Morphodynamic Evolution of a Fringing Sandy Shoal: From Tidal Levees to Sea Level Rise

Abstract

Intertidal shoals are vital components of estuaries. Tides, waves, and sediment supply shape the profile of estuarine shoals. Ensuring their sustainability requires an understanding of how such systems will react to sea level rise (SLR). In contrast to mudflats, sandy shoals have drawn limited attention in research. Inspired by a channel-shoal system in the Western Scheldt Estuary (Netherlands), this research investigates governing processes of the long-term morphodynamic evolution of intertidal estuarine sandy shoals across different timescales. We apply a high-resolution process-based numerical model (Delft3D) to generate a channel-shoal system in equilibrium and expose the equilibrium profile to variations in wave forcing and SLR. Combined tidal action and wave forcing initiate ridge formation at the seaward shoal edge, which slowly propagates landward until a linear equilibrium profile develops within 200 years. Model simulations in which forcing conditions have been varied to reproduce observations show that the bed is most dynamic near the channel-shoal interface. A decrease/increase in wave forcing causes the formation/erosion of small tidal levees at the shoal edge, which shows good resemblance to observed features. The profile recovers when regular wave forcing applies again. Sandy shoals accrete in response to SLR with a long (decades) bed-level adaptation lag eventually leading to intertidal area loss. This lag depends on the forcing conditions and is lowest near the channel and gradually increases landward. Adding mud makes the shoal more resilient to SLR. Our study suggests that processes near the channel-shoal interface are crucial to understanding the long-term morphodynamic development of sandy shoals.Plain Language Summary Intertidal area is the coastal zone that undergoes a rhythm of wet-dry cycles under the influence of tidal action. Intertidal shoals are vital components of the estuarine environment. They have high ecological value but also help in reducing flood risk by wave attenuation. It is important to understand how these shoals will react to sea level rise in order to plan sustainable management strategies. We developed a high-resolution model to investigate the long-term evolution of an intertidal sandy channel-shoal system. The model describes tidal flow, wave action, and associated sediment transports and generates an intertidal sandy shoal in equilibrium. Changes in equilibrium forcing lead to morphodynamic adaptation. A drop in wave height causes the formation of small-scale tidal levees at the channel-shoal edge, which are also observed in reality. High wind-wave activity causes shoal erosion. Shoals accrete in response to imposing sea level rise, but the accretion rate is smaller than the sea level rise rate leading to the loss of intertidal area and increased inundation. Inclusion of mud makes the shoal more resilient to sea level rise. Our study suggests that processes near the channel-shoal edge are crucial to understanding the long-term morphodynamic evolution of sandy shoals...

The impact of wind‐waves and sea level rise on the morphodynamics of a sandy estuarine shoal

“…DeVet et al (2018) showed that a sandy shoal may propagate along the wave direction and wind direction. Elmilady et al (2020) highlight that wave-induced resuspension is essential for distributing sediment supplied from deep channels over the intertidal flat.…”

“…Arafat et al, 2016;Dam et al, 2016;Elmilady et al, 2019;Ganju & Schoellhamer, 2010;Luan et al, 2017). Other studies have focused on exploring morphodynamics of isolated tidal flats by using one-dimensional (1D) profile models (e.g., Maan et al, 2015;Zhou et al, 2015) or twodimensional (2D) models (e.g., DeVet et al, 2018;Elmilady et al, 2020;Fagherazzi et al, 2007;Nguyen et al, 2011). These studies apply a numerical process-based model to study estuarine morphodynamic development and show its increasing potential and reliability.…”

“…Despite sedimentation, intertidal flats cannot keep pace with an exponentially rising sea level, leading to a loss of intertidal area along with increasing inundation frequency (e.g. Elmilady et al, 2019;Elmilady et al, 2020;Ganju & Schoellhamer, 2010;Van Der Wegen, 2013;. Van Goor et al, (2003), Lodder et al (2019), andElmilady et al (2020) suggest that intertidal flats can be maintained under a limited, linear SLR after a decadal adaptation time during which intertidal area is lost.…”

“…Elmilady et al, 2019;Elmilady et al, 2020;Ganju & Schoellhamer, 2010;Van Der Wegen, 2013;. Van Goor et al, (2003), Lodder et al (2019), andElmilady et al (2020) suggest that intertidal flats can be maintained under a limited, linear SLR after a decadal adaptation time during which intertidal area is lost. This maintenance only exists when the accretion rate of the intertidal flats reaches the SLR rate.…”

“…This maintenance only exists when the accretion rate of the intertidal flats reaches the SLR rate. This usually requires a long adaption timescale and depends strongly on the local conditions of tides, waves, and sediment supply (Elmilady et al, 2020;Van Goor et al, 2003). This synopsis reveals some intriguing knowledge gaps.…”