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Net sediment transport is predominantly seaward in fluvial‐dominated estuaries worldwide. However, a distributary branch in the Changjiang Estuary, the North Branch, undergoes net landward sediment transport, which leads to severe channel aggradation. Its controlling mechanism and the role of human activities remain insufficiently understood, although such knowledge is necessary for better management and restoration opportunities. In this study we revisit the centennial hydro‐morphodynamic evolution of the North Branch based on historical maps, field data, and satellite images and provide a synthesis of the regime change from ebb to flood dominance. The North Branch was once a major river and ebb‐dominant distributary channel. Within which alternative meandering channels and sand bars developed. Deposition of river‐borne sediment leads to infilling of the branch, while tidal flat embankment reduces the bankfull width and modifies the channel configuration, resulting in a profound decline in the sub‐tidal flow partition rate. The North Branch then becomes tide‐dominant with an occurrence of tidal bores and elongated sand ridges. Once tidal dominance is established, extensive tidal flat reclamation enhances the funnel‐shaped planform, amplifying the incoming tides and initiating a positive feedback process that links tidal flat loss, sediment import, and channel aggradation. Overall, the shift in branch dominance is a combined result of a natural southeastward realignment of the deltaic distributary channels and extensive reclamation. One management option to mitigate channel aggradation is to stop the aggressive reclamation and allow tidal flats to build up, which might reduce the sediment import and eventually lead to a morphodynamic equilibrium in the longer term. Understanding the impact of tidal flat reclamation is informative for the management of similar tidal systems under strong human interference. [ABSTRACT FROM AUTHOR]

• Storm intensity and sediment availability control tidal flat evolution during a storm. • Storms enhance sediment supply for the studied flats undergoing long-term erosion. • A moderate storm offsets rather than accelerates tidal flat erosion. Accretion rates of estuarine tidal flats have strongly decreased globally due to the combined impacts of sea-level rise, reduction in sediment supply, and human interventions. Storms and associated high-energy activities influence sedimentary processes on tidal flats. A better understanding of the role of storm events on the evolution of tidal flats can facilitate better coastal management. This study assesses the impacts of storms on intertidal sedimentary processes. We monitored tidal water levels, tidal currents, waves, suspended sediments, and morphological changes of the eroding Diaokouhe lobe in the Yellow River Delta during and after a storm event. The estimated bed shear stress during the storm event (average 1.60 N/m2) was much higher than that during fair-weather conditions (average 0.71 N/m2). However, surface erosion of tidal flats was negligible during the storm, which was ascribed to the high critical bed shear stress owing to the compacted bed surface. The tidal flats were inundated for a longer time with a higher water depth and higher suspended sediment concentrations (average 1.13 g/L) during the storm, indicating sediment supply from the subtidal zone and adjacent sea. These conditions, namely moderate storm strength, high sediment compaction, and high sediment availability, led to deposition rather than erosion on the intertidal flats. Overall, the results imply that storm-induced erosion is less likely to occur on tidal flats with rich sediment supply and high erosion resistance. [ABSTRACT FROM AUTHOR]

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. • High SSC during flood tides in the studied marsh creeks is attributed to advection of sediment rather than local erosion. • Ebb tides were limited in suspended sediment by various mechanisms, reducing the export of sediment via marsh creeks. • Roles of marsh creeks in sediment delivery depend on the sediment supply from mudflats and hydrodynamic forcing in creeks. [ABSTRACT FROM AUTHOR]

A tidal creek represents a typical morphologic unit in an intertidal flat. The development and migration of a tidal creek can affect the mass transport, ecological environment, and geomorphologic evolution of the flat. By using field observations, this study links hydrodynamics, sediment transport processes with short-term changes in topography at a typical tidal creek system located at the Chongming Island of the Yangtze River estuary. Hydrodynamic and sediment transport associated with varying tidal cycles across both wet (flood) and dry seasons were measured through the field campaign. The wet and dry seasons represent higher and lower discharges of Yangtze River, respectively. The results indicated that most of the suspended sediment becomes entrained at the beginning of a flood tide. At a fixed point, 7.2 times of suspended sediments, which were entrained out of the creek in wet season, began to be transported along the creek compared to dry season. In the dry season, high flow velocity and shear stress conditions occurred in the tidal creek because the water level was below the top of the flat. In summary, the tidal creeks were found to serve as effective conduits for the transportation of sediments in the wet season, and the secondary flow enhanced the development of tidal meandering. Seasonal variations in creek morphological changes were also continuously monitored over two years at intervals of two months. The change of creek morphology varied from the high level to low level, and tidal meandering was strongly associated with flood and ebb tides. • Water flow and sediment transport were studied in a meso tidal creek system. • The tidal flat tended to receive sediment deposits from April to October, and to be eroded in the other months. • Tidal creek morphology was stable at the high creek level, and it meandered at the low level. • The results contribute to a better understanding of the development mechanism of tidal creeks. [ABSTRACT FROM AUTHOR]

Highlights • We monitor in-situ sedimentary processes in a combined mudflat-creek system. • Significant accretion occurs in mudflats during spring tides. • Sediment resuspended from mudflats deposits in landward creeks in neap tides. • Mudflat-creek systems provide crucial inherent links in intertidal environments. Abstract Intertidal environments, including bare mudflats, tidal creeks, and vegetated salt marshes, are of significant physical and ecological importance in estuaries. Their morphodynamics are closely linked by mudflats and creek networks. Understanding water motion and sediment transport in mudflats and tidal creeks is fundamental to understand intertidal morphodynamics in intertidal environments. To explore dynamic interactions between tidal creeks and mudflats, we conducted field campaigns monitoring water depths, tidal currents, waves, suspended sediments, and bed-level changes at sites in both mudflats and tidal creeks in the Eastern Chongming tidal wetland in the Yangtze Delta for a full spring-neap tidal cycle. We saw that under fair weather conditions, the bed-level changes of the tidal creek site displayed a contrary trend compared with those of the mudflat site, indicating the source-sink relationship between tidal creek and mudflat. During over-marsh tides, the tidal creek site with relatively high bed shear stresses (averagely, 0.37 N/m2) was eroded by 35 mm whereas the mudflat site was accreted by 29 mm under low bed shear stresses (averagely, 0.18 N/m2). To the contrast, during creek-restricted tides, deposition occurred in the tidal creek site by 20 mm under low bed shear stresses (averagely, 0.09 N/m2) whereas erosion occurred in the mudflat site by 25 mm under relatively high bed shear stresses (averagely, 0.21 N/m2). Over a spring-neap tidal cycle, the net bed level changes were −15 mm (erosion) and 4 mm (deposition) in tidal creeks and mudflats, respectively. These results suggested that there were alternated erosion-deposition patterns in spring and neap tides, and a sediment source and sink shift between mudflats and creeks. We found that the eroded sediments in mudflats were transported landward into tidal creeks and deposited therein in neap tides, and these newly deposited sediments would be resuspended and transported to surrounding marshes (over-marsh deposition) at spring tides. The coherent sediment transport and associated erosion-deposition pattern within the mudflat-creek system at spring-neap tidal time scales thus played a fundamental role in intertidal morphodynamic development. These findings suggest that management and restoration of intertidal ecosystem need to take the entire mudflat-creek-marsh system as a unit into consideration rather than focusing on single elements. [ABSTRACT FROM AUTHOR]

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. • De-reclamation cannot fully eliminate the impact of previous reclamation projects. • Tidal flats experience severe erosion shortly after de-reclamation. • This severe erosion can gradually be mitigated under adequate sediment supply. [ABSTRACT FROM AUTHOR]

Quantification of tidal flat morphological changes at varying timescales is critical from a management point of view. High-resolution tidal flat morphology data, including those for mudflats and salt-marshes, are rare due to monitoring difficulty by traditional methods. Recent advances in Terrestrial Laser Scanner (TLS) technology allow rapid acquisition of high-resolution and large-scale morphological data, but it remains problematic for its application on salt-marshes due to the presence of dense vegetation. In this study, we applied a TLS system to retrieve high-accuracy digital elevation models in a tidal flat of the Yangtze Estuary by using a robust and accurate Progressive Morphological filter (PM) to separate ground and non-ground points. Validations against GPS-supported RTK measurements suggested remarkable performance. In this case the average estimation error was about 0.3 cm, while the Root Mean Square Error (RMSE) was 2.0 cm. We conducted three TLS surveys on the same field including salt-marshes and mudflats at the time points 5 days before, 3 days after, and 45 days after a typhoon event. The retrieved data showed that the mudflats suffered from profound erosion while the salt-marshes slightly accreted during the typhoon period. The average elevation change of the total area was about − 4 cm (− 0.28 cm per day). However, both the mudflats and salt-marshes deposited in the post-typhoon period and the accretion over salt-marshes occurred at a higher rate than that during the typhoon. The elevation of the total area increased by 15.9 cm (0.37 cm per day), suggesting fast recovery under calm conditions. Quantification of the erosion and deposition rates was aided by the high quality TLS data. This study shows the effectiveness of TLS in quantifying morphological changes of tidal flats at an event (and post-event) timescale. The data and analysis also provide sound evidence on vegetation impact in stimulating salt-marsh development and restoration, shedding lights on bio-morphological interactions. [ABSTRACT FROM AUTHOR]