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.

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.

The erodibility of sediment mixtures is a key factor in sediment dynamic processes and morphological evolution in coastal environments. However, it remains insufficiently understood. In the current study, the critical shear stress of sediments is analyzed with different mud contents and consolidation degrees from experimental results and previous studies. The results indicate that the critical shear stress increases with clay content, peaking at 30% clay content, and then gradually decreasing. Compared to the solid volume fraction of mud (clay and silt), the solid volume fraction of clay shows a higher relation with the critical shear stress of sand-mud mixtures. The role of the consolidation degree in the erodibility of sediment mixtures was quantified through consolidation experiments, revealing an exponential relation between critical shear stress and consolidation coefficient. An empirical equation for the critical shear stress is proposed to consider the mud content, the solid volume fraction of clay, and the consolidation degree. This equation is applicable to mixed sediment over the full range of mud content and varying consolidation degrees. It has a simple form, is easier to apply, and outperforms other empirical equations (RMSE = 0.62; R² = 0.73).

Wave nonlinearity plays a critical role in modulating energy dissipation and sediment transport in vegetated coastal zones, influencing shoreline stability and ecosystem-based defenses. This study analyzes 45 d of wave observations from the Yangtze Estuary, including data collected during Typhoon Khanun, to investigate its spatial variability and underlying mechanisms of wave nonlinearity across a mudflat–vegetation transect. Wave skewness and asymmetry varied within tidal cycles, increasing at low tide and decreasing at high tide. During typhoon conditions, nonlinearity intensified significantly, with skewness increasing by up to 346% and asymmetry shifting toward more forward-leaning waveforms, both closely linked to elevated Ursell numbers. Bispectral analysis at five stations across the transect revealed distinct energy transfer mechanisms: sum interactions dominated over mudflats, whereas difference interactions prevailed within vegetated zones, indicating vegetation-induced modification of nonlinear wave dynamics. Further analysis shows that shoaling and vegetation exerted opposing influences, amplifying and damping wave nonlinearity, respectively. Empirical formulas proposed by Zhao et al. (Coastal Engineering 2024; 192:104543) from laboratory data were evaluated against the field data, demonstrating reasonable performance under extreme conditions. These findings improve mechanistic understanding of wave–vegetation interactions and support the development of nature-based strategies for coastal resilience and sediment management.

Channel deepening and narrowing are common anthropogenic modifications in estuaries, but their combined effects on estuarine circulation, stratification, and sediment transport remain insufficiently understood. This study investigates these combined impacts in the North Passage of the Changjiang Estuary, where large-scale deepening and narrowing have significantly altered hydrodynamic and sediment processes. Our analysis demonstrates that channel deepening intensifies estuarine circulation by strengthening the landward near-bed flow, thereby enhancing sediment import. Contrary to initial expectations that narrowing would promote sediment flushing, our results indicate that narrowing increases stratification, steepens along-estuary salinity gradients, and suppresses vertical mixing. Intensified stratification further reinforces estuarine circulation, promoting sediment trapping at the saltwater intrusion limit. Additionally, enhanced tidal pumping driven by increased velocity and suspended sediment concentration gradients extends the estuarine turbidity maximum both upstream and downstream, a process often overlooked in engineered estuaries. These findings challenge conventional assumptions regarding the sedimentary impacts of narrowing, emphasizing instead its critical role in amplifying estuarine circulation and sediment trapping. Our results provide new insights into sediment dynamics in river-dominated estuaries, with significant implications for estuarine management, dredging operations, water quality control, and long-term morphological stability.

Decomposing turbulence from waves remains challenging due to frequency overlap and wave-turbulence interactions. Existing decomposition methods, e.g., moving average, energy spectrum analysis, and synchrosqueezed wavelet transform (SWT), produce inconsistent turbulence estimates. Here, we introduce a rotating-coordinate-based method (RoCoM), founded on two assumptions: (1) the energy spectrum in the cross-wave direction remains unaffected by wave orbital velocities, and (2) wave-wise and vertical turbulence spectra are linearly proportional to the cross-wave spectrum, with proportional constants derived from frequencies higher than the wave-dominated frequency range. Both assumptions were validated with observational data collected from the Changjiang Estuary. Comparative analyses using both in-situ observations and controlled laboratory experiments show RoCoM avoids the energy trough problem inherent in the moving average and SWT methods, yielding the most accurate power spectra and turbulent kinetic energy (TKE) estimates. In-situ data reveal that the relative errors of RoCoM are approximately 16 % for total TKE and about 6 % for TKE within the wave-dominated frequency range. Laboratory experiments further confirm its superior accuracy, demonstrating relative errors of approximately 14 % for total TKE and about 7 % for wave-band TKE. RoCoM holds significant implications for marine material transport and coastal energy development by providing robust and precise turbulence and wave energy estimates. Nonetheless, its application is currently best suited for scenarios with predominant wave propagation from a single direction, while SWT remains advantageous in environments characterized by broader directional wave spreading.

Tidal flats are critical coastal ecosystems, with their geomorphic characteristics traditionally understood to be primarily influenced by tidal, wave, and storm forces. This study investigates the impact of rainfall on the morphodynamics of upper tidal flats by combining hydrodynamic-sediment data, meteorological rainfall records, and video monitoring at the Chongming Dongtan tidal flat in the Yangtze River Estuary, China. We show that rainfall significantly increases suspended sediment transport and accelerates tidal channel elongation. Notably, rainfall events—though occurring during only 25 % of observed tidal inundation periods—accounted for 62 % of cumulative net sediment transport. This disproportionate efficiency compared to tidal forcing stems from the rainfall-induced hydraulic connectivity between expansive supratidal areas and tidal channels, where concentrated runoff convergence intensifies scour dynamics. These findings challenge the traditional view of tidal flat dynamics, suggesting that rainfall is a more influential driver of morphodynamic change than previously recognized.

Marsh creeks are perceived as important conduits for transporting water and sediment between mudflats and marshes. In order to advance the understanding of the transport mechanisms in creeks, the source and ultimate sink of sediment which moves between mudflats and marshes through creek channels need further investigation. Therefore, two field campaigns were conducted in two intertidal systems with varying sediment availability. The water depth, flow velocity, suspended sediment concentration, and bed level change were measured simultaneously in a marsh creek and on the adjacent mudflat in Chongming Island (China) and in Paulina Saltmarsh (the Netherlands). Paulina Saltmarsh is much smaller, more frequently flooded, and has lower sediment concentration than Chongming. These contrasting conditions allow for a comparison of transport mechanisms and functioning of the creek. Both systems first show that the high suspended sediment concentration (SSC) measured in marsh creeks is mainly the consequence of sediment advection rather than local erosion. In addition, erosion in marsh creeks is usually limited during ebb tides, reducing the export of sediment through these creeks. However, differences have been observed between two systems. The measured SSC was highly asymmetric between flood and ebb tides in Chongming. Large peaks in SSC during the flood period can be observed for most tidal cycles. The marsh creeks in Chongming therefore function as conduits for sediment import. Additionally, there are distinct overbank and underbank tides in Chongming. Sediment was trapped and retained in creeks during underbank tides, which can then be eroded and transported to the marsh during subsequent overbank tides. We also observed that mudflats in Chongming quickly recovered after erosion. These mechanisms have not been observed in Paulina Saltmarsh, where net sediment export via the marsh creek was observed due to a lack of abundant sediment in suspension during flood tides. Furthermore, the remaining bed surface of mudflats after an erosion event was stronger than before, limiting further erosion in Paulina Saltmarsh. These findings from the two systems indicate that the role of creeks in sediment import/export depends on the availability of sediment from mudflats, shedding light on nourishment strategies for salt marshes.

The survival of salt marshes, especially facing future sea-level rise, requires sediment supply. Sediment can be supplied to salt marshes via two routes: through marsh creeks and over marsh edges. However, the conditions of tides and waves that facilitate sediment import through these two routes remain unclear. To understand when and how sediment is imported into salt marshes, 2-month measurements were conducted to monitor tides, waves, and suspended sediment concentration (SSC) in Paulina Saltmarsh, a meso-macrotidal system. The results show that the marsh creek tends to import sediment during neap tides with waves. A tidal cycle with a small tidal range result in weaker flow in the marsh creek during ebb tides, reducing the export of sediment. Waves enhance sediment supply to the marsh creek by eroding mudflats. However, strong waves can directly resuspend sediment in marsh creeks during spring tides when the water level is above the marsh canopy, enhancing sediment export through creeks. Net sediment import over marsh edges requires the opposite tidal and wave conditions: spring tides with weak waves. Spring tides provide stronger hydrodynamics, facilitating sediment import over the marsh edge. Increased SSC during the ebb phase can occur with strong waves over the marsh edge, resulting in net sediment export. Therefore, the net import or export of sediment, through the creek and over the marsh edge, depends on the combination of tidal and wave conditions. These conditions can vary between estuaries and even individual marshes. Understanding these conditions is crucial for better management of salt marshes.

Creeks are essential for salt marshes by conveying water and sediment through this geomorphic system. In this paper, we investigate the mechanisms that determine the residual sediment flux using measurements conducted in tidal creeks in salt marshes of the Yangtze Estuary. A main creek and a secondary creek were studied to explore whether the mechanisms determining residual sediment fluxes through the main creek differ from those in the secondary creek. Measurements in creeks were carried out over 5 years, spanning different months. Sediment import was found during most tides, both in the main creek and the secondary creek, implying that creeks in Chongming generally function as a conveyor belt of sediment into the marsh. However, sediment export can occur during certain overbank tides. When comparing the role of creeks in drainage and sediment delivery, the main creek functions more in delivering sediment while the secondary creek primarily serves as a drainage conduit. To better understand the mechanisms behind sediment fluxes, the residual sediment flux was compared with the residual discharge and the sediment differential (differences in sediment concentration between flood and ebb). Overbank tides generally lead to a net outward discharge as more water from saltmarshes can be concentrated into the marsh creek during ebb tides. This net outward discharge tends to export more sediment during ebb tides. However, due to the sediment abundance during the flood phase in the turbid environment, sediment import can be expected even with the residual export of water. Export of sediment was only found for the few tides with a net outward discharge and a small positive sediment concentration differential. Large negative sediment differentials (larger averaged suspended sediment concentration during ebb tides) have not been observed because the sediment supply during ebb is limited. This paper unravels how the sediment differential and residual discharge contribute to the residual sediment flux, providing a better understanding of sediment dynamics in marsh creek systems.

Strong hydrodynamic forces generated by storms are key in shaping coastal tidal flats. Most tidal flats achieve equilibrium by adapting to hydrodynamic conditions and sediment inputs. However, high-energy wave activity during storms disrupts this equilibrium, causing rapid and significant changes, particularly in tidal flats, especially in microtidal flats, which are characterized by low tidal ranges. In this study, we conducted an 11-d field campaign on a microtidal flat in the Yellow River Delta (YRD), capturing data during both stormy and calm weather conditions. We measured tidal currents, wave activity, suspended sediment concentrations and sediment grain sizes. The results demonstrated that the tidal flat maintained equilibrium under calm conditions, with minimal fluctuations in bed level (within ±2 mm). Contrastingly, severe erosion and sediment removal during the storm significantly altered the equilibrium of the area. The storm-induced high shear stresses, ranging from 1.02 to 1.48 N/m ², along with alongshore sediment transport, resulted in an elevation change of −10 mm. Furthermore, the subsequent bed level recovery was minimal and insufficient to offset the erosion. Compared to that of the mesotidal and macrotidal flats, post-storm recovery on microtidal flats was limited due to shorter inundation periods and weaker hydrodynamic forces. Therefore, frequent storms may lead to continuous shoreline retreat on microtidal coasts. Conclusively, the present findings underscore the significant impact of storm-induced erosion on the evolutionary processes of microtidal flats and suggest that greater attention should be given to protecting these areas during storms in the Yellow River Delta. The insights can guide the development of more effective coastal protection strategies, highlighting the need for enhanced measures to mitigate erosion and promote resilience in microtidal regions.

De-reclamation is a common strategy used for the restoration of tidal flats. In this study, we investigate the morphodynamic response of tidal channel networks and tidal flats after de-reclamation initiatives using the Delft3D numerical model. We find that tidal channel networks that have undergone reclamation and retreat projects have a lower drainage density (8.95 km⁻¹) than that of channel networks that formed naturally (11.33 km⁻¹), and the drainage efficiency of natural formed channel networks is almost three times greater than restored channel networks. These findings indicate that de-reclamation alone cannot fully erase the imprinting of the previous reclamation. We also find that the ultimate effectiveness of de-reclamation is affected by the geographical layout and unchanneled path length of the inchoate main creek system. In addition, following the implementation of de-reclamation, the immediate opening of previously enclosed areas amplifies the tidal prism, thereby intensifying tidal scouring and resulting in significant erosion, with erosion rate reaching hundreds of millimeters per day. Such losses can be remediated under sufficient sediment supply and prevented through the construction of artificial channels. However, this severe erosion may escalate under an insufficient sediment supply or a heightened tidal prism, potentially leading to permanent loss. These findings constitute an important reference for future engineering practices that support the safety and sustainability of coastal resources.

A decline of the fluvial sediment supply leads to coastal erosion and land loss. However, the fluvial sediment load may influence not only coastal morphodynamics but also estuarine hydrodynamics and associated saltwater intrusion. Previous studies revealed that suspended sediments influence estuarine hydrodynamics through various flow–sediment interactions. In this contribution, we systematically investigate how changes in fluvial sediment load and other climate-change-induced environmental change influence estuarine hydrodynamics and sediment dynamics. For this purpose, we utilize a well-calibrated fully coupled model in which hydrodynamics, saltwater intrusion, and sediment transport interact with each other, to explore saltwater intrusion in the Yangtze Estuary in response to a decline in the sediment load, modified discharge, and sea-level rise. Model results suggest that a 70% decline in the suspended sediment load weakens the impact of sediments on salinity-induced stratification and thereby reducing saltwater intrusion. Sea-level rise or discharge peak reduction increases saltwater intrusion. However, a fully coupled model accounting for sediment effects predicts a much larger increase in saltwater intrusion compared to noncoupled models. Whether this effect is important depends on estuarine sediment concentrations and therefore the potential role of sediments should be carefully investigated before applying a noncoupled model. This work highlights not only the relevance of a suspended sediment decline but also the use of fully coupled models for predicting saltwater intrusion in turbid estuaries and has broad implications for freshwater resource management in turbid estuarine systems influenced by human interventions and climate change.

The interactions between organic and inorganic particles in the context of flocculation is an on-going topic of research. Most current researches do not distinguish between the effects of EPS (produced by microorganisms) and living microorganisms (like algae). In this study, the effect of salinity, EPS and living algae on sediment flocculation are investigated separately. Several types of measurements were performed, which can be divided into the following categories: sediment at different salinities, sediment in the presence of EPS at different salinities, sediment in the presence of living algae at a given salinity. Results show that increasing salinity enhances slightly sediment flocculation. In the presence of EPS there was hardly any flocculation in demi-water, but the flocculation was significant in saline water. The living algae cells were shown to flocculate with themselves and form large flocs. These algae flocs can bind to sediment particles to form larger flocs, both in demi-water and sea water. Size-wise algae-sediment flocs were largest, EPS-sediment flocs came second, and salt-sediment flocs were smallest.