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Combination of Shoreline Stabilization Approaches Offers Best Long-term Protection in Chesapeake Bay

A waterfront restoration site featuring a marsh area enclosed by orange plastic fencing. The fenced-off section contains green vegetation, supported by wooden stakes and string throughout the marsh grass. The shoreline is composed of mud and stacked rocks or cement blocks and potentially oysters to create "oyster castles" in front of the fenced off marsh grass. A few birds are seen near and on the water standing on a wood plank. In the background, a river or estuary extends towards an urban setting with buildings, trees, and hills visible under a clear blue sky.
Restored smooth cordgrass marsh with additional protection against wave action (in this case, oyster castles). Credit: Beryl Kahn/NOAA

NCCOS-funded researchers found that native and invasive marsh grasses in Chesapeake Bay provide different degrees of flood protection depending on storm severity, but that marshes alone may not protect against coastal flooding in the future. Actions that increase marsh slope and height above sea level may be needed to enable marshes to continue protecting coastlines from strong waves during extreme weather events in the future.

The researchers developed wave-attenuation models of Maryland marshes composed of native smooth cordgrass (Spartina alterniflora) and invasive common reed (Phragmites australis) that predict how much the marshes  reduce waves under a high-intensity hurricane and a low-intensity winter storm. The models explored how marshes dominated by each plant type would decrease storm wave energy under increasing sea level rise scenarios.

Under hurricane conditions, models showed that marshes composed of the invasive common reed decreased wave intensity more effectively than native smooth cordgrass marshes. During more frequent low-intensity storms, researchers found just the opposite: native cordgrasses reduced more wave energy than invasive-dominated marshes. However, the models showed that the differences between how marsh types perform may disappear with projected sea level rise by the year 2100, when both types of marshes are estimated to provide similar protections and only during moderate storms.

A set of four line graphs depicting wave attenuation percentages as a function of distance from the shoreline (0 to 500 meters) for different years: 2010 (a), 2050 (b), 2080 (c), and 2100 (d). Each graph contains two primary lines, one in blue and one in red, with shaded areas surrounding them representing variability or confidence intervals. The red line and shading generally indicate higher wave attenuation, while the blue line and shading indicate lower attenuation. Across all graphs, wave attenuation is highest near the shoreline and decreases with increasing distance. Over time, there is a gradual decline in wave attenuation, suggesting potential reductions in coastal wave energy dissipation in future scenarios.
Percentage of wave activity reduced by invasive common reed (red lines) and native smooth cordgrass (blue lines) during a low-intensity storm, with projected SLR scenarios for the years 2010, 2050, 2080, and 2100.
A set of four line graphs showing wave attenuation percentages as a function of distance from the shoreline (0 to 500 meters) for different years: 2010 (a), 2050 (b), 2080 (c), and 2100 (d). Each graph has two primary lines, one in blue and one in red, with shaded regions around them representing variability or confidence intervals. The blue line and shading generally indicate higher wave attenuation, while the red line and shading indicate lower attenuation. Across all graphs, wave attenuation is highest near the shoreline and decreases with increasing distance. Over time, there is a visible decline in wave attenuation, suggesting reduced coastal protection from waves in future scenarios.
Percentage of wave activity reduced by invasive common reed (red lines) and native smooth cordgrass (blue lines) during a hurricane, with projected SLR scenarios for the years 2010, 2050, 2080, and 2100.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Based on study findings, coastal managers should consider their priorities for salt marsh maintenance and restoration. Salt marshes made up of native cordgrass are better at filtering pollutants and are home to more species than marshes that have been taken over by invasive common reeds. However, if current wave attenuation benefits are the primary focus for a site, managers may decide not to target invasive removal without a plan for wave attenuation, especially given economic and ecological costs of removing invasive common reeds. Living shorelines, sills, and sediment placement, in addition to species management, are long-term considerations to help maximize the protective function and health of salt marsh shorelines in Chesapeake Bay.

The project is led by George Mason University, and is funded through the NCCOS Effects of Sea Level Rise (ESLR) Program. Project partners include the Maryland Department of Natural Resources and The Nature Conservancy. Read the publication here.

Citation: Cassalho, F., de Souza de Lima, A., Coleman, D. J., Henke, M., Miesse, T. W., de A. Coelho, G., & Ferreira, C. M. (2023). Projecting future wave attenuation by vegetation from native and invasive saltmarsh species in the United States. Regional Studies in Marine Science, 68, 103264. https://doi.org/10.1016/j.rsma.2023.103264

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