PERSISTENCE OF SORTED BEDFORM MIGRATION ON THE NAPATREE POINT, RHODE ISLAND SHOREFACE BETWEEN 2020 AND 2021

Sorted bedforms are shore perpendicular features consisting of alternating bands of coarser sediment in topographic depressions and fine-grained sediment as topographic highs commonly occurring on sediment-starved shorefaces. The sorted bedforms studied here are located offshore of the Napatree Point Barrier, a wave-dominated, microtidal barrier spit in southwestern Rhode Island, USA. The mapped 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. Underwater video imagery shows that the coarser troughs have 2-Dimensional ripples ~40-50 cm crest-to-crest spacing, crest height ~10 cm and are composed of medium to coarse sand with shell fragments and some pebble-gravel. The extent was digitized on imagery collected in the summers of 2020 and 2021 using an EdgeTech 4125 side-scan sonar system. Digitized features were then split into 25-meter cross-shore bins and the calculated centroid was used to digitize the center axis of each feature. The center axis of each sorted bedform was buffered by 5 meters for every feature in 2020 to analyze potential migration of the bedforms in 2021. A 5-meter buffer zone was used to account for the positional uncertainty of the side-scan data collection. Information on the dimensions, areas, and ending water depths of the 16 longest sorted bedforms were recorded and compared between the two years. Most (75%) of the features in 2021 showed no movement outside the axis buffer zone. One feature in 2020 that split into multiple features in 2021. Various mechanisms have been proposed for the formation of sorted bedforms, including both longshore and cross-shore processes. Our results show that the sorted bedforms offshore of Napatree Point have not migrated between 2020 and 2021. On-going work is continuing to examine storm conditions (wave heights and storm surge) during the year between surveys and examining changes between 2016 and 2019 surveys with the more recent mapping.