Assessing the role of intertidal habitats on estuarine hydrodynamics

Vegetated intertidal areas play a crucial role in estuarine ecosystems by providing key ecosystem services, including habitat provision, flood protection, and potentially mitigating salt intrusion. Both changes in intertidal areas within the current estuarine system as restoration efforts through managed realignment, and thereby increasing the area of intertidal habits in an estuary, have an effect on these ecosystem services. Through a modeling study, using the depth-averaged NeVla model of the Scheldt estuary, we demonstrated that vegetation on intertidal areas within the current system significantly influences local flow velocities. By integrating bathymetry and vegetation cover snapshots from 1993 and 2016 in the hydrodynamic model, we found that changes in vegetation (species composition, extent, and distribution) affect currents to a similar magnitude as bathymetric changes. However, the spatial extent of vegetation’s impact on current velocities in its surroundings was limited to an area of up to the vegetated zone. Nevertheless, over a timescale of decades, these vegetation-induced alterations in current velocities can influence estuarine biogeomorphology through changes in sediment transport, geomorphology and ecological processes.

In estuarine environments, increasing salt intrusion events pose a challenge to freshwater availability and tidal ecosystems. Nature-based solutions offer a promising approach to addressing these challenges while simultaneously supporting flood protection and biodiversity. While existing vegetated intertidal areas within current estuarine systems may have limited influence on salt intrusion mitigation, restoration efforts through managed realignment could provide a viable solution. Previous studies have shown that intertidal areas impact estuarine salt intrusion, but the direction (increase or decrease in salt intrusion length) and magnitude of this effect depends on estuarine classification (e.g., stratified vs. well-mixed systems) and the geometry, elevation and location of the intertidal area (e.g., near the estuary mouth vs. further upstream).

Although managed realignment is primarily implemented for flood protection and intertidal habitat restoration, its potential influence on estuarine salt intrusion remains poorly understood. Therefore, our study aims to assess the impact of managed realignment on estuarine salt intrusion across multiple estuary classes, considering variations in realignment site geometry, elevation and location. We employ an idealized funnel-shaped estuary geometry and integrate intertidal areas through managed realignment using the Delft3D-FM model. By adjusting river discharge and tidal range, we simulate different estuary classes. Our focus is on short estuaries, of which salt intrusion lengths are comparable to tidal excursion lengths. In such systems, the impact of managed realignment on salt intrusion is expected to be most pronounced.

Ultimately, a holistic approach to intertidal habitat restoration—incorporating ecological, flood protection, and salt intrusion mitigation perspectives—is essential for maximizing estuary-wide benefits. This research contributes to a broader understanding of how restoring intertidal habitats can serve as a multifunctional strategy in estuarine management.