle-lower reaches of the Yangtze River occurred every year, with an annual average frequency and duration of 3.7 times and 25 days, respectively. These events exhibited a fluctuating increasing trend over the past 62 years. [Conclusion] Overall, in the context of climate change, compound extreme climate events in the Yangtze River Basin show a significant increasing trend. The intensification of temperature-related extreme events is more pronounced in the upper reaches, while the intensification of precipitation-related extreme events is more prominent in the middle-lower reaches. Since the beginning of the 21st century, the extremeness of various compound events has increased significantly.
[Objective] Continuous sharp bends in river channels are prone to significant river regime adjustments and abrupt changes under the impact of unsaturated sediment-laden flow, which have far-reaching implications for flood control, navigation, and water resource utilization. This study investigates the hydraulic characteristics of the river section with sharp bends in the lower Jingjiang River and the scour and siltation characteristics of the upstream and downstream bends after the natural cutoff through large-scale physical model experiments, aiming to deepen the understanding of the natural cutoff development process and provide references for the long-term regulation and planning of the river-lake confluence section in the middle reaches of the Yangtze River. [Methods] Taking the reach from Xiongjiazhou to Chenglingji in the middle reaches of the Yangtze River as the research object, a physical model was established with a horizontal scale of 1∶400 and a vertical scale of 1∶100. The model had a total straight-line length of about 70 m, a maximum width of about 40 m, and included two continuous sharp bends and upstream and downstream transition sections. Based on the hydrological data measured at Luoshan Station from 2003 to 2020, the model test water and sediment conditions were set up with different flow conditions of flood, medium, and drought. First, the hydraulic characteristics of the bend section under different flow levels were studied through fixed-bed model tests to identify the most likely flow conditions and locations for natural cutoff. Subsequently, movable-bed scour tests were conducted, applying flow conditions favorable for cutoff to study the cutoff development process. Considering that the flow in the Jingjiang section would be in a severely undersaturated state for a long time after the Three Gorges Reservoir is impounded, the inlet water and sediment conditions in this model test were simplified to clear water. [Results] The model test results showed that after the flow overtopped the bank, the main flow belt in the upstream Qigongling bend section gradually shifted from the main channel to the convex bank side. Three velocity concentration zones were formed at the neck, middle, and leading edge of the flow, with the peak velocity decreasing stepwise from the neck to the main channel. During the natural cutoff process of the Qigongling bend, the most likely location for the breach was between 1 300 m and 1 500 m away from the rear embankment. After 3 days of scouring by the overbank flow, gullies began to form, which developed into a fully connected breach over a period of about 30 days. After cutoff, the bend apex section tends to become narrower and deeper, while the transition section tends to become wider and shallower. [Conclusion] The results provide forward-looking guidance for the governance of the middle reaches of the Yangtze River system.
[Objective] This study aims to identify and quantify the long-term evolution characteristics of overbank flood processes in the lower Jingjiang River in the middle reaches of the Yangtze River (from 1966 to 2023) using high-temporal-resolution data, and to explore their relationships with climate change and upstream reservoir groups, thereby providing a scientific basis for riverbed evolution, river ecological assessment, and ecological restoration and flood management in the lower Jingjiang River. [Methods] Based on the flood element data from the Jianli hydrological station, overbank flood processes were identified using the bankfull discharge as the threshold. A hydrological indicator system was constructed from three dimensions: 1) temporal characteristics, including earliest start date, date of maximum flood, latest end date, and their decadal median values; 2) frequency and duration, including annual occurrence frequency, total annual duration, total annual rising-water duration, total annual falling-water duration, average duration, and maximum duration; and 3) intensity, including average discharge, maximum discharge, average flood volume, maximum flood volume, and total annual flood volume. For each indicator, the decadal median value and frequency distribution were calculated, and the interannual variation trends and phased differences were investigated. [Results] 1) In terms of occurrence timing, the earliest start dates mainly concentrated from late June to early July, showing phased variations. The decadal median value of latest end dates was gradually delayed from September 5 to October 5 during the 1960s-1980s. The largest overbank flood events mostly occurred from early to mid-July, showing an overall delayed trend. 2) In terms of frequency and duration, the annual occurrence frequency was mostly (69%) 2 to 5 times. Fluctuations decreased after the 1970s, dropping to approximately 2 times per year in the early 2020s. The median total annual duration increased from 23 days in the 1960s to 63 days in the 1980s, then decreased rapidly after the 1990s, and dropped to 29 days in the early 2020s. The total annual rising-water duration exhibited a relatively small variation, while the total annual falling-water duration changed consistently with the total duration and was more affected by upstream flood regulation. The average duration and maximum duration increased significantly from the 1960s to the 1990s and then showed minor fluctuations after the 2000s. 3) Regarding discharge and flood volume, average discharge and maximum discharge increased significantly from the 1960s to the 1980s (by approximately 8% and 21%, respectively), and then decreased significantly from the 1990s to the 2020s (by 12% and 36%, respectively). The average flood volume gradually increased from the 1960s to the 1990s (by 1.5 times), significantly decreased in the 2000s (by 33%), and then rebounded rapidly after the 2010s. However, the maximum flood volume did not rebound simultaneously in the 2010s, indicating the “peak-shaving” effect of reservoirs on large floods. The changes from the 1960s to the 1980s were highly consistent with the increase in precipitation. After the 1990s, the flood regulation effect of the upstream reservoir groups dominated the changes in most indicators, exerting significant impacts particularly on the falling-water duration, total annual duration, maximum discharge, and total annual flood volume. However, average duration, maximum duration of overbank flood events, and average flood volume still maintained a good response to precipitation changes and could serve as hydrological indicators of precipitation changes. [Conclusion] This study reveals that the overbank flood processes in the lower Jingjiang River from 1966 to 2023 underwent distinct phased evolution. Precipitation was the dominant driver before the 1980s, while reservoir regulation became the main influencing factor after the 1990s. The combined effects of these two drivers lead to differential responses among different indicators. Average duration, maximum duration, and average flood volume can serve as sensitive indicators for monitoring long-term precipitation changes and assessing climatic impacts. In contrast, total duration, falling-water duration, maximum discharge, and total annual flood volume are highly sensitive to upstream project operations and should be incorporated into the evaluation system for regional water resources and ecological management. Future research needs to further couple high-resolution meteorological precipitation data, river channel morphological evolution, and ecological response data to provide decision-making support for the design of flood regulation schemes with ecological priorities.
[Objectives] In recent years, intense channel evolution in the lower reaches of Minjiang River has negatively impacted channel stability, flood control, water supply, and aquatic ecosystems, thereby constraining socio-economic development in riverside cities. The mainstream of the lower Minjiang River (from Shuikou Dam to Huai’an Diversion Outlet) has been a key sand mining area, with significant riverbed incision. However, previous studies have paid insufficient attention to this phenomenon. This study aims to thoroughly investigate the evolutionary processes and characteristics of riverbed incision in this reach and quantify the impact of sand mining on the riverbed incision. [Methods] Using measured topographic data from seven different years (1999-2020), the evolutionary characteristics of riverbed incision were analyzed in terms of planform changes, cross-sectional profiles, and scouring and deposition variations. Based on river sand mining data, the contribution ratio of sand mining to the riverbed incision was calculated. [Results] (1) the mainstream of the lower Minjiang River experienced intense overall scouring from 1999 to 2020, with scouring erosion occurring across 84.18% of the channel area, resulting in a total scouring volume of approximately 255 million m3. The average incision depth was 5.09 m, while the thalweg exhibited an average incision of 7.87 m, with a maximum incision depth approaching 20 m. (2) The riverbed underwent rapid scouring before 2011, whereas the scouring rate significantly decelerated thereafter. The total scouring volume during 1999-2011 and the average thalweg incision accounted for 82.51% and 76.11% of the respective totals for 1999-2020. (3) Except for a slight overall deposition in the riverbed during 2014-2017, net scouring occurred in all other periods, with the highest mean annual incision rate observed between 2008 and 2011. (4) The most pronounced incision occurred in the 15-30 km reach downstream of Shuikou Dam, where the average incision depth reached 6.92 m during 1999-2020, and significant incision persisted there after 2011. (5) Sand mining was identified as the primary driver of riverbed incision, estimated to contribute over 50% to the riverbed incision. [Conclusions] The findings reveal the processes, characteristics, and trends of riverbed incision in the mainstream of the lower Minjiang River in recent years and identify the most severely incised reaches. For the first time, a quantitative assessment of the impact of sand mining on the riverbed incision is provided, and it is suggested that sand mining has been the dominant factor driving riverbed incision in the mainstream of the lower reaches of Minjiang River in recent years. These results provide fundamental support for channel evolution, river management and sand mining planning, and also offer valuable references for flood control, river channel protection, and river-related project development.
[Objective] This study aims to develop a daily-scale precipitation fusion product (2001-2023) with higher spatiotemporal accuracy covering the Yangtze River Basin by utilizing multi-source data and machine learning techniques, to address the poor quality of existing single or fusion products and to provide reliable data support for related research and applications in this region. [Methods] Multiple types of fundamental geographic data and in-situ measured precipitation data were collected and processed. Based on the aforementioned data, eight machine learning models—RF, CatBoost, KNN, Lasso, DTREE, XGBoost, HGBR, and ETREE—were selected for preliminary training, and their comprehensive capabilities were quantitatively evaluated. Subsequently, nine different ensemble model combinations were constructed based on the single models, and through quantitative evaluation, the seasonal ensemble model ELM4-S with the best overall performance was identified to generate the final daily precipitation fusion product for the Yangtze River Basin at a 0.1° resolution. [Results] (1) Based on multiple evaluation metrics, among the four original precipitation products (ERA5, ERA5-Land, GPM, and CMORPH) in the Yangtze River Basin, GPM exhibited the best overall performance. In terms of the probability of detection (POD), the ERA5 series demonstrated particularly outstanding performance, reaching 0.96. (2) A comparison of the performance of the eight machine learning models (RF, CatBoost, KNN, Lasso, DTREE, XGBoost, HGBR, and ETREE) indicated that RF exhibited the best overall performance. After training, all machine learning models achieved satisfactory results and outperformed the original precipitation products in terms of correlation (R), root mean square error (RMSE), and mean relative bias (MRB). (3) Among the nine ensemble models constructed from combinations of different machine learning models, ELM4-S demonstrated the best overall performance. The fusion precipitation product obtained by ELM4-S was superior to the original precipitation products, incorporating the advantages of different original products. It was numerically reasonable and could reflect the detailed characteristics of precipitation variation with topography in its spatial distribution. [Conclusion] The precipitation fusion product generated based on the ELM4-S model is more accurate than the four original gridded precipitation products adopted. This product not only integrates the advantages of each original dataset but also finely captures the spatial distribution characteristics of precipitation variation with topography, exhibiting outstanding detail. This study successfully develops a high-precision daily precipitation fusion product for the Yangtze River Basin from 2001 to 2023 using an ensemble machine learning approach. This product effectively balances POD and false alarm rate (FAR). It outperforms the original data and single-model results in overall performance and captures more reasonable spatial details of precipitation. It can serve as a reliable data product to widely support various production applications and scientific research within the basin.
[Objective] To address the challenges faced by Horizontal Acoustic Doppler Current Profilers (H-ADCP) in online discharge monitoring applications—specifically, the difficulty in selecting index velocity (feature cells), the insufficient non-linear expressiveness of traditional calibration models, and the poor generalization ability and high computational complexity of existing machine learning models under complex hydrodynamic conditions such as tides and engineering regulations—this paper aims to develop a new H-ADCP online discharge monitoring model that can automatically optimize velocity features, integrate the advantages of multiple algorithms, and improve model accuracy. This model is designed to address the complex non-linear mapping problem between high-dimensional velocity data and cross-sectional discharge, thereby enhancing the accuracy, stability, and automation of discharge monitoring. [Methods] A Feature Adaptive Optimization (FAO) model for H-ADCP online discharge monitoring was developed. The technical framework of this model comprised three core components: (1) feature dimensionality reduction: Principal Component Analysis (PCA) was applied to conduct initial dimensionality reduction on the high-dimensional velocity data from up to 128 cells generated by the H-ADCP, reducing subsequent computational complexity while preserving the main velocity distribution characteristics. (2) Multi-model parallel mapping: five machine learning models—Backpropagation (BP) Neural Network, Elman Neural Network, Radial Basis Function (RBF) Network, Generalized Regression Neural Network (GRNN), and Support Vector Machine (SVM)—were constructed in parallel to establish the non-linear mapping relationship between the dimension-reduced feature velocities and the measured cross-sectional discharge. (3) Global optimization and adaptive selection: the Particle Swarm Optimization (PSO) algorithm was utilized as a global optimization engine, with the Root Mean Square Error (RMSE) as the fitness function, to search within the feature subspace and model space through iterative optimization and adaptively determine the optimal combination of velocity cells, the best machine learning model, and its corresponding parameters. To validate the model’s performance, the Luohu Hydrological Station, which is affected by both tides and backwater effects from confluence and has a complex hydrological regime, was selected as the study area. The model was calibrated and verified using measured H-ADCP velocity data and comparative discharge data from a moving-boat ADCP for the years 2019 and 2023. [Results] (1) The FAO model demonstrated superior performance: during the 2019 model verification period, the discharge predictions of the FAO model showed a high degree of agreement with the measured values, with a RMSE of 6.06 m3/s and a Coefficient of Determination (R2) reaching 0.93. This was significantly better than the traditional linear regression model and any single machine learning model. In simulating extreme discharges such as flood peaks, the FAO model also demonstrated a greater ability to capture them, with an annual maximum discharge error of 1.56%. (2) The feature optimization was effective: the model successfully and automatically selected an optimal combination of 11 feature cells ({5,9,12,15,17,19,21,24,26,28,35}) from 40 velocity measurement cells, eliminating invalid data affected by riverbanks and blind zones. The distribution pattern of the selected cells was highly consistent with hydraulic characteristics, demonstrating the physical interpretability of the model’s feature selection. (3) The model showed strong stability: when validated with data from the entire year of 2023, the FAO model performed stably, with an RMSE of 6.02 m3/s and an R2 of 0.91, and effectively fitted the entire annual discharge process, especially for maximum and minimum values. [Conclusion] The proposed FAO model, by organically integrating PCA, multiple machine learning algorithms, and the PSO optimization algorithm, successfully addresses the key technical challenges in H-ADCP online discharge monitoring. The model exhibits powerful self-learning and self-adaptive capabilities, enabling it to automatically find the optimal velocity features and computational model based on data samples, while ensuring computational accuracy and significantly reducing data processing complexity. The application case under complex hydrological conditions demonstrates that the FAO model has high accuracy, good stability, and strong adaptability, providing an efficient and intelligent solution for H-ADCP online discharge monitoring.
[Objective] To investigate the variation patterns of pollutant fluxes in the Yangtze River mainstream before and after the operation of the Three Gorges Project, this study analyzes the annual variations in water quality, runoff, and pollutant fluxes at various control sections based on monitoring data from 2000 to 2023, and conducts a correlation analysis of the fluxes. [Methods] The interannual variation characteristics of pollutant concentration, runoff, and pollutant flux along the Yangtze River mainstream before and after the impoundment of the Three Gorges Project were analyzed. Meanwhile, the Spearman correlation analysis was used to evaluate the correlation between pollutant flux and runoff before and after the operation of the Three Gorges Project. [Results] From 2000 to 2023, the interannual variations in the concentrations of major pollutants including the permanganate index, ammonia nitrogen, and total phosphorus, along each section of the Yangtze River mainstream showed an overall downward trend. The year 2013 was an important turning point for the water quality change in the Yangtze River mainstream. During the “Twelfth Five-Year Plan” period, comprehensive and leapfrog progress was achieved in water pollution control across the Yangtze River Basin, leading to noticeable improvements in water quality. A higher consistency was observed between the variation trends of annual runoff and annual pollutant flux characteristic values at the sections upstream of the Three Gorges Dam, indicating a significant interception effect of the Three Gorges Project on pollutants in the Yangtze River mainstream. Among the main pollutant indicators in the Yangtze River mainstream, the permanganate index showed a highly significant correlation with runoff. The interception effect of the Three Gorges Dam amplified the influence of sediment on the total phosphorus concentration in the water body at the upstream sections, thereby reducing the impact of runoff and resulting in a non-significant correlation between total phosphorus and runoff at these upstream sections. The buffering effect of the Three Gorges Reservoir on the incoming flow from the upstream led to relatively small interannual variations in ammonia nitrogen concentration in the water body at the downstream sections, which further confirmed that the operation of the Three Gorges Project significantly impacted the variations in major pollutant fluxes in the Yangtze River mainstream. [Conclusion] The research findings can serve as a scientific basis for the management and protection of the water environment in the Yangtze River Basin.
[Objectives] Under the background of a new round of technological revolution and industrial transformation, boron is one of the important mineral sources required by global strategic emerging industries. China’s industrial development has a high demand for boron resources, but high-quality reserves are insufficient. As an important area of liquid boron reserves, the Qaidam Basin has great potential for boron resource development. The Nalenggele River watershed, located on the southern margin of this basin, is not only a typical area where hot springs, rivers, and salt lakes coexist and are hydrologically connected, but also a hotspot for studying the enrichment and mineralization of boron in the mountain-basin transition zone. [Methods] In this paper, the chemical characteristics of boron and the advantages and limitations of boron isotope tracing were systematically expounded. The boron enrichment features and hydrochemical consistency of regional geothermal water, river water, and salt lake brine water were also analyzed. Based on prior studies, the sources of high boron content in the rivers and salt lakes were discussed. [Results] 1) Boron isotopes were effective and sensitive tools for source identification,but their quantitative application in process assessment was inherently constrained by necessary preconditions due to its fractionation-prone property. 2) Geothermal waters,river waters,and salt lake waters in the Nalenggele River Basin were uniformly characterized by high boron concentrations,which was in contrast to other surface waters in the northern margin of the Eastern Kunlun Mountains. 3) Although multiple geochemical processes influenced the chemical composition of surface waters,geothermal water input constituted a dominant control on boron enrichment in the Nalenggele River Basin. [Conclusions] Based on current research on boron enrichment and mineralization in the study area,identifying the boron sources of riverine systems,quantifying weathering contributions during source-to-sink processes,and developing quantitative tracers are key research priorities.These results are expected to advance the understanding of surface boron cycling within the “hot springs-rivers-salt lakes” system in the Qinghai-Tibet Plateau.
[Objective] The Three-North Shelterbelt Program represents a pivotal ecological conservation initiative in China. However, over the past decade, varying degrees of degradation and functional decline have been observed in shelterbelt forests across the region. Further research is warranted to explore the process by which water affects tree degradation and the overall decline of the Three-North Shelterbelts. [Methods] To elucidate the role of water availability in driving tree degradation and to unravel the mechanisms underlying shelterbelt decline, the Populus simonii shelterbelts in northern Hebei Province were taken as the research subjects. Tree-ring samples of P. simonii with different degradation degrees from the same area were collected. Using tree basal area increment (BAI) and multi-year meteorological data, the degradation processes of P. simonii shelterbelts with different degradation degrees and their differential responses to drought events were analyzed. Based on tree-ring carbon isotope technology, the changes in intrinsic water use efficiency (iWUE) of P. simonii were examined. Additionally, combined with the ecosystem resilience index (ERI), the possible reasons for why some poplar trees in the same area grew normally while others degraded were explored. [Results] (1) The growth period of P. simonii in Zhangbei could be divided into three stages: rapid growth stage (1976-1995), slow growth stage (1996-2005), and declining growth stage (2006-2017). During the rapid growth stage, P. simonii grew stably with a slight increase, and the impact of the external environment on its growth was relatively small. In the slow growth stage, the growth of P. simonii tree-ring width showed a fluctuating trend, and growth differentiation gradually occurred. In the declining growth stage, the tree-ring width of P. simonii showed a steady increase at a gradually decreasing rate, with growth differentiation becoming obvious. (2) ERI quantified the resistance, recovery capacity, and resilience level of trees under external disturbances. Significant differences in the resistance index and resilience index were observed among P. simonii with different degradation degrees: the resistance index and resilience index of non-degraded P. simonii were higher than those of slightly degraded and severely degraded individuals, with these differences being particularly significant during the slow growth stage and declining growth stage of P. simonii. (3) The iWUE, an important physiological indicator for drought adaptation of trees, displayed significant variation across P. simonii with different degradation degrees, and all presented a significant upward trend. Severely and slightly degraded P. simonii demonstrated significantly higher iWUE values than non-degraded trees, while severely degraded P. simonii exhibited significantly higher iWUE values compared to slightly degraded ones. Among P. simonii with different degradation degrees, severely degraded trees experienced the most severe drought stress. [Conclusion] Insufficient and infrequent short-term precipitation, combined with climate warming and drying may be the main causes of shelterbelt degradation. The extreme drought in 1997 and prolonged dry spell from May 1999 to June 2001 likely initiated the decline of P. simonii shelterbelts in Zhangbei County, while persistent drought between September 2006 and August 2012 further exacerbated this deterioration. Differences in the inherent resistance of trees played a critical role in the observed partial degradation and partial non-degradation. Poplars are fast-growing and highly water-consuming species, which mainly cope with water stress by consuming water stored in heartwood. During the rapid growth process of P. simonii, the trees have larger vessels and higher water conductivity. When water stress occurs, some poplar trees with poor resistance suffer from hydraulic failure, which in turn leads to growth decline. In contrast, non-degraded plants may delay the depletion of stored water through the utilization of deep water sources, more flexible stomatal regulation, and more efficient resource allocation.
[Objective] Under the influence of climate change, the rainfall pattern in the Poyang Lake Basin has changed, potentially affecting local soil erosion and subsequently threatening water and ecological security in the basin. However, current research on soil erosion within the basin mostly focuses on historical periods, with relatively limited research on future soil erosion predictions. The lack of clarity regarding future soil erosion dynamics under climate scenarios constrains local soil and water conservation planning and erosion management. To address this gap, future soil erosion predictions are conducted for the Poyang Lake Basin to elucidate its spatiotemporal evolution characteristics under climate change impacts, thereby providing scientific references for local soil and water conservation planning and soil erosion management. [Methods] The SSPs-USLE coupled model at the Poyang Lake Basin scale was developed by integrating the Shared Socioeconomic Pathways (SSPs) and the Universal Soil Loss Equation (USLE) in this study. Tailored to the specific conditions of the Poyang Lake Basin, the SSPs-USLE coupled model was applied to simulate soil erosion both during the historical period (2000-2022) and under three future climate scenarios (SSPs2-4.5, SSPs4-6.0, and SSPs5-8.5) for the period 2030-2060. Through statistical and comparative analysis of the results, the spatiotemporal evolution characteristics of soil erosion under different future scenarios were elucidated. The underlying causes were further analyzed based on existing literature, thereby providing targeted measures and recommendations. [Results] Temporally, the total area affected by soil erosion in the Poyang Lake Basin was projected to decrease in the future, while the total soil erosion volume was expected to increase. Specifically, under the SSPs2-4.5, SSPs4-6.0, and SSPs5-8.5 scenarios, the soil erosion area decreased by 906 km2/a, 916 km2/a, and 912 km2/a, respectively, predominantly occurring in areas with slight intensity erosion. Meanwhile, the total soil loss increased by 2 022.60×104 , 2 098.61×104 and 3 154.35×104 t/a under the same scenarios, mainly affecting areas with moderate or lower erosion intensity. Spatially, the northeast of Poyang Lake Basin in the historical period exhibited severe soil erosion, with a higher proportion of areas classified as strongly eroded or above. In the future, the northwestern and southern regions would experience a significant expansion of strongly eroded or above areas, leading to a more severe overall soil erosion problem in the basin. Among the different socioeconomic scenarios, the SSPs5-8.5 scenario resulted in the most severe soil erosion within the basin, with a total soil erosion volume of 17 247.61 × 104 t/a. The overall erosion intensity shifted from moderate to strong, indicating substantial pressure for future soil erosion control. In contrast, under the SSPs2-4.5 scenario, the basin experienced the least soil erosion, with a total soil erosion volume of 16 115.86×104 t/a. The overall erosion intensity transitioned from moderate to mild, suggesting lower pressure for subsequent soil erosion control. [Conclusion] Future soil erosion in the Poyang Lake Basin is predicted by developing an SSPs-USLE coupled model at the watershed scale, thereby addressing a critical data gap in future soil erosion in this region. The model results show that with the impact of climate change, soil erosion within the Poyang Lake Basin will exhibit an intensifying trend in the future, particularly with increased erosion volumes at areas with slight to moderate erosion intensities. This phenomenon is predominantly attributed to terrain and rainfall, with significant exacerbation concentrated primarily in localized heavy rainfall centers and mountainous/hilly terrain. Besides, among the three future scenarios, the SSPs2-4.5 scenario has the lowest degree of soil erosion deterioration in the watershed, which is a more desirable outcome for the future. Based on this, the local soil erosion control work should be taken seriously, and attention should be paid to addressing slight-to-moderate soil erosion areas in the Poyang Lake Basin. Multiple measures should be implemented in coordination to alleviate the impact of climate change on soil erosion in the basin, and promote social development towards SSPs2-4.5 or even better scenarios.
[Objective] Water-level fluctuation zone (WLFZ) represents a key challenge for managing carbon emissions from global reservoirs. The Three Gorges Reservoir (TGR) has become a key focus for investigating the emission of greenhouse gases, such as carbon dioxide (CO2). Previous studies have not yielded consistent findings regarding the correlation between different elevations and soil carbon release — a gap that limits an accurate understanding of carbon cycling mechanisms in the reservoir’s WLFZ and hinders effective carbon emission management. [Methods] This study explored soil carbon emission characteristics in the near-dam WLFZ of TGR under fluctuating water levels. Soil respiration rates were measured using the Li-8100 Automated Soil CO2 Flux System. Two representative WLFZs—Longtanping and Lanlingxi—were selected, and within each area, three elevation intervals were established: below 160 m, 160-170 m, and above 170 m. This design ensured that the data would reflect the impact of water level fluctuations on soil carbon emissions across different WLFZ segments. One-way analysis of variance (ANOVA) in SPSS 25.0 was applied to examine differences in soil respiration across elevations and seasons. [Results] No positive correlation was found between elevation and soil respiration. Instead, as elevation increased, soil respiration across the entire study area exhibited a trend of first rising and then falling, with the maximum rate observed under moderate flooding stress. Specifically, the peak soil respiration rate reached 3.91 μmol/m2/s in Longtanping and 2.69 μmol/m2/s in Lanlingxi, with an average of 3.30 μmol/m2/s. This suggested that moderate flooding created optimal conditions for soil microbial activity and organic matter decomposition—two processes that drove carbon emission—whereas excessive or insufficient flooding inhibited these biological activities, reducing respiration rates. When the two WLFZs were analyzed comprehensively and Lanlingxi individually, no significant difference in soil respiration was found between the below-160 m and above-170 m intervals. However, in Longtanping, soil respiration above 170 m was slightly higher than that below 160 m. This regional discrepancy might be attributed to differences in local environmental factors, such as soil texture, organic matter content, vegetation coverage, or microbial community composition. Soil respiration exhibited significant temporal variability. Overall, the seasonal trend showed rates in July and August being highest, followed by September, June, and May. Minor differences existed between the two WLFZs: Longtanping showed a pattern where rates in July and August were highest, followed by September and June, and then May, while Lanlingxi displayed a pattern where rates in July, August, and September were equal and higher than June and May. Nevertheless, both areas recorded their peak soil respiration in August, with the highest rates occurring in the 160-170 m interval: 6.97 μmol/m2/s in Longtanping and 4.58 μmol/m2/s in Lanlingxi. The elevated summer respiration rates (especially in July and August) were primarily linked to vigorous vegetation growth and metabolic activity during this period. Vegetation contributed to carbon emission by releasing organic matter through root exudation and litterfall (providing substrates for microbes) and enhancing soil aeration via root respiration (facilitating microbial decomposition). [Conclusion] Moderate dry-wet alternation (i.e., moderate flooding stress) maximizes soil carbon emissions in the study area, while extreme flooding (either too high or too low) suppresses emission intensity. Summer, characterized by robust vegetation growth and metabolism, shows significantly higher soil respiration than other seasons—with July and August showing particularly high rates, and the moderately flooded zones in August recording the peak. The findings of this study have both theoretical and practical value. Theoretically, they enhance the understanding of carbon cycling in large reservoir WLFZ and contribute to global carbon cycle research. Practically, they provide a scientific basis for the quantitative analysis of carbon emissions in the Three Gorges Reservoir’s WLFZs and support future studies on carbon cycling following WLFZ ecological restoration. This information can further guide water level management strategies to regulate soil carbon emissions, aiding global carbon neutrality efforts and the sustainable development of the reservoir ecosystem.
[Objective] In hydraulic projects, flood-discharge structures operating under compound water inlet conditions often exhibit complex vertical-axis vortices at the inlet in which the approach flow direction differs from the mainstream river direction. Traditional vortex elimination measures perform poorly under such conditions, and severe air-entraining vortices may threaten structural safety and operational efficiency. Based on a pumped-storage power station, this study aims to clarify vortex mechanisms through physical modelling and to develop an efficient and economical vortex elimination measure that completely suppresses air-entraining vortices, thereby providing design guidance for similar projects. [Methods] A 1∶50 undistorted clear-water physical model was built under Froude similarity to ensure geometric, kinematic, and dynamic similitude. Several operating conditions (check, design, energy dissipation, and scour protection) were simulated to reproduce actual operation. Vortex characteristics were observed (air-entraining vortices up to 7.5 cm diameter) without any suppression measures. Five vortex elimination schemes were then tested: Scheme 1—conventional vertical vortex elimination beams with an optimized inlet; Schemes 2-4 —addition of 30° triangular vortex elimination plates to the beams, adjusting beam spacing and width, and reducing the inlet angle α; Scheme 5—a comprehensive optimization scheme using monolithic concrete to simplify the structure. Digital imaging and thin-walled triangular weirs were used to quantitatively analyze vortex elimination effect and flow improvement of each scheme. [Results] (1) Cause of vortices: superposition of lateral and longitudinal inflows under compound conditions markedly increased initial circulation, generating strong vortices within 30° of the inlet (100% type-F vortices at stage 3).(2) Comparison of vortex elimination performance: conventional beams (Scheme 1) merely downgraded type-F to type-D vortices without eliminating air entrainment. Adding 30° vortex elimination plates and optimizing beam spacing (Schemes 2-4) reduced vortex diameter from 7.5 cm to 0.1-0.3 cm, leaving only minor concave vortices (type B) at stage 3. Scheme 5 (preferred) completely eliminated air-entraining vortices while simplifying construction and maintaining smooth flow under all operating conditions. (3) Innovations: 30° triangular vortex elimination plates conformed to the inlet geometry, reducing the inflow angle α and suppressing initial circulation. Widening the central vertical beam enhanced vortex interception and disrupted vortex structure. Scheme 5 replaced complex components with a monolithic concrete pour, balancing effectiveness and constructability. [Conclusions] Vortex intensity under compound water inlet conditions is directly linked to the inlet angle α and initial circulation. Lateral flow amplifies complexity and hazards. Combining vortex elimination beams with 30° triangular vortex elimination plates completely eliminates air-entraining vortices, reducing vortex size by over 98% and remaining effective under all operating conditions. The optimized Scheme 5 balances vortex elimination performance with economy and offers a transferable design paradigm. The findings overcome the limitations of traditional vortex elimination measures and provide valuable guidance for high-head, multi-directional hydraulic projects.
[Objective] As an important part of the side inlet/outlet of a pumped storage power station, the length of the adjustment section directly affects the hydraulic characteristics of the inlet/outlet under bidirectional flow conditions as well as the project cost. This paper aims to investigate the effect of different adjustment section lengths on the hydraulic characteristics of side inlet/outlet, focusing on changes in head loss, velocity distribution, and discharge allocation, and to recommend an appropriate range of adjustment section lengths that meets design codes. [Methods] A three-dimensional mathematical model of the side inlet/outlet of the Lianghekou pumped storage power station was established. The Reynolds Stress Model (RSM) was adopted and validated against physical model test results. The velocity curves obtained from numerical simulation showed good agreement with the experimental measurements, confirming the applicability of the selected turbulence model to the study of inlet/outlet hydraulic characteristics. [Results] For the power generation condition (outflow), increasing the adjustment section length reduced inlet/outlet head loss, homogenized the velocity distribution at the trash rack section, and kept the discharge non-uniformity across openings within 5%. For the pumping condition (inflow), increasing the adjustment section length also reduced inlet/outlet head loss and maintained discharge non-uniformity within 10%. When the adjustment section length L relative to the diffusion section length T satisfied L≥0.3T, the bidirectional hydraulic characteristics of the inlet/outlet were favorable; when 0.1T≤L<0.3T, the bidirectional hydraulic characteristics were slightly poorer but still met code design requirements. [Conclusion] The research results provide support for the design and optimization of adjustment sections.
[Objective] Current research on the damping ratio evolution of silty fine sand under varying saturation conditions remains relatively limited, and in particular, systematic investigations revealing the mechanisms by which saturation variations influence damping ratios within the framework of unsaturated soil mechanics are still insufficient. To address this gap, typical loose silty fine sand from the Hangzhou Bay area is selected as the research subject. Utilizing a self-improved unsaturated soil resonant column testing system, the damping ratio evolution characteristics and underlying mechanisms along the desaturation path are thoroughly investigated. The findings aim to provide a scientific basis for analyzing dynamic properties and engineering applications of unsaturated soils. [Methods] An improved unsaturated soil resonant column apparatus was employed to conduct small-strain dynamic property tests under controlled drainage paths and saturation conditions. Test specimens were initially prepared with varying saturation levels and stress states. A stepwise desaturation process under constant net stress conditions was then applied to establish a series of samples with different saturation states. Subsequently, shear modulus and damping ratio measurements were performed across varying saturation levels and net stress conditions via resonant column loading. By integrating theoretical analyses of soil-water characteristic curves (SWCC) and pore water distribution states, distinct stages of damping ratio evolution were identified, and damping mechanisms induced by saturation variations were elucidated. [Results] The experimental results indicated that: (1) under identical saturation conditions, the damping ratio of silty fine sand gradually decreased with increasing net stress. (2) During saturation evolution, the damping ratio exhibited distinct stage-wise variations: when the soil approached full saturation, the damping ratio remained relatively stable. As saturation declined, the damping ratio rapidly attenuated and reached a minimum near the optimal saturation Sropt. Further saturation reduction led to a slight rebound in the damping ratio. (3) Based on the SWCC and pore water morphology evolution, the saturation-dependent damping ratio variations were categorized into three stages: boundary effect stage, transition stage, and residual saturation stage. [Conclusion] This study elucidates the damping ratio evolution mechanism of loose silty fine sand along desaturation paths. It demonstrates that the damping ratio is influenced not only by net stress but also closely associated with pore water distribution states, and that its minimum damping ratio occurs at the optimal saturation Sropt. The results reveal that saturation-dependent damping ratio changes can be categorized into three stages, each corresponding to distinct pore water morphology evolution processes. By employing the improved unsaturated soil resonant column, the complete damping ratio evolution process with saturation variation is observed. The principle of minimum damping ratio under “optimal saturation” control is proposed, and the three-stage evolution mechanism is revealed through SWCC analysis. These findings provide a foundation for refining unsaturated soil dynamic models and enhancing the scientific rigor of related engineering designs.
[Objective] Large-scale excavation unloading above an existing subway tunnel, resulting from activities such as river dredging and widening or foundation pit excavation, induces unloading deformation of the soil mass, which threatens the stability of the tunnel structure. Therefore, measures are required to limit the soil deformation. Given the limited research on the impact of river dredging and excavation on existing tunnels, this paper, taking the Nanjing Jiuxiang River Widening Project as a case study, investigates the impact of the reinforcement project’s foundation pit excavation on the shield tunnel under different working conditions. It also predicts the effectiveness of the reinforcement structure in suppressing tunnel uplift, thereby providing a reference for subsequent construction. [Methods] A pressure slab structure, consisting of a slab, bored piles, and a diaphragm wall, was proposed to protect the tunnel, and the preliminary design parameters and construction plan for these components were determined. Based on this, the Finite Element Method (FEM) was employed to predict the impact of the reinforcement measures on the existing tunnel section. First, a three-dimensional finite element analysis model was established based on the fundamental assumption of compatibility between tunnel displacement and soil deformation. The parameters for the reinforcement zone and the structure were determined based on engineering experience and field-measured data. Then, a finite element analysis of the vertical displacement of the tunnel segments was conducted under various working conditions, and the contour maps of segment vertical displacement were obtained. Subsequently, the longitudinal deformation of the shield tunnel was analyzed using the calculation results from the most unfavorable working condition. A curve fitting regression analysis was performed using the Gaussian function on the tunnel segment deformation to establish its longitudinal distribution pattern. Finally, a verification calculation of the internal forces within the tunnel structure was performed. [Results] (1) Working Condition 5 was identified as the most unfavorable case, with a maximum uplift deformation of the tunnel segments of 17.8 mm. (2) The tunnel’s vertical displacement peaked near the intersection of the river and tunnel centerlines. This displacement decreased rapidly as the distance from the intersection increased, diminishing to nearly zero at a distance of approximately 90 m. (3) For Working Condition 5, the maximum negative bending moment per linear meter of the tunnel segment was 179.2 kN·m with a corresponding axial force of 724 kN, and the maximum positive bending moment was 161.3 kN·m with a corresponding axial force of 556 kN. The calculated crack widths were 0.170 mm and 0.187 mm, respectively. [Conclusions] (1) The pile-wall-slab structure is proven to be effective in restraining the tunnel’s uplift deformation. (2) The performance of the pile-wall-slab structure is closely related to the excavation method of the foundation pit. Adopting a strip excavation by sections can effectively control ground disturbance and, in turn, tunnel deformation. (3) Overall ground reinforcement is significantly effective in suppressing tunnel uplift deformation, whereas the effect of using counterweights inside the tunnel is relatively limited.
[Objective] In recent years, with frequent extreme climate events, the northwestern region of China has experienced large diurnal temperature difference in summer, with daytime high temperatures continuously rising and persisting for extended periods. The resulting water vapor migration significantly affects the subgrade moisture content, thereby influencing the engineering performance of the subgrade. Investigating the water vapor migration patterns in loess subgrades under sealed pavement structures subjected to high temperatures holds substantial practical significance and application value for scientifically predicting the engineering performance of loess subgrades and ensuring their long-term stability. [Methods] A self-developed one-dimensional soil column test apparatus was employed, with compacted loess columns from the suburbs of Xi’an as the study subjects. The MTD15 temperature and moisture sensors were utilized to monitor the temperature and moisture variations within the loess columns under a boundary heating condition of 55 ℃. The water vapor migration patterns in one-dimensional sealed soil columns were analyzed under two heating modes: diurnal temperature cycling and continuous heating. Furthermore, the effects of different heating modes on the temperature and moisture distribution characteristics of the soil columns were explored. [Results] Boundary heating caused the temperature of the soil columns to rise, with a faster increase in the upper part and a slower increase at the bottom. During the heating phase, the temperature distribution along the height exhibited an approximately linear pattern. Under the cyclic heating of diurnal temperature difference, the internal temperature of the soil columns dropped rapidly after the heating was stopped. The cooling rate in the upper part was significantly higher than that in the middle and lower parts. By 08:00 the next day, the soil column temperature ranged between 22-25 ℃, with the middle part slightly warmer than the two ends. At 20:00 each day and 8:00 the following day, the temperature distribution along the depth of the soil columns remained basically the same. Under continuous heating, the soil column temperature reached dynamic equilibrium after 10 days of heating, exhibiting a two-segment, piecewise linear distribution. The variation trends in the moisture content distribution curves of the soil columns under the two heating methods were basically the same—namely, a decrease in moisture content in the upper part, an increase in the middle, and a decrease in the lower part, with these trends becoming more pronounced as the experiment progressed. However, within the upper 22.5 cm of the soil columns, the moisture content under the cyclic heating of diurnal temperature difference was higher than that under continuous high-temperature heating. [Conclusion] Under boundary heating conditions, the moisture of sealed soil columns is primarily governed by the combined effects of liquid water vaporization and moisture migration within the pores, water vapor migration driven by vapor concentration gradients, and condensation of vapor in the pores when the vapor pressure exceeds the saturated vapor pressure. These mechanisms collectively result in an inverse “S”-shaped moisture distribution. For subgrades with cover layers, 30 days of cyclic heating of diurnal temperature difference during summer induces moisture at 5 cm depth to fluctuate between -2% and +2% of the initial moisture content. This cyclic fluctuation of the upper subgrade moisture induced by diurnal temperature difference will significantly affect the mechanical properties of the subgrade. Under extreme climate conditions characterized by continuously rising heating temperatures and prolonged duration, the moisture content variations in the subgrade become more pronounced. These significant variations in moisture will lead to dehydration, shrinkage, and cracking in the upper subgrade layers, thereby compromising the service performance and lifespan of the subgrade and pavement structures.
[Objective] This paper aims to investigate the mechanical properties and damage evolution law of lime-solidified saline soil under freeze-thaw cycles, with a focus on analyzing the effects of lime content, curing age, and the number of freeze-thaw cycles on its unconfined compressive strength (UCS). In addition, a damage constitutive model based on statistical distribution is established to predict the stress-strain response and performance degradation of solidified saline soil under freeze-thaw conditions. [Methods] Saline soil from Zhenlai County in western Jilin Province was selected as the research object. Lime was added at proportions of 3%, 6%, and 9% as the curing agent, and specimens were prepared under the optimum moisture content (20%) and a degree of compaction of 90%. The specimens were cured for 7 days and 28 days and subjected to 0-60 freeze-thaw cycles. Unconfined compressive strength (UCS) tests and scanning electron microscopy (SEM) analyses were conducted to examine the macroscopic mechanical properties and microstructural changes. Based on the Weibull distribution function, a damage evolution model was established using the experimental data. [Results] The optimum lime content was 6%, and the unconfined compressive strength reached 835.01 kPa after 28 days of curing, which was more than four times that of the untreated soil. Both untreated and solidified saline soils exhibited strain-softening behavior and brittle failure after freeze-thaw cycles. With the increase in freeze-thaw cycles, the strength of the solidified soil gradually decreased but still remained significantly higher than that of the untreated soil. SEM images showed that lime treatment effectively reduced crack development and improved the integrity of the microstructure. The established Weibull damage model accurately simulated the entire stress-strain process under different numbers of freeze-thaw cycles, and the fitting accuracy improved with an increasing number of cycles. [Conclusion] Lime solidification significantly enhances the strength and freeze-thaw resistance of saline soil, with the optimum effect achieved at a 6% lime content and 28 days of curing. The damage model based on the Weibull distribution can effectively characterize the mechanical behavior and damage evolution of solidified saline soil during freeze-thaw processes. The research findings provide theoretical and technical support for the solidification treatment of saline soil in cold regions. The innovation lies in correlating microstructural changes with macroscopic mechanical responses and establishing a statistical damage prediction model suitable for freeze-thaw conditions.
[Objective] The identification and grouping of rock mass structural planes are essential prerequisites for conducting stability analysis. Existing clustering methods are highly sensitive to initial cluster centers, resulting in suboptimal grouping results and relatively low efficiency. To overcome these limitations, this study proposes a rock mass structural plane clustering method based on an improved sparrow search algorithm (KS-SSA). [Methods] First, SPM chaotic mapping was used to initialize the sparrow population, and then the step factor of the SSA was modified to improve the optimization speed. The improved SSA algorithm was used to optimize the initial cluster centers, achieving a global optimal solution. Subsequently, the K-medoids algorithm was applied for the final grouping of structural plane orientation data. Silhouette coefficient (SC) was used to evaluate the clustering performance. Four sets of artificial structural planes were generated using Monte Carlo simulation to validate the proposed algorithm, and comparative analyses were conducted with classical KPSO, FCM, spectral clustering, and traditional sparrow algorithms. [Results] The differences among different algorithms were primarily concentrated at the boundary points. The proposed algorithm could accurately identify all boundary data points, demonstrating good clustering performance with higher recognition accuracy and better clustering results. Using the silhouette coefficient, the structural planes could be accurately divided into four groups, which was consistent with the actual conditions. Furthermore, the sparrow population size and the number of iterations were identified as two key parameters for clustering analysis. Therefore, based on the artificial data, the optimal population size was determined to be 50 by analyzing the variation of fitness curves. Setting the number of iterations to 50 was appropriate while ensuring computational efficiency. The application of the proposed method to the publicly available Shanley dataset further validated its effectiveness. The new algorithm was applied to 201 structural plane data from a rock slope in the Nujiang River. By calculating the silhouette coefficients, three dominant structural plane groups were identified, which were generally consistent with the field investigation results. In addition, the KPSO algorithm was also employed in the field structural plane data analysis, and computational efficiency of the new method was discussed. The KS-SSA algorithm achieved stable clustering results within just 50 iterations, whereas the KPSO algorithm required 110 iterations for convergence. The runtime of the KS-SSA and KPSO algorithms was 45.97 s and 69.95 s, respectively, indicating that the new method had significantly higher computational efficiency. [Conclusion] The KS-SSA algorithm initializes the sparrow population based on SPM chaotic mapping, effectively preventing the clustering results from falling into local optima. Meanwhile, the step factor of the sparrow algorithm is modified, enabling the KS-SSA algorithm to dynamically adjust the step size and improve the optimization speed. Comparative analysis of three case studies demonstrates that compared with traditional algorithms, the proposed method exhibits superior performance in boundary data identification and improved robustness. This study discusses and clarifies the parameter selection and computational efficiency of the new method. The new method demonstrates promising application prospects and can be applied to large-scale data processing and analysis, providing a theoretical basis for the numerical simulation of three-dimensional networks of rock mass structural planes and rock mass stability analysis.
[Objective] During tunnel excavation, mining roadway development, or other underground construction processes, the rock mass of underground engineering is typically subjected to multi-stage permanent dynamic and repetitive stress disturbances. Moreover, cyclic stress amplitude is recognized as one of the primary factors influencing the occurrence of dynamic disasters in underground engineering. This study aims to investigate the influence of stress amplitude on the mechanical behavior and fracture modes of red sandstone under multi-stage constant-amplitude cyclic loading. [Methods] Utilizing an MTS-816 rock mechanics servo-testing system and real-time acoustic emission (AE) monitoring technology, the strength and deformation behaviors of red sandstone specimens during multi-stage cyclic loading were analyzed in detail. The evolution patterns of AE parameters under varying cyclic stress amplitudes and the final failure modes of the red sandstone specimens were elucidated. [Results] Lower cyclic stress amplitudes were found to impart a certain strengthening effect on red sandstone specimens, whereas higher stress amplitudes induced more severe deterioration. The increase in cyclic stress amplitude exhibited a hardening effect on the sandstone. Under multi-stage constant-amplitude cyclic loading, the elastic modulus and elastic strain energy of the red sandstone specimens were generally enhanced. Within a single fatigue loading stage, the AE signals gradually transitioned from active to stable with the increase in loading cycles. Under lower stress amplitude conditions, the red sandstone specimens exhibited a higher number of AE ring-count events during the loading process, and the cumulative AE curve showed only a sudden increase at the moment of failure. In contrast, for specimens under higher stress amplitudes, the AE activity remained relatively stable in the early loading stage but became more intense at the time of failure. An increase in stress amplitude led to the formation of larger axial primary cracks at specimen failure, and the secondary cracks and local spalling phenomena caused by crack coalescence became more severe. Specimens under high stress amplitudes were more prone to splitting failure dominated by overall tensile cracks. [Conclusion] The cyclic stress amplitude has a significant impact on the safety and stability of underground engineering. Therefore, the dynamic cyclic stress amplitude should be emphasized as a key factor during the operation and maintenance of underground engineering.
Hydraulic concrete structures are prone to various types of defects during construction and service. These defects are primarily categorized into three major types: apparent defects, internal defects, and defects at structure-foundation connections. Apparent defects, such as surface cracks, spalling, and cavitation, are primarily induced by thermal stress, shrinkage deformation, and environmental erosion. Specifically, the scouring and abrasion from sediment-laden flow can lead to surface spalling, while high-velocity water flow tends to cause cavitation. Internal defects mainly manifest as honeycombs and voids, which are primarily caused by issues like entrapped air bubbles and grout leakage from formwork due to the difficulties in underwater vibration. These issues are particularly prone to occur in areas with dense reinforcement or in mass concrete. Defects at structure-foundation connections primarily include misalignment and differential settlement, mainly resulting from the combined effects of multiple complex factors, such as repeated hydraulic pressure, uneven foundation settlement, and temperature variations. For concealed and hard-to-access underwater structural defects, non-destructive testing (NDT) technologies demonstrate distinct advantages. This study systematically reviews the research progress on four major NDT methods: optical imaging, sonar scanning, sub-bottom profiling, and impact-echo method. Optical imaging can effectively identify apparent defects through image analysis. However, affected by the optical properties of water, it suffers from problems such as poor image quality and limited identification accuracy. Sonar scanning can overcome the limitations of turbid water and achieve large-scale detection. However, its imaging resolution is relatively low, and it lacks a systematic correspondence between defect features and image features. Sub-bottom profiling, based on the strong penetration capability of low-frequency sound waves, shows potential in detecting internal defects of underwater structures and foundation conditions. However, its application research in the hydraulic engineering field remains relatively limited. The impact-echo method enables the detection of internal defects by analyzing the propagation characteristics of stress waves, unaffected by water and steel reinforcement. However, it still faces challenges in signal interpretation and quantitative evaluation. Based on the analysis and discussion of current research on NDT technologies for underwater structural defects, future development of these technologies should focus on the following directions: (1) establishing a deep learning-driven multi-source data fusion framework to enhance the capability of defect feature recognition; (2) developing opto-acoustic collaborative detection technologies that integrate the detailed resolution capability of optical imaging with the environmental adaptability of sonar; (3) developing more advanced stress-wave signal processing algorithms and quantitative evaluation models to improve the detection accuracy of the impact-echo method. Through multi-technology integration and intelligent development, it is expected that more comprehensive and accurate detection and assessment of underwater structural defects can be achieved, thereby providing strong technical support for the safe operation of hydraulic engineering projects.
[Objective] Dynamic groundwater changes represent one of the key controlling factors in the initiation of soil landslides. Investigating their response characteristics and mechanisms under rainfall is crucial for understanding landslide stability and evolutionary processes. [Methods] Taking the typical thick soil landslide—Tanjiawan landslide—in the Three Gorges Reservoir area as the research subject, this study systematically analysed the influence mechanism of groundwater level dynamics under rainfall by relying on multi-year continuous high-precision GNSS surface displacement data, automated groundwater level monitoring data, regional rainfall records, and information obtained from repeated field geological surveys on landslide geological structure, sliding mass structure characteristics, and groundwater recharge and discharge conditions. [Results] Deformation of the Tanjiawan landslide was concentrated in the mid-front and left-side areas and was closely related to rainfall events. Antecedent cumulative rainfall, to a certain extent, determined the slope's deformation response to a subsequent single heavy rainfall event. When cumulative rainfall was sufficient, even moderate single rainfall intensity may induce significant deformation. Groundwater level changes and rainfall infiltration showed distinct spatiotemporal correlation. The rate of water level rise was influenced by rainfall infiltration conditions and jointly controlled by both the antecedent effect and the concurrent effect of rainfall intensity. After rainfall infiltration, dynamic groundwater migration followed two main paths: one along route AB rapidly converged on the frontal area, generating strong hydrodynamic pressure; the other migrated slowly within the sliding mass, producing a cumulative effect on overall water content and pore water pressure. The hydrodynamic pressure generated along route AB, when coupled with local topographic conditions, directly drove deformation of the I-1 sliding mass, triggering local accelerated deformation or even failure. [Conclusions] Slope deformation trends are significantly controlled by dynamic groundwater changes, whereas slope stability is, to a certain extent, constrained by the duration of peak groundwater level. Prolonged high water levels markedly reduce slope stability. Moreover, monitoring data shows a certain lag between groundwater level rise and slope deformation rate, a characteristic that provides important reference value for early identification and early warning of thick soil landslides. This study provides a theoretical basis for research on the deformation mechanism, early identification, and early warning of soil landslides under rainfall conditions.
[Objective] A systematic investigation is conducted on the detrimental effects of high geothermal temperatures (up to 60 ℃) encountered during Sichuan-Tibet Railway tunnel construction on the performance of shotcrete mortar. The primary objectives are to elucidate the impacts of elevated temperature on hydration kinetics, mechanical strength development, and microstructural evolution, with a focus on understanding the paradoxical phenomenon of early-age strength enhancement versus long-term performance degradation. The research aims to identify critical temperature thresholds and underlying mechanisms responsible for material deterioration, providing insights essential for developing durable shotcrete formulations in geothermal environments. [Methods] Cement mortar specimens that simulated shotcrete (Ordinary Portland Cement, water-cement ratio of 0.5, sand-cement ratio of 1.5) incorporating a commercial alkali-free accelerator (8% dosage) were prepared and cured under controlled humidity at 20 ℃ (as a reference), 40 ℃, and 60 ℃. A multi-technique experimental approach was employed. Isothermal calorimetry was used to track hydration heat flow. The compressive strength was measured at 1, 3, 7, and 28 days. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) were employed to identify phase composition and transformation. Low-field nuclear magnetic resonance (LF-NMR) was utilized to quantify pore structure parameters. Scanning electron microscopy/backscattered electron (SEM/BSE) imaging was applied to characterize the microstructural morphology and interfacial transition zones between cement paste and aggregate. [Results] Elevated temperatures significantly accelerated early hydration. The 60 ℃ specimens achieved 1-day compressive strength that accounted for 64% of their 28-day reference strength, which was more than double the ratio observed at 20 ℃. Calorimetry revealed intensified and earlier heat release peaks at higher temperatures, indicating rapid C3A dissolution and AFt formation. However, temperatures ≥60 ℃ triggered severe microstructural degradation: XRD/FT-IR confirmed the destabilization of ettringite (AFt) and its conversion to weaker monosulfate (AFm), with AFm content increasing by 300% at 80 ℃. LF-NMR analysis demonstrated pore structure coarsening, with harmful pores (≥50 nm) increasing by 45% at 60 ℃ due to disordered hydration product precipitation and moisture loss. SEM/BSE imaging revealed thermal stress-induced microcracks (10-50 μm wide) at paste-aggregate interfaces and excessive CH crystallization. These synergistic effects caused pronounced strength regression—specimens at 60 ℃ peaked at 1-day strength (21.8 MPa) but declined to 34.9 MPa by 28 days, retaining only 78% of the strength exhibited by 20 ℃ controls. [Conclusion] This study identifies 60 ℃ as the critical threshold for irreversible shotcrete degradation in geothermal environments. Its principal innovation lies in decoupling the dualistic temperature response: while temperatures ≤40 ℃ enhance early strength through accelerated hydration, ≥60 ℃ induces multi-scale deterioration via AFt→AFm conversion, moisture loss-induced porosity, thermal microcracking, and pore coarsening. The quantified phase transformations and pore structure evolution provide a mechanistic explanation for long-term strength regression. These findings underscore the necessity of developing thermal-stable accelerators, optimizing binders with SCMs to suppress AFm formation, and implementing active cooling strategies for tunnels exceeding 60 ℃. The study establishes a scientific basis for designing high-performance shotcrete capable of withstanding extreme geothermal conditions in major infrastructure projects like the Sichuan-Tibet Railway.
[Objective] The Code for Design of Hydraulic Concrete Structures (NB/T 11011—2022) is the main reference for the design of reinforced concrete structures in hydropower projects. However, certain provisions in the standard, such as those concerning the design reference period and partial factors, remain difficult to apply or insufficiently clear, which may cause deviations in engineering practice. [Method] Drawing on many years of design experience with hydraulic structures of large and medium-sized hydropower stations, we analyze and discuss the issues related to the design reference period, service life, target reliability index, variable action values, partial factor setting, and seismic combinations. We also conduct cross-industry comparisons with building construction, highway, railway, and port codes, parameter back-analysis, and engineering practice verification. [Result] The structural target reliability index βt in the hydropower sector was set at a relatively low level compared with that of other domestic industries, and that the statistical data on which it was based were outdated, making it difficult to reflect the current reliability level of hydraulic structures. Moreover, relevant provisions on the design reference period of statistical variable actions were missing, while the prescribed live load values for main and auxiliary powerhouse buildings and platforms of hydropower stations were found to match more appropriately with large and medium-sized plants designed for a 100-year reference period. Although the categories of partial factors differ from those in other sectors, the variability of the basic variables and their interactions that these factors account for is essentially consistent, and inclusions and overlaps exist in their conceptual definitions. The current values of partial factors for persistent design situations cannot always ensure that the safety level of hydraulic concrete structures is higher than that of building structures. In addition, treating seismic action solely as an accidental effect is inappropriate, as it ignores the fact that small- and medium-scale earthquakes are now statistically analyzable, which may lead to insufficient reliability in seismic combination limit states. [Conclusions] In response to these problems, we suggest that 1) future revisions of the standard improve the setting of the target reliability index β, differentiate the values of β according to the design reference period, and strengthen reliability research as well as statistical studies for different structure types such as dams, tunnels, powerhouses, and frame structures, so as to gradually achieve differentiated β settings across structure types; 2) specify a single design reference period for hydraulic concrete structures, with 100 years set as the benchmark in line with the service requirements of most major structures and a corresponding β value to match current engineering practice; 3) limit the design reference period, remove the specification of controllable variable actions, and adjust partial factor values to improve the safety level for persistent design situations. Finally, the paper outlines the equivalence relationship between partial factors in hydropower projects and those in other industries. The conclusions are intended to serve as a reference for engineering design practice and for future code revisions.
[Objective] To enhance the understanding of Poyang Lake’s flood characteristics and provide a reference for optimizing flood regulation strategies, we conducted an in-depth investigation into the spatiotemporal patterns of flood propagation in Poyang Lake under the backwater effect from the Yangtze River by comprehensively analyzing differences in flood propagation time, spatial distribution characteristics, and changes in dynamic storage capacity. It further reveals the characteristics of lake flood propagation and its impact on flood control in surrounding areas. [Methods] By integrating hydrological statistical analysis and 2D hydrodynamic simulation methods, we constructed a flood evolution model for the Yangtze River-Poyang Lake system. [Results] 1) Differences in flood propagation time: The annual maximum lake outflow was primarily controlled by floods from its catchment area, whereas the annual highest lake water level was mainly determined by floods from the Yangtze River. The peak discharge generally reached the lake outlet within 48 hours, while the propagation of the peak water level was slightly longer, at 54 hours. The lake’s storage effect tended to shorten the arrival time of the discharge peak, but when encountering floods from the Yangtze River, the backwater effect became more significant. 2) “Five Rivers” and their impacts: The flood propagation paths of different inflowing rivers varied significantly. The flood from the Xiushui River had the shortest propagation distance, reaching the lake outlet the fastest. In contrast, the flood from the Fuhe River had the longest propagation distance and significantly impacted the southern core lake basin. For the “Five Rivers,” the average propagation time for peak discharge was 48 hours, and for peak water level was 54 hours. The peak water levels from the Fuhe, Raohe, and Xinjiang rivers all took 56 hours to reach the lake outlet. 3) Characteristics of dynamic storage capacity: The backwater effect of the Yangtze River was found to significantly influence the water surface slope of Poyang Lake. When the backwater intensity f exceeded 6, the lake surface became as gentle as a reservoir area, and the error in water volume calculation using the stage-storage curve was small. During major basin floods, the water level difference across the lake increased sharply; the average dynamic storage capacity reached 840 million m3, with a maximum of up to 2.2 billion m3. Changes in dynamic storage capacity directly affected the accuracy of flood forecasting and the evaluation of flood control benefits. The traditional stage-storage curve produced considerable errors during the main flood season (May-June), necessitating correction for dynamic storage capacity. [Conclusion] Flood propagation in Poyang Lake is significantly influenced by the comprehensive river-tributary-lake interaction. The lake outflow is predominantly governed by floods from the catchment area, while the water level at the lake outlet is markedly affected by river-lake interaction. Differences in the propagation paths of inflowing rivers lead to variations in flood propagation times and impact areas, with the Fuhe River flood having the most significant effect on the lake’s water level rise. The backwater effect of the Yangtze River significantly affects the lake’s water surface slope and dynamic storage capacity, and the latter is critically important for flood forecasting and the evaluation of flood control benefits during the main flood season. This study’s innovative integration of a “water level correlation” setting for the open boundary condition effectively simulates the flood behavior in river confluence areas and improves the model’s accuracy and reliability. Future research can further investigate flood propagation characteristics under various actual flood combinations, offering valuable insights for regions facing similar flood threats globally.
[Objective] The aim of this study is to analyze the impact of hydrological alteration on water level. [Methods] The Hydrological Alteration Diagnosis System was first used to identify the types of hydrological alteration. Then, based on the identified alteration types, change points in the water level series were detected.The water level series was divided into two periods using the earliest and latest change points as boundaries: the period before the earliest change point, representing natural conditions unaffected by changing environments, and the period after the latest change point, representing current conditions affected by changing environments. The water level series under natural and current conditions, which met the consistency requirements, could be used to analyze water level changes and predict trends. Using measured water level data as the evaluation standard, the analysis results incorporating hydrological alteration were compared with those ignoring hydrological alteration to quantitatively evaluate the deviations and shortcomings of conventional analytical methods. [Results] Using the monthly water level series from key stations (Xingzi and Hukou) in Poyang Lake as the research object, it was found that when hydrological alteration was considered, the fitting efficiency coefficients of the water level series from January to April at both stations were generally higher than those without considering hydrological alteration. Furthermore, the prediction accuracy of the water level at Xingzi station was improved. Greater differences in the types of hydrological alteration led to more significant improvements in prediction accuracy. [Conclusion] Compared to the length of the time series, hydrological alteration has a greater effect on improving the accuracy of water level prediction. This finding is not consistent with the results of some previous studies, indicating that further in-depth research is necessary. Therefore, hydrological alteration should be considered during model training to improve prediction accuracy.
[Objectives] This paper aims to analyze the recent river evolution characteristics of the Yangtze River-Poyang Lake confluence section, and to provide reference for the erosion and deposition evolution trend of the river-lake confluence section under the new water and sediment situation. [Methods] By reviewing domestic and international literature, we systematically summarize the historical evolution of the confluence reach of Yangtze River-Poyang Lake. By collecting data on recent topography, typical cross-sectional morphology, distribution of thalweg, shoal shoreline morphology, and changes in erosion and deposition in the confluence reach of Yangtze River-Poyang Lake, we focused on analyzing riverbed evolution in this reach for the past 30 years. Besides, we explored the factors influencing recent channel evolution in this reach, focusing on changes in water and sediment inflow, the impact of node control, channel regulation projects, and artificial sand mining in the lake area. [Results] Recent river evolution characteristics include: (1) The river regime of the mainstream was generally stable, and only the head of Zhangjiazhou, Zhangbei waterway inlet section and the head of Guanzhou had a certain degree of inter-annual collapse and siltation. The thalweg swing of the inlet section of Zhangbei waterway was significant, and the maximum swing was 900 m. The thalweg swing of the inlet section of Zhangnan waterway and the end of Guanzhou was less than 400 m. (2) The middle and upper sections of the Zhangbei waterway were silted up on the left bank and scoured on the right bank, and the lower section was alternately scoured and silted. In addition to the overall siltation of Guanzhou, the whole Zhangnan waterway was mainly scoured. The maximum scour depth was 8 m, and the scour intensity was higher than that of Poyang Lake. (3) After the impoundment of the Three Gorges Reservoir, the sharp decrease (-70%) of sediment from the main stream was the main factor causing the erosion of the Zhangjiazhou reach. The natural node control and sand mining in the lake area are important reasons for the abnormal distribution of local water and sediment in the lower reaches of the lake. The difference in erosion between the mainstream and the lake area will affect the water retention capacity of Poyang Lake. [Conclusions] We systematically sorted out the recent river evolution characteristics and influencing factors of the confluence section of Yangtze River-Poyang Lake. This deepens the understanding of the river evolution law of the confluence section of large-scale lakes connected to the Yangtze River under the comprehensive influence of new water and sediment conditions and manual intervention.
[Objective] The Yangtze River and the Poyang Lake exhibit complex interactions. Existing studies predominantly focus on the impacts of Three Gorges Reservoir’s flood regulation during extreme hydrological events. In reality, the backwater interaction in the Yangtze-Poyang confluence is inevitably altered by the differentially adjusted flow processes resulting from dam operations across distinct periods. Moreover, various indicators proposed by previous studies to characterize backwater effects were based on water balance or energy conservation mechanisms, but their theoretical assumptions remain insufficient, making it difficult to accurately depict the interactions between the river and the lake. Given this background,the objective of this study is to investigate the influence of regulated flow processes on the backwater effects in a large river-lake system.[Methods] Based on long-term hydrological data from the Yangtze-Poyang confluence area before and after the operation of the Three Gorges Reservoir (TGR), we propose a backwater intensity index and a mainstream-tributary confluence ratio function to reveal how large-scale reservoir regulation alters the intensity of the backwater effect from Poyang Lake’s outflow on the Yangtze River mainstream.[Results] (1) The backwater intensity of the Poyang Lake’s outflow on the Yangtze River first increased and then decreased within a year, with peak occurring from late March to late June, followed by a rapid decline. After TGR operation, the backwater effect weakened overall, with the most significant reduction occurring in April-May and September-November. (2) The operation of TGR and the reservoir group at the Poyang Lake’s five major tributaries altered the flow regimes of both the Yangtze River mainstream and the inflows from the tributaries, consequently adjusting the confluence patterns between the mainstream and Hukou’s outflow. During dry season (December, January-April), drawdown period (May-June), and post-flood storage period, the mainstream-tributary confluence ratio significantly decreased and exhibited a more concentrated distribution, but remained relatively stable during flood season. (3) Changes in the mainstream-tributary confluence ratio of the Yangtze-Poyang confluence system showed a positive correlation with the backwater intensity. When the confluence ratio was below 0.15, the backwater effect from the lake to the river was minor. When the ratio exceeded 0.2, the backwater effect became pronounced and intensified rapidly with increasing ratio, peaking near a ratio of 0.4. After TGR operation, the reduced confluence ratio during dry season, drawdown period, and post-flood storage period led to a overall weakening of the backwater effect from the Poyang Lake outflow during these periods. [Conclusion] The flattening of flow processes in the Yangtze mainstream was the primary driver of confluence ratio changes in the Yangtze River-Poyang Lake system following TGR operation, thereby playing a key role in the overall weakening of the backwater intensity on the Yangtze River. Compared to the changes in the flow processes of the mainstream and tributaries and their encounter conditions, the topographic adjustments had a relatively limited impact on the backwater effect within the Yangtze-Poyang confluence system. The findings can enhance the understanding of the Yangtze-Poyang interaction mechanisms under altered hydro-sedimentary conditions and provide scientific support for river-lake system management.
[Objectives] Water ecological security assessment serves as a crucial tool for evaluating the safety and sustainability of aquatic ecosystems. However, few studies have combined dimensionality reduction techniques with ensemble machine learning methods to identify key driving factors influencing water ecological security. Accordingly, this study aims to fill this gap by: 1) constructing a comprehensive water ecological security assessment based on the pressure-state-response (PSR) model; 2) quantitatively assessing the evolution characteristics of water ecological safety in Poyang Lake from 2014 to 2022; and 3) systematically analyzing critical factors affecting aquatic ecosystems. It is expected to provide scientific evidence and management recommendations for risk mitigation and ecological restoration in the Poyang Lake, thereby enhancing ecological sustainability. [Methods] First, 29 parameters were established based on the PSR model, encompassing hydrological, water quality, socioeconomic, and ecosystem health indicators. The indicators were normalized, and the expected values, thresholds, and indicator weights were determined to explore the interactions among human pressure, environmental state variables, and ecological responses. Next, comprehensive evaluation scores were calculated annually (2014-2022), enabling a longitudinal assessment of ecological safety levels. To disentangle the complex influence of multiple variables and reduce information redundancy, principal component analysis (PCA) was employed to identify the latent structures underlying the indicator set. Finally, random forest, a robust non-parametric ensemble learning technique, was used to rank variable importance and elucidate nonlinear contributions of key factors. [Results] (1) The comprehensive water ecological security scores revealed that from 2014 to 2022, Poyang Lake generally maintained a state from “moderate” to “relatively safe”, with basic stability but a slight downward trend in its ecosystem, particularly in the years marked by intensified human activities and altered hydrological conditions. (2) The PCA results demonstrated the effectiveness of dimensionality reduction and highlighted the indicator correlations, with the first few components strongly associated with mean annual water level, eutrophication status, and socioeconomic development. (3) The RF model revealed a consistent and interpretable ranking of variable importance, identifying the top five factors influencing the water ecological security as: mean annual water level, per capita GDP, urban wastewater discharge, eutrophication status, water quality compliance rate. [Conclusion] Stable mean annual water levels are foundational to ecological security. Economic activities, while beneficial for development, must be coupled with strict pollution control to prevent further ecological degradation. Continuous eutrophication monitoring and adaptive response strategies are essential. Infrastructure for pollution control should be strengthened, with a particular focus on upstream cities and agricultural areas. Ecological restoration projects should be advanced, with emphasis on rehabilitating degraded wetlands and tributaries. These findings offer scientific evidence and decision-making support for improving the water ecological security of Poyang Lake and can be extended to assess similar large freshwater lake systems facing comparable challenges.
