Wildland-urban Interface Fire Research Articles

Progress to Develop Globally Harmonized International Testing Standards for Large Outdoor Fires, Including Wildland‐Urban Interface Fires

ABSTRACTThe wildland‐urban interface (WUI) fire problem continues to perplex governments around the world. The WUI fire situation is not becoming better across the earth and globally harmonized test standards are required to help lessen the destruction in the event of WUI fire disasters. In this brief overview, a review of activities undertaken in ISO TC92/WG14, the Large Outdoor Fires and the Built Environment Working Group, under ISO TC92, is provided. Specifically, the genesis of how this working group came to be formed is provided as well as an overview of current documents that have been published by ISO TC92 and developed by ISO TC92/WG14. The review closes with recently added new work items in ISO TC92/WG14 and some possible future challenges.

Integrating an urban fire model into an operational wildland fire model to simulate one dimensional wildland–urban interface fires: a parametric study

Background Wildland fires that occur near communities, in the wildland–urban interface (WUI), can inflict significant damage to urban structures. Although computational models are vital in wildfires, they often focus solely on wildland landscapes. Aim We conducted a computational study to investigate WUI fire spread, encompassing both urban and wildland landscapes. Methods We developed a 1D landscape-scale semi-physical model by integrating a semi-physical urban fire spread model into an Eulerian level set model of wildfire. The model includes ignition and spread through radiation, direct flame contact and ember deposition. Key results Through a parametric study, we compare the relative change of spread rate from various structural properties and landscape layouts represented by model parameters, highlighting the significant impact of fire-resistant structure materials over surface treatments. Layout configurations play a pivotal role in fire spread, with isolated islands of combustibles effective in reducing spread rate, aligning with existing mitigation strategies. Conclusion Despite using a 1D domain and limitations on spatial and temporal variability, our model provides insights into underlying phenomena observed in WUI fires and their mitigation. It offers early-stage development of strategies for managing structure materials and landscape layouts. Implications Our model and findings provide insights into WUI fire dynamics, paving the way for advanced mitigation strategies.

Molecular Characterization and Photoreactivity of Organic Aerosols Formed from Pyrolysis of Urban Materials during Fires at the Wildland-Urban Interface.

Fires at the wildland-urban interface (WUI) are increasing in magnitude and frequency, emitting organic aerosol (OA) with unknown composition and atmospheric impacts. In this study, we investigated the chemical composition of OA produced through the 600 °C pyrolysis of ten urban materials in nitrogen, which were subsequently aged under UV light for 2 h. The analysis utilized ultrahigh-performance liquid chromatography (UHPLC) separation, coupled with a photodiode array (PDA) detector and a high-resolution mass spectrometer (HRMS) for molecular characterization. Hierarchical clustering analysis demonstrated that lumber-derived OA was the most diverse and distinct in composition. Unaged and aged OA (for each urban material) did not significantly differ in chemical identities. Potential aromatic brown carbon (BrC) chromophores (based on their degree of unsaturation) constituted 13-42% of all assigned compounds. PDA chromatograms revealed multiple BrC chromophoric species that were either enhanced or degraded as a result of UV aging, providing insights into specific BrC chromophores responsible for photobleaching and photoenhancement of the overall absorption coefficient. Thirty-six BrC chromophores were identified across the ten OA types, and their structures were confirmed using reference standards. Components of plasticizers and resins, such as phthalic and terephthalic acids, were structurally confirmed in the samples. We present potential species for WUI fires as components of resins, epoxies, dyes, and adhesives commonly used in manufacturing urban materials. Photolysis did not significantly impact the chemical composition of OA emitted from the burning of specific WUI materials.

Ignition limits of pine wood building material under the coupling effects of thermal radiation and cross winds

The ignition of timber buildings in the context of wildland-urban interface (WUI) fires has emerged as a significant challenge amidst the backdrop of global climate change. However, to the best knowledge of the authors, little attention was paid to the ignition conditions of wood building materials in WUI fires especially in thermal-fluid coupling environments. This study experimentally investigated the smoldering and flaming ignition characteristics of wood under the coupling effects of thermal radiation and cross winds. Surface temperature, internal temperature, mass loss rates, and time to ignition of wood samples were measured to study their variation with the above factors and the threshold between smoldering and flaming ignition phenomena. The results showed that wood exhibited reduced susceptibility to flaming ignition under low radiant heat flux conditions, and the introduction of cross winds posed more challenges for flaming ignition. In addition, both the critical surface temperature and mass loss rate varied with radiant heat flux and cross-wind speeds, so it was not feasible to determine the occurrence of flaming ignition by any of the two factors directly. This is because the coupling effects of thermal radiation and cross winds would result in a competition between radiant heating and convective cooling on wood surfaces. In light of this fact, two dimensionless quantities, namely Damköhler number and heat transfer ratio, were introduced to distinguish between smoldering and flaming ignitions. Damköhler number was used as an indicator of the potential for gas-phase flaming ignition, while the heat transfer ratio, namely the ratio of convective heat loss to radiant heat flux, was used to measure the net thermal energy received by wood surfaces. It has been demonstrated that the two quantities were effective to distinguish between smoldering and flaming ignitions from the perspectives of gas and solid phases with 20–40 kW/m2 in radiant heat flux and 0–1.0 m/s in cross-wind speed. The data and theories disclosed in this study could help predict the ignition behaviors of timber buildings under thermal-fluid coupling conditions, thus providing useful guidance for WUI fire mitigation and control.

Wildland-Urban Interface fire exposure of rural settlements: The case of Montesinho Natural Park

Coexistence with wildfires has always been part of human life. However, the severity of these events has intensified due to the combined effects of climate change and interventions on the environment. Rural settlements, especially those with heritage values, are vulnerable, leaving their existence under constant threat. Montesinho Natural Park, characterised by many settlements with remarkable vernacular buildings, exemplifies this situation. In response to this challenge and added to the wildfire hazard, it was chosen to apply the Wildland-Urban Interface Index (WUIX) methodology to assess the exposure of the wildland-urban interface (WUI) to wildfires in three of the villages: Aveleda, Montesinho, and Rio de Onor. The WUI represents the boundary between the natural landscape and the community and might serve as a path for the percolation of fire. The study focused on evaluating the evolution of WUI over time for those three villages by taking advantage of the two open data national orthophotos of 2018 and 2021. The initial stage of vegetation recognition was changed to an automated process, relying on a Python script that captures the image for the database, extracts the vegetation, converts, and prepares the files for applying WUIX, and again converts and georeferenced the results for analysis. This approach makes the application a faster, reliable and replicable process. As a result, maps of WUIX affectation were generated, visually and quantitatively pointing to the village of Aveleda as the one with a higher level of exposure. While the results alone cannot support the entire vulnerability assessment of those villages, they serve as a complement to efforts to define proactive measures for safeguarding the rural heritage.

Comparing Ignition of Fuel Beds to Firebrand Showers to Ignition of Fuel Beds by Non-Reacting Heaters

ABSTRACT The combustion of various fuels results in the release of firebrands during wildland-urban interface (WUI) fires. Firebrands represent a hazard, since the release of firebrands may lead to multiple ignitions at locations distant from the original combustion source. A newly developed experimental protocol using a non-reacting heater was used to simulate a firebrand. A direct comparison of this simplified non-reacting heater setup was undertaken to realistic fuel bed ignition to actual firebrand showers. Firebrand showers were generated using a custom experimental setup developed for this investigation. The novelty of this research is a detailed comparison for the first time to fuel bed ignition from non-reacting heaters to actual firebrand showers, for the same fuel bed type. The focus here is on smoldering ignition (SI) of the fuel beds, as this is seen as a complex and least understood aspect of firebrand ignition of actual WUI fires. Results indicate that the overall phenomenology of the ignition process is different between a non-reacting cartridge heater and a shower of firebrands. Complicated smoldering ignition phenomena of fuel beds by firebrand showers cannot be simulated by a simple non-reacting heater.

Environmentally persistent free radicals and other paramagnetic species in wildland-urban interface fire ashes

Wildland-urban interface (WUI) fires consume fuels, such as vegetation and structural materials, leaving behind ash composed primarily of pyrogenic carbon and metal oxides. However, there is currently limited understanding of the role of WUI fire ash from different sources as a source of paramagnetic species such as environmentally persistent free radicals (EPFRs) and transition metals in the environment. Electron paramagnetic resonance (EPR) was used to detect and quantify paramagnetic species, including organic persistent free radicals and transition metal spins, in fifty-three fire ash and soil samples collected following the North Complex Fire and the Sonoma-Lake-Napa Unit (LNU) Lightning Complex Fire, California, 2020. High concentrations of organic EPFRs (e.g., 1.4 × 1014 to 1.9 × 1017 spins g−1) were detected in the studied WUI fire ash along with other paramagnetic species such as iron and manganese oxides, as well as Fe3+ and Mn2+ ions. The mean concentrations of EPFRs in various ash types decreased following the order: vegetation ash (1.1 × 1017 ± 1.1 × 1017 spins g−1) > structural ash (1.6 × 1016 ± 3.7 × 1016 spins g−1) > vehicle ash (6.4 × 1015 ± 8.6 × 1015 spins g−1) > soil (3.2 × 1015 ± 3.7 × 1015 spins g−1). The mean concentrations of EPFRs decreased with increased combustion completeness indicated by ash color; black (1.1 × 1017 ± 1.1 × 1017 spins g−1) > white (2.5 × 1016 ± 4.4 × 1016 spins g−1) > gray (1.8 × 1016 ± 2.4 × 1016 spins g−1). In contrast, the relative amounts of reduced Mn2+ ions increased with increased combustion completeness. Thus, WUI fire ash is an important global source of EPFRs and reduced metal species (e.g., Mn2+). Further research is needed to underpin the formation, transformation, and environmental and human health impacts of these paramagnetic species in light of the projected increased frequency, size, and severity of WUI fires.

After the Smoke Clears: Wildland-Urban Interface Fires and Residues in Nearby Homes.

Scientists are scrambling to fill major research gaps about the types of pollutants released when wildfires burn not just vegetation, but also structures and vehicles-and how nearby residents can protect themselves.

Predicting Wildfire Ember Hot-Spots on Gable Roofs via Deep Learning

Ember accumulation on and around homes can lead to spot fires and home ignition. Post wildland fire assessments suggest that this mechanism is one of the leading causes of home destruction in wildland urban interface (WUI) fires. However, the process of ember deposition and accumulation on and around houses remains poorly understood. Herein, we develop a deep learning (DL) model to analyze data from a series of ember-related wind tunnel experiments for a range of wind conditions and roof slopes. The developed model is designed to identify building roof regions where embers will remain in contact with the rooftop. Our results show that the DL model is capable of accurately predicting the position and fraction of the roof on which embers remain in place as a function of the wind speed, wind direction, roof slope, and location on the windward and leeward faces of the rooftop. The DL model was augmented with explainable AI (XAI) measures to examine the extent of the influence of these parameters on the rooftop ember coverage and potential ignition.

Firebrand burning under wind: an experimental study

Background Spot fires play a significant role in the rapid spread of wildland and wildland–urban interface fires. Aims This paper presents an experimental and modelling study on the flaming and smouldering burning of wood firebrands under forced convection. Methods The firebrand burning experiments were conducted with different wind speeds and firebrand sizes. Key results The burning rate of firebrands under forced convection is quantified by wood pyrolysis rate, char oxidation rate and a convective term. The firebrand projected area is correlated with firebrand diameter, char density, wind speed, and flaming or smouldering burning. A surface temperature model is derived in terms of condensed-phase energy conservation. We finally establish a simplified firebrand transport model based on the burning rate, projected area and surface temperature of firebrands. Conclusion The mass loss due to wood pyrolysis is much greater than that due to char oxidation in self-sustaining burning. The burning rate is proportional to U1/2, where U is wind speed. The projected area for flaming firebrands decreases more rapidly than that for smouldering ones. The firebrand surface temperature is mainly determined by radiation. Implications Knowledge about firebrand burning characteristics is essential for predicting the flight distance and trajectory in firebrand transport.

Assessing trends in wildland-urban interface fire research through text mining: a comprehensive analysis of published literature

Assessing trends in wildland-urban interface fire research through text mining: a comprehensive analysis of published literature

Global expansion of wildland-urban interface (WUI) and WUI fires: insights from a multiyear worldwide unified database (WUWUI)

Fires in the wildland-urban interface (WUI) are an important issue globally. To understand the change of WUI, we develop a 9 km worldwide unified wildland-urban interface database for 2001–2020 with Random Forest models and satellite data. We find that WUI has been increasing in all populated continents from 2001 to 2020 and the global relative increase is 24%, with the largest relative increase (∼59%) over Africa. Global total fire counts decrease by 10% from 2005 to 2020, whereas the WUI fraction of fire counts increases by 23%. The global total burned area decreases by 22% from 2005 to 2020, whereas the WUI fraction of burned area increases by 35%. These are mainly due to the expansion of WUI area. On all the populated continents, the WUI fractions of fire counts are higher than the WUI fractions of burned area, implying that WUI fires tend to have smaller sizes than wildland fires. We also project future WUI changes for the years 2030 and 2040, together with the projection of future fire burned area under different shared socioeconomic pathways (SSP) scenarios in the Community Earth System Model version 2 (CESM2). The projected global WUI fraction (excluding Antarctica and the oceans) is 5.9% in 2040 compared to 4.8% in 2020. The global WUI fraction of burned area is projected to increase from now to 2040 under most scenarios analyzed in this study, unless the WUI area stays at the 2020 level together with the projected burned area under SSP4-4.5. This study is a first step to understanding the changes of WUI fires at the global scale and demonstrates a growing importance of WUI fires. The global multi-year WUI and WUI fire datasets developed in this study can facilitate future work quantifying the impacts of WUI fires on air quality and climate.

Rapid Measurement of Magnetic Particle Concentrations in Wildland–Urban Interface Fire Ashes and Runoff Using Compact NMR

Wildfires are increasing in size, frequency, and intensity, releasing increased amounts of contaminants, including magnetic particles, into the surrounding environment. The aim of this paper is to develop a sensing method for the detection and quantification of magnetic particles (MPs) in fire ash and fire runoff using a compact Time-Domain Nuclear Magnetic Resonance (TD-NMR) system. The system is made up of custom NMR electronics with a compact and rugged permanent magnet array designed to enable future deployment as an in situ sensor. A signal-to-noise ratio of 25 dB was measured for a single scan, and sufficient data can be acquired in one minute. A linear relationship with an R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> value of 0.9699 was established between transverse relaxation rates and MP concentrations in ash samples. This was validated by testing known dilutions of pure magnetite particles and showing that they fit within the same linear curve. The developed approach was then applied to detect MPs in surface water, where changes in the relaxation rates as high as 400% were observed before and after a wildfire event. MPs were removed from the surface water using a magnetic particle separator to confirm that observed changes were solely due to the presence of MPs. The compact NMR system can be used as a simple and rapid approach to track and quantify the concentrations of magnetic particles released from fire ashes and also from other sources such as discharges from coal ash and other combustion ashes.

Firebrands Generated During WUI Fires: A Novel Framework for 3D Morphology Characterization

The goal of the present work is to establish a framework for firebrand morphology characterization. Central to this framework is the development of a simple firebrand shape classification model using multi-dimensional particle shape descriptors. This classification model is built upon a series of synthetically generated 3D particles whose shapes and sizes are chosen to be representative of actual firebrands typically encountered during vegetative and structural fuel burns. Principal Component Analysis (PCA) is applied to the synthetic dataset and used to structure the classification model. The model is then verified using 3D digital representations of real-world particles (firebrands collected during tree burns and unburned bark pieces from oak trees). The classification model, which will allow meaningful comparisons of firebrand morphological features by shape class, is expected to be gradually refined as more datasets are made available throughout the Wildland–Urban Interface (WUI) fire research community.

Examining Exposure Fires from the United States National Fire Incident Reporting System between 2002 and 2020

Fires resulting from antecedent fires, known as exposure fires, can manifest across diverse environments, including suburban, urban, and rural areas. Notably, exposure fires represented by structure-destroying fires within the wildland–urban interface (WUI) can extend into non-WUI suburban and urban regions, presenting significant challenges. Leveraging data from the United States National Fire Incident Reporting System (NFIRS) spanning 2002 to 2020, this study investigates 131,739 exposure fire incidents impacting 348,089 features (incidents). We analyze reported economic costs, affected feature types, and property utilization patterns for these exposure fires. We also compare these exposure fires to information documented in other databases. Finally, we examine structure separation distance at residential dwellings and describe ignition pathways for selected fires. Reported property losses for some fire incidents amounted to USD 5,647,121,172, with content losses totaling USD 1,777,345,793. Prominent fire incident categories include buildings, vehicles, and natural vegetation fires, predominantly occurring in residential, outdoor, and storage areas. While the NFIRS lacked information on most major structure-destroying WUI fires, highlighting this analysis’s lack of statistical representation, it did provide insights into less extensive exposure fires, both WUI and non-WUI, unrecorded elsewhere. Our study reveals significant distinctions in the distribution of separation distances between damaged-to-damaged structures (average separation of 6.5 m) and damaged-to-not-damaged structures (average separation of 18.1 m). Notably, 84% of the incidents in exposure fires involved fire suppression defensive actions. These defensive actions contributed to the differences in structure separation distance distributions, highlighting the often-neglected role of these measures in assessing structure responses during WUI fires. We examined ignition pathways at select exposure fires, highlighting some common features involved in fire spread and challenges in documenting these pathways. Finally, we propose a set of idealized attributes for documenting exposure fires, accentuating the inherent difficulties in collecting such data across expansive geographical areas, particularly when striving for statistical representation. Our findings yield valuable insights into the multifaceted nature of exposure fires, informing future research and database development to aid in mitigating their impact on vulnerable communities.

A Field Study of an Extinguishing Material for use in Forest and Wildland-Urban Interface Fires

A Field Study of an Extinguishing Material for use in Forest and Wildland-Urban Interface Fires

Quantification of Bioaccessible and Environmentally Relevant Trace Metals in Structure Ash from a Wildland-Urban Interface Fire.

Wildfires at the wildland-urban interface (WUI) are increasing in frequency and intensity, driven by climate change and anthropogenic ignitions. Few studies have characterized the variability in the metal content in ash generated from burned structures in order to determine the potential risk to human and environmental health. Using inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS), we analyzed leachable trace metal concentration in soils and ash from structures burned by the Marshall Fire, a WUI fire that destroyed over 1000 structures in Boulder County, Colorado. Acid digestion revealed that ash derived from structures contained 22 times more Cu and 3 times more Pb on average than surrounding soils on a mg/kg basis. Ash liberated 12 times more Ni (mg/kg) and twice as much Cr (mg/kg) as soils in a water leach. By comparing the amount of acid-extractable metals to that released by water and simulated epithelial lung fluid (SELF), we estimated their potential for environmental mobility and human bioaccessibility. The SELF leach showed that Cu and Ni were more bioaccessible (mg of leachable metal/mg of acid-extractable metal) in ash than in soils. These results suggest that structure ash is an important source of trace metals that can negatively impact the health of both humans and the environment.

How to define the wildland-urban interface? Methods and limitations: towards a unified protocol

In recent decades, the risk of Wildland-Urban Interface (WUI) fires has increased due to urban growth, particularly in regions with a Mediterranean climate. The identification of the WUI is crucial for formulating fire prevention and management measures. However, there is no unified methodology for defining the WUI and it is not clear if proposals that emerge from scientific research are implemented by fire management agencies. Our objectives were to identify, describe, and compare the methods and criteria used by land and fire management agencies to define the WUI in Mediterranean-climate countries. We conducted a review of laws and fire prevention and management plans and protocols on the official websites of administrative bodies and agencies of the USA, Spain, Portugal, France, Italy, Greece, South Africa, Australia, Chile, and Argentina. Each document was read and analyzed and we conducted searches for the terminology used to name the WUI, the methodology and criteria used for defining the WUI, the fire prevention and management actions implemented in the WUI, the level of territorial organization and the responsible agencies for implementing the actions, and the presence of a methodology and a map at national scale. We found no consensus on the terminology for the WUI, the most common terms used being: wildland-urban interface, urban-rural interface, and urban-forest interface. With the exception of the USA and Portugal, there is no unified methodology at the national scale. We identified three general methods for defining the WUI: considering buffer distance for urban and vegetation areas (USA, Italy, Chile, South Africa), employing networks of strips (Spain, Portugal), and delineating risk-prone zones (Australia, France). All countries undertake fire prevention actions (e.g., fuel reduction and firebreak creation) often implemented at the municipal level. There is almost no interaction between academia and fire management agencies. Our review addresses the gap in the methods to define the WUI effectively implemented by fire management agencies. We highlight the need to implement actions aimed at enhancing the interaction between fire scientists and fire managers, which is essential for formulating and implementing effective strategies for fire prevention and optimizing resources.

Wildland-urban interface wildfire increases metal contributions to stormwater runoff in Paradise, California.

The 2018 Camp Fire was a large late-year (November) wildfire that produced an urban firestorm in the Town of Paradise, California, USA, and destroyed more than 18 000 structures. Runoff from burned wildland areas is known to contain ash, which can transport contaminants including metals into nearby watersheds. However, due to historically infrequent occurrences, the effect of wildland-urban interface (WUI) fires, such as the Camp Fire, on surface water quality has not been well-characterized. Therefore, this study investigated the effects of widespread urban burning on surface water quality in major watersheds of the Camp Fire area. Between November 2018 and May 2019, 140 surface water samples were collected, including baseflow and stormflow, from burned and unburned watersheds with varying extent of urban development. Samples were analyzed for total and filter-passing metals, dissolved organic carbon, major anions, and total suspended solids. Ash and debris from the Camp Fire contributed metals to downstream watersheds via runoff throughout the storm season. Increases in concentration up to 200-fold were found for metals Cr, Cu, Ni, Pb, and Zn in burned watersheds compared to pre-fire values. Total concentrations of Al, Cd, Cu, Pb, and Zn exceeded EPA aquatic habitat acute criteria by up to 16-fold for up to five months after the fire. To assess possible transport mechanisms and bioavailability, a subset of 18 samples was analyzed using four filters with nominal pore sizes ranging from 0.22 to 1.2 μm to determine the particulate size distribution of metals. Trace and major metals (Al, Ba, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, and Zn) were found mostly associated with larger grain sizes (>0.45 μm), and some metals (Al, Cr, Fe, and Pb) also included a substantial colloidal phase (0.22 to 0.45 μm). This study suggests that fires in the wildland-urban interface increase metal concentrations, mainly through particulate driven transport. The metals with the largest increases are likely from anthropogenic disaster materials, though biomass ash also is a major contributor to water quality. The increase in metals following WUI burning may have adverse ecological impacts.

Integrating dynamic wildland fire position input with a community fire spread simulation: A case study of the 2018 Camp Fire

Wildland fire simulation models can be used to inform wildfire risk assessment and mitigation strategies. However, existing models tend to simulate fire spread only inside the wildland or the wildland-urban interface (WUI) communities, but not in both. As a result, there is a need to integrate methodologies to enable seamless simulations of events that ignite in the wildland and continue spreading both across the wildland and inside WUI communities. This paper investigates a systematic methodology to provide a WUI fire spread model with the capacity to assimilate dynamic wildland fire position inputs. The paper uses the streamlined wildland-urban interface fire tracing (SWUIFT) model to simulate WUI fire spread, weather radar data to track the fire line inside the wildland, and the 2018 Camp Fire event as a case study. The work proposes and evaluates a methodology based on a ‘Three-Domain Solution’ (Wildland, Transition, Community) for a smooth fire transition between wildland and community settings. Alternative approaches are examined to describe the boundaries of a Community Domain for simulation purposes. Existing WUI definitions are studied, and considering the analysis results, a non-WUI neighborhood-based housing density (NBHD) method is proposed. This work establishes a systematic approach for a unified wildland-WUI fire simulation.