STORM DRIVEN MIGRATION (1883 – 2014) OF THE NAPATREE BARRIER, RHODE ISLAND, USA

Napatree Point is a 1.5 km long headland bounded barrier spit (i.e. welded barrier) extending across the mouth of Little Narragansett Bay. Historic shoreline change data (1883 to 2014) was compiled from NOAA Hydrographic surveys (T-Sheets), georeferenced historic vertical aerial photographs and digital orthophotographs. The results show an average of ~10 m total retreat between 1883 and 1939 (-0.2 m yr^-1), followed by a period of rapid shoreline change between 1939 and 1975 with average rates between -1.4 m yr^-1 and 1.9 m yr^-1. Interestingly, this period excludes change in shoreline position directly induced by the 1938 Hurricane. Shoreline position was relatively static between 1975 and 2014 (+0.1 m yr^-1). This mirrors what was reported by the USGS National Assessment of Shoreline Change, with a short-term (1975 – 2000) rate of +0.28 m yr^-1. The 1938 Hurricane overwashed the barrier completely as shown in oblique aerial photographs taken 3 days after landfall. While this storm did not appear to produce significant retreat of the shoreline, the working hypothesis here is that the lowering of the profile and erosion of the foredune increased susceptibility of overwash. The barrier was likely overtopped by subsequent storms in 1945, 1950, 1954 and 1962, among others, leading to rapid ‘roll over’ of the barrier, and aerial photographs from this period show washover fans are prevalent. The result is the translation of the barrier one width landward in <50 years. Aerial photographs from 1975 show that vegetation had reestablished along the barrier, likely indicating recovery of the foredune. Overwash since 1975 including Superstorm Sandy (2012) has been limited and localized to lower elevation areas (i.e. walking trails). Widespread debate exists between the role of sea level rise in shoreline change and barrier migration, however for this location storms dominated shoreline change over the last century. Increased storm frequency coupled with sea level rise is a likely consequence of climate change, creating a feedback loop of increased overwash of barriers and reduced intervals for recovery. The pattern of increased storm frequency shown here, even in the absence of rapid sea level rise provides at least a partial analog of future behavior of a barrier spit.