Geomorphology of a Harbor-Breakwater Beach along a High Sand-Supply, Wave-Dominated Great Lakes Littoral Cell

Abstract

Mattheus, C.R.; Diggins, T.P.; Boyce, C.; Cockrell, J.; Kruske, M., and VanWinkle, M., 2019. Geomorphology of a harbor-breakwater beach along a high sand-supply, wave-dominated Great Lakes littoral cell. Journal of Coastal Research, 35(1), 41–55. Coconut Creek (Florida), ISSN 0749-0208.The geomorphology of a breakwater beach along Lake Superior's high-energy SE margin is investigated from aerial photographs, historic nautical charts, and ground-penetrating radar imagery to provide information on littoral dynamics. The study goal was to address shoreline behavior and depositional architecture of the expanding beach in the context of hydrodynamic changes over the last century. Understanding influences of variables such as wave climate, lake level, and winter-ice cover on breakwater-beach development has many coastal management applications, particularly along highly modified coastlines characterized by fragmented littoral cells. Subsurface architectural models of the wedge-shaped Grand Marais beach and shoreline reconstructions are evaluated in the context of hydrologic data. Statistical analyses suggest that although event-based shoreline changes can create lasting depositional architectures, multiyear to decadal patterns of beach development are driven largely by hydrodynamic trends that affect littoral sediment delivery, alter the beach face directly, or both. Periods of above-average wave-height maxima coincide with increased beach growth phases, while high lake levels favor a reduction therein. The direct geomorphic effects of increased erosion through beach inundation and stronger wave climate appear to outweigh influences of higher littoral sediment inputs from increased bluff erosion upstream and accelerated rates of longshore transport. This dynamic stands in contrast to that observed for systems along the southern Lake Erie margin, which are highly sensitive to upstream changes in sediment input. Winter-ice-cover extents and durations are statistically nonsignificant or marginally convincing drivers (low R2) of beach geomorphology.