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 (>102 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.

Beach nourishment, a method for mitigating coastal storm damage or chronic erosion by deliberately replacing sand on an eroded beach, has been the leading form of coastal protection in the United States for four decades. However, investment in hazard protection can have the unintended consequence of encouraging development in places especially vulnerable to damage. In a comprehensive, parcel-scale analysis of all shorefront single-family homes in the state of Florida, we find that houses in nourishing zones are significantly larger and more numerous than in non-nourishing zones. The predominance of larger homes in nourishing zones suggests a positive feedback between nourishment and development that is compounding coastal risk in zones already characterized by high vulnerability.

7Storm-driven overwash is a sediment-transport process fundamental to the 8 evolution of low-lying coastal environments. Physical insight into overwash 9 morphodynamics is crucial for improved risk assessment and hazard forecasting in 10 vulnerable coastal zones. Spatially extended observations of washover deposits have 11shown that back-barrier shoreline planforms can be quasi-periodic. These rhythmic 12 patterns have been attributed to the influence of a forcing template in bathymetry or 13 topography, or inherent in the forcing itself. With an alternative to this prevailing 14 explanation, we present results of a physical experiment and numerical model in which 15 quasi-periodic patterns in washover deposits are self-organized, arising from interactions 16 between barrier topography, routing of overwash flow, and sediment flux. 17

Despite decades of regulatory efforts in the US to decrease vulnerability in developed coastal zones, exposure of residential assets to hurricane damage is increasing-even in places where hurricanes have struck before. Comparing planview footprints of individual residential buildings prior to and long after major hurricane strikes, we find a systematic pattern of "building back bigger" among renovated and new properties. Storm impacts on developed coastlines are expected to increase with climate change 1. In coastal counties around the United States, policies intended to mitigate coastal risk are competing with population growth and development pressures 1-5 that render places more vulnerable and less resilient to major storm events. Research into the repercussions of hurricane impacts has examined regional-and localscale socioeconomics and demographics 6-8 , housing stock and types 8,9 , planning and design requirements (and variances from them) 10-13 , tax and insurance policy 3 , and real-estate market recovery 14. But one indicator of increasing vulnerability in hurricane zones is especially enigmatic: residential footprints are growing even in places with legacies of past impacts, including a systematic pattern of "building back bigger" among renovated and new properties. Here, we investigate broad development trends in hurricane alleys. We measure changes over 5-14 years in residential building footprints at five locations on the US Atlantic and Gulf Coasts that have been struck by one or more hurricanes since 2003 (Fig. 1). Each location occupies a developed coastal barrier in a different state, is characterised primarily SUPPLEMENTARY INFORMATION

Abstract. Despite interventions intended to reduce impacts of coastal hazards, the risk of damage along the US Atlantic coast continues to rise. This reflects a long-standing paradox in disaster science: even as physical and social insights into disaster events improve, the economic costs of disasters keep growing. Risk can be expressed as a function of three components: hazard, exposure, and vulnerability. Risk may be driven up by coastal hazards intensifying with climate change, or by increased exposure of people and infrastructure in hazard zones. But risk may also increase because of interactions, or feedbacks, between hazard, exposure, and vulnerability. Using empirical records of shoreline change, valuation of owner-occupied housing, and beach-nourishment projects to represent hazard, exposure, and vulnerability, here we present a data-driven model that describes trajectories of risk at the county scale along the US Atlantic coast over the past 5 decades. We also investigate quantitative relationships between risk components that help explain these trajectories. We find higher property exposure in counties where hazard from shoreline change has appeared to reverse from high historical rates of shoreline erosion to low rates in recent decades. Moreover, exposure has increased more in counties that have practised beach nourishment intensively. The spatio-temporal relationships that we show between exposure and hazard, and between exposure and vulnerability, indicate a feedback between coastal development and beach nourishment that exemplifies the “safe development paradox”, in which hazard protections encourage further development in places prone to hazard impacts. Our findings suggest that spatially explicit modelling efforts to predict future coastal risk need to address feedbacks between hazard, exposure, and vulnerability to capture emergent patterns of risk in space and time.