Masked Shoreline Erosion at Large Spatial Scales as a Collective Effect of Beach Nourishment

Sea-level rise along low-lying coasts of the world's passive continental margins should, on average, drive net shoreline retreat over large spatial scales (>10(2) km). A variety of natural physical factors can influence trends of shoreline erosion and accretion, but trends in recent rates of shoreline change along the U.S. Atlantic Coast reflect an especially puzzling increase in accretion, not erosion. A plausible explanation for the apparent disconnect between environmental forcing and shoreline response along the U.S. Atlantic Coast is the application, since the 1960s, of beach nourishment as the predominant form of mitigation against chronic coastal erosion. Using U.S. Geological Survey shoreline records from 1830-2007 spanning more than 2,500 km of the U.S. Atlantic Coast, we calculate a mean rate of shoreline change, prior to 1960, of -55 cm/year (a negative rate denotes erosion). After 1960, the mean rate reverses to approximately +5 cm/year, indicating widespread apparent accretion despite steady (and, in some places, accelerated) sea-level rise over the same period. Cumulative sediment input from decades of beach-nourishment projects may have sufficiently altered shoreline position to mask true rates of shoreline change. Our analysis suggests that long-term rates of shoreline change typically used to assess coastal hazard may be systematically underestimated. We also suggest that the overall effect of beach nourishment along of the U.S. Atlantic Coast is extensive enough to constitute a quantitative signature of coastal geoengineering and may serve as a bellwether for nourishment-dominated shorelines elsewhere in the world. Plain Language Summary Sea-level rise over decades to centuries should, on average, drive shoreline erosion. However, analysis of shoreline change over recent decades along the U.S. Atlantic Coast indicates a pronounced decrease in erosion rates. We examine this enigmatic pattern of shoreline behavior in the context of beach nourishment, which, after 1960, became the predominant form of coastal erosion mitigation in the United States. We find that before beach nourishment became widespread, shoreline change along the U.S. Atlantic Coast was, on average, strongly erosional. We suggest that if coastal erosion has been comprehensively masked by beach nourishment, then historical rates of shoreline change, calculated from shoreline surveys prior to 1960, are likely more representative of underlying erosion hazard.