PERSISTENCE OF SORTED BEDFORM MIGRATION ON THE NAPATREE POINT, RHODE ISLAND SHOREFACE BETWEEN 2016 AND 2021

Sorted bedforms are shore perpendicular features consisting of alternating bands of coarser grained topographic depressions and finer grained topographic highs. The sorted bedforms in this study are on the shoreface of the Napatree Point Conservation Area (NPCA), a wave dominated microtidal barrier spit in southwestern Rhode Island, USA. The sorted bedforms range from 2 to 25 meters in width and from 200 to 1,000 meters in length, with ending water depths ranging from 4.3 to 9.1 meters. The extent was digitized on side-scan sonar records collected between 2016 and 2021 using an EdgeTech 4125 side-scan system and supplemented with USACE 2018 topobathymetric LiDAR. Polygons were converted into raster datasets for each year with cell size 1 m² to assess the persistence of features. A value of 1 was assigned to cells within sorted bedforms; a value of 0 was assigned to cells outside the sorted bedforms. The surfaces were then summed using Raster Calculator in ArcMap to create a single raster dataset with each cell displaying how many years a sorted bedform was present. A value of 5 represents a location where sorted bedforms were present for all 5 surveys; a value of 0 would represent no sorted bedforms present over time. Digitized features were split into 25-meter cross-shore bins and the centroid calculated for each polygon and we used the centroids to digitize a center axis for each feature providing a semi-objective approach. The axes were buffered by 5 meters to account for potential positional error in the collection in the original side-scan sonar data. The buffered axes were also converted to rasters and summed. Our results show that 8 sorted bedforms display a stable location over the 5 years of data collected. The spacing between stable features ranges from <30 to >200 meters. Determination of stable position was based on cells having at least 3 sorted bedform surveys within. The area containing at least 3 sorted bedform surveys consisted of 62.7% of the total area containing only 1 feature over the 5 years of study. Water level data collected from Block Island, RI USA, show that there were no major storm events over the 0.7 meter MHHW threshold between the surveys. Differences in sorted bedform appearances were observed within the 2019 survey, where thin features located in the mid-shoreface were too distorted to be digitized correctly.