Response of a sand-limited urban pocket beach to repeat decadal lake-level rise events, Southwestern Lake Michigan

Engineered beaches along fragmented urban Great Lakes littoral zones are important recreational areas to large population centers, yet few studies have examined how beaches respond to decadal changes in water level. This paper addresses the evolution, over similar meter-scale lake-level rise events, of Chicago’s Rainbow Beach, southwestern Lake Michigan. Located along the littoral downdrift end of a fully engineered Great Lakes coastline, the study site is characterized by relatively low sand supply and thus offers a useful contrast to higher sand-supply systems like the previously studied Montrose Beach. Geomorphic patterns of change at Rainbow Beach during the mid-1960s through mid-1970s and 2013 through 2020 lake-level rise events were of comparable magnitude and rate. No substantial net change in sand volume occurred between 2012 and 2020, and shoreline reconstructions suggest identical patterns of geomorphic change during similar lake-level rise events decades apart. These are linked to consistent wave and current modification by the beach-bounding infrastructure. While some urban coastal embayments have high capacity for sand storage and benefit from sediment influx during lake-level rise, the more highly enclosed Rainbow Beach is decoupled from direct exchanges with the lake-margin littoral environment. Sand redistribution across the embayment with decadal variances in lake level thus occurs in predictable ways, influenced by controls of groins and groin-attached breakwaters on storm hydrodynamics. Insights from this study can be used to develop beach-specific management strategies for coastal resiliency that take geomorphic change patterns/trajectories and sand-supply dynamics into account.