Morphological Impacts and Vegetation-Induced Attenuation of Wind and Vessel Generated Waves

Rising sea levels due to climate change and increasing economic activity along coastal areas necessitate coastal defensive mechanisms. Ships laden with heavy cargo can generate large wave groups when passing through navigation channels, disrupting delicate shorelines and altering local morphology. Pea Patch Island, the home of Fort Delaware and a major wading bird nesting site located on the Delaware River, suffers from beach erosion and wetlands loss. The Delaware River is a major shipping route for Camden, Philadelphia, Wilmington, and Trenton, attracting a vast array of large vessels importing and exporting materials. Riprap installed around part of the island absorbs and reflects wave energy including ship wakes, but increases erosion at riprap endpoints, likely due to reduced sediment movement alongshore and wave diffraction. Vegetation along shorelines attenuates wave energy by inducing a drag force in the water, thus protecting coastal and wetland areas from erosion. Two studies were conducted simultaneously from June 6 to July 9, 2018 on Pea Patch Island: one along a transect on the beach on the north side of the island and one along a transect into a patch of vegetation on the marshy south side of the island. Ultrasonic distance meters, pressure sensors, and current meters were deployed at each site to obtain water depths, cross-shore, and alongshore velocities. Time lapse cameras recorded daytime footage of ship passages and ship wakes. The relative effects of ship direction, speed, size, and loading on ship wakes that impact the sites will be considered. Phragmites australis and Schoenoplectus pungens make up the vegetation of interest at the marshy site; characteristics of these plants and site elevation profiles will be used to explore the effects on hydrodynamics along the southern transect. Wind speed and wind direction were collected from a local weather station to determine the relative impacts of wind and ship generated waves at the sites. Analyses of wave parameters, velocities, ship wake correlation with wave conditions, and vegetation wave attenuation will be presented.