[Objective] After the operation of the Three Gorges Reservoir, the ratio between siltation volume in the Three Gorges Reservoir and channel scour volume in the middle and lower reaches of the Yangtze River does not conform to the value (1∶0.5) determined in previous studies, mainly due to the substantial reduction in sediment inflow into the Three Gorges Reservoir. This study aims to analyze the characteristics of sediment deposition in the Three Gorges Reservoir and the scouring variations in the middle and lower reaches of the Yangtze River, and to investigate the correlation between sediment retention by the Three Gorges and other reservoirs and the resulting channel scouring in the middle and lower Yangtze River. [Methods] Measured hydrological data and topographic data from 2003 to 2021 were utilized. The sediment transport method, cross-sectional method, and low-water-level verification were comprehensively adopted for the analysis. [Results] The ratio between basin-scale sediment retention and downstream channel scour was 1∶0.48, which was generally consistent with that of typical reservoirs in China such as Sanmenxia Reservoir and Danjiangkou Reservoir. 1) Regarding siltation volume in the Three Gorges Reservoir, the cumulative siltation from 2003 to 2021 was calculated as 2.381×10⁹ t, which was equivalent to 2.238×10⁹ m³, using the sediment transport method. The cross-sectional method estimated 1.784×10⁹ m³ of siltation in the mainstream of the reservoir area. The difference between the two methods was 25.4%, due to differences in calculation principles, but the results were generally consistent. Siltation was mainly concentrated in the perennial backwater zone, totaling 1.853×10⁹ m³. Siltation within the flood control storage accounted for only 0.67%, indicating a relatively small loss of effective storage capacity. 2) Regarding channel scour in the middle and lower reaches of the Yangtze River, the cumulative scour in the Yichang-Datong reach was calculated as 8.99×10⁸ m³ using the sediment transport method. The cross-sectional method yielded 3.146×10⁹ m³. The minimum scour volume inferred from the decline in water level under low-flow discharge conditions was 2.160×10⁹ m³, demonstrating that the cross-sectional method results were more reasonable. The discrepancy was mainly attributed to the underestimation of fine-grained sediment in the sediment transport method and the exclusion of human activities, including sand mining of 6.64×10⁸ m³ and channel dredging of 3.08×10⁸ m³. The cumulative scour of the entire river reach reached 5.03×10⁹ m³. Continuous scour persisted in the Jingjiang reach with no reduction in intensity, and scour in the downstream reaches significantly intensified after 2013. 3) At the basin scale, the total reduction in sediment volume from 2003 to 2021 was 8.497×10⁹ m³. After excluding the influence of human activities, the ratio between sediment retention and scour was 1∶0.48, which was consistent with the sediment retention-scour pattern of large reservoirs. Siltation in the Three Gorges Reservoir itself was 2.238×10⁹ m³, which directly caused 1.074×10⁹ m³ of downstream scour and accounted for 26.5% of the total scour. The contribution in the first decade was significantly higher than that in the second decade, indicating that its impact gradually weakened with the enhanced sediment retention effect of upstream reservoir groups. [Conclusion] It is necessary to optimize sediment release scheduling of reservoir groups to improve the sediment flushing ratio. Measures such as dredged sediment resource utilization and sediment replenishment should be combined to mitigate channel scour, while strengthening sand mining management and long-term monitoring of scour and deposition.

[Objective] After the construction of reservoirs, the sediment concentration in discharge flow has significantly decreased. The along-reach recovery of sediment leads to scour adjustments in the downstream river, potentially impacts downstream flood control, water resource utilization, and aquatic ecological environments. Meanwhile, this topic remains one of the key challenges in river sediment dynamics. [Methods] This paper summarizes the existing research results in the characteristics of sediment recovery and theoretical research, analyzes the deficiencies in the current research, and preliminarily discusses the future research directions. [Results] (1) The sediment recovery processes and patterns in the downstream reaches of different reservoirs show significant differences.The recovery varies among different grain-size groups of sediment.Factors such as riverbed composition, inflow of water and sediment from lakes and tributaries all have an impact on the recovery rates of different grain-size groups of sediment. However, there is no consensus on whether there is an exchange between coarse and fine sediment during the along-reach recovery process. (2) Scholars derive the sediment recovery saturation coefficient through back-calculation based on measured data or constructed empirical formulas. The calculation results show a high degree of consistency with actual data. However, due to the differences in measured data, the applicable scope of the empirical formulas is limited. There is not yet a unified understanding on its physical meaning, calculation formulations, and value selection. Among them, some parameters in the expression derived by Han Qiwei are relatively complex, and some key parameters still rely on empirical formulas or empirical values, which to a certain extent restricts its wide application in mathematical models. (3) Abundant achievements have been made in key influencing factors of the sediment recovery saturation coefficient.Theoretical formulas and improved formulas for the vertical distribution of equilibrium sediment transport concentration. Basically meet the current requirements of calculation accuracy. The main difficulty of the research lies in the vertical distribution of sediment concentration under the unsaturated flow condition. Although scholars such as Han Qiwei have proposed effective methods, the calculation and value selection of some key parameters, such as the unsaturated coefficient c, the weight coefficient μ, and the bottom sediment concentration S_b, have not yet been well resolved.[Conclusion] Future research can be advanced in the following aspects: 1) strengthen prototype observations, particularly targeted observations of the vertical distribution of sediment concentration, and utilize advanced technologies such as artificial intelligence to integrate and analyze observational data; 2) use advanced and precise measuring instruments to conduct in-depth flume experiments, provide accurate, comprehensive and systematic observational data for theoretical research;3) combine the analysis of measured data and the results of flume experiments for research on the basic theory of along-reach sediment recovery,and deepen research on directions such as the vertical distribution of sediment concentration in unsaturated flows.

[Objective] The Wudongde Reservoir is the uppermost cascade reservoir among the Wudongde, Baihetan, Xiluodu, and Xiangjiaba cascade reservoirs in the lower reaches of the Jinsha River. To support the operation scheduling of sediment discharge in the Wudongde Reservoir, a one-dimensional unsteady flow and sediment mathematical model for the mainstream and tributaries of the Wudongde Reservoir is established. The feasibility of the model is verified using measured data after impoundment, and then the sediment discharge characteristics of typical water and sediment processes during the initial impounding period of the Wudongde Reservoir are simulated. [Methods] The water and sediment processes in July and August of 2020, 2022, and 2023 were selected as typical water and sediment processes for model calculation. Among them, 2020 was a high-flow year, while 2022 and 2023 were low-flow years. [Results] 1) During July and August, the Wudongde Reservoir had strong sediment discharge capacity when the inflow volume was large. In July 2020, due to large inflow volume, the sediment discharge ratio corresponding to a water level of 945-975 m reached 32%-14%. During the impounding period in August, the impact of initial water level on sediment discharge capacity was not significant. In August 2020, due to large inflow volume, the sediment discharge ratio corresponding to an initial water level of 952-975 m was relatively large, ranging from 24.8% to 18.8%. During July and August, the Wudongde Reservoir could utilize high water for sediment discharge. 2) Due to significant reduction in sediment inflow, the siltation volume and proportion in the fluctuating backwater area of the Wudongde Reservoir during July and August were relatively small, providing favorable conditions for optimal operation of the reservoir. During the flood season in July, when the inflow volume was large and reservoir water level was >965 m, the siltation proportion in the fluctuating backwater area gradually increased but the siltation amount was small. When the inflow volume was small, the siltation pattern in the fluctuating backwater area was similar but the corresponding reservoir water level was 960 m. 3) At the beginning of August during the impounding period, when the initial water level was raised and the inflow volume was large, the impact on the siltation volume and proportion in the fluctuating backwater area was small. When inflow volume was small, the siltation proportion in the fluctuating backwater area increased correspondingly with the rise of the initial water level, but the siltation volume remained small. When the inflow sediment volume was relatively small, the inflow water volume was the main factor affecting the siltation proportion in the fluctuating backwater area in July. When the inflow sediment volume was relatively large, the inflow sediment volume was the main factor affecting the siltation proportion in the fluctuating backwater area in August. When the inflow sediment volume was relatively small, the inflow water volume was the main factor affecting the siltation proportion in the fluctuating backwater area in August. [Conclusion] With the significant reduction in inflow sediment volume after the impounding of the Wudongde Reservoir, even though the siltation proportion in the fluctuating backwater area increases, the increase in siltation volume in the fluctuating backwater area remains small, indicating room for further optimization of reservoir operation. Therefore, when studying the optimal operation of the Wudongde Reservoir, it is not sufficient to only consider relative indicators such as the sediment discharge ratio and the siltation proportion in the fluctuating backwater area. It is also necessary to consider absolute indicators such as the outflow sediment volume, the total siltation volume in the reservoir area, and the siltation volume in the fluctuating backwater area. This requires further long-term scouring and deposition calculations based on typical series years. Further in-depth research is needed on the optimization space for reservoir operation and specific regulation indicators under the permissible siltation principles of the Wudongde Reservoir.

[Objective] This study aims to quantitatively identify the main hydrological driving factors of channel morphological changes in the middle reaches of the Tarim River; to be specific, the effects of discharge, daily discharge variation, sediment concentration, and daily sediment concentration variation on channel erosion-deposition dynamics. It also aims to quantify the seasonal relationships between key driving factors and morphological responses in different hydrological periods (dry season before flood, medium-water season before flood, flood season, and post-flood season), and to clarify the differentiated morphological evolution patterns of straight, naturally braided, and artificially straightened reaches under different discharge conditions. [Methods] A high-resolution two-dimensional flow and sediment transport model was established for a typical 8.41 km reach at the Tatilike Village section of the mainstream of the Tarim River. A high-precision initialization method was innovatively adopted. The airborne laser bathymetry was carried out in the dry season (April 1, 2023) to minimize the interference of suspended sediment, and a high-precision digital elevation model with a resolution of 3 m was generated as the initial riverbed topography. One-dimensional and two-dimensional models were coupled, and the validated one-dimensional model provided boundary conditions for the two-dimensional model, including the temporal discharge series at the inlet boundary and the temporal water depth series at the outlet boundary. With integrated spatiotemporal analysis, this model simulated daily morphological changes from April 1, 2023 to November 30, 2023, and output data at 21 characteristic cross-sections, cross-sections in braided reaches, and cross-sections in artificially straightened reaches. [Results] Compared with discharge, sediment concentration, and daily sediment concentration variation, daily discharge variation was the main hydrological factor affecting the intensity of erosion and deposition at the thalweg of channel cross-sections. In different hydrological periods, including the dry season before flood, the medium-water season before flood, the flood season, and the post-flood season, the correlation between daily discharge variation and daily thalweg elevation variation showed an increasing trend, with the strongest correlation in the post-flood season. The erosion and deposition process of the river channel exhibited significant seasonal differentiation. The influence of different periods on the evolution of the main channel followed the order: post-flood season>flood season>medium-water season before flood >dry season before flood. [Conclusion] Daily discharge variation is the main driving factor controlling daily riverbed elevation changes and exhibits clear seasonal characteristics. Their correlation gradually increases from the dry season before flood to the post-flood season and reaches the strongest relationship and maximum net erosion intensity after the flood peak. Given the significant impact of the post-flood season on channel reshaping, management strategies should prioritize monitoring during this period and prepare for possible interventions. The established evolution intensity sequence (post-flood season > flood season > medium-water season before flood>dry season before flood) provides a framework for predicting periods of high geomorphic activity and related ecological impacts, such as habitat gain and loss, and bank slope stability. The differentiated evolution trajectories of braided channels and the intensive erosion-dominated adjustment of artificially straightened reaches indicate that management and restoration measures must be tailored to specific reach types. This study provides a scientific basis for river planning and for responding to water diversion, climate change, or engineering interventions, and will effectively support the sustainable restoration practices of the Tarim River ecosystem.

[Objective] This study aims to investigate the differentiated effects of various driving factors on the stage-specific characteristics of hydrological drought in the Jitai Basin under the influence of climate change and intensive human activities. [Methods] We collected meteorological and hydrological data from 1959 to 2023 from three typical watersheds (the Shushui, Wujiang, and Tongjiang Rivers) in the Jitai Basin, and adopted the Pettitt test to divide the study period into a baseline period, a transition period, and a change period. By using the improved two-parameter monthly water balance model, we analyzed the drought characteristics and the quantitative effects of driving factors in each stage, and clarified the dominant role of different driving factors as well as their nonlinear regulation mechanisms. [Results] 1) The runoff generation mechanisms of the three typical watersheds shifted around 1980 and 2008. Specifically, in baseline period (1959-1980), the underlying surface conditions of the watersheds were relatively stable, hydrological processes were dominated by natural climate drivers, and runoff variations were directly controlled by the precipitation-evaporation balance, with no obvious disturbance from human activities. In transition period (1981-2008 ), the intensifying regional human activities began to alter the original runoff generation mechanisms of the watersheds. In the change period (2009-2023), underlying surface modification and water conservancy project regulation became dominant factors. The runoff coefficient α increased significantly in transition period and then declined in the change period, which also indicated that the regulatory intensity of human activities on runoff exceeded natural fluctuations. In terms of drought characteristic variations, drought severity and duration decreased notably in the transition period compared with the baseline period, but rebounded in the change period; the average drought severity of the three typical watersheds increased by 46.2%, 26.9% and 25.9% respectively relative to the transition period. 2) By introducing a regulating coefficient of parameter C during wet and dry periods, the improved two-parameter monthly water balance model effectively improved the overall simulation accuracy of runoff series, especially for low-flow and drought months. The Nash-Sutcliffe efficiency coefficient (NSE) was higher than 0.7 and the correlation coefficient R exceeded 0.85 in both the baseline and transition periods, with the water balance error controlled within ±1%. In the change period, the measured runoff series was heavily disturbed by human activities, which increased the difficulty of simulating monthly-scale runoff series. Parameter calibration results showed that the value of C for each typical watershed in transition period was lower than that in baseline period, but rebounded in the change period, reflecting regular variations in the precipitation-evaporation relationship of the watersheds across different stages. High temperature and low rainfall in summer and autumn were critical driving factors of hydrological drought in the Jitai Basin, with strong sensitivity to drought severity. A 10% reduction in precipitation during this period led to an increase of 0.14-0.23 in drought severity, accompanied by a marginally diminishing effect. Nevertheless, compared with the baseline period, human activities played a dominant role in the change period, causing greater variations in runoff depth and drought severity than climate change factors. [Conclusion] The multi-stage quantitative method for driving factors constructed in this study reveals the nonlinear regulatory effects of climate change and human activities on hydrological drought severity in the Jitai Basin at different stages, clarifies the influence intensity, sensitivity and stage characteristics of each driving factor, and identifies the nonlinear regulation of these factors on drought severity.

[Objective] The operation of cascade reservoirs in the lower reaches of Jinsha River has significantly altered the inflow flood characteristics of the Three Gorges Reservoir (TGR), posing new requirements for the adaptability of existing flood control scheduling. This study aims to reveal changes in inflow flood processes and flood propagation characteristics within the TGR after the impoundment of cascade reservoirs in the lower reaches of Jinsha River, providing a theoretical basis and data support for scientific flood control scheduling of the reservoir. [Methods] Based on the measured hydrological and topographic data of the TGR from 2003 to 2020, mathematical statistical methods were employed to analyze changes in inflow flood characteristics before and after the impoundment of cascade reservoirs. A one-dimensional hydrodynamic model was adopted. After calibration and validation, multiple comparative scenarios were designed to conduct simulation and analysis of flood peak propagation processes in the TGR area. [Results] Statistical analysis of the measured data showed that around 2013, the average inflow flood volume of the TGR decreased by approximately 9.9%, while the rising and falling durations decreased by 4.9% and 9.9%, respectively, and the average rising and falling rates increased by 10.3% and 9.8%, respectively. These results indicated that the flood recession became faster, the flood peaks occurred earlier, the average peak discharge decreased, and the risk of extreme floods increased. The mathematical model results showed that, in terms of discharge characteristics, the flood peak propagation time was positively correlated with the flood peak discharge, while the influence of changes in baseflow was not significant. In terms of hydrograph shape, the flood peak propagation time was negatively correlated with the rising duration and positively correlated with the falling duration. In terms of boundary conditions, the flood peak propagation time was negatively correlated with the water level upstream of the dam. [Conclusion] The operation of cascade reservoirs significantly reduces inflow flood peak, and the attenuation effect on the flood hydrograph also leads to a significant increase in flood duration. The flood peak propagation is faster under conditions of lower peak discharge, longer rising duration, shorter falling duration, and higher water level upstream of the dam.

[Objective] The Hanjiang River Basin, as the core water source area of the Middle Route of the South-to-North Water Diversion Project, has runoff variations that are of great significance to water supply security and regional sustainable development. This study aims to identify the main driving factors of runoff evolution in the upper, middle, and lower reaches of the Hanjiang River Basin. By following the framework of “pattern identification-hydrological modeling-attribution analysis of runoff changes,” this study quantitatively and qualitatively assesses the impacts of climate change and human activities on runoff variations, thereby providing a scientific basis for rational water resources utilization and management decisions in the river basin. [Methods] Statistical methods, including linear regression, moving average, rescaled range (R/S) analysis, cumulative anomaly, Mann-Kendall trend test, and sliding t-test, were employed to identify the evolution patterns of hydrological elements. A distributed Xin’anjiang model was constructed to simulate and reconstruct natural runoff, and the contribution rates of climate change and human activities to runoff changes were quantified. [Results] The results showed that from 1959 to 2019, precipitation in the upper reaches exhibited a decreasing trend at a rate of -0.59 mm/a, whereas precipitation in the middle and lower reaches showed increasing trends at rates of 0.24 mm/a and 0.05 mm/a, respectively. Temperature differences among the three reaches were minimal. Runoff exhibited significant interannual variability, with an abrupt change occurring in 1990. The decline in runoff at the Xiantao station in the lower reaches was significantly larger than that at the Huangjiagang station in the middle reaches and the Shiquan station in the upper reaches. Climate change contributed to a reduction in runoff at Shiquan station by 75.97 mm, accounting for 59.98% of the total change. Human activities led to runoff reductions of 83.32 mm at Huangjiagang station and 78.45 mm at Xiantao station, with contribution rates of 54.89% and 77.20%, respectively. [Conclusion] The impact of human activities on runoff evolution is gradually intensifying and becomes more pronounced in the downstream areas. The upper reaches, characterized by higher elevation and dominated by forest and grassland with relatively limited human activities, experience a smaller degree of anthropogenic influence. In the middle reaches, higher population density and economic development drive greater water demand, while regulated water transfer from hydraulic engineering has led to an overall decline in runoff. In the lower reaches, intensive human modifications to the underlying surface, frequent human activities, high water demand, and large-scale regulated water transfers collectively result in a significant reduction in runoff. The findings of this study provide valuable insights for water resources development, utilization, and watershed planning in the Hanjiang River Basin.

[Objective] Promoting the natural spawning of fish through reservoir operation is an important and effective measure for enhancing the ecological benefits of reservoirs. From 2011 to 2025, the Three Gorges Reservoir (TGR) conducted 24 ecological operation tests over 15 consecutive years to promote the spawning of the four major Chinese carps (FMCC) and facilitate the recovery of fish resources. However, early-stage fish resource monitoring data indicate that the responses of FMCC to each ecological operation test vary, with spawning effects being pronounced in some cases but suboptimal in others. Therefore, attention should be focused on the schooling behavior of the parent fish of FMCC in the Yichang river section, and appropriate hydrodynamic conditions constitute a key ecological basis for their spawning. [Methods] To determine the appropriate ecological flow for FMCC parent fish schooling in the Yichang river section, this study began with the ecohydraulics requirements of the parent fish schooling, proposed a hydrodynamic suitability curve based on previous monitoring results and existing literature, established a three-dimensional hydrodynamic model and a Physical Habitat Simulation Model (PHABSIM) of the spawning site, analyzed the hydraulic habitat suitability of FMCC under different flow conditions, and identified the ecological flow range suitable for parent fish schooling. [Results] By simulating the weighted usable volume (WUV) of FMCC spawning schooling habitats under different outflows from the TGR, the spatial response characteristics of the schooling habitat were quantitatively evaluated. Overall, WUV first increased and then decreased as the flow of the spawning site increased. At a flow of 14 000 m³/s, WUV reached its maximum, accounting for 71.7% of the percent usable volume (PUV) of the study area. Under this flow, conditions were favorable for forming a larger, more concentrated, and well-connected schooling-suitable habitat. Under high flow conditions exceeding 22 000 m³/s, the increased flow velocity and intensified vertical velocity gradient hindered the formation of stable FMCC spawning schooling, significantly reducing the suitable habitat volume. Therefore, to promote parent fish schooling, flow should not be too high. The optimal flow range should be 10 000-20 000 m³/s, accounting for more than 64.3% of PUV, which was conducive to forming a three-dimensional habitat suitable for FMCC schooling and maintaining high ecological stability. The suitable flow velocity and depth ranges for parent fish spawning schooling in the Yichang river section were 0.9-1.4 m/s and 3.0-5.0 m, respectively. FMCC habitat suitability in the study area exhibited distinct vertical and horizontal spatial distribution patterns. At a flow of 14 000 m³/s, the average velocity in the upper layer was 1.10 m/s, while in the lower layer it was 0.75 m/s. For the upper-middle layer, flow velocity, depth, and comprehensive suitability index were 0.90, 0.91, and 0.84 respectively, whereas for the bottom layer, they were 0.89, 0.65 and 0.57. The upper-middle layer outperformed the bottom layer, particularly in terms of depth and comprehensive suitability index, indicating that the upper-middle layer provided more stable and suitable habitat conditions for FMCC. [Conclusion] Using an unstructured irregular triangular mesh, a three-dimensional hydrodynamic model of the FMCC spawning site in the Yichang river section is established. The model accuracy meets the requirements, enabling accurate simulation of the hydrodynamic characteristics of the spawning site under different flow conditions. Before spawning, parent fish tend to occupy the upper-middle layer, and their behavior is more sensitive to flow conditions. The flows of 10 000-20 000 m³/s favor parent fish schooling, with the most suitable range being 12 000-16 000 m³/s. The findings provide a scientific basis for the TGR to precisely regulate ecological water management during key life history stages, such as parent fish schooling, thereby enhancing the ecological benefits of the reservoir.

[Objective] Soil erosion and non-point source pollution have become increasingly prominent in the Danjiangkou Reservoir area, affecting the water quality of inflow rivers and posing a threat to the water security of the Danjiangkou Reservoir area. Existing studies have not thoroughly analyzed the influence of landscape patterns at different scales on water quality. This study aims to supplement research on the differentiated effects of landscape pattern indices at different scales on water quality in the Danjiangkou Reservoir area, thereby providing a reference for developing scientifically sound water quality protection and improvement measures. [Methods] We took the Shibanhe small watershed in the Danjiangkou Reservoir area as the research object. Based on the spatial pattern characteristics of land use, we adopted spatial analysis, mathematical statistics and Mantel test to reveal the spatial differentiation characteristics of different water quality indicators, investigate the main landscape pattern indices affecting water quality at scales such as subwatershed, river buffer zone, and circular buffer zone, and explore the mechanisms of landscape background on surface water quality. [Results] (1) The concentrations of total nitrogen (TN) and nitrate nitrogen ( ${NO}_{3}^{-}-N$) in the surface water of the Shibanhe small watershed were relatively high, mainly distributed in the central and southeastern parts of the watershed where cultivated land and residential areas were concentrated. Orchards near the river also served as important potential sources of river water pollution. (2) At different scales, the proportions of cultivated land and forest land had a significant impact on river water quality, and their correlation was more significant at scales of subwatershed, 100 m river buffer zone, and 200 m river buffer zone. At the subwatershed scale, patch density (PD) had a highly significant correlation with TN and ${NO}_{3}^{-}-N$. Landscape pattern indices, such as contagion index (CONTAG), cohesion index (COHESION), Shannon’s diversity index (SHDI), and Shannon’s evenness index (SHEI), had certain effects on water quality at all scales, and their effects were greater at subwatershed and 100 m rive buffer zone scales. (3) The effects of landscape background on river water quality had a significant scale effect. The subwatershed and 100 m river buffer zone scales were the key scales affecting river water quality, where landscape structure and landscape pattern exerted the greatest influence. [Conclusion] Management and regulation of landscape structure and landscape patterns at scales of subwatershed and 100 m river buffer zone—especially strengthening the monitoring and management of landscape fragmentation—are of great significance for improving river water quality in the Danjiangkou Reservoir area. Future research should strengthen long-term water quality monitoring to reveal dynamic change patterns of regional water quality and seasonal differences in their driving factors.

[Objective] Algal polysaccharide is used as a curing agent in place of traditional cement, with the addition of corn fiber, water-retaining agents, and ecological fertilizers, to improve the ecological restoration performance of the substrate. In previous studies, the algal polysaccharide composite substrate has been preliminarily applied in the ecological restoration of slopes in red soil regions. However, there is still a lack of sufficient research on improving the erosion resistance of the algal polysaccharide composite substrates in red soil. This study aims to address the issues caused by the use of cement as a curing agent in sprayed vegetation techniques, including environmental pollution, difficulty in degradation, and restricted plant growth. [Methods] We selected the red soil slope of a power transmission and transformation project in southern China as the research subject. From previous 16 sets of orthogonal experiments, the best three types of substrates were selected for indoor simulated rainfall experiments. Rainfall intensities were set at 60, 90, 120 mm/h, and the slope gradients were set at 20° and 30°. With a bare slope treated only with clean water as the control group, a total of 72 rainfall experiments were conducted (2 slope angles×3 rainfall intensities×4 slope surfaces×3 repetitions), and the substrate with the best erosion resistance was ultimately selected. [Results] 1) Experimental results showed that on a 20° slope, when the rainfall intensity increased from 60 mm/h to 120 mm/h, the initial runoff yield time for substrates 1, 2, and 3 was shortened by 19.48%, 16.67%, and 30.43%, respectively, compared to the control group. The corresponding stable runoff yield rates were in the ranges of 0.56-2.08, 0.68-2.15,0.61-2.09 L/(min·m²), while the sediment yield rates at rainfall intensities of 60, 90, 120 mm/h were 0.18-0.22 L/(min·m²), 0.41-0.75 L/(min·m²), and 1.12-1.44 g/(min·m²), respectively. On a 30° slope, the initial runoff yield time was shortened by 25.80%, 48.48%, and 40.41%, respectively. The stable runoff yield rates were in the ranges of 0.63-2.21, 0.86-2.38,0.78-2.24 L/(min·m²), and the sediment yield rates under the corresponding rainfall intensities were 0.18-0.39 L/(min·m²), 0.68-0.86 L/(min·m²), and 1.27-1.77 g/(min·m²). 2) The drainage efficiency of the three substrates ranged from 16.35% to 52.73%, with substrate 2 showing the best performance, while the soil conservation efficiency remained stable at 97% to 98%. With increasing rainfall duration, both runoff yield rate and sediment yield rate exhibited a trend of first increasing and then stabilizing. The runoff yield rate of substrate 2 was higher than that of substrates 1 and 3, while the differences in sediment yield rates among the substrates were small. Both the runoff yield rate and sediment yield rate were significantly correlated with runoff shear force (p<0.05). As runoff shear force increased, both the runoff rate and sediment yield rate showed a linear increasing trend. Runoff shear force had the greatest impact on both runoff yield rate and sediment yield, with a significantly higher correlation compared to runoff power, resistance coefficient, and other factors, indicating that it was the primary controlling factor for runoff and sediment yield on the slope. [Conclusion] Algal polysaccharides significantly reduce the initial runoff yield time on substrate slopes, with the reduction becoming more pronounced as the slope angle or rainfall intensity increases. The runoff and sediment yield processes for different substrates under various slope gradients and rainfall intensities are generally consistent. Compared to bare slopes, runoff yield rates of substrate-covered slopes significantly increase, while the sediment yield rates markedly decrease. A significant positive correlation is observed between the runoff yield rate and the hydrodynamic parameters, especially with runoff shear force, which exhibits the highest correlation. However, except for runoff shear force, no significant correlation is found between sediment yield rate and other hydrodynamic parameters. Among the three test substrates, substrate 2 demonstrates superior erosion resistance and is recommended for engineering applications. The optimal formulation for this substrate is algal polysaccharide∶fiber∶fertilizer∶water-retaining agent=1%∶0.8%∶3%∶0.9%. The findings of this study can provide new materials and technical support for similar slope protection projects, promoting the development of green engineering and the high-quality advancement of soil and water conservation.

[Objective] This study aims to understand the pollution characteristics and sources of heavy metals in the riparian soil of the lower reaches of the Fenhe River. [Methods] We collected 32 soil samples from 8 sections in the lower reaches of the Fenhe River and analyzed the content and distribution characteristics of heavy metals. The degree of heavy metal pollution and potential ecological risks were evaluated using the single-factor pollution index method and the potential ecological risk index method. The sources of heavy metals were analyzed through principal component analysis (PCA) and positive matrix factorization (PMF). [Results] 1) The content of heavy metals in the riparian soils of the lower reaches of the Fenhe River was generally low, but the enrichment effects of Pb and Hg were significant. The points with higher contents of Cu, Zn, Cr, Pb and Hg were mainly distributed in urban suburbs and the industrial areas concentrated along the riverbanks. The content of heavy metals in the Yellow River estuary area was relatively low. 2) The potential ecological risk index (PERI) was the highest downstream of Linfen City, at river confluences, and in Hejin City. Hg and Pb were at moderate-severe and moderate ecological risk respectively. This may be affected by the emission of Hg vapor from coal-fired power plants and coking plants downstream of Linfen City and Hejin City, as well as the sedimentation of smoke dust from lead smelters. The river confluences were areas where substances precipitated and accumulated, which was conducive to the enrichment of heavy metals. Industrial parks were scattered in areas such as Xiangfen County and Jishan County, resulting in a higher content of heavy metals in the middle of the lower reaches than in areas flowing into the Yellow River. In the areas with moderate ecological risks of heavy metals Hg and Pb, the contribution rates of Hg and Pb were 54.92% and 31.28%, respectively, indicating that Hg and Pb were the key elements for controlling soil heavy metal pollution in the lower reaches of the Fenhe River. 3) The correlation analysis results among heavy metals showed that there was a significant correlation between Zn and Pb, with a correlation coefficient of 0.671, indicating the influence of human activities on heavy metals in the soil. The high content of Pb in the soil may be caused by the waste gas and waste residue from surrounding industrial and mining enterprises. Dust and wastewater from human activities also caused Zn to enter the soil. The results showed that agricultural production was an important source of Zn and Pb. The unreasonable use of chemical fertilizers and pesticides for agricultural irrigation in the lower reaches of the Fenhe River led to the long-term accumulation of Zn and Pb in the soil, and the exhaust emissions from agricultural machinery also caused the accumulation of Pb in the soil. The dust and waste gas from mining, smelting, and electroplating production of minerals in the study area were deposited into the soil after rainfall, resulting in an increase in the contents of Zn and Pb in the soil. 4) According to PMF simulation, the main source of Hg was industrial production. Cu in the soil had multiple sources, including industrial production, agricultural activities, and transportation. Cr was relatively stable and less affected by human activities. The changes in Cr content were mainly influenced by soil parent materials and geological activities. [Conclusion] In summary, the main sources of heavy metal pollution in the soil along the lower reaches of the Fenhe River are human activities and industrial production. Controlling heavy metal sources at the origin can gradually reduce the heavy metal pollution in basin soils. At the same time, it can also reduce the heavy metal content in the tributary basins of the Yellow River, which is conducive to the overall ecological protection of the Yellow River.

[Objective] The Poyang Lake floodplain wetland ecosystem plays an irreplaceable role in regulating the hydrology of the Yangtze River Basin and maintaining biodiversity. However, due to the combined effects of climate change and human activities, problems such as water quality deterioration and vegetation community degradation have become increasingly prominent, highlighting the urgent need for systematic investigation. This study aims to address two core scientific questions by constructing a multi-element collaborative analytical framework: (1) to reveal the spatiotemporal change patterns and driving mechanisms of wetland water quality under the dual stress of climate change and human activities; (2) to analyze the combined impact of hydrological regime shifts and land use pattern changes on vegetation community structure and benthic species communities. [Methods] This study integrated multi-source data fusion and spatiotemporal coupling analysis techniques. Six typical monitoring sites were selected, covering three types of habitats: dish-shaped lake areas, tailing areas, and flood detention areas. Water quality parameters (e.g., DO and TP, six indicators in total) and benthic species data were obtained through continuous observations during three hydrological periods (normal, flood, and dry seasons) in 2024. Remote sensing image interpretation was used to obtain land use changes and vegetation conditions. Based on water quality index (WQI), Pinkas’ index of relative importance (IRI), and other methods, the spatiotemporal change characteristics of the wetland ecosystems were systematically analyzed. [Results] (1) The WQI of the Poyang Lake floodplain wetlands was influenced by both temporal and spatial scales. Temporally, it exhibited a migration pattern of pollutants characterized by “dilution in the flood season and enrichment in the dry season”. Spatially, it exhibited a gradient pattern of “poorer conditions in the upper and middle reaches and better conditions in the lower reaches”, with the best water quality observed in the dish-shaped lake areas, while the overall wetland water quality reached the “good” level (60<WQI≤80). Changes in land use patterns led to water quality deterioration during the dry season, and the expansion of farmland in the western region was spatially coupled with areas of water quality deterioration. (2) The vegetation community structure of the Poyang Lake floodplain wetlands deteriorated from 2023 to 2024. The high-density vegetation coverage area (NDVI≥0.7) nearly disappeared in 2024, and the area of the medium-density zone (0.2≤NDVI<0.5) decreased by 12.7%. Vegetation degradation was significant, while overall vegetation coverage increased by 38%. The benthic species communities were significantly influenced by hydrological conditions. Except during the dry season, there were notable interannual variations in the dominant benthic species. Spatially, it was affected by human activities, with community distribution in areas of frequent human activities being lower than that in natural ecological areas. [Conclusion] This study identifies three types of ecological problems in the Poyang Lake floodplain wetlands. The first is the “water quality imbalance under the combined effects of hydrological regimes and human activities”, characterized by significant seasonal variations in pollutants, following a pattern of “dilution in the flood season and enrichment in the dry season”. The second is “degradation of biological community structure and decline in ecological functions”, characterized by deterioration of vegetation community structure, disappearance of high-density areas, reduction of medium-density areas, and lower biomass of benthic species in areas with frequent human activities compared with natural ecological areas. The third is “conflict between land use and ecological functions”, characterized by adverse changes in land cover, forming a positive feedback chain of “farmland encroachment-non-point source pollution-vegetation degradation”. A coordinated governance system of “hydrological regulation-ecological restoration-social participation” is proposed to address these three types of problems. In future, the monitoring scope can be further expanded, and long-term research on the response of wetland ecology to climate change can be strengthened to provide more comprehensive scientific support for sustainable wetland management in the context of the Yangtze River protection strategy.

[Objective] River basins with complex terrain and sparse ground-based rainfall observations often fail to fully capture the spatial heterogeneity of rainfall. This introduces substantial uncertainties into hydrological runoff modeling, flood disaster risk management, and related applications. The Nujiang River Basin upstream of the Gongshan station in southwestern China exemplifies this challenge: while it covers a vast area with highly rugged topography, it is plagued by a scarcity of rainfall gauges and their uneven distribution. This study aims to address the challenge of design storm estimation in such data-sparse river basins by exploring the use of high-resolution reanalysis rainfall data. [Methods] A systematic performance assessment was first conducted for three precipitation datasets: the China Meteorological Forcing Dataset (CMFD), the Multi-Source Weighted-Ensemble Precipitation product (MSWEP), and ERA5. Daily rainfall values from each dataset were compared with available gauge observations using four statistical indicators: Pearson correlation coefficient (R), relative bias (BIAS), root mean square error (RMSE), and mean absolute error (MAE) to identify the dataset with the best overall performance. Subsequently, for the assessed optimal dataset, the L-moment method was applied in a basin-wide, grid-scale batch processing approach to compute design storms. The specific steps included: (1) determining the most appropriate probability distribution type for each grid point through goodness-of-fit tests; (2) validating the reliability of the method using leave-one-out cross-validation based on observed data. In contrast to conventional analyses typically conducted separately at individual stations, this grid-scale batch processing approach enabled consistent treatment across the entire spatial domain of the river basin, thereby providing a more intuitive reflection of the spatial distribution characteristics of rainfall. Then, annual maximum rainfall series for 1-day, 3-day, and 7-day durations were extracted for each grid cell to compute grid-based design storm values for specified return periods according to the locally optimal probability distribution type and parameters. Finally, spatial analysis was conducted on the design storm values to reveal their spatial distribution patterns. [Results] The evaluation of daily areal average rainfall revealed substantial performance differences among the three reanalysis datasets. ERA5 tended to significantly overestimate rainfall in the Nujiang River Basin, with a BIAS of up to 56%, indicating substantial deviation from ground observations. MSWEP performed better than ERA5. CMFD showed the best performance, displaying a strong correlation with observed station data (R=0.82), an exceptionally low BIAS (1%), and the smallest RMSE (1.38 mm) and MAE (0.78 mm) among the three datasets. The accuracy assessment results at each station further confirmed the superiority of CMFD. Nevertheless, all three datasets exhibited their largest significant errors at Gongshan station—an outcome consistent with previous studies indicating that both reanalysis and merged products struggled to maintain accuracy in areas of steep relief and high spatial rainfall variability. The derivation of design storms using the grid-scale L-moment method driven by CMFD exhibited significant reliability, specifically manifested in the following aspects.At the station scale,the design storm values under the 5% design frequency,estimated by the L-moment method based on CMFD grid data,were highly consistent with results obtained by the Pearson Type Ⅲ distribution’s visual curve-fitting method. At the areal scale, for the 1-day, 3-day, and 7-day annual maximum design storms in the river basin, under the four return periods of 0.1%, 1%, 2%, and 5%, the relative errors between the calculation results of the CMFD-based grid method and the areal design storm values of the river basin derived by the traditional visual curve-fitting method were within 10%. Spatial analysis of the CMFD-derived design storm maps revealed three distinct high-value zones of design storms across the river basin. The high-value zone surrounding Gongshan station was corroborated by observed data, confirming its reliability. The other two potential high-intensity zones were located in areas lacking adequate ground observations and thus required further verification through targeted field campaigns. [Conclusion] The grid-based high-resolution design storm estimation framework in this study overcomes the limitations of traditional station-based methods in data-scarce basins. By integrating optimally evaluated reanalysis precipitation data with the L-moment method applied in a spatially explicit manner, the approach yields detailed precipitation extreme maps that preserve local variation. Compared with the conventional approach that relies solely on limited station data to produce basin-average or single-station design storm estimates, the generated design storm maps deliver substantially enhanced spatial detail and accuracy.

[Objective] To meet the demands of constructing a modern reservoir operation and management matrix and to further enhance the flood forecasting and early warning capabilities of the Danjiangkou Reservoir, this study simulates the over-standard flood discharge scenario of the Danjiangkou Reservoir, and analyzes the evolution and inundated conditions of the downstream flood, aiming to improve the accuracy of the flood forecasting visualization and emergency planning. In doing so, the study seeks to provide stronger support for ensuring the safety of the Danjiangkou Reservoir Project and the security of water supply. [Methods] Based on the principles of the continuity equation and the momentum equation, the MIKE series model software was used to establish a two-dimensional hydrodynamic model of the downstream river channel and floodplain. Fully considering the impact of flood evolution on key cities downstream, a computational grid was constructed based on DEM and measured terrain to simulate the flood evolution of the downstream areas during over-standard discharge from the Danjiangkou Reservoir under seven scenarios. The arrival time of flood and peak flood in downstream areas under different scenarios was analyzed. The relationships between peak flood water level, peak flood discharge, design water level, and maximum allowable flow at typical cross-sections were compared. Based on GIS software, the inundation range and inundation depth in the downstream areas under different scenarios were analyzed. [Results] Under the regulation and storage effect of the project, the flood reached Xiangyang and its upstream areas within 2 to 10 hours, the Yicheng-Shayang cross-section within 10 to 24 hours, and the Xiantao area approximately two days later. The arrival time of the flood peak at each typical cross-section exceeded 68 hours. Under the condition of concrete dam failure, the flood spread pattern in the mountainous areas near the dam was similar to that under the regulation and storage scenario, while the flood evolution speed in the plain areas was generally more than 3 hours slower than that under the engineering regulation and control scenario. In the case of sudden over-standard water inflow and earth dam failure, the flood and flood peak in the areas upstream of Zhongxiang arrived within 12 and 18 hours, respectively, while in the areas downstream of Zhongxiang, they arrived after 13 and 31 hours, respectively. There were high risks of embankment overflow in the section from Danjiang to Yicheng. Under the conditions of verification flood, design flood, dead water level failure of the concrete dam, and failure of the elevated part of the concrete dam, the inundation depth of most areas from the dam site to Fancheng, Xiangyang was 16-20 meters, while that downstream of Tianmen was less than 3 meters. In the case of a severe flood in which all soil and rocks collapsed, the inundation depth from the dam site to Fancheng, Xiangyang was approximately 35 meters, while that of the Danjiangkou section exceeded 50 meters. The flood in Shayang County caused the inundation of over 70% of the area in Qianjiang City, with the overall inundation depth below 2 meters. [Conclusion] Based on the above results, this study provides strong support for the emergency preparedness for the safety management of the Danjiangkou Reservoir Dam, the development of a visualized flood rehearsal, and the construction of a modern reservoir operation management matrix.

[Objective] At present, the calculation methods for the discharge capacity of polygonal-line low-head practical weirs at sluices (such as the discrimination of the submergence limit and the calculation of the submergence coefficient) are still incomplete, which brings considerable inconvenience to related engineering design. This study aims to investigate the submergence limit discrimination and submergence coefficient calculation of polygonal-line low-head practical weir flow, and to propose corresponding discrimination criteria and calculation methods. [Methods] Hydraulic normal model tests on the discharge capacity of polygonal-line low-head practical weirs at sluices were conducted. The model conditions included a weir height T≤2.0 m (T=0, 0.5, 1.0, 1.5, and 2.0 m), a ratio of weir height to crest length T/δ≤0.4, and a submergence ratio h_s/H₀≤0.96, where h_s was the downstream water depth referenced to the weir crest, and H₀ was the total upstream head above the weir crest. The model scale was 35.5. [Results] (1) Under the experimental conditions of this study, the critical submergence limit (h_s/H₀) of the weir flow ranged from 0.748 to 0.787, which was lower than that of the broad-crested weir (0.8), indicating that the polygonal-line low-head practical weir at a sluice was more susceptible to submergence than a broad-crested weir. As the weir height T and T/δ increased, the critical submergence limit decreased correspondingly, and the difference between the critical submergence limit of the polygonal-line weir and that of the broad-crested weir increased accordingly. (2) Under the same submergence ratio h_s/H₀, the submergence coefficient σ of the polygonal-line low-head practical weir was smaller than that of the broad-crested weir σ₀. The absolute value of the relative error η between the two increased with increasing T, T/δ, and h_s/H₀. Compared with the geometric parameters of the polygonal-line low-head practical weir, namely the weir height T and the crest length δ, the variations in weir height T had a relatively greater influence on the relative error η. (3) When h_s/H₀ ranged from 0.8 to 0.96, the relative error η ranged from -1.5% to -7.6%. (4) Based on the weir height T and submergence ratio h_s/H₀, the relative error η can be obtained from the η-T-h_s/H₀ relationships shown in Table 4 and Figure 8. The corresponding submergence coefficient of the polygonal-line low-head practical weir could then be calculated using the formula and applied to discharge capacity calculation. (5) The results were validated by multiple hydraulic model test results of sluice projects. The discharge values calculated based on the present results were in good agreement with the measured values, with the absolute value of the relative error between the two being less than 1.5%. [Conclusion] The findings of this study can be applied to the calculation of discharge capacity of polygonal-line low-head practical weirs at sluices under the conditions of T≤2.0 m, T/δ≤0.4, and h_s/H₀≤0.96. Further in-depth studies are still required on the discharge characteristics of polygonal-line low-head practical weirs with T>2.0 m and larger ranges of T/δ. The proposed method has relatively high calculation accuracy and convenient application, and can provide a reference for the design and operation of similar projects.

[Objective] Ensuring safe and efficient downstream fish passage for hydraulic engineering structures remains a critical and less-solved challenge in eco-hydraulics. This review aims to systematically synthesize global research on the behavioral characteristics and hydrodynamic requirements of fish during downstream migration. The primary objectives are: (1) to consolidate the known hydrodynamic thresholds (e.g., velocity, turbulence, shear stress) for various fish species; (2) to evaluate the design and efficacy of different downstream passage facilities in relation to these hydrodynamic needs; and (3) to propose an integrated framework for future research and facility design that moves beyond single-parameter approaches to a holistic “fish-flow system” perspective.[Methods] Literature Synthesis and Categorization: literature was categorized along three primary themes: (1) empirical studies on fish behavior (e.g., active vs. passive descent) in response to hydrodynamic factors (velocity, turbulence, shear, acceleration, pressure); (2) technological assessments of different downstream passage facilities (bypasses, turbines, fish collection systems, specialized channels); and (3) advancements in research methodologies, encompassing in-situ field monitoring, laboratory flume experiments, and numerical modeling techniques like Computational Fluid Dynamics and individual-based models. Comparative Analysis and Critical Evaluation: quantitative data (e.g., preference velocity thresholds, injury limits) was synthesized into consolidated summaries (Table 1) and the strengths, weaknesses, and operational challenges of different passage technologies were assessed. Special attention was paid to inconsistencies, research gaps, and the applicability of findings across different fish species and geographies. [Results] 1) First, fish downstream behavior is systematically categorized into active descent (head-first, efficient) and passive descent (counter-current, inefficient), with transitions between these states triggered by specific hydrodynamic conditions. Preference and injury thresholds for critical hydrodynamic parameters have been quantified for several species. For instance, juvenile grass carp exhibit a preference velocity of 0.19-0.49 m/s, while Atlantic salmon smolts prefer 0.38-0.73 m/s. Turbulent kinetic energy (T_KE) below 0.03 m²/s² is generally preferred, with higher T_KE leading to disorientation and inefficient passage. Crucially, injury thresholds for shear strain rate vary significantly among species, from 500 s^-1 for trout to 2 179 s^-1 for juvenile Chinese carps. Similarly, pressure change gradients exceeding 50 kPa/s during descent or 15 kPa/s during ascent can cause barotrauma. 2) The critical evaluation of passage facilities reveals distinct performance profiles. Surface-oriented bypasses and specialized downstream channels (e.g., fish slides) generally offer a higher survival rate by leveraging fish surface-orientation and providing low-turbulence pathways. Despite improvements that can increase fish survival rates to over 97% in fish-friendly turbines, they still represent the highest-risk passage route compared to alternative passage routes. Collection and transportation systems are effective for high dams and complex terrain conditions, but require intensive operational maintenance and incur high costs. 3) One of the most significant result of this review is the proposal of an innovative, three-dimensional framework for defining and designing hydrodynamic conditions for downstream passage. This framework posits that effective passage requires the simultaneous satisfaction of three interconnected criteria: Necessity, Safety, and Timeliness. Necessity: Hydrodynamic conditions (e.g., specific “preference velocities” and “low-turbulence windows”) that actively attract fish and initiate downstream movement into the facility. Safety: Conditions that prevent injury or mortality, defined by rigid thresholds for damaging forces (e.g., shear strain rate, pressure gradient) throughout the entire passage route. Timeliness: Conditions that promote efficient and timely passage by minimizing behavioral delays (e.g., “non-direct descent” caused by adverse acceleration flows), ensuring fish are not energetically depleted or exposed to predators for extended periods. [Conclusion] Prevailing approach often focuses on isolated hydrodynamic parameters, which is insufficient for designing highly effective downstream passage facilities. The path forward requires a paradigm shift towards an integrated “fish-flow system” approach, as embodied by the proposed Necessity-Safety-Timeliness (N-S-T) framework. The ideal downstream passage is not merely a conduit with non-lethal hydraulic conditions; it is a system where the entrance flow (Necessity) seamlessly connects with a guiding, efficient internal flow (Timeliness), all while rigorously excluding hazardous forces (Safety). Future research can prioritize: (1) multi-factorial studies that explore the synergistic effects of coupled hydrodynamic parameters on fish behavior; (2) expanded research on adult fish and a wider range of non-salmond species, particularly those prevalent in Asian rivers; (3) the systematic development of a comprehensive, open-access database of hydrodynamic requirements; and (4) the integration of advanced monitoring technologies (e.g., AI-powered sonar, drones) and predictive numerical models for both design optimization and post-construction evaluation. By adopting this holistic perspective, the goal of harmonizing hydropower generation with sustainable fish conservation becomes increasingly attainable.

[Objective] This study aims to reveal the spatiotemporal distribution patterns and controlling mechanisms of bend circulation in a macrotidal estuarine environment. The specific objectives are as follows: (1) to conduct an in-depth analysis of the dynamic variations in longitudinal velocity, transverse water surface slope, and circulation intensity in the bend during flood and tidal periods; (2) to clarify the controlling role of strong tidal dynamics in the formation and evolution of bend circulation and to reveal its differences from those in conventional rivers. [Methods] Based on the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM) framework, a large-scale three-dimensional hydrodynamic model covering the Qiantang River Estuary from the hydropower station to Ganpu was established. The spatiotemporal distributions of longitudinal velocity, transverse water surface slope, and circulation intensity in the bend were simulated during both flood and tidal periods. The model employed an unstructured grid, with local refinement in the Wenyan bend to accurately resolve the channel topography and flow structures. Model driving conditions included astronomical tidal levels at the offshore open boundary and river discharge at the upstream boundary. Recent synchronous field measurements obtained in the Wenyan reach during spring tide and flood periods, including water level, flow velocity, and flow direction, were used for model calibration and validation. [Results] (1) The SCHISM model showed high applicability and remarkable computational efficiency for this complex application. Under the same parallel computing environment, its computational speed was nearly two orders of magnitude higher than that of conventional explicit Euler-type models (approximately 88 times faster). (2) The transverse water surface slope in the bend varied drastically during the flood tide acceleration stage, and a distinct bimodal pattern was identified for the first time. This behavior was markedly different from the unimodal or gradual variation patterns commonly observed in estuaries with moderate tidal ranges or in unidirectional rivers. In contrast, the variation in the transverse slope during ebb tide was much milder. Quantitative analysis showed that the maximum transverse slope during flood tide reached 5.3 times that during ebb tide, indicating pronounced tidal asymmetry. (3) During both flood and tidal periods, the bend circulation intensity exhibited a spatial pattern characterized by larger values near the bend apex and smaller values toward both ends. The core zone of maximum circulation intensity was consistently located at the cross-section on the downstream side of the bend apex. During the flood period, the maximum circulation intensity was approximately 0.08-0.12. Notably, the maximum circulation intensity during the flood tide of the autumn spring tide (0.09) was slightly greater than that during the 50-year return-period flood and was much greater than that during ebb tide. [Conclusion] (1) The SCHISM model is a powerful and efficient tool for simulating complex hydrodynamic processes in macrotidal estuaries. Its ultrafast computational capability (nearly 100 times faster than explicit models) represents a major technical advantage and has great potential for scientific and engineering applications requiring numerous simulation scenarios. (2) The hydrodynamic phenomena in bends of macrotidal estuaries are highly distinctive. The newly identified bimodal transverse slope pattern during flood tide, together with the pronounced tidal asymmetry, provides an important extension to conventional bend hydrodynamic theory and highlights the fundamental differences between macrotidal estuaries and ordinary rivers. (3) Tidal dynamics are an important factor controlling the generation and development of bend circulation, and their influence may even exceed that of extreme flood events.

[Objective] The particle size distribution of fine-grained loess is a key indicator for evaluating its engineering mechanical properties. The traditional hydrometer method is limited by theoretical assumptions, experimental procedures, and dispersion effects, leading to insufficient measurement accuracy and long testing periods, which makes it difficult to meet the requirements for precise determination of particle size distribution of fine-grained soil. To address these issues, this study takes fine-grained loess from Lanzhou, Gansu Province, as the research object, and proposes a wet sieving test method suitable for fine-grained soil by modifying standard test sieves and using nylon filter cloth as a fine sieve medium. The study systematically investigates the determination of key test parameters and micromorphology. [Methods] After modifying the standard test sieves, multiple comparative tests were conducted focusing on key indicators including soil mass, water volume, test duration, and soil mass loss rate. Scanning electron microscopy (SEM) was employed to observe the micromorphology of particles in different size groups after separation. On this basis, comparative tests between the wet sieving method and the hydrometer method were conducted under conditions of full gradation and single-particle-size groups. [Results] The test results of key indicators determined the optimal test parameters as 30 g of soil and 4 000 mL of water. The duration of a single test was approximately 12 h, with the soil mass loss rate stably controlled within 3%. In contrast, the hydrometer method required approximately 48 h, indicating a fourfold improvement in efficiency. SEM results showed that the wet sieving method could achieve physical separation of fine-grained loess particles. The short-axis dimension of particles was identified as the key parameter controlling the sieving process. A high proportion of particles in the corresponding size groups after separation verified the reliability of the method. Comparative test results showed that the contents of clay and colloidal particles measured by the wet sieving method were significantly higher than those obtained by the hydrometer method, indicating that the traditional hydrometer method markedly underestimated the content of particles smaller than 0.005 mm in fine-grained loess. Further tests on single-particle-size groups confirmed that the hydrometer method exhibited large deviations in particle size determination for fine fractions, whereas the proposed method could directly and accurately reflect the particle size distribution. [Conclusion] This study indicates that the modified wet sieving method based on nylon filter cloth can accurately determine the particle size distribution of fine-grained loess and effectively compensate for the limitations of the traditional hydrometer method in fine particle testing. This method provides reliable technical support and experimental basis for precise geotechnical testing, engineering disaster prevention, and structural safety assessment in loess regions.

[Objective] Among numerical simulation methods for blasting fragmentation, the continuous medium simulation method has high efficiency, but its mechanical mechanisms are not rigorous and errors are significant when dealing with discontinuous problems; the discontinuous deformation analysis (DDA) method performs well for discontinuous problems, but when the fragment size becomes too small, excessively long computation time and non-convergence are likely to occur. This study aims to propose a numerical simulation method for blasting fragmentation that considers both computational efficiency and mechanical rationality. [Methods] Field tests were conducted to reveal the formation characteristics of blasting fragmentation, and the necessity of selecting appropriate numerical simulation methods for different fragment size ranges of blasting fragmentation was clarified. A continuous-discontinuous numerical simulation method for blasting fragmentation based on LS-DYNA+DDA coupling was proposed. In the near-field region of the blast hole, a continuous medium numerical simulation based on LS-DYNA was used to improve the computational efficiency of the crushing zone. In the middle- and far-field regions, a discontinuous method based on DDA was used to achieve discontinuous characterization of blasting fragmentation. The accuracy of using stress and velocity components as coupling parameters was compared. Finally, the LS-DYNA+DDA coupling method was validated based on the mining and blasting practice of Zhoushan Green Petrochemical Mine. [Results] Through field experiments and numerical simulation, it was determined that small-sized fragments were mainly concentrated within a very small range near the blast hole. The continuous medium method could efficiently simulate the distribution of small-sized fragments while ensuring accuracy. It was more reasonable to use DDA method to simulate the fragmentation of medium- and large-sized fragments. Using peak velocity as the coupling parameter between different methods could reduce the pressure loss during computation transmission. [Conclusion] Based on the measured results, comparison and validation between existing numerical simulation methods and the proposed LS-DYNA+DDA coupling method show that the proposed method improves the accuracy of blasting fragmentation prediction and has advantages in balancing the mechanical rationality of fragmentation mechanisms and computational efficiency. However, this method is currently applied in limited engineering scenarios, and its prediction efficiency needs further summary and optimization for different lithologies and blasting parameters.

[Objective] Research on predicting the hydraulic conductivity of cohesive soils is relatively lacking. The classical Kozeny-Carman equation provides an effective method for estimating the hydraulic conductivity of coarse-grained soils, but it performs poorly in predicting the hydraulic conductivity of cohesive soils. This study aims to improve the Kozeny-Carman equation and establish a method for calculating the hydraulic conductivity of cohesive soils. [Methods] We first constructed a relationship between bound water content and liquid limit (LL) in cohesive soils using statistical methods based on their correlation analysis. With this relationship as a bridge, we established a correlation between the total void ratio and the effective void ratio of the soil. Accordingly, the Kozeny-Carman equation was modified to develop a method for calculating the hydraulic conductivity of cohesive soils. Considering that parameter C in the modified equation was difficult to obtain in engineering practice, we developed a calculation model for specific surface area of cohesive soils, incorporating bound water, free water, and soil particles, in order to establish an engineering-friendly equation. A semi-empirical equation relating the specific surface area (S_s) of soil particles to liquid limit was derived, leading to a formula that calculated parameter C based on specific surface area. Data of 105 cohesive soils from published literature were employed to calculate hydraulic conductivity using both the original and modified equations, and the results were compared with measured values. After predicting the saturated hydraulic conductivity of cohesive soils using the improved model, we further evaluated the model’s predictive performance using two error metrics: Mean Absolute Error (MAE) and Root Mean Square Error (RMSE). Subsequently, the sensitivity of each input parameter was analyzed using the cosine amplitude method. Finally, the influence of the main clay mineral types and the order of magnitude of the measured hydraulic conductivity values on the model’s predictive performance was analyzed. [Results] (1) As the water content increased in cohesive soils, the hydration of clay minerals proceeded sequentially through tightly bound water, loosely bound water, and free water phases. A strong linear correlation existed between the ineffective void ratio and the logarithm of liquid limit (lgLL), with a coefficient of determination (R²) of 0.98. A discernible linear correlation was observed between the reciprocal of specific surface area (1/S_s) and the reciprocal of liquid limit (1/LL), with R²=0.83. Parameter C in the Kozeny-Carman equation exhibited a power-law relationship with soil specific surface area, with R²=0.85. The prediction reliability of the classical Kozeny-Carman equation was 56.2%, while that of the improved equation achieved 81.9%, representing a 25.7% improvement in accuracy. However, predictions exhibited divergence, primarily due to the heterogeneity of the experimental data sources, the error propagation from the indirect estimation of specific surface area data, and the fact that the improved formula relied solely on void ratio and liquid limit, potentially neglecting factors like particle size distribution and pore channel tortuosity. (2) Sensitivity analysis revealed that both void ratio and liquid limit were the primary parameters affecting the prediction accuracy of hydraulic conductivity. The model’s performance metrics for the database were MAE=0.29 and RMSE=0.36. For kaolinite-dominated clay, prediction reliability reached 72.4% (MAE=0.29, RMSE=0.38); that of montmorillonite-dominated clay achieved 94.4% (MAE=0.30, RMSE=0.32); and that of illite-dominated clay showed 77.8% (MAE=0.35, RMSE=0.38). Overall, the type of clay mineral had little influence on model performance. When the measured hydraulic conductivity value was within the 10^-9 m/s order of magnitude, the prediction reliability was 88.2% (MAE=0.24, RMSE=0.29);when it was within the 10^-10 m/s order of magnitude,the prediction reliability was 93.3% (MAE=0.20,RMSE=0.25);when it was within the 10^-11 m/s order of magnitude, the prediction reliability was 65.9% (MAE=0.42,RMSE=0.47). [Conclusion] These results show that the prediction reliability of hydraulic conductivity at the 10^-11 m/s order of magnitude is significantly lower than at the 10^-9 and 10^-10 m/s order of magnitude, with the errors and divergence much higher for the 10^-11 m/s order of magnitude. Therefore, the magnitude of hydraulic conductivity has a great impact on model performance, and the model has better applicability for predictions within the 10^-9 to 10^-10 m/s order of magnitude. The modified Kozeny-Carman equation proposed in this study provides a reliable theoretical reference for estimating the hydraulic conductivity of cohesive soils in geotechnical engineering practice.

[Objective] This study aims to investigate the mechanical softening behavior of cataclastic granite under different in-situ stress conditions through true triaxial compression tests, thereby providing an experimental basis and theoretical references for the safe construction and long-term stability assessment of deep soft rock engineering. [Methods] Using the self-developed TAXW-5000 multi-field coupled true triaxial simulation system, true triaxial compression tests on cataclastic granite under different minimum principal stresses (σ₃) were conducted for the first time. The effects of the minimum principal stress on the mechanical response, post-peak softening behavior, and shear-dilation characteristics were systematically analyzed.Furthermore, in combination with 3D-CT scanning technology, the internal crack structures of the failed specimens were extracted and reconstructed, revealing the spatial distribution characteristics of the cracks. The damage evolution patterns of cataclastic granite under different in-situ stress environments were discussed. [Results] As the minimum principal stress (σ₃) increased from 1 MPa to 10 MPa, the peak stress of the cataclastic granite rose from 48.98 MPa to 80.42 MPa, and the residual stress increased from 27.17 MPa to 75.67 MPa. Meanwhile, the softening modulus decreased from 25.67 GPa to 4.81 GPa. During the softening stage, the shear-dilation coefficient decreased from 1.37 to 0.21 with increasing σ₃. In the residual stage, the shear-dilation coefficient first increased and then decreased with higher σ₃, reaching a maximum value of 1.21 at σ₃=5 MPa. CT scanning results indicated that a lower σ₃ led to a greater number and larger apertures of internal cracks in the failed specimens. The damage factor, calculated based on crack statistics, decreased from 0.29 to 0.12 as σ₃ increased from 1 MPa to 10 MPa. [Conclusion] With an increase in the minimum principal stress (σ₃), the softening modulus of cataclastic granite shows a negative correlation with σ₃. The shear-dilation coefficient in the residual stage is relatively high overall and exhibits a trend of first increasing and then decreasing with rising σ₃. In contrast, the shear-dilation coefficient in the softening stage decreases significantly, indicating a transition in the rock deformation mechanism from brittle to ductile failure. A higher σ₃ environment leads to more pronounced softening behavior in cataclastic granite, resulting in a lower degree of crack development and lower damage levels after failure, thereby enhancing the overall engineering stability.

[Objective] The non-filter membrane straw drainage body (NSD) offers significant advantages in vacuum preloading treatment of dredged sludge, such as eliminating the need for filter membranes and preventing clogging, making it a promising solution for practical applications. However, the permeability characteristics and pore structure of the straw filter layer are not yet well understood, which limits its widespread adoption. [Methods] Laboratory permeability tests were conducted to investigate the variation in the permeability coefficient of the non-filter membrane straw drainage body with vacuum preloading time, filter layer thickness, and the initial water content of the dredged sludge. CT scanning was also used to further explore the influence of pore structure evolution under different treatment conditions on the permeability characteristics. [Results] (1) Initially, the NSD contained vertically and horizontally interconnected fissure drainage channels, and these channels formed a continuous seepage network through the connection of pores, which endowed the NSD with superior permeability. The voids occupied 31.42% of the total volume of the NSD, and the volume of fissure structures was 14 times greater than that of the pore structures. (2) The permeability performance of the straw filter layer was superior to that of conventional geotextile filter membranes. The permeability coefficient of the NSD-Reverse (NSD-R) filtration system decreased rapidly and then stabilized with increasing vacuum preloading time, ultimately reaching a stable value on the order of 10^-5 cm/s after 30 minutes, which was one order of magnitude higher than that of the reverse filtration system using geotextile filter membranes for clay (around 10^-6 cm/s). An increase in the filter layer thickness led to a decrease in the permeability coefficient of the filtration system, while a higher initial water content of the dredged sludge corresponded to a larger permeability coefficient of the NSD-R filtration system. (3) As vacuum preloading time increased, the pore structure of the straw filter layer in the NSD-R filtration system evolved. With the increase in preloading time, the porosity of the NSD filter layer decreased rapidly, with the proportion of centimeter-scale fissure structures significantly reduced, effectively blocking the continuous migration of fine particles. Subsequently, the proportion of millimeter-scale pore structures increased, enhancing soil retention while ensuring water permeability, thereby achieving a balance between soil retention and water permeability. [Conclusion] These findings provide theoretical support for the engineering application of NSD in environmentally sustainable stabilization of dredged sludge.

[Objective] Vegetation concrete in alpine regions is prone to structural loosening and mechanical performance degradation after freeze-thaw cycles, which in turn limits the effectiveness of slope ecological restoration, while the coupled effects of soil type and biochar content on the freeze-thaw characteristics of vegetation concrete under unidirectional freeze-thaw conditions remain insufficiently understood. To address the above issues, this study investigates the effects of soil type and biochar content on the freeze-thaw characteristics of vegetation concrete, reveals the underlying mechanisms, and provides theoretical support for the optimization design of frost-resistant mix proportions in alpine regions. [Methods] Sandy soil and cohesive soil collected from Yichang were selected as planting substrates. Vegetation concrete specimens using sandy soil (VC-SS) and cohesive soil (VC-CS) were fabricated, respectively. Unidirectional freeze-thaw tests were conducted. The temperature field changes at different depths of the specimens were monitored in real time, and frost heave deformation data were collected using displacement sensors. The layered water content before and after freeze-thaw cycles was determined using the oven-drying method. The effects of soil type and biochar content on freezing temperature, frost heave amount, and water migration patterns of vegetation concrete were systematically analyzed, and the mechanisms were interpreted from the perspectives of thermal conduction, pore structure, and water transport. [Results] 1) Soil type had a significant effect on the freeze-thaw characteristics of vegetation concrete. Under the same biochar content, the freeze-thaw resistance of VC-SS was significantly better than that of VC-CS. The freezing center temperature of VC-SS was 0.2 ℃-1.8 ℃ lower than that of VC-CS, the maximum frost heave amount reduced by 5.6-7.0 mm, the water migration amount decreased by 0.2%-1.3%, and VC-SS reached the frost heave peak earlier. 2) During the freeze-thaw process, the water content of both types of specimens exhibited an “inverted C-shaped” distribution pattern. In freezing stage, water showed a unidirectional upward migration pattern from bottom to top, with the water content in the deep layer decreasing to 15.2%-19.83% and that in the shallow layer increasing to 20.2%-22.6%. In thawing stage, the water migration pattern shifted to bidirectional migration. The surface layer water content decreased by 0.05%-1.3%, the middle layer increased by 0.02%-1.4%, and the deep layer showed an overall decreasing trend of 0.4%-1%. 3) The effect of biochar content on the freeze-thaw characteristics of vegetation concrete exhibited a nonlinear pattern. With increasing biochar content, the freezing center point temperature, frost heave amount, and water migration amount of VC-SS and VC-CS all showed a trend of first decreasing and then increasing, with 0.5% being the optimal content. [Conclusion] Under unidirectional freeze-thaw conditions, sandy soil with low fine-particle content combined with 0.5% biochar content can significantly improve the freeze-thaw resistance of vegetation concrete and is an optimal scheme for mix proportion design in alpine regions. This mix proportion has relatively high thermal conductivity and low thermal insulation performance. Therefore, plant species with low-temperature germination characteristics should be selected in engineering applications to ensure the effectiveness of slope ecological restoration. The innovation of this study lies in the systematic clarification of the coupled regulatory mechanisms of soil type and biochar content on hydrothermal migration and frost heave deformation of vegetation concrete under unidirectional freeze-thaw action for the first time. This study clarifies the internal mechanisms of freeze-thaw deterioration under different mix proportions, and addresses the insufficient understanding of unidirectional freeze-thaw characteristics of vegetation concrete in alpine regions. The findings provide key theoretical support for frost-resistant design of vegetation concrete in slope ecological restoration of water conservancy and transportation engineering in alpine regions. Future studies can further investigate the evolution of geotechnical properties of vegetation concrete under freeze-thaw cycles to improve the engineering application system.

[Objective] Owing to its abundant availability, aeolian sand can be used in concrete production. To investigate the influence of curing age on the pore structure of aeolian sand concrete, concrete specimens are prepared by replacing river sand with varying amounts of aeolian sand, and the effects of aeolian sand content and curing age on the development of pore structure and compressive strength are explored. [Methods] In the experiments, compressive strength was used as a macroscopic indicator for aeolian sand concrete specimens with different aeolian sand contents at different curing ages. At the microscopic level, nuclear magnetic resonance (NMR) was employed to analyze the internal pore structure, including T₂ spectrum, porosity, free fluid saturation (MFFI), and bound fluid saturation (BVI). Scanning electron microscopy (SEM) was used to observe the internal microstructure and monitor structural changes in the concrete. [Results] Results showed that the internal porosity of concrete gradually decreased with increasing curing age, while the compressive strength increased correspondingly. At the same curing age, the compressive strength of concrete initially increased and then decreased as aeolian sand content increased. The optimal improvement occurred at 25% aeolian sand replacement. At 28 d age, the compressive strength of the concrete with 25% aeolian sand replacement was 1.02, 1.08, 1.11, and 1.19 times that of ASC-0, ASC-50, ASC-75, and ASC-100, respectively. SEM observations showed that microcracks and pores gradually decreased with curing age, and ASC-25 exhibited superior compactness among all mixtures. The T₂ spectrum displayed three to four peaks, with the first peak as the dominant component. As the curing age increased, the proportion of the first peak area gradually increased in all groups. At 14 d age, the first peak proportions of ASC-0, ASC-25, ASC-50, ASC-75, and ASC-100 were 89.7%, 92.68%, 88.74%, 86.66%, and 86.13%, respectively. The proportion of harmless pores gradually increased in each group. For ASC-25, the harmless pore proportion was 43%, 48%, 60%, 63%, and 66% at 7, 14, 28, 56 d, and 84 d, respectively. The internal pore structure of the concrete gradually became denser. For ASC-25, the bound fluid saturation values were 91.51%, 92.72%, 94.53%, 95.21% and 96.37% at 7, 14, 28, 56 d, and 84 d, respectively, with corresponding porosities of 3.00%, 1.65%, 1.25%, 1.22%, and 1.04%, respectively. [Conclusion] The gray correlation analysis indicates that bound fluid saturation and the proportion of harmless pores are strongly correlated with compressive strength, with correlation coefficients exceeding 0.8. The compressive strength predicted by the GM(1,3) model closely matches the experimental results, with a maximum residual of 0.44 MPa, and the relative errors are all within 5%. The established GM(1,3) model can effectively predict the compressive strength of aeolian sand concrete at different curing ages, providing a reference for practical engineering applications.

[Objective] With the rapid development of the iron and steel industry, steel slag, a by-product of iron and steel smelting, has become a key focus for environmental protection and sustainable development. However, its low activity can lead to a reduction in the early strength of concrete. Calcium silicate hydrate (C-S-H) not only shortens or even eliminates the induction period through homogeneous nucleation but also provides an excellent physical filling effect without negatively affecting later strength. This feature gives C-S-H broader application potential and greater prospects compared with traditional early-strength agents. However, C-S-H tends to agglomerate due to its small particle size and large specific surface area, which reduces its accelerating effect. [Methods] In this study, calcium silicate hydrate/polycarboxylate ether (C-S-H/PCE) materials were synthesized, and the effects of different dosages of C-S-H/PCE on the setting time, compressive strength, pore structure, and cement hydration of steel slag cement mortar were systematically investigated. The setting time of the cement mortar was measured following the standard test methods for water consistency, setting time, and stability of cement (GB/T 1346). Following the cement mortar strength test method (GB/T 17671-2021), the compressive strength of cement mortar was measured at 6 h, 8 h, 12 h, 18 h, 1 d, 3 d, 7 d, and 28 d. The porosity of the cement mortar after 1 d, 3 d, and 7 d of curing was determined using an Autopore IV 9520 mercury porosimeter. The thermal stability of the mortar at 3 d, 7 d, and 28 d was analyzed using a NETZSCH STA 2500 thermogravimetric analyzer. The mineral composition of steel slag cement mortar with varying C-S-H/PCE content was qualitatively analyzed using a Shimadzu XRD-6100 X-ray diffractometer. [Results] The adsorption of Ca²⁺ by C-S-H/PCE followed the Langmuir adsorption model. The results indicated that Ca²⁺ adsorption by C-S-H/PCE was monolayer, with a maximum adsorption capacity (Qmax) of 26.19 mg/g. Incorporating an appropriate amount of C-S-H/PCE into steel slag cement mortar effectively accelerated its setting time. The reduction in setting time was proportional to the C-S-H/PCE dosage. Higher C-S-H levels led to shorter times for the cement mortar to reach both initial and final setting. Adding C-S-H/PCE enhanced the compressive strength of steel slag cement mortar, particularly at early stages (within 1 day). Higher C-S-H/PCE content resulted in greater early compressive strength, while the rate of compressive strength increase decreased with curing age. Incorporating an appropriate amount of C-S-H/PCE effectively improved the compactness of steel slag cement mortar and refined its pore structure. This effect was particularly pronounced at early stages of mortar curing, during which C-S-H/PCE significantly reduced porosity. As C-S-H/PCE content increased, the number of macropores decreased significantly, while the proportion of gel pores and mesopores increased, promoting a denser and finer pore structure. [Conclusion] C-S-H/PCE not only stimulates the formation of additional hydration products and accelerates the overall hydration process but also does not alter the types of hydration products, ensuring the stability and controllability of cement properties, and providing a new approach for optimizing steel slag cement mortar performance. This study provides a solid theoretical foundation and technical guidance for the scientific and rational utilization of steel slag in concrete, promoting its practical application and the sustainable development of steel slag resources.

[Objective] This study investigates the fracture performance of ultra-high performance concrete (UHPC) reinforced with steel fibers and steel-polyvinyl alcohol (PVA) hybrid fibers through combined experimental tests and extended finite element method (XFEM) simulations. The objective is to determine an optimal hybridization strategy that enhances fracture resistance and cost efficiency, thereby providing theoretical support and practical guidance for engineering applications. [Methods] Notched beam specimens were tested using the three-point bending method. The program included one control group, five groups with varying steel fiber dosages (0.5-2.5% by volume), and five groups reinforced with hybrid steel-PVA fibers, maintaining a total fiber volume of 2.5% while adjusting PVA replacement ratios from 0 to 1.0. P-CMOD (load-crack mouth opening displacement) curves were used to evaluate flexural strength, initiation toughness, unstable toughness, and fracture energy. Parallel XFEM simulations were developed in ABAQUS, where fracture initiation was governed by maximum principal stress criterion and crack growth was modeled with energy-based softening laws. Experimental and numerical outcomes were compared to assess the predictive accuracy of XFEM. [Results] 1)The addition of fibers transformed the fracture behavior of UHPC from brittle through-crack failure to ductile non-penetrating fracture. Three distinct modes were identified: brittle single-crack, ductile single-crack, and ductile multi-crack. Steel fibers mainly provided bridging and anchorage, delaying unstable crack growth and enhancing energy dissipation, whereas PVA fibers controlled micro-crack initiation and dispersed stresses effectively, often rupturing instead of pulling out. This complementary mechanism revealed a clear division of roles, highlighting a “synergistic hybrid effect” that improved toughness and post-cracking performance. 2)Quantitatively, increasing steel fiber dosage yielded significant improvements. At 2.5% steel fibers, the initiation load, peak load, initiation toughness, unstable toughness, and fracture energy increased by 146.55%, 60.94%, 145.13%, 56.28%, and 45.58%, respectively, compared with specimens containing 1.0% steel fiber. Hybrid reinforcement further optimized performance. At a total fiber content of 2.5%, replacing 20% of steel fibers with PVA (γ=0.2) increased initiation toughness by 6%, while unstable toughness decreased by only 2%, representing the most favorable balance between toughness and economy. In contrast, higher PVA replacement ratios (γ>0.2) reduced flexural strength and fracture energy due to fiber agglomeration and uneven dispersion within the UHPC matrix. 3)Cost analysis further emphasized the advantages of hybridization. Copper-coated steel fibers cost approximately 6.5 RMB/kg, whereas PVA fibers were about twice as expensive. By replacing 20% of steel fibers with PVA at 2.5% total content, material costs were reduced by 11.6% compared with 2.5% steel fiber UHPC, without compromising fracture resistance. This finding underscored the engineering value of hybrid design, particularly for large-scale applications requiring both high durability and economic efficiency. 4)XFEM simulations closely reproduced experimental outcomes. Simulated P-CMOD curves were generally enveloped within the experimental results, and predicted crack paths matched observed failure modes. Average relative errors were 4.21% for peak load, 3.88% for unstable toughness, and 13.62% for initiation toughness, which were within acceptable limits. Moreover, XFEM captured the delayed crack penetration behavior in hybrid fiber specimens, showing how fiber synergy effectively slowed crack growth. This predictive capability demonstrated the suitability of XFEM for analyzing complex hybrid fiber systems, reducing experimental workload while offering mechanistic insights into crack evolution. [Conclusion] Steel-PVA hybridization significantly enhances UHPC fracture behavior and reduces cost, confirming the benefits of a synergistic reinforcement approach. The main conclusions are as follows: 1) Fibers convert UHPC failure from brittle through-crack rupture to ductile failure characterized by irregular, non-penetrating cracks, improving structural integrity and durability. 2) Increasing steel fiber dosage enhances toughness and ductility, with contents above 1.5% yielding substantial improvements in fracture parameters and shifting the load-bearing response from brittle to ductile. 3) A replacement ratio of γ=0.2 is optimal, strengthening crack initiation resistance and sustaining fracture toughness while reducing material costs by 11.6%. Excessive replacement (γ>0.2) negatively affects strength and fracture energy, highlighting the need for balance in hybrid design. 4) XFEM effectively simulates crack initiation, propagation,and post-cracking responses, achieving strong agreement with experiments.The method offers a reliable tool for predicting fracture performance in hybrid UHPC and can support performance-based design with reduced reliance on extensive laboratory testing.

[Objective] This study aims to analyze research literature in the field of water ecological health assessment in the Yangtze River Basin using bibliometric software, systematically summarize development trends, frontier hotspots, knowledge clusters, and evolutionary pathways of research in recent years, and review research progress and main contents, thereby providing reference for the future research and protection of water ecological environment in the Yangtze River Basin of China. [Methods] Bibliometrics and knowledge graph methods are adopted. Using VOSviewer and CiteSpace software, this study systematically analyzes the research literature related to water ecological health assessment in the Yangtze River Basin from 2000 to 2024 in the Web of Science Core Collection (WoSCC) database (561 papers) and China National Knowledge Infrastructure (CNKI) database (364 papers), and identifies the clustering of research hotspots and the evolution patterns of burst keywords. [Results] The results show that: (1) the research development exhibits three-stage characteristics. In the early stage, the annual average number of publications was less than 10. In the middle stage, the annual number of publications gradually increased driven by policies. After 2017, the annual number of publications increased rapidly, showing a significant coupling relationship with the implementation of ecological protection policies in the Yangtze River Basin. (2) WoSCC research focuses on the pollution-driven ecological risk assessment mechanisms, while CNKI research emphasizes innovation in assessment methods and the diagnosis of biological integrity thresholds. The former mainly addresses micro-level mechanisms, while the latter focuses on macro-level management. (3) In the new era, research priorities have shifted toward multidimensional comprehensive assessment of water ecosystems, supporting precise health assessment of the river basin and facilitating the fundamental shift of the “14th Five-Year Plan” river and lake management objectives toward “water ecosystem health evaluation”. [Conclusion] Future research should focus on deepening interdisciplinary integration, further strengthening the capacity building of water ecological monitoring, promoting the deep integration of intelligent monitoring technology and multi-interface coupling models, developing comprehensive evaluation indicators of multi-group biological indicators, improving the monitoring indicators and standard system for water ecological health assessment, and providing scientific support for the protection and research of water ecological health in the Yangtze River Basin.