Controls on blowout evolution in southern Portugal: A 49-year analysis

Abstract

Blowouts are wind-formed depressions that help maintain the sediment budget and enhance biodiversity in coastal dunes. However, the drivers controlling their evolution and the temporal scales associated to their genesis, development and decay phases remain unclear. To address this, the morphometric characteristics of a series of blowouts on the Ancão Peninsula (South Portugal) were digitized using imagery from 1972 to 2021, and used to analyse changes in the number of blowouts, total area, morphometric characteristics (width, length, orientation), and elongation rate over time. These data were compared with metocean time series and human activities, allowing the identification of blowout phases, drivers, and associated temporal scales. This work revealed that the blowout genesis phase primarily arised from the impact of physical external factors (e.g., non-storm low-to-moderate winds blowing out sand from dune scarp irregularities formerly created by extreme wave events), creating incisions across the foredune crest, and lasted 1 or 2 years. The blowout development phase, still ongoing, was characterized mainly by blowout expansion and rotation of large blowouts from North-northeast (NNE) to the East-northeast (ENE) controlled by external physical forces at specific times (e.g., low-to-moderate winds) and blowout internal factors (e.g., size and orientation). Complete blowout decay phases were not observed, except the complete artificial sealing of some blowouts due to fencing, which lasted 4 years. These findings suggest that a complete and natural blowout genesis-development-decay cycle could likely take more than five decades, with complex and spatiotemporally variable ecogeomorphic feedbacks driving their evolution. The only phase reversal documented was the reactivation of the artificially sealed blowouts, due to storm impacts. Allowing the dune and blowouts to evolve naturally appears to be the current best approach for the dune management at the studied area