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

Abstract

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.

Correlation Between Shoreline Change and Planform Curvature on Wave‐Dominated, Sandy Coasts

“…On the timescales considered in our analysis, multiple nourishment episodes can obscure diffusional signals from waves. This result (Figure ) resonates with the previous finding concerning the highly developed coasts of the Mid‐Atlantic: Shorelines which in historic times experienced shoreline erosion are now exhibiting net accretional shoreline change signals resulting from the cumulative impact of nourishment projects (Armstrong & Lazarus, ; Hapke et al, ). Over long timescales, the natural erosional signal appears completely obscured by human activity.…”

“…Journal of Geophysical Research: Earth Surface cumulative impact of nourishment projects (Armstrong & Lazarus, 2019;Hapke et al, 2013). Over long timescales, the natural erosional signal appears completely obscured by human activity.…”

“…Along low‐lying, wave‐dominated, sandy coastlines, a variety of physical processes, affect shoreline change across a wide range of spatial and temporal scales. Despite their vulnerability to storms and sea‐level rise—event‐driven and chronic natural hazards—these environments tend to be intensively developed (Wong et al, ), motivating efforts to quantify present and historical rates of shoreline change and assess erosion risk, in the United States (Armstrong & Lazarus, ; Fletcher et al, ; Gibbs & Richmond, ; Gornitz et al, ; Hapke et al, ; Hapke et al, ; Hapke et al, ; Hapke & Reid, ; Morton et al, ; Morton et al, ; Morton & Miller, ; Ruggiero et al, ) and internationally (e.g., Coelho et al, ; Nicholls & Vega‐Leinert, ; Shaw et al, ). Related to this empirical work are efforts to explain past and predict future trends in shoreline behavior with numerical models of coastal processes and environmental conditions (Ruggiero et al, ; Gutierrez et al, ; Hapke et al, ; Plant et al, ; Vitousek et al, ; Yates & Le Cozannet, ).…”

Defining Coastal Resilience

“…In coastal settings, specifically, exploratory numerical modelling suggests that developed coastal barriers with engineered protections against hazard impacts (i.e., chronic erosion, inundation during major storms) exhibit complex dynamical behaviours with distinct attractors, including oscillatory boom-bust cycles in which coastal development intensifies until the costs of protection become unsustainable and the area is abandoned [116][117][118][119]. Quantitative empirical tests of this theoretical work, however, are only just emerging [117,[120][121][122].…”

“…The variety of possible dynamical attractors for coastal human-environmental systems remains largely unknown. If a boom-and-bust oscillator is potentially one attractor, then a trajectory on that attractor may be the tendency for coastal risk to intensify through a feedback between hazard protection and investment in development [102,103,116,117,120,[122][123][124][125]. Beyond its promise of short-term financial gain in coastal real estate markets, this is not necessarily a preferred trajectory, or attractor, to be locked into.…”