Sediment sorting and bedding dynamics of tidal flat wetlands: Modeling the signature of storms

Abstract

Field studies suggest that storms can considerably affect the morphology, sedimentology and bedding structure of tidal flat wetlands because of their high-energy, while numerical modeling studies unravelling the underlying mechanisms remain rare. With a tidal flat of the Changjiang Delta, China as a reference site, this study explores the role of storms on sediment sorting and bedding behaviors using a biomorphodynamic model that couples hydrodynamics, sediment transport, marsh dynamics and morphological change. Model results indicate that storms can leave clear signatures on tidal flats in both horizontal and vertical sedimentary features, in agreement with field observations. The high bed shear stress induced by storms can initiate the motion of relatively coarse sediment which can be brought shoreward during the flood and settle on the upper tidal flats (or saltmarshes), altering the typically observed cross-shore “landward fining” phenomenon. The storm-entrained coarse sediment on the higher flat is usually hard to be brought offshore during calm weather, and hence leaving an evident sand-dominated layer (SDL) within neighboring mud-dominated layers (MDL). Storms can also destroy tidal rhythmites formed under calm weather. With the increase in storm intensity and duration, the thickness and the sand fraction of the SDL increase accordingly. The SDL thickness generated by a weaker storm during the spring tide is larger than that of a stronger storm during the neap tide. Storm chronology also plays an important role. In particular, the impact of two overlapping or temporally close storms is larger than the sum of the two individual storms. Overall, this study results in a more in-depth mechanistic understanding of storm impacts on tidal flat biomorphodynamics, providing a scientific facet to make sustainable management strategies for coastal wetlands.