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

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

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

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

[Objective] The Emeizhou braided channel in the lower section of Anqing is situated in the lower reaches of the Yangtze River. Due to the significant variations in scouring and silting in the braided channel of the Anqing section, the variations in diversion ratio exhibit certain periodicity under the new water-sediment regime. This study aims to investigate the variations in the diversion ratio of the left branch and the stability of the main navigation channel conditions under the new water-sediment regime, as well as the potential impacts on water-related projects in the Mawo area of the left branch that may arise from near-bank siltation at the confluence of the braided channels in the near future. [Methods] By integrating a two-dimensional hydrodynamic model and utilizing the latest field observation data on water and sediment, this study tracked and analyzed the periodic evolution characteristics, influencing factors, and flow characteristics of this braided channel. Additionally, recommendations for the future management direction of the Anqing section were proposed. [Results] In recent decades, the thalweg swing amplitude in the Emeizhou braided channel was relatively small overall, and the river regime remained relatively stable, except for areas near the diversion point of Emeizhou, the head of the right branch, and certain localized river sections of the middle branch. After the impoundment of the Three Gorges Reservoir, the left branch of Emeizhou has experienced slight scouring, while the right branch was expected to maintain a shrinking trend for some time. Numerical simulation results of the flow characteristics of the braided channel showed that from 1998 to 2006, the middle branch developed rapidly, characterized by increased flow velocity and a shift of the main flow towards the Emeizhou on the right bank, while the flow velocities in the left and right branches decreased slightly. From 2006 to 2021, under the guidance and regulation of river (navigation) channel engineering, the diversion ratio of the Emeizhou braided channel remained basically stable, the development of the middle branch slowed down, and changes in flow velocity were mainly concentrated in the lower section of the middle branch. [Conclusion] We recommend to closely monitor the effectiveness of the existing river regulation projects in the Anqing section and to implement relevant measures when necessary.

[Objective] Submerged dams, as an important component of waterway regulation projects, can prevent from river and coastal erosion and also alter river flow patterns and improve river habitat environments. This study selects juvenile crucian carp as the target species to investigate the effects of W-shaped submerged dams with different particle size compositions on flow velocity distribution and the aggregation of target fish. It aims to clarify the habitat improvement effectiveness of W-shaped submerged dams and provide a theoretical basis for the construction of ecological conservation zones in cut-off river sections. [Methods] Crucian carp, a dominant fish species in the Pinglu Canal, were selected as the experimental species. Experiments on the influence of W-shaped submerged dam groups on flow structure and typical fish aggregation behavior were conducted in an annular flume. The W-shaped submerged dams were set with three particle size compositions ([5,10] mm, [5,20] mm, [10,20] mm), and four flow rate conditions (0.03, 0.045, 0.06, 0.09 m³/s) were set up. Under each test condition, 30 juvenile experimental crucian carp were placed, and the entire experimental process was recorded using a Nikon D7500 camera. Tracker software was used to collect and record the positions, frequencies, and total time where the experimental fish stayed for more than 1 minute. The average fish aggregation rate was used to evaluate the attraction effect of the W-shaped submerged dams on fish, and the flow pattern diversity index was used to quantify changes in flow patterns. [Results] Under the action of W-shaped submerged dams of the same particle size, as the flow rate increased from 0.03 m³/s to 0.09 m³/s, the area and flow velocity of the backwater zone upstream of the weir in the study area continuously increased, while the area of the triangular recirculation zone formed downstream of the weir gradually decreased. Under the action of W-shaped submerged dams of different particle sizes, those composed of larger particle sizes (10-20 mm) exhibited better permeability, smaller velocity gradients compared to smaller particle size groups, and higher flow pattern diversity. Analysis of the influence of W-shaped submerged dam groups on the aggregation behavior of experimental fish showed that at a flow rate of 0.09 m³/s, the W-shaped submerged dams composed of 10-20 mm particle sizes had the highest number of stays in fish aggregation areas, reaching 85 times. These dams composed of larger particle sizes (10-20 mm) had high habitat diversity, providing better habitat and shelter for experimental fish. As the flow pattern diversity index increased, the average fish aggregation rate also showed an increasing trend. The river flow pattern diversity index and the average fish aggregation rate exhibited a linear relationship. [Conclusion] W-shaped submerged dams composed of larger particle sizes have high habitat diversity and can provide better habitat and shelter for experimental fish. In the construction of ecological conservation zones in cut-off river sections of artificial canals, it is recommended to select large-particle-size submerged dam schemes with better permeability effects. A good correlation is observed between the average fish aggregation degree and the flow pattern diversity index, enabling quantitative assessment of habitat diversity and providing theoretical support for research on river habitat heterogeneity and ecological optimization design. In the future, the long-term effects of different submerged dam design parameters on the ecosystems of target river sections can be further explored, and more influencing factors can be incorporated to investigate the ecological development of canals.

[Objective] This study aims to systematically characterize the spatiotemporal distribution patterns and ecological risk levels of persistent organic pollutants (POPs)—including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and organochlorine pesticides (OCPs)—in the Jiangsu section of the Beijing-Hangzhou Grand Canal. Seasonal variations between wet and dry periods were examined, and pollutants posing significant ecological threats were identified to support water quality assessment, pollution-control prioritization, and risk management in artificial canal systems. [Methods] Water samples were collected from designated monitoring sites during both wet and dry seasons to capture POPs’ spatial distribution under distinct hydrological conditions. After standardized pretreatment, concentrations of individual POPs were quantified using a 7890-5975C gas chromatography-triple quadrupole mass spectrometry system (GC-MS/MS). Pollution loads and longitudinal variation trends were subsequently analyzed. Ecological risks of PAHs, PCBs, and OCPs were evaluated using the risk quotient method, enabling the identification of high-risk pollutants at both category and compound levels. [Results] During wet season, total concentrations of 16 PAHs ranged from 44.34-56.01 ng/L (median: 45.92 ng/L), increasing substantially to 99.05-198.04 ng/L (median: 128.08 ng/L) in the dry season, nearly tripling and indicating a pronounced seasonal accumulation effect. Total concentrations of 18 PCBs exhibited a similar pattern, increasing from 94.06-123.04 ng/L (median: 95.09 ng/L) during the wet season to 120.75-137.79 ng/L (median: 124.66 ng/L) in the dry season. For OCPs, total concentrations ranged from 417.86-676.68 ng/L (median: 453.74 ng/L) in the wet season and increased to 560.39-673.11 ng/L (median: 617.21 ng/L) in the dry season, indicating comparatively higher and persistent contamination relative to PAHs and PCBs. Despite the relatively narrow longitudinal variation along the canal, pronounced seasonal differences were observed. The recurrent elevation of pollutant concentrations in the dry season suggests that hydrological regulation is a key driver of POPs dynamics in the artificial canal system. Reduced river discharge weakens dilution capacity, resulting in the concentration and retention of hydrophobic pollutants in the water column. Lower water levels and intensified sediment-water interactions during the dry season may enhance sediment resuspension, further releasing historically deposited POPs into the overlying water. Additionally, lower winter temperatures inhibit photolytic and microbial degradation, prolonging the environmental persistence of POPs. Reduced hydrodynamic dispersion in the dry season also limits downstream transport, promoting local accumulation of pollutants. Anthropogenic activities, such as increased domestic heating and industrial energy demand during colder months, may also contribute additional pollutant inputs to the system. Comparison with other rivers globally revealed distinct contamination profiles for different POP categories. PAH concentrations in the study area were generally lower than those reported for many industrialized or highly urbanized rivers, reflecting limited direct emissions from combustion sources in the region. PCB concentrations fell within the intermediate range reported internationally, indicating residual sources associated with economically developed regions, intensive industrial activities (e.g., textile, electronics), and historical industrial zones along the Jiangsu section of the canal, which likely contribute to elevated PCB levels. In contrast, OCP concentrations were relatively high compared with both domestic and international rivers, likely due to Jiangsu Province being a major agricultural production area, where fertilizers and livestock wastewater carry OCP residues. Collectively, these findings indicate that the aquatic ecosystem of the canal remains under non-negligible environmental pressure. Ecological risk assessment revealed an overall moderate risk level, yet significant differences existed across compound classes and seasons. A total of 15 PAHs, 11 PCBs, and 15 OCPs exhibited moderate-to-high ecological risk during at least one sampling period. The number of high-risk compounds increased during the dry season, consistent with the observed elevation in pollutant concentrations. Among all detected compounds, benzo[b]fluoranthene, PCB180, PCB189, and endosulfan I contributed disproportionately to the total ecological risk. These compounds share characteristics of strong hydrophobicity, high chemical stability, resistance to degradation, and high bioaccumulation potential, collectively making them the primary drivers of ecological risk. [Conclusion] This study provides a comprehensive assessment of POP pollution characteristics, seasonal dynamics, and ecological risk profiles within an artificial canal system. The identification of key risk pollutants and the elucidation of hydrological control mechanisms offer valuable guidance for water quality assessment, pollution control prioritization, and ecological risk mitigation in artificial waterways, while providing a transferable framework for POP risk management in similar engineered systems.

[Objective] To address the high cost and limited sustainability of conventional river-dredging techniques, this study proposes a novel, environmentally friendly, and energy-saving dredging approach—a pressure-difference-driven passive rotating flow turbulence device. It aims to clarify the mechanisms by which different blade types (curved, twisted, and flat) of the device affect local scouring and sediment transport, and to identify the blade type with the optimal scouring-silting performance, thereby providing an innovative technical method and structural optimization basis for targeted and directional river dredging. [Methods] The study was conducted through flume-based physical model experiments. Passive rotating flow turbulence devices with three blade types (curved, twisted, and flat) were designed and fabricated, with a single-pile device (without blades) as the control group. Tests were conducted under constant flow (13.62 L/s) and water depth conditions, with a movable bed paved using fine sand (median grain size d₅₀=0.16 mm) having a gradation similar to the prototype sand of the Tarim River. Three-dimensional laser scanning technology was employed to accurately measure topography, and obtain the area, volume, and morphological characteristics of scouring and silting. An acoustic Doppler velocimeter (ADV) system was used to measure the time-averaged flow velocity and turbulence intensity distribution in the flow field around the device. By comparatively analyzing the scour hole development process, flow velocity field structure, turbulence characteristics, and final scouring-silting equilibrium state (with the scouring-silting ratio K as the core evaluation indicator), the dredging performance of devices under different blade types was assessed. [Results] The experimental results demonstrated that blade type significantly affected the device’s rotation characteristics, flow field disturbance capability, and ultimate scouring-silting performance. (1) Flow field characteristics: the curved-blade device achieved stable continuous rotation. Due to the Magnus effect, the main flow was noticeably deflected toward the side rotating with the current (right bank), with the bottom flow velocity at the right bank increasing by about 49.6% compared to the left bank, which effectively accelerated the initiation of bed sediments. The peak bottom turbulence intensity increased by about 89.47% compared to the flat-blade device, with the most intense flow field disturbance. The twisted-blade device rotated discontinuously, while the flat-blade device remained largely stationary. Both exhibited weaker flow field disturbance capability and less asymmetry than the curved type. (2) Scouring/silting morphology and performance: the curved-blade device yielded the highest scouring-silting ratio (K=94.6%), 48.5% and 16.3% greater than that of the twisted (K=46.1%) and flat (K=78.3%) types, respectively. The curved blade produced a distinctly asymmetric scour hole biased toward the right bank, with a large affected area and an interwoven pattern of scouring and silting zones, which facilitated downstream sediment transport. (3) Scour hole development: the scouring process could be divided into four stages: initial, transition, disturbance, and stabilization. The curved-blade device exhibited the fastest scour-depth development rate across all stages, particularly during the disturbance stage (device rotating continuously), efficiently disturbing bed sediments and enlarging the scour hole. [Conclusion] Among the passive rotating flow turbulence devices, the curved blade demonstrates the best comprehensive performance in scouring and sediment transport, owing to its ability to induce stable continuous rotation, produce a significant Magnus effect, and maximize the enhancement of bottom turbulence and flow velocity deflection, with a scouring-silting ratio significantly higher than other types. The device has a simple structure and requires no external power, offering a novel, environmentally friendly, and energy-saving approach for targeted river dredging. The findings clarify the critical influence of blade type, thereby providing a direct theoretical and experimental basis for further optimization of the device’s structural parameters and engineering deployment schemes.

[Objective] This study centers on the dike fields of the spur dike group in the Yangtze River Estuary, a typical tidal estuary where complex water-sediment dynamics and diverse dike layouts jointly shape deposition processes. Its core objectives are twofold: first, to unravel the coupling mechanism through which dynamic factors (e.g., runoff, tides) and geomorphic parameters (e.g., dike spacing, initial water depth) jointly regulate sediment deposition intensity in tidal estuarine dike fields; second, to develop a reliable empirical formula for predicting such deposition intensity. By addressing the gap in existing research—where the integrated effects of dynamic and geomorphic factors are often overlooked—this study aims to provide robust theoretical support for optimizing the design of spur dike groups and enhancing the accuracy of deposition forecasting in the Yangtze River Estuary and analogous tidal estuarine systems worldwide. [Methods] The dike fields of the spur dike group in the north passage of the Yangtze River Estuary, a key area of the Yangtze River Estuary Deepwater Channel Regulation Project, were selected as the research focus. Long-term, systematic measured data were analyzed, including dike field topographic surveys, hydrological observations, and sediment monitoring records. Correlation analysis was first performed to examine how deposition intensity relates to key dynamic factors (upstream runoff from the Datong Hydrological Station, suspended sediment concentration, offshore tidal range, suspended sediment particle size) and critical geomorphic parameters (relative spacing of spur dikes, initial water depth of dike fields, spur dike length, dike field depth). Using dimensional analysis and the Buckingham π theorem, a comprehensive dynamic parameter was constructed by integrating the four dynamic factors, synthesizing their combined influence on water-sediment transport and deposition. Simultaneously, a set of geomorphic parameters was established, incorporating spur dike spacing, length, and dike field depth to quantify the impact of spur dike group layout and dike field topographic features on local flow patterns and sediment trapping. A power function model was then used to quantify the coupling relationship between the comprehensive dynamic parameter and geomorphic parameters, and an empirical formula for deposition intensity was derived. Finally, the formula was validated using measured data from representative dike fields, including those unaffected by subsequent engineering and those influenced by phased projects. [Results] 1) As the Yangtze River Estuary Deepwater Channel Regulation Project advanced through three phases, total sediment deposition in dike fields increased significantly (from 15.48×10⁶ m³ in Phase I to 128.01×10⁶ m³ in Phase II), confirming the spur dike group’s strong sediment-trapping effect. 2) Deposition intensity was positively correlated with runoff (higher runoff carries more sediment to dike fields) and sediment concentration (more available sediment for deposition), but negatively correlated with tidal range (larger tidal range strengthens ebb currents, enhancing offshore sediment transport) and sediment particle size (coarser particles settle before reaching dike fields or are easily resuspended by strong flows). 3) Among geomorphic parameters, initial dike field water depth showed a strong positive linear correlation with deposition intensity (deeper water provides more deposition space and reduces flow velocity, favoring sediment settlement), while spur dike relative spacing had weak correlation (R²=0.44), due to interactions with factors like flow blockage (too small spacing) or uneven energy distribution (too large spacing). 4) The comprehensive dynamic parameter correlated highly (R²=0.94) with annual deposition in undisturbed dike fields (TS1, TS2), effectively capturing dynamic drivers of deposition; geomorphic parameters correlated strongly (R²=0.96) with initial deposition, clearly distinguishing differences between dike fields in the same spur dike group. 5) The empirical formula showed excellent agreement with measured data: it matched well with the measured deposition intensity of TS1, TS2, and TS8 (used for fitting analysis) and effectively reflected the deposition intensity of TN7, TN8, and TN9 (used for validation in the second-phase project). Even for dike fields affected by phased engineering or new structures (e.g., a 21 km sediment barrier), the formula still successfully captured the overall deposition trend. [Conclusion] This study makes three key contributions: it innovatively integrates dynamic factors and geomorphic parameters into a unified analytical framework for Yangtze River Estuary spur dike group dike fields, overcoming the limitations of previous single-factor research; the constructed comprehensive dynamic parameter and geomorphic parameters effectively quantify the combined effects of water-sediment dynamics and dike layout/topography on deposition, making complex processes interpretable; the empirical formula, with high applicability and accuracy, offers a reliable tool for tidal estuarine dike field deposition prediction.

[Objective] The Jingjiang-Dongting Lake region, a critical river-lake system in the middle reaches of the Yangtze River, is crucial for flood mitigation, sediment regulation, and ecological conservation. This study aims to develop and apply an enhanced 2D water-sediment model incorporating a morphological acceleration factor (MF) to efficiently predict the bed deformation and sediment redistribution over a 30-year period, thereby providing a scientifically sound and computationally feasible tool for long-term morphodynamic prediction and sustainable management of large river-lake systems. [Methods] A 2D depth-averaged water-sediment model was established to solve shallow water equations and multi-fraction suspended sediment transport equations coupled with a bed deformation module. The governing equations were discretized using an unstructured finite volume method, with advection terms solved by the Euler-Lagrange method (ELM) to enhance numerical stability. To overcome the computational bottleneck of long-term simulations, the MF was introduced, linearly scaling the bed evolution per hydrodynamic time step to accelerate morphodynamic simulations while maintaining physical realism. The model domain covered the lower Jingjiang reach and the Dongting Lake and was discretized into unstructured cells with refined resolution along the main channel. Sediment was grouped into three size classes for suspended load and four for bed material, with gradation initialized from field surveys from 2003 to 2012. MF was determined at 7, 15, and 24 for sensitivity analysis. [Results] (1) The MF markedly improved computational performance. With MF values of 15 and 24, simulation time decreased to 42% and 30% of that for MF=7, corresponding to speed-up factors of 2.38 and 3.33, respectively. The spatially distributed erosion-deposition patterns remained consistent across different MF values, confirming the robustness of the approach. The total sediment deposition in Dongting Lake varied by less than 5% across scenarios, while the total erosion volume along the main stem exhibited higher sensitivity, with a maximum deviation of 9.1% between MF=24 and MF=7. These deviations were primarily localized and did not alter the long-term trends or magnitudes. (2) The mainstream of the Jingjiang River experienced sustained incision, with a total scour volume of 462 million m³ and an average bed incision of 1.86 m over 30 years. Local deposition occurred along convex banks due to curvature-induced secondary flows. Dongting Lake exhibited net deposition of 276 million m³, with an average siltation thickness of 0.09 m. Notably, the annual deposition rate in the lake decreased significantly over time—from 20 million m³ to about 6 million m³—representing an approximate 70% reduction and indicating a gradual approach toward a new morphodynamic equilibrium. Significant spatial variability in sediment redistribution was observed. The Jingjiang mainstream was dominated by scour, particularly in the deep channel, while point bars developed in its meandering segments. Within Dongting Lake, distinct patterns emerged. The western Dongting area was near equilibrium with no clear trend, the southern Dongting experienced significant deposition along floodways from the Three Outlets with thicknesses up to 4 meters, and the eastern Dongting exhibited complex patterns with both deep scour pits infilled by sediment and a depositional bar at the Zhuzikou inlet. Furthermore, the outlet channel in the lake-river confluence zone experienced upstream erosion and downstream deposition, gradually flattening the longitudinal slope. Regarding model validation, the simulated results closely aligned with previous studies and field observations. The deviation in total deposition was 17.0% compared to a 1D model over 30 years, and the deviation was approximately 14.2% at 10 and 20 years compared to an earlier 2D lake model. Notably, the predicted average annual deposition of 89.3 million m³ from 2011 to 2020 closely matched the measured data, further supporting the model’s reliability. [Conclusion] This study demonstrates that integrating a morphological acceleration factor into a 2D water-sediment model enables efficient and accurate simulation of decadal-scale morphodynamics in large river-lake systems. The MF method decreases computational time for a 30-year simulation by about 60%-70% while maintaining prediction errors within 5% for total lake deposition. The Jingjiang reach is projected to undergo continued incision, while Dongting Lake will experience slight net deposition at a strongly declining rate, indicating adjustment toward a new dynamic equilibrium. Spatially heterogeneous erosion-deposition patterns highlight the need for targeted management strategies.

[Objective] Illegal sand mining in river channels has caused riverbed incision and led to eco-environmental problems such as permafrost degradation, bank slope instability, and destruction of aquatic ecosystems, threatening the ecological health of plateau rivers and lakes in the Xizang Autonomous Region. This study aims to analyze the current situation and problems of river sand mining management in Xizang, propose management and protection measures, and provide references for promoting the standardization of river sand mining management in the Xizang Autonomous Region. [Methods] Methods including investigation, data statistics, empirical analysis, qualitative analysis, and comprehensive analysis were employed to systematically review the current status of river sand mining management in the Xizang Autonomous Region and analyze problems in the construction of laws and regulations, river sand mining planning, preparation of annual sand mining implementation plans, and management of engineering sand mining. [Results] River sand mining management in the Xizang Autonomous Region faces problems such as incomplete laws and regulations, insufficient operability and specificity of some institutional requirements, and the presence of blind spots in management. In some areas, river sand mining plans and annual sand mining implementation plans are not prepared according to relevant technical guidelines or standards, resulting in insufficient scientific basis and compliance of the plans or annual implementation plans. The mechanism for defining engineering sand mining requires further improvement. In daily supervision, the management initiative of water administrative departments in some areas needs to be strengthened, with limited regulatory measures and relatively weak technical capacity. These problems affect the efficiency of management and supervision of river sand mining. [Conclusion] This study proposes the following countermeasures and recommendations. (1) Efforts in legislative research, local regulations, and legal constraints should be strengthened in full consideration of the management situation of river sand mining in the Xizang Autonomous Region. (2) By refining the management requirements for river sand mining and formulating standards for illegal activities and penalties, scientific river sand mining plans should be developed, taking into account the characteristics of river sections, current status of sand mining, and ecological management needs of rivers and lakes in the planning period for each planned reach. In light of the widespread distribution of glaciers and permafrost, long vegetation recovery cycle, and chain reactions like riverbed incision and shoreline collapse triggered by sand mining activities that threaten water conservation functions, annual river sand mining plans and technical standards for the comprehensive utilization of dredged sand should be formulated to strengthen the top-level design of river sand mining management. (3) For special areas such as uninhabited areas, differentiated supervision plans should be developed. The river chief system platform should be fully utilized to strengthen joint law enforcement operations among multiple departments, carry out joint law enforcement at the junctions of administrative regions, enhance targeted governance in areas with weak regulatory capacity, and strengthen on-site supervision for permitted sand mining sites by scale and region, thereby improving daily supervision mechanisms. (4) A complete management system for river sand and gravel mining and transportation should be established, an integrated monitoring and perception system for river sand mining should be gradually developed, and the construction of intelligent supervision should be promoted. These recommendations can provide theoretical support and practical reference for the supervision of river sand mining with plateau ecological characteristics.

[Objective] Poyang Lake is the largest lake connecting to the Yangtze River. A complex river-lake relationship is observed between the Yangtze River and Poyang Lake, manifested as hydrodynamic processes such as water level uplift and flow field changes. In recent years, extreme drought in Poyang Lake and floods in the Yangtze River have become increasingly frequent amid repeated flood-drought alternations, resulting in significant changes in the river-lake relationship. This study aims to clarify this relationship for the Yangtze-Poyang confluence system. [Methods] To simulate the natural flow state and typical inflow conditions, we established a large-scale generalized physical model of about 80 meters in length and 45 meters in width. The model consisted of the Yangtze River section (from Jiujiang to Balijiang) and the waterway of Poyang Lake (from Xingzi to Hukou). A total of nine flow conditions representing the combined hydrological characteristics of the Yangtze River and Poyang Lake were selected for the experiments. The cases covered typical natural flow conditions such as floods and droughts in the Yangtze River and Poyang Lake, as well as confluence ratios ranging from 1 to 60 at the Yangtze River-Poyang Lake confluence. [Results] (1) When the confluence ratio of the Yangtze River and Poyang Lake was 10, the jacking coefficient α was greater than 1, and the energy coefficient F_e was less than 0. When the flow discharge of the Yangtze River further increased and the confluence ratio reached 60, α and F_e reached their maximum and minimum values. When the confluence ratio was about 0.5, the river-lake relationship exhibited opposite effects. (2) The evolution of the river-lake relationship was closely related to the flow conditions of the Yangtze River and Poyang Lake. The jacking coefficient α was nonlinearly positively correlated with the confluence ratio of the two flows, and weakly linearly positively correlated with the velocity ratio. The energy coefficient F_e showed a nonlinear negative correlation with the confluence ratio, and a weak linear negative correlation with the velocity ratio. (3) When jacking effect occurred, the water level difference between Xingzi and Hukou in Poyang Lake decreased, and the flow velocity distribution tended to be uniform. At the confluence, the Yangtze River water flowed upward from the middle and lower parts and interacted with the Poyang Lake water. As the Yangtze River discharge further increased, the water level drop decreased. There was also a backflow on the right side of the confluence, causing Yangtze River water to flow back into Poyang Lake. When the flow rate of Poyang Lake reached 25 000 m³/s, there was a significant increase in the water level drop and cross-sectional flow velocity. The water flow of Poyang Lake mainly entered the Yangtze River near the water surface. [Conclusion] When the confluence ratio exceeds 45, it is recommended to regulate the outflow of reservoirs on the Yangtze River in advance to reduce the water level and mitigate backflow. Under extreme low water conditions where the confluence ratio is less than 0.5, a water level gradient threshold (0.45‰) is used as an indicator. The scheduling of water conservancy projects is suggested to be combined with these measures to increase the water replenishment flow and reduce the water level gradient in the Poyang Lake waterway, thereby alleviating the water level reduction effect.

[Objective] Continuous sharp bends in river channels are prone to significant river regime adjustments and abrupt changes under the impact of unsaturated sediment-laden flow, which have far-reaching implications for flood control, navigation, and water resource utilization. This study investigates the hydraulic characteristics of the river section with sharp bends in the lower Jingjiang River and the scour and siltation characteristics of the upstream and downstream bends after the natural cutoff through large-scale physical model experiments, aiming to deepen the understanding of the natural cutoff development process and provide references for the long-term regulation and planning of the river-lake confluence section in the middle reaches of the Yangtze River. [Methods] Taking the reach from Xiongjiazhou to Chenglingji in the middle reaches of the Yangtze River as the research object, a physical model was established with a horizontal scale of 1∶400 and a vertical scale of 1∶100. The model had a total straight-line length of about 70 m, a maximum width of about 40 m, and included two continuous sharp bends and upstream and downstream transition sections. Based on the hydrological data measured at Luoshan Station from 2003 to 2020, the model test water and sediment conditions were set up with different flow conditions of flood, medium, and drought. First, the hydraulic characteristics of the bend section under different flow levels were studied through fixed-bed model tests to identify the most likely flow conditions and locations for natural cutoff. Subsequently, movable-bed scour tests were conducted, applying flow conditions favorable for cutoff to study the cutoff development process. Considering that the flow in the Jingjiang section would be in a severely undersaturated state for a long time after the Three Gorges Reservoir is impounded, the inlet water and sediment conditions in this model test were simplified to clear water. [Results] The model test results showed that after the flow overtopped the bank, the main flow belt in the upstream Qigongling bend section gradually shifted from the main channel to the convex bank side. Three velocity concentration zones were formed at the neck, middle, and leading edge of the flow, with the peak velocity decreasing stepwise from the neck to the main channel. During the natural cutoff process of the Qigongling bend, the most likely location for the breach was between 1 300 m and 1 500 m away from the rear embankment. After 3 days of scouring by the overbank flow, gullies began to form, which developed into a fully connected breach over a period of about 30 days. After cutoff, the bend apex section tends to become narrower and deeper, while the transition section tends to become wider and shallower. [Conclusion] The results provide forward-looking guidance for the governance of the middle reaches of the Yangtze River system.

[Objective] This study aims to identify and quantify the long-term evolution characteristics of overbank flood processes in the lower Jingjiang River in the middle reaches of the Yangtze River (from 1966 to 2023) using high-temporal-resolution data, and to explore their relationships with climate change and upstream reservoir groups, thereby providing a scientific basis for riverbed evolution, river ecological assessment, and ecological restoration and flood management in the lower Jingjiang River. [Methods] Based on the flood element data from the Jianli hydrological station, overbank flood processes were identified using the bankfull discharge as the threshold. A hydrological indicator system was constructed from three dimensions: 1) temporal characteristics, including earliest start date, date of maximum flood, latest end date, and their decadal median values; 2) frequency and duration, including annual occurrence frequency, total annual duration, total annual rising-water duration, total annual falling-water duration, average duration, and maximum duration; and 3) intensity, including average discharge, maximum discharge, average flood volume, maximum flood volume, and total annual flood volume. For each indicator, the decadal median value and frequency distribution were calculated, and the interannual variation trends and phased differences were investigated. [Results] 1) In terms of occurrence timing, the earliest start dates mainly concentrated from late June to early July, showing phased variations. The decadal median value of latest end dates was gradually delayed from September 5 to October 5 during the 1960s-1980s. The largest overbank flood events mostly occurred from early to mid-July, showing an overall delayed trend. 2) In terms of frequency and duration, the annual occurrence frequency was mostly (69%) 2 to 5 times. Fluctuations decreased after the 1970s, dropping to approximately 2 times per year in the early 2020s. The median total annual duration increased from 23 days in the 1960s to 63 days in the 1980s, then decreased rapidly after the 1990s, and dropped to 29 days in the early 2020s. The total annual rising-water duration exhibited a relatively small variation, while the total annual falling-water duration changed consistently with the total duration and was more affected by upstream flood regulation. The average duration and maximum duration increased significantly from the 1960s to the 1990s and then showed minor fluctuations after the 2000s. 3) Regarding discharge and flood volume, average discharge and maximum discharge increased significantly from the 1960s to the 1980s (by approximately 8% and 21%, respectively), and then decreased significantly from the 1990s to the 2020s (by 12% and 36%, respectively). The average flood volume gradually increased from the 1960s to the 1990s (by 1.5 times), significantly decreased in the 2000s (by 33%), and then rebounded rapidly after the 2010s. However, the maximum flood volume did not rebound simultaneously in the 2010s, indicating the “peak-shaving” effect of reservoirs on large floods. The changes from the 1960s to the 1980s were highly consistent with the increase in precipitation. After the 1990s, the flood regulation effect of the upstream reservoir groups dominated the changes in most indicators, exerting significant impacts particularly on the falling-water duration, total annual duration, maximum discharge, and total annual flood volume. However, average duration, maximum duration of overbank flood events, and average flood volume still maintained a good response to precipitation changes and could serve as hydrological indicators of precipitation changes. [Conclusion] This study reveals that the overbank flood processes in the lower Jingjiang River from 1966 to 2023 underwent distinct phased evolution. Precipitation was the dominant driver before the 1980s, while reservoir regulation became the main influencing factor after the 1990s. The combined effects of these two drivers lead to differential responses among different indicators. Average duration, maximum duration, and average flood volume can serve as sensitive indicators for monitoring long-term precipitation changes and assessing climatic impacts. In contrast, total duration, falling-water duration, maximum discharge, and total annual flood volume are highly sensitive to upstream project operations and should be incorporated into the evaluation system for regional water resources and ecological management. Future research needs to further couple high-resolution meteorological precipitation data, river channel morphological evolution, and ecological response data to provide decision-making support for the design of flood regulation schemes with ecological priorities.

[Objectives] In recent years, intense channel evolution in the lower reaches of Minjiang River has negatively impacted channel stability, flood control, water supply, and aquatic ecosystems, thereby constraining socio-economic development in riverside cities. The mainstream of the lower Minjiang River (from Shuikou Dam to Huai’an Diversion Outlet) has been a key sand mining area, with significant riverbed incision. However, previous studies have paid insufficient attention to this phenomenon. This study aims to thoroughly investigate the evolutionary processes and characteristics of riverbed incision in this reach and quantify the impact of sand mining on the riverbed incision. [Methods] Using measured topographic data from seven different years (1999-2020), the evolutionary characteristics of riverbed incision were analyzed in terms of planform changes, cross-sectional profiles, and scouring and deposition variations. Based on river sand mining data, the contribution ratio of sand mining to the riverbed incision was calculated. [Results] (1) the mainstream of the lower Minjiang River experienced intense overall scouring from 1999 to 2020, with scouring erosion occurring across 84.18% of the channel area, resulting in a total scouring volume of approximately 255 million m³. The average incision depth was 5.09 m, while the thalweg exhibited an average incision of 7.87 m, with a maximum incision depth approaching 20 m. (2) The riverbed underwent rapid scouring before 2011, whereas the scouring rate significantly decelerated thereafter. The total scouring volume during 1999-2011 and the average thalweg incision accounted for 82.51% and 76.11% of the respective totals for 1999-2020. (3) Except for a slight overall deposition in the riverbed during 2014-2017, net scouring occurred in all other periods, with the highest mean annual incision rate observed between 2008 and 2011. (4) The most pronounced incision occurred in the 15-30 km reach downstream of Shuikou Dam, where the average incision depth reached 6.92 m during 1999-2020, and significant incision persisted there after 2011. (5) Sand mining was identified as the primary driver of riverbed incision, estimated to contribute over 50% to the riverbed incision. [Conclusions] The findings reveal the processes, characteristics, and trends of riverbed incision in the mainstream of the lower Minjiang River in recent years and identify the most severely incised reaches. For the first time, a quantitative assessment of the impact of sand mining on the riverbed incision is provided, and it is suggested that sand mining has been the dominant factor driving riverbed incision in the mainstream of the lower reaches of Minjiang River in recent years. These results provide fundamental support for channel evolution, river management and sand mining planning, and also offer valuable references for flood control, river channel protection, and river-related project development.

[Objectives] Bank collapse is a major form of planform deformation of alluvial riverbeds and one of the major natural disasters in the middle and lower reaches of the Yangtze River. However, due to multiple influencing factors and complex mechanisms of bank collapse, its accurate prediction and early warning remain challenging. After the construction and operation of the Three Gorges and upstream cascade reservoir groups, the Jingzhou section of the Yangtze River in Hubei Province shows long-distance and long-term scour trends, with significantly increased bank collapse risks, seriously affecting flood control, navigation, and socio-economic development along the river. This study aims to develop a method for predicting bank collapse under continuous scour conditions in the middle reaches of the Yangtze River, providing technical support for the establishment and practical application of a multi-indicator bank collapse risk assessment model. [Methods] A comprehensive bank collapse risk evaluation indicator system was developed for the middle reaches of the Yangtze River based on the analytic hierarchy process, encompassing three dimensions: current bank collapse status, substrate conditions, and near-bank variations, with a total of six characteristic indicators. On this basis, a comprehensive bank collapse risk assessment model in the middle reaches of the Yangtze River was established. Three-level early warning classification criteria for bank collapse were proposed, and they were applied to predict bank collapse risks in the Jingjiang and Honghu sections of the Yangtze River mainstream. [Results] Following the operation of the Three Gorges Project, most of the bank collapses and areas with high bank collapse intensity in the Jingzhou section of the Yangtze River mainstream were largely associated with local river regime adjustments. In addition to collapse occurring in unprotected bank sections, many failures occurred in the weak parts of protected sections or lightly protected sections, with a notable increase in sudden bank collapse events. In 2024, the Jingzhou section of the Yangtze River mainstream in Hubei had nine bank sections predicted to be at high risk of collapse with a red warning level. The majority of the high-risk bank collapse sections were distributed in natural unprotected sections, though some protected sections still had relatively high early warning levels for bank collapse. Among them, the total lengths of the Level I and II warning bank sections were 3.54 km and 16.76 km, respectively. [Conclusions] Based on the evaluation results of typical bank sections including protected, unprotected, and mainstream-adjacent banks, the bank collapse risk assessment model constructed in this study demonstrates certain applicability for bank collapse prediction in typical sections of the middle reaches of the Yangtze River. The characteristic indicators show certain sensitivity to variations in different bank conditions, and the proposed classification criteria for bank collapse early warning levels are reasonably sound.