Morphodynamic Resilience of Intertidal Mudflats on a Seasonal Time Scale

Abstract

Intertidal mudflats are morphodynamic features present in many estuaries worldwide. Often located between vegetated shores and deep channels they comprise valuable ecosystems and serve to protect the hinterland by attenuating waves. Although mudflats are persistently present on yearly to decadal time scales, little is known on their morphodynamic adaptation to short-term variations in forcing such as storms, spring-neap tidal cycles, and sediment supply. This study aims to explore the morphodynamic resilience of mudflats to seasonal variations in forcing. First, we compare transects observed in South Bay, California, at 3-to 6-monthly intervals. Second, we present the results of a process-based, morphodynamic profile model (Mflat). Mflat is an open source, Matlab code that describes both cross-shore and alongshore tidal hydrodynamics as well as a stationary wave model. An advection-diffusion equation solves sediment transport while bed level changes occur by the divergence of the sediment transport field. Mflat reproduces the observed South San Francisco Bay profile in equilibrium with significant skill. Short-term variations in hydrodynamic forcing and sediment characteristics disturb the profile mainly at the channel-shoal edge. The modeled profile disturbance is consistent with observations. The modeled profile is remarkably resilient since it recovers to the equilibrium profile within weeks to months. The model results suggest that 3-monthly observation intervals are probably too long to discriminate processes responsible for the profile disturbance. These processes may include variations in sediment supply, mudflat erodibility, and wave action as well as the spring-neap tidal cycle.Plain Language Summary Intertidal mudflats are morphodynamic features present in many estuaries worldwide. Often located between vegetated shores and deep channels, they comprise valuable ecosystems and serve to protect the hinterland by attenuating waves. Mudflats seem to be quite stable features, but their bed level may change due to variations in hydrodynamic forcing, like waves and tides. In this research we explore how a mudflat reacts to changes in forcing like high waves, changing sediment supply, or variations in tides. We first explored observed mudflat profile changes in South San Francisco Bay at 3-to 6-monthly intervals. Next, we developed a model (Mflat) to reproduce observed mudflat profiles. We then imposed varying forcing conditions on the equilibrium profile. Mflat skillfully reproduces observed mudflat profiles in equilibrium. Imposing variable forcing conditions such as storms or periods without waves leads to bed level changes that resemble observed changes. The main bed level changes occur at the channel-shoal edge, while the profiles restore their equilibrium once normal forcing conditions prevail again. Our validated model can be used in future studies to explore the impact of sea level rise and the management of shoals by extra sediment supply.

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

“…Also, the mixture profile reaches equilibrium faster than the base-case sand profile. This trend provides good agreement with previous studies (Maan et al, 2018;van der Wegen et al, (2017van der Wegen et al, ( , 2019. In our model, this is attributed to the combined effect of a larger sediment supply and, more importantly, to the ability of mud fractions to remain in suspension for a longer time than sand so that landward locations are reached faster.…”

“…Our modeling approach is based on the morphological equilibrium concept. Similar to previous modeling studies (Maan et al, 2018;Roberts et al, 2000;van der Wegen et al, 2017van der Wegen et al, , 2019, the shoal evolve over time toward a state of reduced tide-residual sediment transport maintained by a balance between sediment supply, tidal forcing, and wave action. This state might imply the loss of shoal dynamics.…”

“…de Vet (2020) and Zhu et al (2017) show that, in most instances, intertidal flats were able to recover from storm events. Van der Wegen et al (2019) highlight that this resilience also holds for mudflats in response to seasonal forcing changes. Similar to the Galgeplaat, the largest morphological activity occurred at the mudflat edge and channel slope while it notably decreased toward the landward section.…”

“…Other studies implemented a process-based approach without underlying assumptions of equilibrium (e.g., Pritchard et al, 2002;Pritchard & Hogg, 2003). Roberts et al (2000) and van der Wegen et al (2017van der Wegen et al ( , 2019 showed modeled 1-D profiles similar to observed mudflat profiles; 2-D/3-D processed-based models provided skillful reproduction of observed morphodynamic developments in entire estuaries such as San Francisco Bay (Ganju et al, 2009;van der Wegen et al, 2011;Elmilady et al, 2019), Western Scheldt (Maan et al, 2018), and Yangtze Estuary (Guo, 2014;Luan et al, 2016Luan et al, , 2017.…”

“…Although the previous section suggests that shoal formation is primarily associated with tidal currents, waves play an important role when shoals become shallow and intertidal (e.g., Kohsiek et al, 1988). Observations and modeling studies suggest that tidal flats tend to evolve toward a morphologically steady-state maintained by a balance between sediment supply, wave action, and tidal forcing (Friedrichs, 2011;Friedrichs & Aubrey, 1996;Hu et al, 2015;Maan et al, 2015Maan et al, , 2018Roberts et al, 2000;van der Wegen et al, 2017van der Wegen et al, , 2019. Tidal forcing is usually associated with deposition while erosion tends to be associated with wind-wave activity (e.g., Allen & Duffy, 1998;Christie et al, 1999;Janssen-Stelder, 2000;Yang et al, 2003).…”