Modeling the Morphodynamic Response of Estuarine Intertidal Shoals to Sea‐Level Rise

Abstract

Intertidal shoals are key features of estuarine environments worldwide. Climate change poses questions regarding the sustainability of intertidal areas under sea‐level rise (SLR). Our work investigates the SLR impact on the long‐term morphological evolution of unvegetated intertidal sandy shoals in a constrained channel‐shoal system. Utilizing a process‐based model (Delft3D), we schematize a short tidal system in a rectangular (2.5 × 20 km) basin with a high‐resolution grid. An initial, mildly sloping, bathymetry is subjected to constant semidiurnal tidal forcing, sediment supply, and small wind‐generated waves modeled by SWAN. A positive morphodynamic feedback between hydrodynamics, sediment transport, and morphology causes the emergence of large‐scale channel‐shoal patterns. Over centuries, tide‐residual sediment transport gradually decreases leading to a state of low morphological activity balanced by tides, waves, and sediment supply. Tidal currents are the main driver of the SLR morphodynamic adaptation. Wave action leads to wider and lower shoals but does not fundamentally change the long‐term morphological evolution. SLR causes increased flood dominance which triggers sediment import into the system. Shoals accrete in response to SLR with a lag that increases as SLR accelerates, eventually causing intertidal shoals to drown. Seaward shoals near the open boundary sediment source have higher accretion rates compared to landward shoals. Similarly, on a shoal‐scale, the highest accretion rates occur at the shoal edges bounding the sediment suppling channels. A larger sediment supply enhances the SLR adaptation. Waves help distribute sediment supplied from channels across shoals. Adding mud fractions leads to faster, more uniform, accretion and muddier shoals under SLR.

Reclamation of Tidal Flats Within Tidal Basins Alters Centennial Morphodynamic Adaptation to Sea‐Level Rise

“…Furthermore, the excluded wind-waves may enhance sediment resuspension and help to stimulate sand bar merging and reduce the number of distributary channels in the ebb-tidal delta, as have been reported over river deltas (Geleynse et al, 2011) and tidal basins (Elmilady et al, 2022). Future study is also needed regarding the sensitivity to the lateral tidal flat profile slope, quantification of the morphodynamic adaptation scale, and the morphodynamic equilibrium restoration under the sediment import regime.…”

Emulating the estuarine morphology evolution using a deep convolutional neural network emulator based on hydrodynamic results of a numerical model

“…The first step consists in the implementation of the hydro-morphodynamic numerical model to generate input data for the AI model. This task was based on the work developed by Elmilady et al (2022). In their study, a tidal basin with an open seaward boundary and a landward boundary was implemented in the D3D software to investigate SLR impact on the long-term morphodynamics.…”

“…For the sediment transport, the software solves a 3D advection-diffusion equation for suspended sediments (Deltares 2018). It considers the Van Rijn approach for sand transport, including bed and suspended loads, and the formulation also considers the combined effects of currents and waves (Elmilady et al 2022). The implementation of these kinds of models is based on the definition of a numerical grid where the transport equations are solved, depending on the boundary conditions.…”

“…The numerical model used in this paper was implemented by Elmilady et al (2022), whose work investigated the SLR effects on the evolution of long-term morphodynamics of a sandy channel-shoal system dominated by intertidal sandy shoals, considering the effects of wind and waves in similar conditions to those that occur in the Wadden Sea. The numerical model had an outer coarse grid with 100Â200 m of spatial resolution and an inner fine grid with 33Â66 m resolution.…”

“…The depth-averaged current velocity and the bottom shear stress variables were used as inputs, and the erosion and sedimentation results were obtained as outputs. The numerical model was implemented in the Delft3D (D3D, https://oss.deltares.nl/web/delft3d) software by Elmilady et al (2022), which studied the SLR impact on the long-term morphological evolution of intertidal sandy shoals in a constrained channel-shoal system.…”