Bluff retreat induced by wave action on a tropical beach, in Espírito Santo, Brazil

Abstract

Interest in understanding sea bluff erosion along the coasts around the world has grown in recent years, especially when in proximity to urban expansion. To prevent economic, social and environmental losses, it is necessary to understand the factors behind the sea bluff erosion. This study analyzed temporal and spatial changes in wave exposure over 60 years (1948-2008) and the corresponding impacts of these variations on three sea bluffs with similar lithology over 55 years (1953-2008). Erosion distances and annual erosion rates were obtained from aerial and satellite images for the time intervals 1953-1970, 1970-1995 and 1995-2008. Analysis of bluff retreat and wave exposure compared annual wave behavior, extreme wave occurrences and modeled spatial and temporal variations in wave energy to yearly retreat rates obtained from imagery analysis and assessed for each of the three intervals. For the three bluffs, the total erosion distance decreased northward - 51, 43 and 18 m, resulting in recession rates of 0.8, 0.69 and 0.43 m/year, respectively. Overall, annual erosion rates appear to slow down during long periods (63 years) compared to the short-term (8-25 years). The shift in modeled wave energy distribution is mainly in accordance with the longshore variability of erosion rates. However, the relationship between bluff recession and wave energy shows that periods of high energy are not necessarily related to intense bluff recession. The erosion process induces debris deposition, which may be able to protect the bluff toe from wave action for a while. The bluffs in our study receded at different rates, despite proximity, in response to spatial and temporal differences in wave energy distribution. Based on results, we propose a bluff erosion cycle model for the studied area and infer current erosion vulnerability.