DOCUMENTING STORM IMPACTS ON A COASTAL BARRIER SYSTEM: NAPATREE POINT CONSERVATION AREA, RHODE ISLAND

The Napatree Point Conservation Area (NPCA) is a 2 km barrier spit in southwest Rhode Island that was mapped after three extratropical storm events in December 2023/January 2024. Water levels reached 1.1 m NAVD88 on 12/18, 1.3 m NAVD88 on 1/10, and 1.4 m NAVD88 on 1/13 and offshore significant wave heights reached 8.9 m, 7.5 m, and 5.1 m respectively. The landward extent of overwash was mapped using differential GPS and compared to the dune crest and volume from spring 2022 USGS LiDAR using the Digital Shoreline Analysis System within ArcMap. A clear correlation emerged between the dune height and response; where the dune height >5 m NAVD88, the response was predominantly collisional. The dune was overwashed where the crest was <5 m NAVD88, and washover fans extended >40 m landward of the dune crest. Drone LiDAR imagery was collected in March 2024 to analyze cumulative storm impacts. Transects (n = 5), measured since 2013 and surveyed pre/post storms using RTK-GPS as well as additional elevation transects (n = 10) extending across the barrier were mapped using RTK-GPS following the 12/18 and 1/10 storms, and resurveyed on 8/12/24 to assess dune recovery. A surge channel and washover fan were mapped with RTK-GPS to compare the volumes before/after two of the storm events. The washover fan showed >200 m3 of net deposition to the bay side of the barrier following the 12/18 and 1/10 storms. The transects show some sediment deposited on the back barrier as washover fans with an overall net erosion of the dunes. Washover fans on the back barrier buried or partially buried Ammophila breviligulata (American Beachgrass), and other vegetation. Four surveys conducted monthly from May-August 2024 assessed the rate of regrowth or emergence using the Daubenmire cover scale. Revegetation occurred rapidly and the washover fan deposition was almost undetectable by August 2024. A camera positioned near one washover fan provided time lapse imagery of revegetation. Continued research on barrier systems improved understanding of changing barrier morphology and volumes, how timing between storm events contributes to dune erosion and barrier mitigation, and how dune vegetation is resilient to minor overwash events. Understanding storm impacts on barriers over different timeframes will help to predict future storm impacts on coastal barriers.