Factors controlling blowout morphodynamics and evolution in southern Portugal

Wind-induced airflow acceleration over irregular foredunes promotes the genesis of sandy depressions named blowouts. During their development, they transfer sediment to the back-dune helping to maintain the available barrier sediment budget while promoting eco-geomorphological feedbacks that regulate the plant community and biodiversity. However, the mechanisms controlling their dynamics and evolution are still not well understood and demand further research, as these are key landforms for the future management of coastal dunes under rising seas and climate change. This work aims to identify the internal (e.g. blowout morphometric characteristics) and/or external factors (e.g. metocean conditions, human interventions) influencing the migration and spatiotemporal evolution of a series of blowouts present in the foredune of a coastal stretch of 1.3 km situated in Ancão Peninsula, South Portugal. To achieve this, their morphometric characteristics (area, orientation, width, length, width-length ratio and centroid position) were mapped and their changes analysed over time, together with the storm frequency, wave power, dune toe location and anthropogenic interventions in the area. The previous was done using a 49-year set of historical aerial photos, orthophotos, and Google Earth images as well as time series of metocean conditions (from 1972 to 2021). The estimated Kendall’s bivariate correlation coefficients showed that, with 0.1 significance level, blowout migration rates were positively dependent on blowout area, width, length, orientation and dune toe retreat. Besides, fastest migration rates occurred in narrower and lower dune crest areas as these offer less resistance to erosion. It was not possible to obtain certainties on the statistical dependencies with the metocean conditions due to the low temporal image resolution at the beginning of the study period. Nevertheless, two main phases of significant dune toe retreat (1996-2001 and 2008-2011) were concomitant with the impact of several extreme storm clusters reported in the literature (1998-2000 and 2008-2009). The role of extreme events in the study area is threefold: (1) shoreline erosion and dune scarping, which further debilitates the foredune, (2) increase in the total blowout area as the widths and lengths of the medium and large blowouts increase, and (3) disappearance of small blowouts as well as blowout genesis. Lastly, the jetty construction updrift (finished by 1972) seemed a very likely trigger of the initial dune instability in the area while foredune fencing promoted the artificial sealing of blowouts, which showed approximate sealing times of 4 years after fencing implementation.