Abstract
Abstract
Living shorelines are increasingly promoted as ecologically preferable substitutes for hard armouring, but the great majority of the performance evidence comes from low-energy, microtidal, ice-free estuarine settings in the mid-Atlantic and southeastern United States. Whether that evidence transfers to high-fetch, seasonally ice-affected, friable-bedrock coasts is largely untested. We exploited an unusual natural experiment on the south shore of Prince Edward Island (PEI), Canada, where a single 320 m property presents three contiguous, comparably exposed coastline sections: a 2009 limestone-conglomerate armour stone seawall, a brush-and-biomass living shoreline installed in August 2015, and an unmanaged control. Bluff-edge retreat was quantified over one year (September 2015 – September 2016) using a peg-line survey at eight monitoring stations and shoreline delineation of four georeferenced sUAS orthomosaics along transects spaced at 5 m (n = 61 usable transects). Mean bluff-edge retreat differed significantly among sections (Kruskal–Wallis H = 16.09, p < 0.001). The armour stone section retreated 0.06 ± 0.07 m (mean ± SE; n = 20), a value not distinguishable from zero (Wilcoxon p = 0.56), whereas the living shoreline (0.50 ± 0.08 m; n = 25) and the unmanaged control (0.45 ± 0.07 m; n = 16) both retreated significantly (p < 0.001) and did not differ from one another (p = 1.00 after correction). The Hodges–Lehmann median difference between the living shoreline and the armour stone was 0.42 m. Archival shoreline analysis showed that the section subsequently armoured had been the fastest-eroding part of the property before 2009 (0.15 m yr⁻¹ versus 0.01–0.02 m yr⁻¹) and the slowest afterwards, while the transect immediately downdrift of the wall recorded the highest post-installation rate on the property (0.75 m yr⁻¹), a pattern consistent with flanking. A site revisit in January 2017 found the western third of the living shoreline structurally removed by winter storms. We conclude that, in this single-site, first-year case study and in this wave climate, the biomass-based living shoreline design delivered no measurable erosion benefit relative to no intervention. The result should not be read as a general indictment of nature-based shoreline protection, but as evidence that design typologies developed for sheltered estuarine fetches cannot be transplanted uncritically to open-coast Gulf of St. Lawrence conditions, and that wave-energy screening should precede installation. These findings carry direct implications for coastal conservation planning and permitting decisions in Atlantic Canada, where nature-based shoreline designs are increasingly promoted without regionally validated siting criteria.