ABSTRACT
Mattheus, C.R., 0000. Longshore sand-wave migration and variable beach morphodynamic response to meter-scale water-level rise, southwestern Lake Michigan, U.S.A.
This paper addresses how the downdrift migration of a longshore sand-wave can modify beach morphodynamic response to subdecadal meter-scale lake-level rise. It presents results of a geomorphological monitoring study along ∼1.5 km of natural, unobstructed shoreline at Illinois Beach State Park, SW Lake Michigan. Federal LIDAR datasets from 2008, 2012, and 2020 and more than 5 consecutive years of seasonal topobathymetric surveys (2018–23) provide unparalleled insights into alongshore sediment-transport processes and cross-shore topobathymetric profile adjustments. Meter-scale variances in base water level have occurred over this time, affecting geomorphic developments along this wave-dominated coastline. Migrating at <300 m/y, a pronounced shoreline protuberance of ∼100 m amplitude and >1 km wavelength influenced the following during >1.5 m of lake-level rise from 2013 through 2020: shoreline retreat and overwash into backshore environments along the stoss-ward side and beach-profile accretion and progradation about its apex. Different geomorphic trajectories, although ultimately prompted by the change in lake level and executed by storms, were greatly influenced by the relative position of the migrating sand-wave. Although insights from previous Great Lakes studies point to the significance of migrating sand-waves as a sand-supply mechanism to evolving foredune areas and conceptually frame shoreline morphodynamic behaviors as a function of base-level and sand-supply regime, this paper provides much-needed data constraints. Better understanding littoral sand-transport dynamics on multiyear coastal geomorphic development has tangible implications for effective shoreline management. Models forecasting future coastal change, whether conceptual or numerical, must address the significance of cross-shore vs. along-shore sediment-transport components and their spatiotemporal variances with lake-level change.
