Morphodynamic Response of Intertidal Shoals to Sea Level Rise

Abstract:

Intertidal shoals are key components of the estuarine environment. In contrast to mudflats, sandy shoals have drawn limited attention in recent research. Inspired by the channel-shoal systems in the Western Scheldt Estuary and Wadden Sea (The Netherlands), our work investigates the mechanisms that drive the long-term morphological evolution of sandy shoals including the morphological impact of sea-level rise (SLR).

We apply a high resolution (35×65 m), process-based model (Delft3D) to simulate the evolution of a sandy channel-shoal system in a schematized (20×2.5 km) tidal basin. An initially sloping bathymetry is subjected to constant (M₂) tidal forcing with wind-generated waves (10-20 cm) modeled by SWAN. Initial bed-level perturbations trigger a positive morphodynamic feedback between hydrodynamic forcing and morphology leading to the emergence of channel-shoal patterns (e.g. see figure). Over centuries, a near-equilibrium morphological state develops by a balance between tidal forcing, sediment supply, and wave action.

SLR triggers sediment import from the seaward boundary. Shoals accrete due to the drop of wave-induced shear stresses associated with the increased water depth. Seaward shoals react faster with higher accretion compared to landward shoals. Also, the highest accretion occurs at the shoal edges and gradually decreases towards central locations. Waves help redistribute the sediment supplied from the channels over the shoals. Incorporating mud fractions leads to faster accretion and muddier shoals under SLR scenarios.

Intertidal flat accretion rates remain less than the SLR rate, eventually leading to intertidal area loss and increased shoal inundation. Implementing a large-scale, high-resolution approach, allowed for highlighting a notable spatial variation in the morphological response. Short-term shoal dynamics by wind waves can be of high relevance for the long-term morphological adaptation to SLR.