Hydropower dams induce downstream sediment starvation, influencing fluvial morphology. With the focus commonly on morphological changes, an aspect of sediment starvation that has received much less attention is the impact of changes in the sediment grain size distribution (GSD) on these morphological changes. In this study, we investigate the effects of the Three Gorges Dam (TGD) on the multi-fraction sediment transport and bed recovery in the middle-lower Yangtze River. Based on long-term field data (1987–2021), we evaluate how fine (d < 0.031 mm), medium (0.031–0.125 mm), and coarse (d > 0.125 mm) fractions differentially respond to dam regulation. Our findings reveal a progressive coarsening of suspended sediment and identify three distinct dam-induced sediment regimes: static armored gravel bed, active bed armoring, and strong erosion. Within the first ~350 km downstream of the TGD, erodible sediments, especially fine and medium fractions, have been almost entirely depleted. In contrast, the subsequent 750 km reach has emerged as the dominant sediment source, increasingly characterized by medium and coarse fractions over time. In addition, tributaries now supply fine-grained sediment during the wet season, whereas lakes, acting as long-term sediment storage zones, release previously deposited material during the dry season. Both sources are playing an increasingly important role in modulating the GSD of the middle-lower Yangtze. These findings shed lights on the dam-induced multi-fraction sediment recovery, offering valuable guidance for the sustainable management of river systems influenced by upstream dams.

Many estuaries worldwide have been significantly modified by land reclamation, influencing hydrodynamics and ultimately bed level changes. However, detailed observations documenting how hydrodynamic and morphological changes interact, especially revealing positive feedback mechanisms strengthening the original intervention, remain scarce. This study investigates the morphodynamic response in the macro-tidal, highly turbid Hangzhou Bay (China) over the past three decades to interventions, using unique datasets of annual bathymetry (1991–2019) and synchronous hydrography before and after intensive reclamation (1999, 2019). The analysis reveals severe accretion within the inner bay, with an average bed level rise of 1.21 m (total volume ∼ 2.2 × 10⁹ m³) since 1999, concurrent with a drastic 54.6% reduction in intertidal storage volume. Two key morphological parameters, the ratio of intertidal storage volume to channel volume (Vs/Vc) and the relative tidal amplitude (a/h) systematically decreased by 43% and increased by 6.5%, respectively, thereby enhancing flood dominance. All key hydrodynamic and sediment transport parameters, current velocity, suspended sediment concentration (SSC), tidal volume and sediment flux, showed strong correlations with the tidal range at the bay mouth. Despite a ∼ 30% reduction in mean current velocity and tidal volume from 1999 to 2019, SSC paradoxically increased by 47–133%, and sediment flux rose by ∼44%. The changes are driven by a positive feedback mechanism: the loss of accommodation space and increasing flood dominance promote sediment import and accelerated deposition, which in turn further reduces channel volume and reinforces the tidal asymmetry. This work reveals a key feedback loop accelerating the transition of macro-tidal estuaries towards flood dominant, rapidly infilling and highly turbid systems in response to land reclamation. Such knowledge is key to sustainable sediment management in turbid macrotidal estuaries.

Tidal rivers and estuaries may experience high levels of suspended particulate matter (SPM), which impacts water quality and ecosystem functioning. The processes controlling the development of estuarine turbidity maxima (ETM) are fairly well understood. However, predicting the maximum SPM concentration in an estuary based on aggregated parameters (estuarine dimensions, river discharge, tidal range) remains, up to now, impossible without extensive in-situ measurements and/or numerical models. This study introduces an approach that links the strength of the ETM to the tidal, river, and morphological characteristics of a system. Using in-situ data from contrasting meso- to macro-tidal estuaries, we found a consistent pattern of maximum SPM concentrations within a two-dimensional parameter space. The resulting turbidity diagram reveals a high SPM hotspot in estuaries with specific forcing conditions, corresponding to intermediate relative tidal amplitudes and freshwater Froude numbers. This multi-site research advances our predictions of ETM intensity in tide-dominated estuaries, offering a straightforward method to explore potential turbidity trajectories under various human pressures.

The Ems estuary faces rising turbidity and increased flood risk due to sea-level rise. We investigated three Nature-based Solutions (NbS) through hydro-morphological modeling to address these issues by 2100: converting a polder to wetland, facilitating salt marsh growth with brushwood groynes, and re-using dredged sediment. Without NbS, turbidity is projected to increase, especially with sea-level rise. Reconnecting a polder can help reduce turbidity in the Dollard while expanding facilitating new wetland, but may not fully counteract the increased sediment import projected with sea-level rise. Extracting mud from the Delfzijl harbour is the most effective measure in reducing turbidity, while marsh expansion with brushwood groynes does not significantly affect turbidity. Nevertheless, groynes facilitate both marsh expansion as well as significant local flood risk reduction.

The Ems Estuary faces existential challenges including flood risk, increasing turbidity, and biodiversity loss, all of which may intensify under future climate scenarios and require transboundary collaboration between the Netherlands and Germany. Addressing these challenges requires compliance with EU, national, and local regulations. Simultaneously each nation pursues socioeconomic benefits from the restoration through a holistic, system-based approach. This study synthesizes the key processes driving flood risk, hyper-turbidity, and salinization within the Ems Estuary. From this understanding the paper catalogues the planned and implemented pilot measures from both countries to advance their climate adaptation plans. Both nations share a common vision of leveraging the high turbidity of the estuary as an asset in climate adaption, e.g. for land raising, dyke reinforcement or habitat creation. Building on the pilot projects and shared visions, three transboundary upscaling strategies involving sediment management are proposed: (A) land elevation using dredged sediment; (B) multifunctional flood defences incorporating nature-based solutions; and (C) habitat creation and restoration to enhance ecological resilience. The Ems Estuary offers valuable insights for global transboundary estuarine management, illustrating how innovative sediment management and transboundary cooperation can be achieved to support climate adaptation and sustainable development. The study underscores the need for harmonized governance, standardized success metrics, and cross-border planning to enable effective upscaling.

Storm surges pose significant threats to coastal populations and livelihoods, and rapid, accurate spatiotemporal forecasting is crucial for mitigating their impacts. In recent years, deep learning models have shown strong forecasting potential, offering new solutions for storm surge prediction. In this study, we developed a storm surge prediction model based on the Adaptive Fourier Neural Operator. The model effectively captures spatiotemporal relationships between meteorological forcing and storm surge dynamics at a spatial resolution of 0.05°. Unlike models that rely solely on historical data, our approach incorporates continuously updated typhoon information as input, allowing it to respond dynamically to changes in storm track and intensity. When tested on Tropical Storm Pulasan (2024) and Typhoon In-Fa (2021), the model achieved 48-h mean spatial root-mean-square errors (RMSEs) of 0.28 m and 0.27 m, respectively, with correlation coefficients (CORRs) exceeding 0.69 and 0.85 relative to numerical simulations used as the training reference. In addition, validation against observations from multiple tide gauges yielded station-averaged RMSEs of 0.35 m and 0.34 m, CORRs of 0.74 and 0.78, and peak surge errors of 0.18 m and 0.28 m, respectively. These results demonstrate the model's strong potential for operational storm surge forecasting and emergency management.

Tidal flats provide essential ecosystem services but are increasingly threatened by reduced sediment supply and human activities, requiring close monitoring and understandings in estuaries. We focus on the four tidal flats with a total area of 1800 km²in the Yangtze Estuary and systematically evaluate their morphodynamic evolution based on consistent bathymetry data over 60 years (1958–2022). While fluvial sediment supply has declined since the mid-1980s, all four tidal flats in the estuary sustained accretion until 2010, demonstrating a lag of 20–30 years in estuarine morphological response to sediment decline. However, note that accretion primarily occurs on higher parts of the shoals, whereas erosion dominates in the subtidal zones. This is mainly attributed to the combined impact of saltmarsh expansions, reclamation, and channel scour and dredging. It suggests that part of the eroded sediment from channels deposits on adjacent shoals, leading to a regional sediment budget balance, particularly in the central channel-shoal complex with the navigation channel. Moreover, the initiative of removing Spartina from the shoals, a fast-spreading invasive species that benefits shoal accretion but not native species, might disrupt the ongoing accretion of high shoals and induce overwhelming erosion and sediment loss. One management strategy to counteract these impacts and restore tidal flats is to make beneficial use of the dredged and trapped sediment from the North Passage, an annual amount of approximately 50 million m³, to the adjacent shoals, though how to sustainably manage the sediments remains another concern.

Accurate and efficient prediction of spatiotemporal variations in the distribution of substances in fluids (SIFs) is crucial for various aspects of fluid mechanics related research and applications, involving for instance, material transport quantification, water quality assessment, and engineering condition analysis. This study proposes a framework for resolving the spatiotemporal distribution of SIFs such as salt and suspended sediment based on water levels and flow velocities. The framework incorporates a deep learning model based on a classic neural operator (DeepONet) architecture, which consists of a feature network and a position network to encode the characteristics of input variables and the problem domain. Numerical simulations were performed to generate the needed datasets. The framework was well-validated by predicting salinity and suspended sediment concentration (SSC) distributions in two idealized cases and a real-word case, demonstrating its efficacy and robustness. Time-series validation further demonstrated the prediction accuracy of the framework. The deep learning model is also capable of enhanced-resolution predictions, enabling the generation of high-resolution spatial distributions of SIFs from low-resolution hydrodynamic data. Both bottom and surface layers of the water column were analyzed, revealing that the mapping relationships between hydrodynamics and SIF distributions can be accurately captured throughout the water column, despite variations in correlation coefficients. Due to these capabilities and advantages, additional data sources can be integrated into the framework in the future, highlighting its considerable potential for broader applications in aquatic environments.

Human interventions influence sediment dynamics, and understanding these mechanisms is essential for predicting short-term and long-term estuarine development. The Deep Channel Navigation Project (DCNP) in the Yangtze Estuary is such a large infrastructural intervention that substantially alters sediment exchanges between channels and shoals and may thereby influence this estuarine development. However, the effect of these constructions on channel-shoal sediment exchange is up to now poorly known. In this study, we use an extensive dataset collected both in channels and on shoals and a numerical model to clarify the exchange mechanisms driving sediment transport patterns in a strongly anthropogenically modified environment. The results indicate that the stepwise construction of hydraulic structures leads to gradual changes in sediment exchange. The first phase was characterized by partially blocked sediment exchange with northward sediment transport towards the main channel and to the northern flats (2002–2010). Next, a transition period was characterized by weaker horizontal sediment exchange and reduced sediment supply (2010–2016). Since 2016, more efficient structures blocking sediment exchange further hinder northward transport and promote deposition on the southern flats. These processes point to the important role of engineering works in strengthening the southward growth of the delta. Moreover, data analyses suggest that northward over-jetty flow during high water induces a net sediment flux towards the channel due to water level gradients. The residual flow controls the net sediment transport both in the longitudinal and lateral direction over the tidal flats. Therefore, a clockwise residual circulation cell forms in the channel-shoal system, contributing to the channel siltation. These findings shed important insights into the role of sediment exchange in channel siltation and large-scale hydrodynamic and delta development. Such knowledge is crucial for sustainable future management of delta distributaries.

Physics-informed neural networks (PINNs) are increasingly being used in various scientific disciplines. However, dealing with non-stationary physical processes remains a significant challenge in such models, whereas fluid motions are typically non-stationary. In this study, a PINN-based method was designed and optimized to solve non-stationary fluid dynamics with shallow water equations in a polar coordinate system (PINN-SWEP). It was developed and validated with a classic circular basin case that is well-documented in scientific literature. In the validation case, the wind-induced water surface fluctuations are less than 1 cm, posing challenges in modeling. However, our PINN-SWEP model can accurately simulate such tiny water surface fluctuations and resolve complex fluid motions based on limited and sparse data. A boundary discontinuity problem associated with the use of a polar coordinate system is further discussed and improved, thereby enhancing the applicability of PINN in water research. The methodology can provide an alternative solution for numerical or analytical solutions with high accuracy.

This study examines the local, intratidal effects of suspended sediment concentrations (SSCs) on the hydrodynamics and vertical mixing in the Ems Estuary, located on the border between Germany and The Netherlands, during summer and winter seasons when the estuary turbidity maximum (ETM) is located upstream and adjacent to the study site, respectively. Measurements of density, SSCs, turbulent kinetic energy dissipation, and current velocity were collected and analyzed over a semi-diurnal tidal cycle in August of 2018 and January of 2019 as part of the collaborative Ems-Dollard Measurement (EDoM) campaign. During August, the estuary turbidity maximum was located 25 km upstream from the measurement site and local SSCs were low. Results revealed that under these conditions, suspended sediment minimally impacted vertical mixing by stabilizing density near-bottom during flood tide, while typical salinity-induced tidal straining patterns dominated. During January, the ETM was located only 5 km upstream of the measurement site leading to higher local sediment concentrations. Salinity-induced straining of the density occurred on early flood tide, creating stratification that suppressed vertical mixing. The suppression was enhanced by the contribution of vertical gradients in SSC to density, as signified by the gradient Richardson number. Suppression of vertical mixing by sediment-enhanced stratification was most significant within the hour following maximum flood currents when elevated velocity shear occurred. The variability observed between the local dynamics during August and January were attributed to greater sediment concentrations due to the ETM proximity in January. The intratidal asymmetry of vertical mixing observed under higher SSCs likely has implications for sediment transport.

Land reclamations influence the morphodynamic evolution of estuaries and tidal basins, because an altered planform changes tidal dynamics and associated residual sediment transport. The morphodynamic response time to land reclamation is long, impacting the system for decades to centuries. Other human interventions (e.g., deepening of fairways or port construction) will add more morphodynamic adaptation timescales. Our understanding of the cumulative effects of anthropogenic interference with estuaries is limited because observations usually do not cover the complete morphological adaptation period. We aim to assess the impact of land reclamation works and other human interventions on an estuarine system by means of digital reconstructions of historical morphologies of the Ems Estuary over the past 500 years. Our analysis demonstrates that the intertidal-subtidal area ratio altered due to land reclamation works and that the ratio partly restored after land reclamation ended. The land reclamation works have led to the degeneration of an ebb and flood channel system, transitioning the estuary from a multichannel to a single channel system. We infer that the 20th-century intensification of channel dredging and re-alignment works accelerated rather than caused this development. The centennial-scale observations show that the Ems estuary evolution corresponds to a land reclamation response following tidal asymmetry-based stability theory as it moves toward a new equilibrium configuration with modified tidal flats and channels. Considering the long history of land reclamation in the Ems Estuary, it provides an analogy for expected developments in comparable tidal systems where land reclamations were recently carried out.