Long‐term erosional hot spots in the southern Brazilian coast

Abstract

The southern Brazilian coast is a sandy barrier adorned by subtle shoreline perturbations widespread in a rhythmical pattern. Previous works indicated that short‐ and long‐term shoreline changes are not alongshore uniform. They are based on coastline position monitoring, geomorphological evidences, historical cartographic documents, and aerial photography. Moreover, these studies associated the main observed erosional sites with high rates of sediment drift and energy focusing due to wave refraction. Nevertheless, the role of the highly oblique incident waves in driving shoreline changes was not considered. A recently derived sediment diffusion equation showed the possibility of coastal instability induced by incident waves with high obliqueness, as opposed to the traditional approach. Therefore, this paper applied these concepts to wave climate and performed coastal instability analysis to explain shoreline changes at an undulating littoral. Calculations showed wave climate yielding a negative instability index (−0.13) when using a 42‐month‐long time series of wave parameters obtained from a forecasting numerical model. Also, the calculated direction asymmetry on wave climate revealed a net littoral drift. Consequently, the sites between troughs and updrift flanks of shoreline undulations are expected to exhibit erosive processes. This is the case in the most conspicuous long‐term erosional hot spots at the southern Brazilian coast, which was pointed out in earlier studies. Therefore, coastal instability and littoral sediment drift analysis explained the main morphodynamic processes governing shoreline changes at the study area.