SHORT TERM ASSESSMENT OF SEDIMENT TRANSPORT AND CHANGING SYSTEM DYNAMICS OF THE NAPATREE POINT LAGOON, WATCH HILL, RHODE ISLAND

The Napatree Lagoon is located at the west end of the Napatree Point Conservation Area in Watch Hill, RI, and is an important conservation area for various shorebird species, fish and invertebrates such as the horseshoe crab. Understanding the geomorphology of the lagoon system is vital to ensuring the overall health of this unique ecosystem. Tidal current velocities were collected at nine stations and bedform surveys in the inlet and flood-tidal delta capture a complete tidal cycle in the summer of 2019. Velocities were plotted on a composite curve to look at the potential sediment transport capabilities of the tidal currents. The bedform survey data and the elevation data were both imported into ArcGIS for spatial analysis. The bedform survey revealed overall flood dominant tidal currents, as dunes and many ripples remained flood oriented during the ebb tide. Measured current velocities support the bedform survey, showing asymmetric tidal currents within the inlet (max flow: ebb, 0.75 m s^-1; flood, 0.90 m s^-1). Flood currents also potentially transport more sediment during the flood than during the ebb, as there is more time above the assumed minimum transport velocity (0.20 m s^-1) except for the station located in the inlet channel outside of the lagoon. Continuous elevation data of the tidal delta and channel cross sections were also collected in both July of 2019 and January 2020 using RTK-GPS. Elevation data from January 2020 was compared to April 2018 U.S. Army Corps of Engineers LIDAR data. The analysis shows the area exhibited net deposition of of sediment over 15-months. The cross-section of the inlet (January 2020) was calculated to be , smaller than both the 2018 measurement (, and the calculated channel size based on the tidal prism of the lagoon(O’Brien’s relationship) (5.3 m²). This suggests the lagoon inlet could close, separating it from Little Narraganset Bay and bears further monitoring.