EVALUATION OF HISTORIC SHORELINE CHANGE AND SPIT MIGRATION (1939-2018): NAPATREE LAGOON, WATCH HILL, RHODE ISLAND

The Napatree Point Conservation Area (NPCA) is a 2km long barrier spit separating Block Island Sound and Little Narragansett Bay, bounded by glacial till headlands (Watch Hill Point and Napatree Point). The lagoon at the western end of NPCA is separated from Little Narragansett Bay and encloses the 0.04 km² Napatree Lagoon, a globally recognized sanctuary for endangered migratory shorebirds. The Hurricane of 1938 breached Napatree barrier in several places and an inlet formed, separating the adjacent Sandy Point barrier from Napatree Point, leaving a 370 m spit extending off Napatree Point. A hurricane in 1944 further destroyed the 1938 spit shortening the spit to 90 m. Subsequent recovery post 1944 marks the onset of spit growth and lagoon formation. The last high tide swash (LHTS) shoreline was mapped (1939 to 2018) along the spit using georeferenced historic aerial photographs, digital orthophotographs and handheld DGPS. The annualized rate of change, width and length of the spit were measured using ESRI ArcMap. Eight cross-shore profiles on the spit measured using RTK-GPS examine changes in onshore/offshore sediment transport. The spit lengthened a total of 440m between 1945 and 2010 via longshore transport, while migrating south 100 m due to overwash and washover fan deposition. The inlet position switched from an eastern position to a western location in 2011 likely due to Hurricane Irene. Superstorm Sandy (October 2012) overwashed and breached the spit, opening a new inlet to the east. The spit retreated 15 m between 2012 and 2014 (significantly higher than the annualized migration rate of the spit (1939 to 2017) of 0.3m yr^-1). Cross-shore profiles vary across the spit showing dune growth on the western side where the spit remained intact after Sandy and overwash and washover fan deposition on the eastern portion of the spit. The lagoon spit and the processes that shape it serve as a model of natural shoreline change and spit migration on other larger barriers. Continued spit migration is likely and will further reduce the size of the lagoon altering that critical component of the NPCA ecosystem.