Coastal erosion is a ubiquitous hazard for sandy beaches in the Laurentian Great Lakes, especially during periods of more energetic wave climates associated with high lake levels. A fundamental barrier to managing these hazards is the lack of a process-based, quantitative understanding of longshore and cross-shore sediment exchange and connectivity across the entire coastal profile. In this study, a unique multidecadal dataset of beach and nearshore profiles collected at six sandy beaches along the eastern coast of Lake Michigan and contemporaneous hydrodynamic data are utilized to quantitatively identify long-term boundaries of sediment transport zones during accretionary and erosive wave conditions. Data analysis demonstrates that accretionary wave conditions can transport sediment onshore nearly exclusively from the lower reaches of the subaerial beach and from shallow sediment ridges in the inner nearshore while erosive wave conditions can mobilize sediment from all areas of the profile and redistribute it offshore. While sediments stored in deeper nearshore bars in the surf zone are activated by high-energy erosive wave conditions, they likely only function as multidecadal sinks for eroded beach sediment and not as sources of sediment for near-term beach accretion. The results of this study also demonstrate that cross-shore sediment transport has a more dominant role in long-term profile morphology change than longshore sediment transport along this stretch of coastline. Most importantly, the findings of this study suggest that future rates of beach recovery following high lake levels are likely to decrease in the study region as extensive coastal armoring will reduce sediment availability in the narrow profile zones activated during accretionary conditions. By identifying the edges of active sediment transport zones during accretionary and erosive wave conditions, this study is an initial step towards being able to better forecast the likelihood and manage the impacts of coastal erosion and beach recovery in the future.
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