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

Alongshore sediment transport that is driven by waves is generally assumed to smooth a coastline. This assumption is valid for small angles between th e wave crest lines and the shore, as has been demonstrated in shoreline mod els(1). But when the angle between the waves and the shoreline is sufficien tly large, small perturbations to a straight shoreline will grow(2,3). Here we use a numerical model to investigate the implications of this instabili ty mechanism for large-scale morphology over long timescales. Our simulatio ns show growth of coastline perturbations that interact with each other to produce large-scale features that resemble various kinds of natural landfor ms, including the capes and cuspate forelands observed along the Carolina c oast of southeastern North America. Wind and wave data from this area suppo rt our hypothesis that such an instability mechanism could be responsible f or the formation of shoreline features at spatial scales up to hundreds of kilometres and temporal scales up to millennia.