Salt marshes can experience a significant land loss through erosion and retreat of their perimeter edges. Rates of shoreline change between 1957 and 2007 were determined for four salt marshes in a Virginia coastal bay using aerial photographs and the Digital Shoreline Analysis System (DSAS). High average rates of lateral erosion of 1.0-1.6 m year −1 were found at three marshes, while the edge of the fourth marsh, along the mainland edge of the bay, remained stable. Erosion rates were temporally consistent during the 50-year period at the three eroding sites, although there was a significant spatial variation in rates of change along the length of the edges at these sites. A simple parametric wave model and the SWAN (Simulating WAves Nearshore) spectral wave model were used to calculate incident wave energy flux along the marsh boundaries at each of the sites. Values of wave energy flux agreed fairly well between the two models but are sensitive to the manner in which wave energy flux is calculated. A stronger relationship was found between wave energy flux and volumetric erosion rates along the marsh edges than with lateral erosion rates. This is an important consideration when examining the effects of future sea level rise on marsh loss.

Sediment resuspension and related increases in turbidity in shallow coastal bays are strongly controlled by local bed properties. However, knowledge of bed properties in coastal bays is typically sparse at best. In this study, we developed a method to estimate the spatial distribution of bed properties in shallow coastal bays using a combination of bed sediment measurements and residence time calculations that requires neither extensive dedicated modeling nor extensive sampling. We found a strong relationship between water residence times derived from a coastal hydrodynamic model and observed bed grain size fractions in a system of coastal bays and used that relationship to transform maps of residence time to maps of grain size fractions throughout the bays. Because grain‐size fractions are related to other bed properties such as organic fraction, permeability and cohesion, these maps provide valuable information for habitat studies as well as morphodynamic modeling. We used our maps of grain size distributions to initialize a 2‐month‐long model simulation of currents, waves and suspended sediment forced with measured wind and tides. Spatial variations in suspended sediment concentration (SSC) reflected spatial gradients in sand and mud abundance in the bed. Lower SSC in sandier regions of the bays, near barrier islands and inlets, resulted in higher benthic light availability but lower sediment supply for deposition on back‐barrier marshes. Higher SSC in more landward, muddier regions resulted in greater light attenuation and sediment availability for deposition on mainland fringing marshes. The proposed methodology facilitates quantification of these bed‐dependent spatial variations in SSC.

[1] Wave, current, and sediment observations collected in approximately 5 m depth on the muddy Atchafalaya clinoform, LA, USA, are used to study the interaction between near-bed wave-induced turbulent flows and suspended sediment characteristics in a muddy environment. Low wave-bias estimates of near-bed Reynolds stresses are strongly correlated with flow accelerations and suspended sediment concentration, as previously observed on sandy beaches, where accelerations have been associated with bed fluidization and sediment transport. A detailed numerical analysis of the observations is performed, based on a uni-dimensional boundary layer model that accounts for the coupling between the fluid and the cohesive sediment phases. The numerical simulations suggest that sediment-induced stratification effects are of the same order of magnitude as turbulent dissipation, and thus play a significant role in the turbulent kinetic energy balance within the tidal boundary layer. However, inside the wave boundary layer, the ratio of stratification to shear-induced turbulence production (i.e., gradient Richardson number) decreases significantly, and shear-induced turbulence production dominates. For these observations, the vertical structures of currents and Reynolds stresses are relatively insensitive to the exact floc size.

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