VARIABILITY IN OVERWASH FREQUENCY AND MAGNITUDE OF WASHOVER TERRACES AT YEARLY TIME SCALES AND IMPLICATIONS FOR THE EVOLUTION OF TRANSGRESSIVE BARRIER ISLANDS

High-resolution measurements of overwash and associated geomorphic changes are lacking yet necessary to fully understand the role washover deposition plays in barrier island evolution. These measurements are challenging to acquire given the logistical difficulty in forecasting overwash as well as personnel and equipment risks during storms. To address this, we developed a method for remotely-sensing overwash frequency and magnitude using pressure sensors inside shallow wells. We deployed these wells on Onslow Beach, NC over a three-year period to monitor the evolution of two washover terraces (F2 and F2.1) that formed during Hurricane Irene in 2011.

The sensor record and bimonthly terrestrial laser scanning topographic surveys indicate that overwash continued to modify fan morphology years after initial formation. At both sites, little morphologic change occurred in the first year after Irene. In October 2012, Hurricane Sandy passed offshore, transforming the F2 terrace into a fan and minimally impacting F2.1. Several nor’easters during the six months after Sandy resulted in frequent overwash at F2, which increased the volume and landward extent of the washover fan. F2.1 remained stable during this period as well as throughout the rest of the study; likely due to higher barrier elevation and thicker vegetation cover. In the third year after Irene frequent overwash ceased at F2, promoting vegetation recolonization and vertical accretion via aeolian transport. Sustained resistance to overwash appeared imminent, until surge resulting from a nor’easter and the swell from Hurricane Joaquin inundated the fan in October 2015; reactivating washover channels, depositing a large volume of sand across the fan, and increasing the fans landward extent.

Geomorphic models and reconstructions of barrier evolution should consider that washover features may grow in response to both storm and non-storm overwash after an initial disturbance. Our overwash record indicates that the ~200-m-wide fan at F2 formed from the sum of many events rather than one large storm. Overwash vulnerability is sensitive to differences in barrier elevation and backbarrier accommodation, thus underscoring the role of along-shore morphologic variability in the dynamics of barrier evolution.