EVOLUTION OF A DYNAMIC COASTAL LAGOON SYSTEM NAPATREE POINT, RHODE ISLAND

Napatree Point is a mainland welded barrier spit extending partially across Little Narragansett Bay (LNB) in Watch Hill Rhode Island. The focus of this study is a 9.9-acre backbarrier lagoon at the western end of the barrier spit. The lagoon is an important forage habitat for migratory shore birds. The spit enclosing the lagoon lengthened between 1945 and 2011. The inlet closed in 2011 before reopening to the west following Superstorm Sandy. The 2011 inlet has migrated to the eastern side of the lagoon near where the original inlet closed. Vertical aerial photographs and RTK-GPS field surveys show the spit has lengthened >125 m between 2017 and 2023, simultaneously shifting the position and decreasing the size of the inlet. The channel cross-sectional area (above MHHW) decreased from 8.6 m² (2021) to 2.6 m² (2023). Measured ebb and flood current velocities in the lagoon also decreased between 2019 and 2022. Duration of the tidal currents are asymmetric; 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). 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. Net sediment deposition in the ebb platform northeast of the inlet was -1,340 m³ between 2018 and 2023; the sediment here may have migrated from the narrowing spit. Water levels measured using an Onset Hobo Data Logger show that the tidal range (and tidal prism) decreased ~75% from 0.8 m in 2017 to 0.2 m in 2023. The water level records indicate that spring high tide within the lagoon are comparable to LNB; neap high tides in LNB are not high enough to the threshold height to allow water to enter the lagoon resulting in periods of now tidal flow into the lagoon. Taken together, the inlet appears to be closing again, similar to 2011, although it remains unclear how long this will take to close and if a second inlet will open (and when). The inlet already experiences temporary closures due to high amounts of algae accumulating the inlet. Permanent inlet closure will likely alter the lagoon ecosystem and potentially have a negative impact on the surrounding environment.