Post-wildfire slope stability effects and mitigation: a case study from hilly terrains with unmanaged forest

Abstract

The combined effects of wildfires and storms strongly induce slopes stability phenomena, thus requiring mitigation measures to manage long-lasting wildfire-induced risks. In Portugal, thousands of increasingly destructive wildfires occur during the hottest months. In the last two decades, almost 3,000,000 ha of forest, bush and agricultural land burned in Portugal, 20% of which caused by massive wildfires that occurred in 2017. Portuguese central mountain range was one of the most affected areas, namely the study area of Serra do Acor and Serra da Estrela, where extreme rainfall events in 2018 and 2019 caused extensive slope instability phenomena that created a unique opportunity for assessing long-term field performance of treated and untreated burnt areas. The registered phenomena as well as the treatment solutions applied to mitigate instability effects on slopes were studied in two sites. Over 3 years, several field research trips were carried out to scientifically assess the situation over time. By comparing areas with distinct response to the extreme rainfall events and based on the data collected, namely prior to 2017, it was been possible to link the registered instability events and the October 2017 wildfires and qualitatively evaluate the effectiveness of the adopted mitigation solutions. This study not only confirms that the study area is prone to slope instability wildfire-related phenomena, with a 13-year return period between similar events in the study area, but also validates the use of protective barriers as a mitigation measure, contrary to previous studies carried out in other countries.