The future of large‐scale coastline evolution will be strongly coupled to human manipulations designed to prevent erosion. We explore the consequences of this coupling using a numerical model for large‐scale coastline evolution to compare the long‐term, non‐local effects of two generalized classes of shoreline stabilization: 1) beach nourishment (the addition of dredged sand to an eroding beach), and 2) hard‐structures (e.g., seawalls, groynes, etc.) which fix the position of the shoreline without adding sand. In centurial model experiments where localized stabilization is maintained in the context of changing climate forcing, both forms of stabilization are found to significantly alter patterns of erosion and accretion at distances up to tens of kilometers. On a cuspate‐cape coastline similar to the North and South Carolina coast, USA, with stabilization applied to the eroding updrift flank of a single cape, perturbations to coastline evolution are qualitatively similar within ∼20 km for each stabilization scenario, though they differ in magnitude both updrift and downdrift of the stabilized shoreline. The “human” signal in coastline change can extend as far as a neighboring cape (approximately 100 km away), but these long‐range effects differ for each scenario. Nourishment resulted in seaward growth of the stabilized cape, increasing the extent that it blocked sediment flux in downdrift regions of the coast through wave shadowing. When stabilized with a hard structure the cape's initial position remain fixed, decreasing wave shadowing.

Since cuspate coastlines are especially sensitive to changes in wave climate, they serve as potential indicators of initial responses to changing wave conditions. Previous work demonstrates that Cape Hatteras and Cape Lookout, North Carolina, which are largely unaffected by shoreline stabilization efforts, have become increasingly asymmetric over the past 30 years, consistent with model predictions for coastline response to increases in Atlantic Ocean summer wave heights and resulting changes in the distribution of wave-approach angles. Historic and recent shoreline change observations for Cape Fear, North Carolina, and model simulations of coastline response to an increasingly asymmetric wave climate in the presence of beach nourishment, produce comparable differences in shoreline change rates in response to changes in wave climate. Results suggest that the effect of beach nourishment is to compensate for -and therefore to mask -natural responses to wave climate change that might otherwise be discernible in patterns of shoreline change alone. Therefore, this case study suggests that the effects of wave climate change on human-modified coastlines may be detectable in the spatial and temporal patterns of shoreline stabilization activities. Similar analyses of cuspate features in areas where the change in wave climate is less pronounced (i.e. Fishing Point, Maryland/Virginia) and where local geology appears to exert control on coastline shape (i.e. Cape Canaveral, Florida), suggest that changes in shoreline configuration that may be arising from shifting wave climate are currently limited to sandy wave-dominated coastlines where the change in wave climate has been most pronounced. However, if hurricane-generated wave heights continue to increase, large-scale shifts in patterns of erosion and accretion will likely extend beyond sensitive cuspate features as the larger-scale coastline shape comes into equilibrium with changing wave conditions.

Abstract. We investigate the controls upon the shape of freely extending spits using a one-contour-line model of shoreline evolution. In contrast to existing frameworks that suggest that spits are oriented in the direction of alongshore sediment transport and that wave refraction around the spit end is the primary cause of recurving, our results suggest that spit shoreline shapes are perhaps best understood as graded features arising from a complex interplay between distinct morphodynamic elements: the headland updrift of the spit, the erosive "neck" (which may be overwashing), and the depositional "hook". Between the neck and the hook lies a downdrift-migrating "fulcrum point" which tends towards a steady-state trajectory set by the angle of maximum alongshore sediment transport. Model results demonstrate that wave climate characteristics affect spit growth; however, we find that the rate of headland retreat exerts a dominant control on spit shape, orientation, and progradation rate. Interestingly, as a spit forms off of a headland, the rate of sediment input to the spit itself emerges through feedbacks with the downdrift spit end, and in many cases faster spit progradation may coincide with reduced sediment input to the spit itself. Furthermore, as the depositional hook rests entirely beyond the maximum in alongshore sediment transport, this shoreline reach is susceptible to high-angle wave instability throughout and, as a result, spit depositional signals may be highly autogenic.

Abstract. We investigate the controls upon the shape of freely extending spits using a one-contour-line model of shoreline evolution. In contrast to existing frameworks that suggest that spits are oriented in the direction of alongshore sediment transport and that wave refraction around the spit end is the primary cause of recurving, our results suggest that spit shoreline shapes are perhaps best understood as graded features arising from a complex interplay between distinct morphodynamic elements: the headland updrift of the spit, the erosive "neck" (which may be overwashing), and the depositional "hook." Between the neck and the hook lies a downdrift-migrating "fulcrum point" whose trajectory is set by the angle of maximum alongshore sediment transport. Model results demonstrate that wave climate characteristics affect spit growth; however, we find that the rate of headland retreat exerts a dominant control on spit shape, orientation, and progradation rate. Interestingly, as a spit forms off of a headland, the rate of sediment input to the spit itself emerges through feedbacks with the downdrift spit end, and in many cases faster spit progradation may coincide with reduced sediment input to the spit itself. Furthermore, as the depositional hook rests entirely beyond the maximum in alongshore sediment transport, this shoreline reach is susceptible to high-angle wave instability throughout and, as a result, spit depositional signals may be highly autogenic.

Low‐lying, wave‐dominated, sandy coastlines can exhibit high rates of shoreline change that may impact coastal infrastructure, habitation, recreation, and economy. Efforts to understand and quantify controls on shoreline change typically examine factors such as sea‐level rise; anthropogenic modifications; geologic substrate, nearshore bathymetry, and regional geography; and sediment grain size. The role of shoreline planform curvature, however, tends to be overlooked. Theoretical and numerical model considerations indicate that incident offshore waves interacting with even subtle shoreline curvature can drive gradients in net alongshore sediment flux that can cause significant erosion or accretion. However, these predictions or assumptions have not often been tested against observations, especially over large spatial and temporal scales. Here, we examined the correlation between shoreline curvature and shoreline change rates for spatially extended segments of the U.S. Atlantic and Gulf Coasts (~1,700 km total). Where shoreline stabilization (nourishment or hard structures) does not dominate the shoreline change signal, we find a significant negative correlation between shoreline curvature and shoreline change rates (i.e., convex‐seaward curvature [promontories] is associated with shoreline erosion, and concave‐seaward curvature [embayments] with accretion) at spatial scales of 1–5 km alongshore and timescales of decades to centuries. This indicates that shoreline changes observed in these reaches can be explained in part by gradients in alongshore sediment flux acting to smooth spatial variations in shoreline curvature. Our results suggest that shoreline curvature should be included as a key variable in modeling and risk assessment of coastal change on wave‐dominated, sandy coastlines.