# Experimental and Numerical Evaluation of a Wildland-Urban Interface Fire Scenario
## 1 Introduction  
The wildland-urban interface (WUI) refers to the area where human development and wildland or vegetative fuels intermix or overlap. It is characterized by the proximity and potential interaction between structures and flammable vegetation [1, 2, 3, 4, 5, 6, 7]. Nowadays the concepts of intermix and interface are also deeply connected to the definition of WUI, differentiating the areas where human developments and wildland vegetation overlap (intermix) from the areas adjacent to a densely vegetated wildland (interface) [2, 7]. The general trends in WUI dynamics indicate that the expansion of human settlements into previously uninhabited or sparsely populated areas will continue, which may worsen the problem in the near future. This trend is primarily driven by population growth and urban sprawl, land use changes through the conversion of agricultural and/or forest areasinto urban ones, economic and infrastructure developments in rural areas (tourism and the chase for natural amenities), and climate change [1; 7]. Fires in the WUI are becoming increasingly frequent around the world, mainly due to human activities, the continuous expansion of urban areas into forested regions, and the abandonment of rural activities that contribute to increased fuel continuity [1; 2; 3; 4; 5; 6; 7; 8; 9]. Additionally, short-term global climate change scenarios show how they will actively contribute to exacerbating the fire risk due to the increase in recurrent and longer heatwaves and dry periods [10; 11]. Consequently, fires at the WUI and their impacts have increased significantly [12] in the last few decades. The catastrophic events in Portugal in 2017 resulting in 114 fatalities [13; 14] and the 2018 Camp Fire in Northern California resulting in 82 fatalities [15; 16] are a few clear examples of the severe impacts arising from uncontrolled wildfires reaching the WUI. The loss of property in these cases was huge; 458 homes and 521 industrial buildings were damaged in Portugal in 2017 [14; 17; 18], with an estimated cost of EUR 1.5 billion. In 2022, according to the European Forest Fire Information System (EFFIS, [[https://effis.irc.ec.europa.eu/](https://effis.irc.ec.europa.eu/)]([https://effis.irc.ec.europa.eu/](https://effis.irc.ec.europa.eu/)) (accessed on 4 December 2023)), the burned area in Europe was one of the highest ever recorded since 2006, affecting several countries such as Portugal, Romania, and Spain. The increasing frequency of such events leads to the need for a detailed understanding and mapping of the critical WUI areas around the world, employing strategies and methodologies that enable permanent updates [2]. Several studies show that, worldwide, the WUI increased significantly in the last few decades [2; 3; 4; 7], taking into consideration mapping methods relying on building density, vegetation proportion and cover, and distance from buildings to high-density vegetation (zonal approach for mapping the WUI) [2; 7]. [PERSON] et al. [7] presented a comprehensive study characterizing the global WUI, concluding that by 2020, the total WUI area was about 4.7% of the global land area. The majority (3.6%) represented intermix areas, illustrating the territorial sprawd of urban areas. A more focused study was performed in Portugal, adopting a novel strategy for mapping the WUI and defining the concepts of direct (urban areas in physical contact with flammable vegetation) and indirect (urban areas are less than a specified distance away from flammable vegetation) interface [19]. The adopted approach to WUI delineation mapped 29,977 km of direct WUI and 11,192 km of indirect WUI in Portugal and identified substantial regional variations in the length and density of both kinds of WUI, reflecting differences in the degree of urbanization and extent of wildlands, and indicating different levels of urban area exposure to wildfires [19]. [PERSON] et al. [19] also discussed the WUI length and density as the ratio of WUI length to urban area.