Dunes erosion under an extreme high wave energy event on the central and southern coast of Rio Grande do Sul state, Brazil

Abstract

On the oceanic coast of Rio Grande do Sul, many of the coastal segments that are in the process of erosion have been urbanized. The study of the impact of high wave energy events on segments presenting dunes facing urban areas is important in order to consolidate the state of the art in relation to coastal erosion, and at the same time to alert about the problems that can be occasioned by the occupation close to the ocean. This study aimed to characterize frontal dunes retreat as the result of an extreme high wave energy event, which took place in October 2016, in central and southern sectors of the Rio Grande do Sul State, specifically the beaches: Mostardense, Praia do Farol, Praia do Mar Grosso, Cassino, Hermenegildo e Maravilhas. For this, was analyzed vertical and oblique high resolution aerial photography remotely obtained with an unmanned aerial vehicle (drone) before and after the event, as well as, meteo-oceanographic data. This event can be considered as an extreme wave energy event. The significant wave height (Hs) exceeded 5 m close to the coast and in deepper waters the significant wave height exceeded 6 m with maximum height (Hm) above 10 m, such values were not described in the last 40 years on this oceanic coast. The wave period (Ts) was 15 s. This storm generated a medium retraction of the frontal dunes between 4 and 21.8 m in most of the analyzed coastal segments. However, there were variations in the rates found in the six segments analyzed. The highest rates occurred in the two areas to the north (13.8 m in average in Mostardense and 21.8 m in Farol Beach). The natural characteristics of the site, with a south facing coast, and a barrier with a retrogradational tendency and preserved dunes, favored the action of high wave energy on the coast. In areas with a prograded barrier and estuarine influence (Mar Grosso and Cassino), there was also frontal dune retraction but with lower retraction rates than Mostardense and Farol Beach (9.6 m in Mar Grosso and 4 m in Cassino beach). It is speculated that in the southern beaches, the rates (9.9 m in Hermenegildo and 8.8 m in Maravilhas) were not higher due to the presence of erosion containment structures in Hermenegildo and peat outcrops in Maravilhas, which probably contributed to the reduction of the rate of frontal dunes retreat. The description of this intense erosive process is important for the planning of coastal occupation in order to identify that coastline changes, in this case the retraction of the boundary between the beach and the frontal dune, is not a linear process, but in many cases a response to high wave energy events, especially so-called extreme events.