Seawall Fronted Barrier Island Response to Hurricane Forcing under Rising Sea Level

In Bay Head, New Jersey, USA, a buried seawall fronts 1260 m of the barrier island coastline. Once exposed, the seawall significantly reduced wave forces during Hurricane Sandy (2012) (Irish et al., 2013, Coastal Engineering) and prevented complete destruction of the dune system (Smallegan et al., in review). Fifty years earlier under significantly lower mean sea levels, this seawall was also exposed and similarly protected coastal infrastructure during an extratropical storm, indicating a historic trend of seawall exposure. Since this seawall has been repeatedly unburied during past storms, it is of interest to analyze the behavior of the seawall fronted barrier island as sea levels increase. Therefore, this study is a numerical investigation of the protection afforded by the buried seawall at Bay Head under future sea level rise and hurricane conditions. Using XBeach, a numerical morphological model, and assuming a static island profile sustained by human interaction, Hurricane Sandy conditions are simulated for the c1960, c2010 and c2060 (fifty years after Hurricane Sandy) sea levels. A conservative linear sea level rise rate of 4 mm/yr is used based on the historical change over an 80-year period (NOAA Sandy Hook, NJ, tidesandcurrents.noaa.gov). Analyzing the simulated morphological change, the seawall is unburied in every scenario, but the volume of sediment eroded increases as sea level increases. Compared to the c1960 scenario, 28% and 56% more sediment is eroded in the c2010 and c2060 scenarios. The additional sediment transport occurs mostly along the dune peak and in between buildings, increasing in severity (depth and width of channels) as sea level increases. Additional sea level rise scenarios are currently being simulated. Those results are expected to reveal similar trends, indicating an immediate need to reevaluate the protection afforded by buried seawalls and armored dunes along beaches during extreme events in areas subject to sea level rise.