CENTURY SCALE (1883-2018) CHANGE ANALYSIS FOR THE NAPATREE BARRIER SHOREFACE AND LITTLE NARRAGANSETT BAY, WATCH HILL, RHODE ISLAND

The Napatree Point Conservation Area (NTPCA) is a 2.4 km barrier spit at the entrance to Little Narragansett Bay (Watch Hill, RI), and is a vital ecosystem for a variety of endangered/threatened and regionally important species. This study evaluates the transgression of the Napatree barrier (1883 to 2018) as well as analyze the bathymetric changes to Little Narragansett Bay and the adjacent shoreface. A historic hydrographic survey from 1883 was digitized in ESRI ArcMap, and the bathymetric soundings were adjusted for sea level rise (relative to 2018; 0.417 m) and tidal datum (from mean low water to mean lower low water; 0.0426 m). Topographic surfaces were interpolated from the historic soundings using the natural neighbor algorithm in ESRI ArcMap v. 10 were compared to LiDAR data from 2018 to observe significant change in deposition or erosion. The study area was divided into three sections,: the shoreface (SF) – which includes the area on the seaside of the present-day barrier, Little Narragansett Bay (LNB) – which includes the area landward of the barrier and to the east of Sandy Point, and the West of Sandy Point (WSP) which makes up the area to the West of the 1883 extent of Sandy Point. This study focuses on the SF and LNB due to inconsistent data available for WSP. The Napatree shoreface at water depths greater than 5 m (200 to 250 m offshore), showed no significant change in slope or elevation within a reasonable uncertainty (+/- 1 m). The cross-sections observed for this study show the barriers’ transgressive movement from 1883 to 2018. The seaward portion of the barrier on the uppermost shoreface showed a consistent slope with approximately 100 m of landward movement in 135 years. Volumes of sediment deposition and erosion were calculated using the Surface Volume tool within ArcMap 3D Analyst. LNB was found to be a sediment sink, with a net gain in sediment of approximately 5.42 x 10⁶ m³ from 1883 to 2018 (40,100 m³/year), and the SF experienced a net loss in sediment of approximately 137,000 m³ (-1,010 m³/year).