[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.

[Objective] This study aims to investigate the effects of different vegetation arrangements (parallel and staggered) in the junction zone between channel and floodplain in urban rivers on the spatial distribution of bed shear stress, and to reveal how vegetation length and arrangement influence hydrodynamic characteristics, thereby providing theoretical support for ecological revetment design. [Methods] A two-dimensional hydrodynamic model was established using Delft3D-FM and validated with measured water levels and discharges. To account for the effects of vegetation, a vegetation module was incorporated into the hydrodynamic model. The model considered plant height, width, and density, with vegetation resistance simplified as bed roughness. Vegetation zones with lengths of 0.5L, 0.75L, L, 1.25L, and 1.5L(L represents 1 000 m vegetation zone length) were arranged in parallel and staggered patterns in the channel-floodplain junction zone. The two-dimensional hydrodynamic model that accounted for vegetation effects was used to simulate the flow fields under different conditions. Based on the simulation results, the distribution characteristics of bed shear stress in the vegetation zone and its downstream region were analyzed. The effect of vegetation on hydraulic resistance was evaluated using the blockage factor, dimensionless hydraulic radius, and surface area blockage factor (characterizing vegetation zone length). Finally, a dimensionless hydraulic radius function considering the effects of vegetation was proposed to predict the maximum bed shear stress. This function was introduced to quantitatively characterize the influence of vegetation. [Results] (1) In parallel arrangement, vegetation dominated the flow dynamics in the junction zone. The resulting shear stress zones extended from the main channel within the vegetation zone to its downstream end, forming elongated stress zones downstream of the vegetation zone. However, the stress field patterns varied with vegetation zone length, with significant differences observed in the maximum shear stress distribution. In longer vegetation zones, the location of maximum shear stress tended to shift farther downstream from the end of the vegetation zones. With a vegetation length of 0.5L, the maximum shear stress zone was located 0.25L-0.30L downstream from the end. When the vegetation length was L, the maximum stress zone nearly coincided with the downstream end. With a length of 1.5L, the maximum stress zone was 0.25L-0.5L downstream from the end. As vegetation length increased, the location of maximum shear stress zone in the main channel shifted upstream, showing a tendency to move away from the downstream end of the vegetation zone.(2) In staggered arrangement, the shear stress in the main channel reached its maximum within the staggered zone. Under the influence of the bend, the maximum cross-sectional shear stress shifted from the convex bank to the channel center. This indicated that vegetation reduced the effect of centrifugal forces on secondary flow and shear stress in the bend, with the maximum shear stress consistently occurring at the upstream face of vegetation zone. The shear stress in the main channel readjusted according to the vegetation zone distribution and then stabilized to meet the spatial variation of bed shear stress. The bed shear stress varied significantly with the length of the vegetation zone. Regardless of the vegetation zone length, the bed shear stress peaked at the cross-section adjacent to the vegetation units. Notably, the bed shear stress in longer vegetated zones was significantly lower than that in shorter ones at this cross-section. [Conclusion] Vegetation arranged in parallel pattern tends to form larger shear stress zones near the vegetation end and elongated stress zones downstream, with longer vegetation bringing stress concentration zones closer to vegetation zones. Vegetation arrangement and length significantly affect bed shear stress distribution by altering flow structures, with staggered arrangement forming large stress zones at staggered zones and the cross-sections of adjacent vegetation units, where longer vegetation zone results in smaller stresses. The proposed dimensionless hydraulic radius function proves effective for predicting the maximum bed shear stress, providing a basis for optimizing vegetation arrangements in channel-floodplain junction zones.

[Objective] With the release of low-sediment water, the middle and lower reaches of the Yangtze River will undergo long-distance and long-duration river channel scouring. Optimizing the operation strategy of the Three Gorges Reservoir to reduce scouring downstream of the dam is of great significance. [Methods] Based on the statistical analysis of measured data, the response mechanisms of erosion and deposition in the downstream river channel were analyzed. Using mathematical modeling, a preliminary study was conducted on reservoir operation strategies aimed at reducing downstream scouring. The sediment regulation concept of “regulating sediment to reduce scouring, regulating water to regulate sediment, and achieving sediment regulation through water regulation” was proposed. [Results and Conclusion] At representative hydrological stations along the middle and lower reaches of the Yangtze River, sediment load exhibits a good power-law relationship with flow, indicating a strong correlation between sediment transport and flow. Therefore, sediment regulation in the middle and lower reaches can be achieved by controlling corresponding flow processes. Typical years of 2012 and 2013 were selected to study the effects of regulating sediment peaks during the flood season and different regulation modes for small and medium-sized floods on the reservoir’s sediment flushing ratio and downstream river channel scouring. Downstream scouring is influenced by both the sediment released from the upstream reservoir and the volume and process of the incoming flow. Increasing the reservoir’s maximum flow during sediment peak periods and shortening its duration are both beneficial for reducing downstream scouring. Among these, increasing the maximum flow has a more significant effect. Therefore, to mitigate downstream scouring, the flow during flood seasons should not be too low. Based on typical years from 2008 to 2017, the effect of optimized operation schemes on reducing scouring between Yichang and Datong was studied. In terms of total scouring volume across this reach, the implementation of an optimized operation scheme aimed at reducing downstream scouring resulted in increased sediment discharge from the reservoir and a total reduction of 24.47 million m³ in scouring volume, with an average annual reduction of 2.447 million m³. Overall, considering sediment peak regulation during the flood season can reduce downstream river channel scouring to a certain extent. Using hydrological and sediment data from 1991 to 2000 and considering both the joint operation of upstream cascade reservoirs and the optimized operation of the Three Gorges Reservoir for scouring reduction, the long-term water and sediment outflow processes of the reservoir were predicted and used as boundary conditions for long-term simulations of downstream scouring. A one-dimensional hydrodynamic and sediment transport model for the Yichang-Datong reach of the middle and lower Yangtze River was applied to predict the long-term evolution of downstream river channel scouring under optimized operation. The research findings can provide a reference for the optimized operation of the Three Gorges Reservoir.

[Objective] This study aims to investigate the dramatic changes in water-sediment processes within the Jinsha River reservoir area following the impoundment and operation of the Wudongde and Baihetan cascade hydropower stations. Using multi-source observational data, the study reveals the variation patterns of water and sediment fluxes between the two dams, the spatiotemporal distribution characteristics of riverbed erosion and deposition, and their driving mechanisms. The findings provide scientific support for reservoir safety operation, navigation channel management, and ecological conservation. [Methods] The study was conducted using runoff-sediment transport data from 2015 to 2023 at the Wudongde and Baihetan hydrological stations, fixed cross-sectional topographic surveys from 2016 to 2023, and hydrodynamic measurements collected downstream of the Wudongde Dam in 2023. Water-sediment relationship analysis was employed to examine the response patterns between runoff and sediment transport. Erosion and deposition volumes were calculated using the cross-sectional method, with 825 m water level as the reference and the channel storage volume estimated via the frustum formula. Spatial variations of erosion and deposition were quantified by overlaying thalweg line and comparing morphological changes of typical cross-sections (JC199, JC153, JC126). [Results] 1) Water-sediment flux variations: Annual runoff exhibited a slight decrease, 2% at the Wudongde station and 17.8% at the Baihetan station. Annual sediment transport plummeted by more than 90%, primarily due to the “cumulative sediment retention effect” of upstream reservoirs. Intra-annual runoff distribution demonstrated a “peak-shaving and valley-filling” pattern, with a 22%-48% increase in December and a 16%-38% decrease in July. Sediment transport was concentrated from June to October (accounting for over 63%), yet monthly averages dropped by more than 95%. A progressive downstream sedimentation trend was observed in September. 2) Spatiotemporal evolution of erosion and deposition: erosion dominated during dry season (October-May), while deposition dominated the wet season (May-October). From 2021 to 2023, a net deposition volume reached 12.63 million m³, showing an overall cumulative trend. Spatially, a strong erosion zone formed at the reservoir tail driven by the high-kinetic-energy discharges from the Wudongde Dam. The core deposition area in the main reservoir was found 25-75 km upstream of the dam. In the tributary-affected zone, the Heishui River confluence showed prominent deposition. 3) Driving mechanisms of erosion and deposition: In terms of hydrodynamic forces, erosion was triggered by high flow velocities and strong sediment-carrying capacities within 20 km downstream of the Wudongde Dam, while beyond this zone, deposition was promoted by slower flows and weaker sediment-carrying capacities. Regarding tributary replenishment, tributaries such as the Pudu River, Xiaojiang River, and Heishui River contributed an average annual sediment transport of 5.73 million tons (2011-2022), accounting for over 46% of the deposition volume in the reservoir area. [Conclusions] The operation of cascade hydropower stations has restructured the water-sediment process. Although the runoff volume decreased slightly, its intra-annual redistribution was significant, and the sediment transport plummeted by 96% due to the “cumulative sediment retention effect”, with sediment being concentrated in flood season. The erosion and deposition in the reservoir area exhibit a spatial pattern of “erosion at the tail and deposition in the head”. The reservoir tail is eroded by the discharged flow, while the main reservoir experiences deposition due to reduced flow velocity and tributary replenishment, with the confluence of the Heishui River being a key source of deposition. A clear long-term deposition trend is observed, and it is necessary to focus on the high-risk deposition zone 25-75 km upstream of the dam and the sections with drastic morphological changes at tributary estuaries. These findings provide a quantitative basis for the joint operation of cascade reservoirs, navigation channel maintenance, and sediment management.

[Objective]As a critical deep-water navigation channel, the stability of the water-sediment structure in the North Passage of the Yangtze River Estuary directly affects channel maintenance and navigational safety. This study aims to (1) identify the main driving factor behind the baroclinic effects (salinity vs. temperature) caused by water density in the North Passage, (2) quantitatively reveal the differential influence of baroclinicity on flood and ebb current velocities across different water layers (surface vs. bottom), (3) clarify how baroclinic effects reshape the vertical distribution of sediment content (particularly the tidal-averaged sediment content and its vertical gradient), and (4) investigate the similarities and differences in baroclinic effects between the main channel and adjacent shoals, thereby deepening the understanding of the physical mechanisms governing water-sediment dynamics in the North Passage, providing more precise theoretical support for channel management and sediment deposition prediction. [Methods] An advanced three-dimensional high-resolution coupled mathematical model of hydrodynamics-sediment-salinity was established to accurately represent the complex topography, tidal forcing, runoff input, and salt-freshwater mixing processes in the North Passage of the Yangtze River Estuary. Key components of the model included hydrodynamic module, salinity transport module, sediment module, and density calculation. To isolate and quantify the baroclinic effects, a baseline scenario and a baroclinicity-off scenario were designed (in which the baroclinic term induced by density gradients was artificially disabled, while maintaining identical topography, tides, runoff, and sediment parameters). By comparing the velocity fields (especially the vertical structure) and sediment content fields (vertical distribution and tidal average) under the two scenarios, the net influence of baroclinicity on the water-sediment structure in the North Passage was precisely determined. [Results] (1) Salinity difference was the absolute dominant factor in generating water body density gradients and significant baroclinic effects in the North Passage, while the influence of temperature was negligible.(2) Differentiated vertical influence on flow velocity structure: Baroclinic effects significantly enhanced the bottom-layer flood current velocity, with the most pronounced influence observed in the middle and lower sections of the North Passage. In contrast, its influence on surface-layer flood velocity was relatively small or slightly weakening. Baroclinic effects generally weakened the ebb current velocity across all water layers (surface to bottom), with a particularly evident reduction in the bottom layer.(3) Spatial difference: The influence of baroclinic effects on flow velocities (including flood and ebb currents) was significantly stronger in the main channel than in adjacent shoals, indicating that the deep-channel topography amplified the dynamic effect of baroclinicity.(4) Influence on sediment content structure: Baroclinic effects led to a significant decrease in the average sediment content during ebb tides in the bottom layer, while conversely, the average sediment content during flood tides showed an increasing trend. Baroclinic effects profoundly altered the vertical distribution structure of sediment content, enhancing the vertical gradient of average sediment content during flood tides. This indicated that during flood tides, the difference between the bottom layer with high sediment content and the surface layer with low sediment content became more pronounced, intensifying vertical stratification. In contrast, baroclinic effects reduced the vertical gradient of average sediment content during ebb tides, indicating relatively enhanced vertical mixing or reduced differences in sediment content between layers during ebb phases.(5) Mechanistic linkage: The changes in flow velocity structure (enhanced bottom-layer flood currents and weakened ebb currents) served as the direct hydrodynamic driver for the sediment content response (increased sediment content during flood tides and decreased during ebb tides in the bottom layer). The variations in vertical gradients reflected how baroclinic effects influenced the vertical diffusion and stratification of sediment by altering vertical circulation and mixing intensity. [Conclusion] (1) Salinity is the sole key factor driving the baroclinic effects in the North Passage of the Yangtze River Estuary, and for the first time, the detailed influence patterns of baroclinic effects on layered flow velocities and vertical structure of suspended sediment content in the North Passage are systematically revealed.(2) The baroclinic effects substantially restructure the vertical momentum distribution in the North Passage by altering the vertical pressure gradient, manifesting as enhanced flood-tide dynamics and weakened ebb-tide dynamics in the bottom layer. This finding is of great significance for understanding the dynamic mechanisms of the formation and maintenance of the largest turbid zone in estuaries.(3) New insights into sediment response: The influence of the baroclinic effects on suspended sediment content exhibits significant tidal phase dependence and vertical non-uniformity, promoting greater sediment accumulation in the bottom layer during flood tides (increased sediment content and vertical gradient), and enhanced diffusion during ebb tides (decreased sediment content and vertical gradient).(4) The influence intensity of baroclinic effects on hydrodynamics exhibits distinct spatial heterogeneity. The main channel of the deep-water navigation route demonstrates more sensitive and pronounced responses to baroclinic forcing compared to the adjacent shallow shoals on both sides, highlighting the critical role of topography in modulating baroclinic effects.(5) The successful application and verification of the three-dimensional coupled hydrodynamics-sediment-salinity model, combined with the “baroclinic switch” scenario comparison method, demonstrates its effectiveness in complex estuarine studies and provides a reliable approach for precisely isolating the influence of a single physical process (e.g., baroclinicity) within a complex system. This study demonstrates that salinity-induced baroclinic effects serve as a key physical mechanism shaping water-sediment transport and deposition in the North Passage of the Yangtze River Estuary. The revealed detailed response characteristics of flow velocity and sediment content (layered, tidal-phase-specific, and spatially heterogeneous) offer theoretical and practical value for improving the prediction model for the siltation of deep-water channels in the Yangtze River Estuary, optimizing the dredging strategies for channel maintenance, and understanding the morphological evolution of estuaries. Future water-sediment modeling and management in the North Passage must fully consider the refined influence of salinity baroclinicity.

[Objective] A bibliometric analysis is conducted using data from China National Knowledge Infrastructure (CNKI) to examine the application of remote sensing technology in monitoring river and lake morphology and water bodies (including runoff monitoring, sediment monitoring, water level monitoring, water surface monitoring, and water volume estimation). The study focuses on discussing the temporal distribution of research publications, spatial distribution of study areas, types of sensors used, and variations in research methods within China. It summarizes key applications and development trends of remote sensing technology in China’s river and lake evolution and management, and compares them with literature on similar topics published between 2014 and 2023 from the Web of Science (WOS) Core Collection. [Methods] Using the advanced search tool of the CNKI database, 25 topics were selected, including “evolution”, “erosion and deposition”, “sediment”, “turbidity”, “main channel”, “fluvial facies”, “bank collapse”, “river regime”, “shrinkage”, “expansion”, “wetland”, “riparian zone”, “connectivity”, “unmanned aerial vehicle (UAV)”, and others. Using the Advanced Search tool in the WOS, this study retrieved relevant literature from the WOS Core Collection of the past decade on similar topics. After excluding literature irrelevant to “river-lake system evolution”, this study ultimately selected 284 articles from CNKI and 745 from WOS for analysis. [Results] In the CNKI dataset, the quantity of literature on river and lake evolution studies using remote sensing technology has shown fluctuating increase since 2002, peaking in 2023 with annual literature quantity of 33 papers. In the WOS dataset, literature quantity has increased steadily since 2018, reaching its peak between 2020 and 2022. Earlier co-occurring keywords included “wetland” and “sediment transport”, while more recent keywords included “Surface Water and Ocean Topography (SWOT)”, “human activities”, “river morphology”, “bank erosion”, and “Google Earth Engine”. Further statistical analysis of the remote sensing data sources used in these studies reveals that Landsat satellite data were the most commonly used, followed by platforms such as MODIS, Chinese Resources Satellites, Environmental Satellites, Sentinel satellites, and Gaofen series. [Conclusion] The application of remote sensing technology in river and lake evolution studies in China has transitioned from reliance on single-source passive optical sensors (visible to infrared spectrum) to multi-source remote sensing through the integration of optical and microwave multi-satellite synergy. This development overcomes limitations of traditional methods for observation, simulation, and management of river-lake systems. Remote sensing technology provides long-term image data, and with further improvement in image interpretation capabilities, more accurate methods for identifying water bodies, vegetation, and other features are expected to further support research. Leveraging remote sensing to deepen the understanding of river-lake ecosystems is of great significance for achieving integrated watershed management.

[Objectives] The sedimentation of rivers and lakes poses a persistent challenge to water resource management. Dredging, while effective for removing excess sediment and restoring channel capacity, often triggers the resuspension of contaminated bed material, leading to secondary pollution and ecological disturbance. Among various dredging techniques, grab-type dredging is widely used for its adaptability to diverse bed conditions, but its impact on local flow fields and sediment dynamics remains underexplored. This study addresses this gap by employing a full-scale two-dimensional numerical simulation using the FS3M (Fluid-Structure-Sediment-Seabed Interaction Model) to investigate the hydrodynamic and sediment suspension responses during grab bucket descent. The aim is to identify descent strategies that minimize sediment resuspension and contribute to more environmentally friendly dredging operations. [Methods] The simulation framework integrates Large Eddy Simulation (LES) for turbulent flow, a Volume of Fluid (VOF) method for water-sediment interface tracking, and a sediment transport module (STM) for modeling both suspended and bedload sediment processes. A 23 m³ environmentally friendly grab bucket is modeled descending in a symmetric two-dimensional domain that includes a 3-meter-thick sand bed. Multiple descent cases are considered: a baseline with constant velocity (1.0 m/s) and six modified cases where the grab decelerates at different heights (1.0 m, 3.0 m, 5.0 m) above the bed, with secondary descent speeds of either 0.33 m/s or 0.50 m/s. Bed deformation, flow velocity, and sediment concentration distributions are monitored over time to assess each strategy’s environmental performance. [Results] Simulation results show that the grab bucket generates significant flow disturbances during its descent, especially near the sediment bed, causing bed erosion and sediment entrainment. In the baseline scenario, rapid descent leads to high flow velocities at the bed surface and the formation of vortices that promote sediment resuspension and diffusion. In contrast, cases involving velocity reduction prior to bed contact exhibit a marked decrease in sediment disturbance. Specifically: 1)Lowering the descent speed reduces the near-bed flow velocity and suppresses the entrainment of suspended sediment. 2)Starting the deceleration at 3.0 meters above the bed (Case D3) with a reduced speed of 0.33 m/s achieves the best balance between operational efficiency and environmental performance. 3)Cases with deceleration starting at 5.0 meters do not significantly improve sediment control compared to the 3.0-meter point, suggesting diminishing returns for earlier deceleration. 4)The presence of a movable bed significantly alters flow patterns compared to fixed-bed simulations, emphasizing the importance of accounting for sediment feedback in modeling. [Conclusions] This study demonstrates that modifying the descent speed of a grab bucket is an effective way to reduce sediment resuspension during dredging operations. Key conclusions are as follows: 1)Environmental Impact Mitigation: Gradually reducing the grab’s descent speed before it reaches the sediment bed effectively decreases near-bed turbulence and sediment entrainment, thereby mitigating secondary pollution. 2)Recommended Strategy: Decelerating to one-third of the initial speed (0.33 m/s) starting at 3.0 m above the bed is the optimal descent profile among the cases studied, achieving substantial reduction in suspended sediment without compromising operational feasibility. 3)Modeling Advances: The integration of fluid, structural, and sediment dynamics through the FS3M model provides a powerful tool for analyzing complex interactions in dredging scenarios, capturing realistic behavior that conventional monitoring methods cannot resolve. 4)Future Work: Further studies should extend the modeling to include sediment excavation and lifting processes, and explore dynamic descent control strategies based on real-time sediment feedback.

[Objective] Current research on water-sediment characteristics of the Huaihe River mainstream mainly focuses on single channels, while studies on water-sediment characteristics, relationships, and diversion patterns in its bifurcated channels are limited. This study selects the typical bifurcated section from Wangjiaba to Nanzhaoji (hereinafter referred to as “Mengwa section”) in the middle reaches of the Huaihe River, aiming to clarify the variations in water-sediment characteristics and diversion patterns of typical bifurcated channels. [Methods] A combination of cumulative anomaly analysis, Mann-Kendall (M-K) trend test, R/S analysis, and Morlet wavelet analysis was used to study the water-sediment inflow characteristics of the bifurcated channels in the Mengwa section from 1985 to 2020. The driving factors of abrupt changes and variation trends in water and sediment conditions were explored. The water-sediment coordination relationships and sediment transport capacity variations were evaluated using water-sediment relationship curves, and quantitative ratios of flow and sediment diversion across bifurcated channels of the Mengwa section were provided. [Results] The annual runoff at Wangjiaba station (total) showed no significant increasing or decreasing trend, while the sediment concentration displayed a pronounced decreasing trend, stabilizing below 0.15 kg/m³ after 2010 and continuing to decrease in the future. An abrupt change occurred around 1995. Sediment retention by reservoirs, agricultural land use changes altering underlying surfaces, and soil and water conservation measures were the primary driving factors of sediment concentration reduction. The sediment coefficient showed a decreasing trend, with external influence coefficient “a” gradually decreasing and sediment transport fitting coefficient “b” gradually increasing. This indicated a continuous reduction in sediment inflow intensity and an enhancement of the channel’s sediment transport capacity, promoting channel scouring. Meanwhile, the main channel cross-section exhibited sustained expansion, indicating an ongoing erosional state in this river section. The flow diversion ratio of the Menghe River was generally positively correlated with total flow in this section. At 2 000 m³/s (low-to-medium flow level), the main channel on the Huaihe River’s southern branch served as the primary flow passage. As the flow increased, the weight diverted through the Meng River progressively rose. Below the flow level of 6 000m³/s, its diversion capacity slightly declined, while at 6 000m³/s, the flow achieved equitable flow diversion with the mainstream of the Huaihe River. The sediment concentration ratios of each bifurcated channel were approximately equal to the ratios of their respective flow sediment transport capacity. [Conclusion] These findings provide theoretical support for sediment concentration calculation models in bifurcated channels during numerical simulations of water and sediment dynamics in the middle reaches of the Huaihe River, while offering a scientific basis for adopting long-distance dredging schemes in the river’s channel regulation strategies.

In recent years, under the combined effects of intensified human activities, climate change, and sea level rise, estuarine and coastal areas have faced increasing risks of extreme flood-tide damage, posing a serious threat to the safety of estuarine and coastal embankments. Due to the complex and rapidly changing dynamic conditions, multiple disaster-causing factors, and the abrupt and strongly destructive nature of related processes, research on embankment safety risk assessment and disaster early warning has become increasingly challenging. This represents an interdisciplinary research frontier and hotspot in the field of disaster prevention and mitigation that has attracted global attention. Focusing on international research hotspots in embankment safety risk assessment and early warning and strategic needs for disaster prevention and mitigation at the national level, this study systematically reviews the current status and trends of embankment safety risk assessment and early warning technologies both domestically and internationally. Additionally, it identifies the key scientific and technical challenges that require urgent solutions. Based on the limitations of existing research, this study proposes suggestions and future research directions. The study integrates approaches from multiple disciplines, including estuarine and coastal science, coastal dynamics, river dynamics, hydrology, meteorology, engineering geology, geophysics, information engineering, and computer engineering. By employing field surveys, dynamic monitoring, flume experiments, numerical simulation, machine learning, and theoretical analysis, it clarifies three key relationships driven by the evolution mechanism of the flood-tide disaster chains: the “fluid-structure interaction”, “causal relationship between disaster-causing factors and risk assessment”, and “coordination between habitat safety and dynamic early warning”. From the perspective of hydrometeorological conditions, disaster chain evolution, and fluid-structure interaction, this study reveals the evolution mechanism of flood-tide disaster chains under changing conditions and the response mechanism for embankment disasters. Furthermore, from the perspective of multi-element monitoring and multi-indicator dynamic early warning, the study establishes a comprehensive embankment safety risk assessment system and early warning model for estuarine and coastal areas, with technical applications to enhance the accuracy of disaster forecasting and the resilience of embankment protection. The research findings are expected to improve the safety assurance capabilities in estuarine and coastal areas, significantly enhance early warning of embankment disaster risks, and provide critical scientific and technological support for evidence-based decision-making in disaster prevention and mitigation.

[Objectives] Since the operation of the Three Gorges Project and the cascade reservoirs in the upper reaches of the Yangtze River, the new water and sediment regime has caused large-scale and high-intensity continuous erosion in the Jingjiang River section of the middle Yangtze River, resulting in recurrent bank collapse incidents at some revetment sections. To investigate the causes of recent bank collapses at revetment sections and better respond to such dangerous situations, this study examines the sudden bank collapse and its emergency treatment at the “Tianzi-1” revetment section in the lower Jingjiang River in 2023. [Methods] Measured water and sediment data at Jianli station since 1990, annual erosion and deposition data of the lower Jingjiang River since 2003, and measured data on channel topographic changes and geological drilling before and after the bank collapse at the “Tianzi-1” section were used to analyze the causes. [Results] In November 2023, continuous cave-in-type bank collapses occurred at the “Tianzi-1” revetment section in the Hunan segment of the lower Jingjiang River, with collapse lengths of approximately 100 m and 35 m, respectively. The results showed that the bank collapse primarily resulted from continuous riverbed erosion under the clear-water discharge condition, significant changes in local river regime causing deep channel and thalweg migration toward banks, poor geological conditions of the riverbank slope, and the influence of prolonged low-to-medium water levels. Based on the characteristics of the dangerous situation, an underwater stone dumping method was adopted for emergency treatment of the collapsed section and the affected upper and lower reaches. [Conclusions] This recurrent bank collapse at the revetment section highlights that the lower Jingjiang River will continue to face severe threats of bank collapse for the foreseeable future. Without timely reinforcement measures, large-scale damage can occur to existing bank protection structures, seriously threatening flood control safety. Therefore, this study proposes medium-to-long-term governance measures, including establishing institutional frameworks as soon as possible, securing construction funding, strengthening dynamic monitoring of slope toe variations at dangerous sections, enhancing the development of bank failure control technologies, and improving the monitoring, early warning, and emergency response mechanisms for bank collapse.

[Objective] Significant recent scouring of the riverbed has been observed near Biandanzhou on the right bank of the Taiziji Reach, potentially threatening dike safety, influencing river regime control nodes, and altering the flow distribution ratio of downstream branches. Meanwhile, the navigation conditions in the main channel of Donggang on the right branch remain unstable. [Methods] To ensure flood control and navigation safety, this study analyzed the recent riverbed evolution of the Taiziji Reach based on the latest original underwater topographic observation data and long-term measured records. The study also predicted river regime development trends and proposed corresponding preventive and control measures to address existing issues. [Results] The shoreline has remained generally stable over the years. Except for the straight reach from Changhekou to Xingfucun, the thalweg has shown limited variation. The most significant platform migration, with a maximum lateral migration of approximately 1.1 km, was recorded in the Biandanzhou-Xingfucun area. The thalweg elevation generally exhibited a lower upstream and higher downstream pattern, with alternating changes in elevation over time. The -5,-10, and -15 m contour lines showed a slight overall scouring trend in the deep channel. The Tietongzhou branch would continue to exhibit a pattern of left branch and right main channel, with the right branch showing a general scouring trend. Locally, severe scouring near Biandanzhou was evident inter-annually, with the right bank retreating by approximately 700 m between 1981 and 2021 and the maximum local depth of scouring in the riverbed about 12 m. [Conclusions] It is recommended to implement protective measures as soon as possible for the severely scoured sections near Biandanzhou. Continuous enhancement of hydrological and topographic monitoring and follow-up analyses is essential. Once issues are identified, engineering interventions such as river regulation works should be promptly adopted. The results of this study provide critical technical support for future river management strategies in the Taiziji Reach under new flow-sediment conditions.

The Wuhu-Yuxi reach, one of the vital links between cities in the Yangtze River Delta urban area, is located in the lower reaches of Yangtze River. Understanding the evolution of the Wuhu-Yuxi reach under new hydro-sedimentary conditions provides theoretical support for future flood control and river regulation. Observed field data for the Wuhu-Yuxi reach was analyzed to understand the altered flow-sediment regime and erosion-deposition features after the operation of the Three Gorges Reservoir (TGR). Results reveal that the Wuhu-Yuxi reach has remained generally stable. The riverbed is mainly subject to scouring, with the heads of sandbars eroded and retreating. Erosion on convex banks and deposition on concave banks have emerged in the sharp-bend section and the multi-braided section due to the development of the right sub-branch of upstream Qianzhou bar upstream which leads to an increased diversion ratio, and the mainstream chute cutoff resulted from the prolonged duration of medium-low water levels. Short sub-branches have also developed in the multi-braided section because the diversion ratio of the left sub-branch in the multi-braided section of Chenjiazhou bar has increased, which in turn intensifies the scouring of the Chenjie and Caojie waterways.

Curved slopes in natural river channels are highly susceptible to erosion, posing a significant challenge for river management. To address this issue, a comparative study was conducted on the protective effects of sand-and-gravel rip-rap bags of different shapes against erosion based on the concept of ecological slope protection. First, a geometric model of the curved-channel bed was constructed according to the actual river structure. Then, the CFD-DPM method was employed to analyze the bank-protection effects of rectangular, hemispherical, and semi-cylindrical bags placed on the concave bank of the curved channel. Results revealed that under a flow velocity of 0.5 m/s, among the three shapes of protection bags, rectangular bags bring about the least pressure on the concave bank, while semi-cylindrical bags obstruct the largest amount of sediment particles, leading to the smallest Reynolds number of particles and the minimum average particle velocity, 9% lower than that in the model without protection bags. In conclusion, rectangular rip-raping bags offers better protection for the adjacent concave bank, while semi-cylindrical bags provide the most effective sand-blocking function.

To reveal the formation and evolutionary patterns of typical meandering river morphology in the main stream of the Tarim River, this study used remote-sensing images and overlaying techniques to summarize the types of planform changes in the river boundaries of the middle reach of Tarim River from Wusiman to Aqike over the past decade. Moreover, integrating measured cross-sections and hydrological data, river regime analysis was employed to evaluate the stability of river cross-sections at different times. The results are as follows: 1) The planform geometric evolution of the middle reach from Wusiman to Aqike can be categorized into four single patterns (positive shift, negative shift, extension, and contraction) and four combined patterns (positive shift plus extension, positive shift plus contraction, negative shift plus extension, and negative shift plus contraction). Among them, the positive shift plus extension pattern is the most prevalent in the study reach. 2) Morphological parameters, namely the offset degree and widening degree, along with their calculation methods, were proposed to depict the main types of planform evolution in river channels. Annual discharge was found to be the primary factor influencing the offset degree, followed by the annual sediment transport and bedload discharge. Flood was identified as the key factor affecting the widening degree. 3) In recent years, the hydraulic geometric relation coefficient has increased, suggesting an enhanced longitudinal stability of the riverbed. Meanwhile, the transverse stability of the riverbed has also improved along the study reach. This research offers significant reference for flood control engineering and river channel regulation of meandering rivers in the middle reach of the Tarim River.

River confluences are areas where environmental elements such as flow structures, sediment deposition, water quality, and organisms undergo significant changes in river networks, and are the key nodes for flooding and pollutant transport. This review summarizes the basic characteristics of flow structures and pollutant transport at river confluences and their responses to different conditions, along with the important impacts of complex flow structures and habitats on water safety issues in previous field measurements, laboratory experiments, numerical simulations, and theoretical studies. Future research should focus on enhancing the understanding of the hydraulic characteristics of river confluences under unsteady flow conditions, further clarifying the response mechanisms between pollutant transport and hydraulic parameters, and improving the flood safety and the water ecological environmental quality through the combination of engineering layout and hydraulic optimization and regulation.

As an important river morphology, meandering channels widely exist in the middle and lower reaches of the Yangtze River. Due to the complex evolution, the regulation of meandering channels had always been a hot and difficult issue for water conservancy and transportation sectors. We made a review on the evolution rules, the theories and regulation technologies of meandering channels in the middle and lower reaches of the Yangtze River before and after the construction of the Three Gorges Reservoir. On this basis, we propose that the regulation of meandering channels should be in line with the evolution rules and trends of river regime, stabilizing favorable river regime while improving unfavorable river regime. Furthermore, we delve into the directions of future regulation: the long-term evolution trend of meandering channels under low sediment concentration and the demand for multi-objective regulation, the risk of bank collapse under the long-term scouring of near-shore riverbed and the monitoring and early warning of river channel, as well as the comprehensive regulation technology for flood control and navigation and the application of new materials for regulation.

The physical habitat of the middle and lower reaches of the Yangtze River has experienced significant changes after the impoundment of the Three Gorges Reservoir, thereby affecting river functions. Based on data analysis and literature review, this paper examined the alterations in typical physical habitats such as the hydrological condition, river morphology, and vegetation after the TGR operation and their subsequent impacts on flood prevention, navigation, water supply, and typical aquatic organisms. Key areas for further research were identified as follows: 1) monitoring, including systematic and long-term monitoring programs, and the assessment of the effectiveness of river (waterway) management projects and ecological regulation measures; 2) laws and mechanisms of changes in river morphology, vegetation in the main stream and shoals, and the responses and thresholds of flood levels and benthic animals to variations in the physical habitat; 3) methods for predicting medium and long-term trends of hydrological conditions under the influence of multiple factors, channel regulation technologies that adapt to changes in the physical habitat and meet demands, as well as reservoir operation schemes that align with water supply objectives, benthic animal and fish breeding needs. Additionally, integrated research efforts focusing on physical habitat changes, their impacts, and improvement strategies and technologies require increased attention.