Large-Scale Patterns in Hurricane-Driven Shoreline Change

Abstract

The effects of storm events on cross-shore beach profiles have been the subject of concerted examination by nearshore researchers for decades. Because these investigations typically span relatively short (less than a kilometer) shoreline reaches, alongshore patterns of storm-driven shoreline change at multikilometer scales remain poorly understood. Here we measure shoreline position from seven airborne lidar surveys of coastal topography, spanning 12 years (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008), along a continuous~80 km stretch of the northern North Carolina Outer Banks, United States. Two of the lidar surveys were flown in the wakes of Hurricane Bonnie (1998) and Hurricane Floyd (1999), allowing a rare window into storm-related alongshore coastline changes at large scales. In power spectra of shoreline change variance and in calculations of plan view shoreline curvature, we find evidence of transient behaviors at relatively small alongshore scales (less than a kilometer) and an interesting combination of both transient and cumulative shoreline change patterns at larger scales (1-10 km). Large-scale plan view shoreline undulations grow in amplitude during the storm intervals we examined, possibly forced by a large-scale morphodynamic instability. Long-term (decadal) shoreline adjustments, however, trend in the opposite direction, with an overall diffusion or smoothing of shoreline shape at multiple-kilometer scales, probably due to gradients in alongshore sediment transport. Although storms can significantly reshape the coastline across a wide range of scales, those changes do not necessarily accumulate to patterns of long-term change.

Instability and finite-amplitude self-organization of large-scale coastline shapes

“…Using multiple airborne (lidar) surveys of shoreline position along a nearly straight portion of the North Carolina Outer Banks (USA), analyses of patterns of coastline change show that the components of shoreline change with alongshore length scales of a few kilometres or greater exhibit a relationship between shoreline change and coastline curvature that is consistent with predictions of coastline diffusion [29][30][31].…”

Investigating the evolution and formation of coastlines and the response to sea-level rise

“…If shoreline evolution were determined by the decadal storm, the constant accumulation of shoreline smoothing would not be seen. Lazarus et al (2012) find that the diffusive smoothing is key to alongshore shoreline evolution. This implies the importance of the mean climate in the alongshore direction as extreme events might tend to exagerate the coastline rather than smooth it.…”

“…By analyzing storm-driven versus decadal shoreline change along the North Carolina coast, Lazarus et al (2012) suggests that while storms may reshape the shoreline in the short-term, long-term shoreline evolution is controlled by the background wave climate. Gunawardena (2008), when studying 22 years of field-collected beach profiles from Duck, NC, emphasizes importance of looking at cross-shore and alongshore evolution differently, as the timescales that operate on the cross-shore and alongshore range from monthly to annual.…”

Geoengineering Coastlines? From Accidental to Intentional

“…Alongshore gradients in this long-term net sediment flux produce a net gain or loss of sediment locally that tends to shift the shoreline seaward (gain) or landward (loss). Because large-scale coastline shapes (kilometers or larger) tend to create gradients in net alongshore flux (different coastline orientations experience different sets of wave conditions), these gradients then cause long-term shoreline change (Lazarus et al, 2012) that reshapes coastlines. On an approximately straight stretch of coastline oriented in an arbitrary direction, even subtle curvatures can lead to progressive shoreline change, such that on most coastline stretches, convex-seaward areas tend to erode (or erode more rapidly than the large-scale average) while concave-seaward stretches tend to build seaward (or erode less rapidly); see Figure 7.2 (Lazarus et al, 2012;Ashton and Murray, 2006a,b).…”