MORPHODYNAMICS OF AN URBAN POCKET BEACH ALONG THE WAVE-DOMINATED SOUTHWESTERN COAST OF LAKE MICHIGAN: INSIGHTS INTO LACUSTRINE FORCING IN ABSENCE OF LITTORAL CONTRIBUTIONS

Rapid transition from historically low water levels in 2013 to present highs has exacerbated land losses along many stretches of Illinois’ Lake Michigan coastline. Ongoing efforts are addressing the impacts of changing lake levels and other lacustrine parameters (e.g., winter ice) on coastal sedimentary dynamics. Along-shore transport of sand can obscure morphologic signatures associated with the direct impacts of lacustrine forcing. Beach responses to given lake-level changes, for example, are often masked by altered sediment-supply regimes. We herewith present preliminary insights into the evolution of an urban pocket beach that is decoupled from the littoral engine. Its geomorphology should therefore more succinctly reflect the impacts of lake-level changes, variances in wave climate, and winter ice.

Our study is aimed at linking temporal variances in hydrodynamic forcing to geomorphic response. An aggregate dataset combines newly acquired, high-resolution spatial survey data with existing NOAA topo-bathymetric LiDAR for an assessment of North Beach, IL. These data were supplemented with a near-annual coverage of aerial photographs, from which subaerial beach-change metrics were derived. The 300 m-long beach, which was created in the late 1980s as part of marina construction, is flanked on both sides by shore-perpendicular hard structures. It faces nearly 400 km of open lake to the NE, the quadrant from which most high-energy winter-storm winds (and waves) originate. This setup makes North Beach an ideal location for studying hydrodynamic impacts on lacustrine shoreline morphodynamics in absence of a strong littoral overprint.

Beach-change metrics were derived from historic shoreline positions for statistical evaluation against information on hydrodynamics (i.e., metrics on lake levels, ice covers, and wave data), which were binned to match the temporal signature of the former. We anticipate variances in wave climate and ice covers to have comparatively little influence on beach behavior compared to lake-level changes (magnitudes and rates). Investigating multiple pocket beaches along the urbanized Illinois coastal margin may help further refine distinctions between intrinsic (i.e., local) and extrinsic (i.e., regional) controls on shoreline dynamics, allowing influences of other factors such as hard structure design and sand nourishment activity to be evaluated.