Lake-bottom geology of the Chicago nearshore: A sand-distribution context for urban beach morphodynamics along a fragmented littoral zone

Abstract

This paper reports on a high-resolution, regional sand assessment of the Chicago littoral zone, undertaken within ∼ 3 km from shore, to facilitate an improved process-based understanding of pocket-beach morphodynamics between the distinctive North and South sides of the Chicago Lake Michigan shoreline. While past studies have touched upon regional themes of sand limitation and littoral fragmentation by lakefill peninsulas and engineered shoreline infrastructure at these two areas, the integration of digital ‘chirper’ marine seismic reflection data (collected in 2022), geologic information from newly acquired sediment cores and lake-bottom grab samples (from 2023), and a 2020 LiDAR-based topobathymetric DEM provided the basis for geological lake-bottom unit characterization and delineation, a context for morphodynamics. Major findings are that: (1) The sandy lake bottom is distinct in bathymetric character from other compositions/textures, including bedrock, gravel-lag, and muddy sediments; (2) the Chicago North Side is sandier than the Chicago South Side; (3) sand thickness is greatest beneath urban pocket beaches and decreases sharply at the transition from engineered embayment to open lake environment; (4) sand thickness beyond semi-confinement is generally < 1 m, with the toe of the littoral sand wedge generally in > 5 m water depth; and (5) while the Chicago North Side littoral zone is continuously sandy alongshore, with few obstructions impacting unit continuity, that of the South Side is characterized by littoral fragmentation given exposure of bedrock highs. Differences in framework geology between Chicago North and South Sides are further enhanced by engineered shoreline structures and therewith associated littoral fragmentation.