IMPACTS OF A NOVEMBER GALE ON A PROGRADING LAKE SUPERIOR HARBOR-HEADLAND BEACH RESOLVED WITH HIGH-RESOLUTION GPR: IMPLICATIONS FOR STUDYING OLDER SHORELINE ARCHITECTURES.

The shallow stratigraphy of a prograding harbor-headland beach is studied along the southern Lake Superior shoreline using 500 MHz ground penetrating radar to understand how subsurface architectures are generated and preserved. GPR data were gathered along shore-perpendicular transects from lakeward limits of dune structures to the upper swash zone, capturing progradational architectures of subsurface sedimentary units on the up-drift side of the Grand Marais harbor jetty in Alger County, Michigan. A total of three transects, spaced ~100 m apart, were re-surveyed following a strong storm event to help address the role of event-based shoreline changes on long-term beach development. Data were processed with Automatic Gain Control and underwent multiple Hyperbola Velocity Calibration checks to corroborate GPR velocities. A comparison of mid-November and early-December datasets provide insight into changes affiliated with a strong late-November gale.

Storm-induced morphologic changes included a noticeable loss of sand across upper foreshore and lower backshore regions and a redistribution of sand across the proximal nearshore. In GPR these changes are recognized as a loss of shallow radar units extending 12 m inland from the shoreline, manifested as erosional truncation of formerly-mapped strata. Assuming uniform beach response along strike to the storm activity, approximately 24,000 m3 of beach sand are estimated to have been removed from the shore-proximal beach during this event.

The observed changes to the beach system suggest that significant sand loss can be expected during beach-profile adjustment from summer to winter. These changes should be associated with truncation of strata in the landward direction and an offshore redistribution of materials. Which event-based and/or seasonal architectures are preserved is yet to be determined. Continued monitoring of morphologic changes at the shoreline and their stratigraphic manifestation over the seasons should provide a template for a better understanding of which event-scale structures are preserved within overall prograding systems.