PHYSICAL AND METEOROLOGICAL PARAMETERS IMPACTING RECENT TRANSGRESSION AT POCKOY ISLAND, SC

Pockoy Island, SC is one of the most rapidly eroding locations along the coast of South Carolina, with a long-term (1851-2022) rate of erosion of -4.8 m/yr that has increased to an average of -9.10 m/yr over the past 10 years. Pockoy exhibits classic indicators of transgression, including exposure of older underlying geologic units on the beach and a ‘boneyard’ with trees from the adjacent maritime forest. Erosion at Pockoy has occurred in a linear fashion, and SCDNR shoreline change analysis of historic (1949, 1973, 1994) and more recent shorelines do not indicate episodes of progradation. Research sought to track transgression at Pockoy and to address questions pertaining to the physical thresholds that cause erosion and transgression at this location. This presentation details findings from two years of aerial (UAV) surveys and two years of GNSS and transect-based shoreline erosion measurements. Orthomosaics and Digital Surface Models (DSMs) derived from UAV orthoimagery, beach profile data collected using measuring tape from a fixed and measured set of benchmarks, and high-resolution RTK-GNSS beach profile data were used to quantify horizontal transgression and vertical loss/gain over time. Additionally, geophysical (high-resolution sidescan sonar) data collected directly offshore of Pockoy Island in 2021 were used to establish baseline nearshore bathymetry. Episodes of rapid transgression were then identified and compared with meteorological conditions from NOAA and a nearby meteorological station installed by the South Carolina Climate Office. Notable transgressive events are caused by high water levels that can be associated with one or a combination of the following: perigean spring tides (king tides), tropical cyclones, winter storms, high onshore winds (often related to tight pressure gradients), and high winds parallel to the coast. Duration of high water levels is a significant determinant of the severity of erosion. Research also investigated whether the movement of trees on the beach and in the nearshore, detected through analysis of repeat aerial surveys and a geophysical survey conducted in 2021, could be used as proxy for erosive conditions.