EVOLUTION OF A DYNAMIC BACKBARRIER COASTAL LAGOON: NAPATREE POINT, RHODE ISLAND

Napatree Point a welded barrier spit extending partially across Little Narragansett Bay in Watch Hill Rhode Island between the Napatree Point and Watch Hill Point headlands is an important forage habitat particularly for migratory shore birds, such as Piping Plovers (Charadrius melodus) and American Oystercatchers (Haematopus palliatus). The focus of this study is a 9.9-acre backbarrier lagoon at the western formed following the Hurricane of 1938, when an inlet breached, separating the Napatree and Sandy Point barriers. The remnant spit rotated and lengthened enclosing the lagoon. Previous studies show eastward spit growth between 1945 and 2011, eventually resulting in the inlet closing. A new inlet on the western side of the lagoon however, it remains unclear if the new inlet formed prior to the or after the closure of the existing inlet. The 2011 inlet has migrated back to the eastern side of the lagoon following Superstorm Sandy. Vertical aerial photographs and RTK-GPS field surveys show the spit has lengthened by ~70 m between 2017 and 2022 via longshore sediment transport, shifting the inlet to the east while simultaneously decreasing the size of the inlet. The channel cross-sectional area decreased from 2.4 m² (2021) to ~0m² (2022) (relative to NAVD88). Water levels measured using an Onset Hobo Data Logger show that the tidal range (and tidal prism) decreased ~58% from 0.8 m in 2017 to 0.3 m in 2022. Measured ebb and flood current velocities in the lagoon have decreased between 2019 and 2022. Duration of the tidal currents are asymmetrical; flood currents have higher peak velocities over a shorter period (~4 to 5 hours) while ebb currents have lower peak velocities spread over longer periods (~8 hours). Current velocities in the inlet exceed the threshold velocity for medium sand (15 cm s^-1) for 4 to 5 hours of recorded current velocities for both the ebb and flood. Net sediment deposition on the flood-tidal delta between 2018 and 2022 (+1,500 m³) supports the interpretation that this is a flood-dominated system. Taken together, the inlet appears to be closing again, similar to 2011, although remains unclear how long this will take and if a second inlet will open (and when). Inlet closure will likely alter the lagoon ecosystem and potentially have a negative impact on the surrounding environment.