Salt marshes are complex and highly productive ecosystems in coastal areas. They play crucial roles in providing habitats for diverse species, sequestering carbon, and serving as natural buffers against storms. However, salt marshes have been threatened by human activities (e.g. reduced sediment supply) and climate change (e.g. sea-level rise), leading to a potential loss. This loss of salt marshes highlights the importance of an influx of sediment to salt marshes for maintaining their structure and resilience. Mudflats, located adjacent to salt marshes, facilitate sediment transport to these marshes under certain conditions. Therefore, advancing our understanding of sediment transport between mudflats and salt marshes is important, as it can provide valuable insights into effective salt marsh management.

This research aims to unravel the varying sediment transport processes between mudflats and salt marshes under different hydrodynamic and sediment dynamic conditions. Chongming Saltmarsh from the Yangtze Estuary and Paulina Saltmarsh from the Western Scheldt Estuary have been selected as study cases. The distinct differences in hydrodynamic forcing and sediment availability between these two estuaries contribute to differing environments and states of their intertidal systems. These differences enable us to compare the sediment transport processes across divergent systems and explore the mechanisms governing the long-term evolution of salt marshes.

Saltmarsh creeks are recognized as efficient conduits that actively facilitate the exchange of water and sediment between mudflats and salt marshes. To identify the role of marsh creeks in sediment transport between two different intertidal systems, the sediment transport processes in a main creek and on the adjacent mudflat in Chongming Saltmarsh (China) and Paulina Saltmarsh (the Netherlands) have been investigated (Chapter 2). Our findings revealed notable differences and common patterns in sediment transport between the two systems. In Chongming, SSC exhibited significant asymmetry between flood and ebb tides, with large SSC peaks occurring during most flood periods. This asymmetry in SSC caused the marsh creek in Chongming to function as a conduit for sediment import. Furthermore, distinct overbank and underbank tides were observed in Chongming. During underbank tides, sediment was trapped and retained within the creeks, only to be eroded and transported to the marsh during subsequent overbank tides. Additionally, the mudflats in Chongming showed a relatively rapid recovery after erosion events. These mechanisms were not observed in Paulina Saltmarsh, where a net export of sediment through the marsh creek was recorded during calm weather. In both systems, the SSC in marsh creeks showed a slight increase due to local erosion of the creek bed but responded more significantly to the erosion of mudflats, indicating that the main sediment sources of the high SSC result from the sediment advection rather than local erosion. These comparative findings suggest that the role of marsh creeks in sediment import and export is closely linked to the availability of sediment from adjacent mudflats, highlighting the importance of mudflats for the growth of salt marshes.

After recognizing the role of main creeks in sediment transport within turbid systems, the role of creek tributaries in sediment delivery still remains poorly understood. Therefore, field measurements were conducted in a main creek and in a secondary creek within Chongming Saltmarsh. These measurements revealed the dual roles of saltmarsh creek systems in drainage and sediment transport, as well as the mechanisms driving residual sediment flux within saltmarsh creeks (Chapter 3). The results indicated that the main creek played a dominant role in sediment delivery, while the secondary creek, influenced by the presence of vegetation, was more effective as a drainage conduit and contributes less to sediment transport. Additionally, the direction and magnitude of residual sediment flux are influenced by the relative importance of asymmetries in net discharge and sediment concentration. Overbank tides primarily result in an ebb-dominant flow asymmetry, which tends to drive sediment export along with the net outflow. However, the abundance of sediment during flood tides can occasionally counteract this export tendency, mitigating the impact of flow asymmetry on sediment export.

Sediment can be imported from mudflats to salt marshes through marsh creeks and marsh edges. To address how varying tidal and wave conditions affect sediment transport within marsh creeks and over marsh edges (Chapter 4), a two-month field campaign was conducted in Paulina Saltmarsh. Field data revealed that tidal ranges determine the direction of residual sediment flux in the marsh creek, while wave intensity determines its magnitude. Conversely, wave intensity determines the direction of residual sediment flux over the marsh edge, whereas tidal ranges determine the magnitude. Specifically, sediment was imported through the marsh creek during tidal cycles with small tidal ranges and strong waves, whereas sediment was imported through the marsh edge during tidal cycles with large tidal ranges and weak waves. These findings offer deeper insights into sediment transport through marsh creeks and marsh edges under different tidal and wave conditions, which is crucial for effective salt marsh management.

This dissertation explores sediment transport between mudflats and salt marshes in two different systems, providing insights into the roles of marsh creeks and marsh edges in facilitating or impeding sediment import to salt marshes under varying conditions. The findings offer guidance for developing conservation and management strategies to support salt marsh growth in response to decreasing sediment supply and accelerating sea-level rise.@en