Urban pocket-beach morphodynamics along the wave-dominated southwest coast of Lake Michigan: An analysis of shoreline and sand volumetric changes

Abstract

Many of the world’s beaches are embayed, but while a large body of work addresses the geomorphology of pocket beaches in oceanic settings, little is known about urban analogs, especially within the Great Lakes of North America. Groins and jetties shelter these systems from direct interaction with littoral processes, which elsewhere can influence how changes in lake level, winter-ice cover, and wave climate impact beach evolution. We address the direct controls of these forcing parameters on beach morphodynamics over a 33-yr period at North Point Beach, which is confined to an engineered ‘container’ along Lake Michigan’s wave-dominated SW margin. Analysis of near-annual beach change suggests lake-level change is the dominant geomorphic driver over inter-annual to decadal timeframes, with winter ice playing a secondary role. Pocket-beach shoreline positions were found to be unreliable indicators of sand volumetric changes. Lake-level rise facilitated shoreline retreat and overwash-induced beach accretion while high lake levels created the accommodation for additional sands to enter the embayment. This is important for coastal managers to consider when developing mitigation strategies for ongoing lake-level fluctuations and anticipated regional climate impacts. This foundational assessment has implications for embayed beaches of the greater Chicago coastal margin (n > 20), where many other site-specific variables (e.g., groin orientations and shoreline aspect) may factor into nearshore and onshore beach morphodynamics. Continued research into urban pocket beaches of the Great Lakes stands to offer useful information on the impacts of littoral fragmentation on coastal sediment routing during different lake-level phases and degrees of littoral interconnectivity.