USING SINGLE-BEAM BATHYMETRY, RTK-GPS AND ESRI ARCMAP MODEL BUILDER TO EVALUATE SEDIMENTATION IN A COASTAL LAGOON

This study evaluates and aims to better understand sediment deposition within the inlet of Ninigret Pond, a microtidal coastal lagoon (42.26 km²) located in Charlestown, Rhode Island. This inlet/lagoon is important to the local economy, used for fishing, kayaking, and other recreational activities. The inlet has been dredged to maintain a navigation channel, and portions of the flood-tidal delta have been dredged for habitat restoration. Volumes of sediment were examined (within 1.15 km² of the inlet area) to calculate changes in sediment volume overtime. Understanding sediment flux in this inlet improves dredge management of the system and furthers our understanding of the sediment budget for the RISS. Water depths were measured at points along the inlet using a SonarMite single-beam echo sounder coupled with a Trimble R10 RTK GPS at least biannually between 2012 and 2018 (12 surveys). Resulting points were cleaned to remove outliers, and the remaining points (typically n= 2,000 to 3,000) were interpolated into a continuous surface using the Natural Neighbors algorithm in ESRI ArcMap Spatial Analyst. The surfaces were clipped to a consistent extent to directly compare subsequent surveys. The various tools needed for the analysis were input into ArcMap Model Builder to create a single tool streamlining the process for future surveys. Interpolations were recalculated using an additional tool, which randomly subset 95% of data. The resulting surfaces were compared against (5%) known points; average error between interpolation and surveyed points was 0.12 meters with maximum offsets near the survey boundary. Maximum depth of the inlet over the study period was about 4.36 meters below MLW and sediment volumes are measured as the volume above the 4.57 m (15 ft) contour. The average annualized flux, excluding the influence of dredging and Superstorm Sandy is ~10,000 m³ yr^-1 but values range from 1,500 to 29,000 m³ yr^-1. Immediately following (5 days after) Sandy, the survey showed a slight (4,000 m³) decrease in volume, followed by a significant influx (29,000 m³) into the system between November 2012 and May 2013. This highlights the importance of storms to this coastal system and the range of values and unpredictability of storms (timing, magnitude and response) reduce the reliability of an annual estimate of sediment flux here.