Salt marshes are increasingly recognized as resilient and sustainable supplements to traditional engineering structures for protecting coasts against flooding. Nevertheless, many salt marshes face severe erosion. There is a consensus that providing structures that create sheltered conditions from high energetic conditions can improve the potential for salt marsh growth. However, little proof is provided on the explicit influence of structures to promote salt marsh growth. This paper investigates how artificial structures can be used to steer the morphological development of salt marshes. A morphological model (Delft3D Flexible Mesh) was applied, which enabled the analysis of various artificial structures with realistic representation. A salt marsh in the Wadden Sea which has seen heavy erosion (lateral retreat rate of 0.9 m/year) served as case study. We simulate both daily and storm conditions. Hereby, vegetation is represented by an increased bed roughness. The model is able to simulate the governing processes of salt marsh development. Results show that, without artificial structures, erosion of the salt marsh and tidal flat continues. With structures implemented, results indicate that there is potential for salt marsh growth in the study area. Moreover, traditional structures, which were widely implemented in the past, proved to be most effective to stimulate marsh growth. More broadly, the paper indicates how morphological development of a salt marsh can be steered by various configurations of artificial structures.

Estuaries worldwide are susceptible and adapting to climate change (CC) impacts from both the river and coastal boundaries. Furthermore, engineering efforts are undertaken to improve flood safety, to claim land for human use or for port operations, which change estuary morphology. This paper aims to gain an understanding of the combined effects of CC and human interventions on the estuarine‐wide morphological response by analyzing the sediment infilling of highly engineered estuaries. A schematized process‐based morphodynamic model is used (Delft3D‐FM, in 2DH), resembling a highly engineered estuary in the Rhine‐Meuse Delta, The Netherlands. Three types of changes were implemented, both in isolation and in combination: (a) local interventions (changing channel depth or wetland area), (b) upstream human interventions (changing fluvial sediment supply) and (c) extreme CC scenarios (with projections for the future forcings and bathymetry). Results show that a CC scenario can elicit both positive and negative changes in the estuary's sediment budget. The direction and magnitude of the change depend on the local intervention and can align with the effect of the local intervention, intensifying its impact. The combined effects can even reverse the sign of the sediment budget. This stresses the need of analyzing CC impacts in combination with human interventions. Additionally, a relationship was identified which quantifies how a change in peak flow velocity due to both local interventions and sea‐level rise affects the annual sediment budget. These findings can help determine how local interventions affect morphodynamics of engineered estuaries in present and future climates.

Estuaries are continuously adapting to anthropogenic pressure. Because of sea-level rise and reduced fluvial sediment supply, they are at risk of sediment starvation. Contrarily, some estuaries require frequent dredging after artificially deepening the channel to maintain port operations. To optimize current estuarine functions and make estuaries more resilient to future threats, improved understanding of estuarine development after system changes is essential. This paper investigates the estuarine response related to two large-scale human interventions: (1) altering channel depth, following global trends of channel deepening for port navigability; and (2) creating or restoring wetlands, a nature-based solution increasingly explored for its ecosystem services. A schematized 2D-morphological model is set up using Delft3D-FM reflecting a highly engineered estuary in a micro-tidal and wave-dominant environment. Results demonstrate how channel deepening (from 13 m to 17 m, without wetland presence) increased sedimentation in the channel by +31%. Sedimentation rates in the wetland were mostly unaffected by channel depth. After restoring the wetland area (wetland width from 0 km to 1 km, constant channel depth of 15 m), sedimentation within the channel was reduced by $-72$%. The wetland area not only served as sediment sink, but also increased the tidal flow, diminishing sedimentation throughout the estuarine channel. Further analysis showed that restoring wetland areas along a specific segment mostly affected channel sedimentation locally (i.e., at the channel segment along the restored wetland). As such, to alleviate dredging operations at critical locations in the navigation channel, strategic restoration of wetlands can be considered which can provide a sustainable alternative to dredging within highly engineered estuaries.

Estuaries are heavily modified for human use while also being susceptible to sea level rise (SLR), changing estuarine geometries worldwide. In these estuaries, salt water intrudes inland, limiting freshwater availability, particularly during storms or low discharge periods. This paper studies how salt intrusion is influenced by three geometric changes that are prominent in estuaries worldwide in isolation and in combination: (a) channel depth, as channels are often dredged to improve port navigability, (b) intertidal wetland width, representing intertidal reclamation or restoration, and (c) lowering of the intertidal area due to SLR. A 3D schematized process‐based model was employed (Delft3D‐FM), reflecting conditions in the Rotterdam Waterway, the Netherlands. Salt intrusion lengths increased with channel depth, and the stratification generally increased with channel depth and intertidal area. However, results show that the latter dependency diminishes with larger intertidal areas, a novel finding. The salt intrusion length responds ambiguously to increasing intertidal wetland area, attributed to the balance between tidal flow and estuarine circulation flow: increasing intertidal areas, or lowering their elevation due to SLR, increases the tidal flow velocity amplitude as the tidal prism increases. This in turn suppresses the estuarine circulation flow. Consequently, the salt intrusion length decreases in strongly stratified estuaries while the salt intrusion length increases in well‐mixed estuaries. In between these extremes, processes balance out and intertidal area do not influence the salt intrusion length. The findings how increases in tidal flow velocities, caused by intertidal areas, influences salt intrusion processes is novel.

Estuaries around the world are extensively modified to support dense human populations. They trap both fluvial and marine sediments, creating regions with high suspended sediment concentrations (SSCs), called estuarine turbidity maxima (ETM), which affect port navigability and ecological values. The impact of human-induced narrowing and deepening on sediment trapping and flushing is still subject to debate, particularly in strongly stratified estuaries. This study evaluates the effects of channel deepening and intertidal wetland reclamation on sediment trapping and flushing in a highly engineered and stratified estuary. A schematized 3D numerical model, inspired by the Rotterdam Waterway, was developed and validated under present-day conditions. Subsequently, human interventions were simulated in combination with high discharge events which occur every 1, 10 and 100 years. Results indicate that the amount of sediment flushed during high discharges is almost entirely dependent on river discharge and channel depth. Net sediment flushing occurred at Freshwater Froude numbers of 0.2 or higher. Sediment trapping decreased with shallower channels and larger intertidal area: on average, a 1.5m shallower channel reduced trapping by 13%, while a 0.5km wider intertidal area reduced it by 35%. Namely, while both interventions reduced trapping due to tidal flow, they enhanced trapping due to estuarine circulation flow, the dominant trapping mechanism in the system. Finally, a conceptual model is presented, explaining the impact of intertidal wetlands and channel depth on the key processes relevant for fine sediment dynamics in stratified estuaries.

The global loss of intertidal habitats undermines ecosystem services. To restore ecological value and enhance flood protection, managed realignment sites (MRSs) are increasingly implemented along estuaries worldwide. At the same time, estuarine salt intrusion intensifies due to climate change and anthropogenic impacts. While it is known that the extent of the intertidal area influences salt intrusion, the effects of MRS location and configuration remain understudied. We adopt a hydrodynamic model to simulate an idealized funnel‐shaped short estuary, where the salt intrusion length is comparable to the tidal excursion length. Different stratification regimes are simulated by varying the tidal range and river discharge. MRSs vary in along‐estuary location, surface area and inlet width. Results show that the impact of MRSs on salt intrusion depends on both the global stratification regime and site location: a salt‐wedge regime exhibits minimal changes, a partially mixed (PM) regime shows either increases or decreases depending on location, and a strain‐induced periodic stratification regime experiences increases. In a PM regime, coupling an MRS at the baseline intrusion length increases salt intrusion, while positioning it further up‐estuary reduces it. Tide‐averaged stratification decreases in all scenarios, though a local maximum appears near the inlet of the MRS. These effects arise from estuary‐scale shifts in dominant salt transport mechanisms, a changing tidal prism and local tidal trapping, modulated by MRS geometry and location. Our findings demonstrate the key role of along‐estuary location on salt intrusion response for a given estuarine stratification. This underscores the need to explicitly consider salinity dynamics in MRS design.