Permanent nearshore sediment loss and inevitable retreat of cohesive bluff‐backed beaches

Abstract Lake Michigan experiences decadal scale water level fluctuations that cause beaches to alternate from wide during low lake levels to narrow during high lake levels. Narrow beaches offer less protection to dunes and bluffs than wide beaches, increasing susceptibility to erosion by waves during high water levels. Bluff erosion is the main source of sediment to the nearshore in western Lake Michigan, and the perception of coastal managers is that rapid armouring of the shoreline during high lake levels reduced input of sand that typically acts as a buffer from chronic erosion. However, there are no datasets that document the sediment pathways between the onshore and nearshore at high spatial and temporal resolution, and the connections between erosion during high lake levels and buffering potential after lake levels fall remain unclear. This work used repeat surveys at two natural (unarmoured) coastal sites: a dune‐ and bluff‐backed beach to investigate nearshore changes corresponding to shoreline erosion during high lake levels. We include two regional topobathy LiDAR datasets that bracket a period of rising lake levels between 2012 and 2020 and nine high‐resolution sUAS/sonar elevation surveys during falling lake levels (2020–2023). Our data show that a nearly equivalent volume of sediment was moved out of the survey compartment at both sites during high lake levels. However, the dune‐backed system regained an equal volume as lake levels fell, whereas there was a net loss at the bluff‐backed site. Even during periods of high erosion, the sand fraction did not accumulate in a way that buffered against future erosion at the bluff toe. These findings contradict public perception, emphasizing the chronic nature of bluff retreat following a high‐to‐low lake level cycle.