Abstract
Bay–marsh systems, composed of an embayment surrounded by fringing marsh incised by tidal channels, are widely distributed coastal environments. External sediment availability, marsh-edge erosion, and sea-level rise acting on such bay–marsh complexes may drive diverse sediment-flux regimes. These factors reinforce the ephemeral and dynamic nature of fringing marshes: material released by marsh-edge erosion becomes part of a bay–marsh exchange that fuels the geomorphic evolution of the coupled system. The dynamics of this sediment exchange determine the balance among seaward export, deposition on the embayment seabed, flux into tidal channels, and import to the marsh platform. In this work, we investigate the sediment dynamics of a transgressive bay–marsh complex and link them to larger-scale considerations of its geomorphic trajectory. Grand Bay, Alabama/Mississippi, is a shallow microtidal embayment surrounded by salt marshes with lateral erosion rates of up to 5 m year−1. We collected 6 months of oceanographic data at four moorings within Grand Bay and its tidal channels to assess hydrographic conditions and net sediment-flux patterns and augmented the observations with numerical modeling. The observations imply a divergent sedimentary system in which a majority of the suspended sediment is exported seaward, while a smaller fraction is imported landward via tidal channels, assisting in vertical marsh-plain accumulation, maintenance of channel and intertidal-flat morphologies, and landward transgression. These results describe a dynamic system that is responsive to episodic atmospheric forcing in the absence of a strong tidal signal and the presence of severe lateral marsh loss.
Highlights
A common setting in coastal environments is the coupled bay–marsh complex, which can be found along lowenergy coasts, in microtidal and macrotidal regimes, both temperate and high-latitude settings (Allen and Pye 1992), and often near large sediment sources such as rivers (Fagherazzi et al 2012)
The tidal wave is approximately standing, except for within the channels when water levels were within 30 cm of the saltmarsh elevation, when velocities increased by a factor of 2–3
Observed lateral marsh erosion over the past century combined with seasonal-scale measurements of sediment fluxes in channels and bays lead to a conceptual model of sediment dynamics within Grand Bay (Fig. 9)
Summary
A common setting in coastal environments is the coupled bay–marsh complex, which can be found along lowenergy coasts, in microtidal and macrotidal regimes, both temperate and high-latitude settings (Allen and Pye 1992), and often near large sediment sources such as rivers (Fagherazzi et al 2012). Despite an acknowledgement of the importance of such environments, efforts to protect and restore bay–marsh complexes are typically hampered by a lack of knowledge regarding the overall sediment budget of the coupled system. These complexes are subject to physical forcing mechanisms such as sediment deficits from limited external sources, edge erosion from wave attack, and drowning from rising sea levels, three interrelated factors which determine in large part the geomorphic trajectory of bay–marsh systems. Systems lacking a sufficient external sediment source will likely self-cannibalize and lose areal extent as a result
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