Morphodynamics and Sand Volumetric Changes within Engineered Great Lakes Pocket Beaches: Varied Responses to a Meter-scale Lake-level Rise, Chicago Lakefront

Abstract

Lake Michigan’s water level recently rose by >1.5 m, from its historical low (in 2013) to a near-historical high (in 2020). This study evaluates the lake-level rise-associated physiographic and sand volumetric changes at four of Chicago’s engineered urban lakefront embayments. Insights from lowstand and highstand topobathymetric measurements are compared to shoreline morphodynamic trends. The recent low–high phase transition is also compared to that of the mid-1960s through mid-1970s period of lake-level rise, the most similar in terms of magnitude and rate. While embayed beaches of Chicago’s urban lakefront (n>20) responded predictably to lake-level rise, with shoreline recession and overwash into backshore regions a common dynamic, spatial patterns and magnitudes of change were beach-specific and influenced by aspect, infrastructure design, littoral sand availability/supply, and type of beach-management activities. Patterns of shoreline rotation during lake-level rise reflect embayment orientation, groin placement and design, and/or antecedent backshore topographic conditions. Sedimentary dynamics within the enclosed nearshore are predictable along strike and often decoupled from subaerial morphodynamics. This relates, in part, to different sand-supply conditions along the highly fragmented urban littoral system. Sediment interconnectivity between coastal embayments is likely to vary with changes in water level. This topic requires further study. Beach managers should consider local (beach-specific) and regional (sand supply-related) impacts on morphodynamics across subaerial and subaqueous parts of lakefront embayments for climate-change mitigation planning.