Formation of coastline features by large-scale instabilities induced by high-angle waves

Abstract

Along shore sediment transport that is driven by waves is generally assumed to smooth a coastline. This assumption is valid for small angles between the wave crest lines and the shore, as has been demonstrated in shoreline models. But when the angle between the waves and the shoreline is sufficiently large, small perturbations to a straight shoreline will grow. Here we use a numerical model to investigate the implications of this instability mechanism for large-scale morphology over long timescales. Our simulations show growth of coastline perturbations that interact with each other to produce large-scale features that resemble various kinds of natural landforms, including the capes and cuspate forelands observed along the Carolina coast of southeastern North America. Wind and wave data from this area support our hypothesis that such an instability mechanism could be responsible for the formation of shoreline features at spatial scales up to hundreds of kilometres and temporal scales up to millennia.

Littoral steering of deltaic channels

“…CEM assumes a constant 171 shoreface cross-sectional profile such that the divergence of littoral fluxes along the coast 172 corresponds directly to advance or retreat of the shoreline position (Ashton and Murray, 2006a; 173 Ashton et al, 2001). Assuming refraction over shore-parallel shoreface contours, the wave 174 energy and wave direction then drive a sediment flux alongshore (Qs), calculated with the CERC 175 formula for littoral transport (Fig.…”

Modeling shoreline sand waves on the coasts of Namibia and Angola

“…2C). These spits resemble the flying spits described by Ashton et al (2001), who related their formation to the high-angle wave instability acting over very long time periods. The study of Elfrink et al (2003) about the evolution of the Walvis Bay spit confirmed that high-angle wave instability might play an important role on its evolution, given the highly oblique wave climate on this African coast.…”

“…If there is a dominant wave direction sand waves migrate downdrift at hundreds of meters per year. Once formed, they can continue to grow in amplitude and wavelength, eventually developing into spits or capes at larger scales provided that the coast is straight and uninterrupted long enough and the generating mechanism prevails sufficiently long (Ashton et al, 2001). Even if the coastline is stable because high angle waves are not dominant, the instability mechanism may significantly weaken shoreline diffusivity so that sand waves generated by other mechanisms can migrate a long time with little decay.…”

“…However, sand waves can also emerge from small irregularities of an otherwise rectilinear coast in absence of any forcing at its wavelength. This can occur if the wave climate is dominated by high-angle waves, i.e., waves with a high incidence angle relative to the shore normal, because the rectilinear coast becomes unstable (Ashton et al, 2001, Ashton andMurray 2006a;Falqués et al, 2011). We will hereinafter refer to the mechanism as high angle wave instability and to the resulting shoreline pattern as self-organized or free sand waves.…”

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

“…This dependence between breaking-wave angles and heights complicates an analysis of how alongshore sediment flux depends on coastline orientation that focuses on breaking-wave characteristics. Ashton et al [10] present a more parsimonious analysis based instead on the characteristics of offshore waves-before they are affected by near-shore bathymetry.…”