VARIATIONS IN BARRIER-ISLAND RESISTANCE TO OVERWASH AT MONTHLY TO DECADAL TIME SCALES

Long-term (months to decades) data on barrier-island overwash is lacking, but necessary for the proper parameterization of models addressing island response to rising sea levels, increased storminess and anthropogenic changes. Here, we examine the evolution of two washover fans that formed within 300 m of each other on the southwestern end of Onslow Beach, NC during Hurricanes Fran (Sept. 1996, Fran Fan) and Irene (Aug. 2011; Irene Fan). Both washover fans experienced frequent overwash immediately after initial formation, which caused landward transport of sand and vertical accretion of the distal portions of the washover fans. Cores through the Fran Fan sampled multiple fining-upward sand beds above backbarrier marsh sediment, interpreted to have been deposited during individual subsequent overwash events. The Fran Fan extended landward in the northeast an additional 120 m during the period between 1998 and 2002 when the barrier was impacted by multiple tropical and extratropical storms, including Hurricane Bonnie in 1998. Vegetation cover and dune elevation increased significantly at the Fran Fan from 2007-2011, which caused seaward movement of the fore-dune toe and increased resistance to overwash. The Fran Fan did not experience overwash during the 2011-2015 period. Using water-level loggers, we recorded 43 distinct overwash events at the Irene Fan from October 2012-2013, 14 months after that fan initially formed, which expanded the washover fan an additional 200 m landward. During that year, there was a shift in overwash intensity reflecting both changing water levels and changing barrier morphology, which was constrained from high-resolution digital elevation models created from terrestrial laser scanning data. By 2013, the elevation of the Irene Fan increased to the point where the frequency of overwash decreased to monthly, which allowed vegetation cover to increase; however, it could take another decade for the Irene Fan to be as resistant to overwash as the Fran Fan is today. The time-series data presented here show that it can take decades for a washover fan to naturally become resistant to overwash from a storm that has a yearly recurrence interval.