Coastal geomorphic changes near groin structures under fluctuating water levels in Lake Michigan

Abstract

Coastal geomorphic changes near groins under fluctuating water levels are important for environmental management, hazard mitigation, and socioeconomics. In this study, geomorphic changes of shorelines, beaches, and bluffs were examined within a groin field and two pocket beaches on a high bluff coast in Lake Michigan (at Sheridan Park, Wisconsin), and compared under low, mean, and high lake levels. Distinct geomorphic responses were observed. In the pocket beaches, shorelines were crenulate-shaped and responded fairly linearly with fluctuating lake levels. In comparison, shorelines in the groin field tended to be straight during high lake levels and curved (cusp and saw-toothed) at low lake levels and changed nonlinearly, with abrupt retreat from mean to high lake levels. Beaches were typically wider within the groin field than the pocket beaches when lake levels were low; however, the opposite was found during high water levels, suggesting decreased effectiveness of the groin field at maintaining beach integrity during high water. Increased sediment supply from eroding bluffs likely contributed to wider pocket beaches during high lake levels. Bluff toe retreat was observed in the pocket beaches during high water levels but was not apparent within the groin field. Accelerated bluff recession was observed downdrift of the terminal groin, as has been reported on Lake Michigan and oceanic coasts but not along the regularly nourished pocket beaches of Chicago. Overall, distinct geomorphic changes near groins unique to a high bluff coast under fluctuating water levels were revealed. These insights can help inform coastal management and decision-making in the Great Lakes.