QUANTIFYING SEDIMENT TRANSPORT FROM COASTAL BLUFFS AND CORRESPONDING NEARSHORE CHANGES IN THE GREAT LAKES

Bluffs composed of unconsolidated glacial sediments are important for coastal dynamics in the North American Great Lakes because they represent the primary source of nearshore sediment. Previous work has established that sediment contribution from coastal bluffs is nonlinear in space and time, however these studies have focused on the subaerial coast only, and changes to the subaqueous nearshore are understudied. Here we quantify both the sediment liberated from the bluffs and corresponding changes in offshore morphology to examine the sediment transport and supply capacities. Three surveys were conducted at Lion’s Den State Natural Area, a 1.5 km stretch of coastal bluffs, at the beginning and end of a quiescent transport period (summer), and before and after a significant storm event. From this, the relative contributions of different wave climates can be quantified. Topographic surveys were carried out using Structure from Motion photogrammetry from a Phantom4 RTK drone with 3 cm of precision in the final models. Shore-perpendicular nearshore transects spaced at 20-30 m were measured using a Bathycat RC boat equipped with a single-beam echosounder and Emlid RTK GPS, recording at 10 hz. Contour maps were created from the transects using triangulated irregular network (TIN) interpolation. Difference maps between each topobathy survey pair were executed in ArcMap. Preliminary results suggest that differences in the subaerial topography were minimal over both study periods, whereas the nearshore was more active. The nearshore changes over a summer period (~months) that was devoid of a large storm were of the same magnitude as changes related to a much shorter period (~weeks) but that contained a storm event.