Wildfire behavior regimes can be broadly characterized as either fuel-dominated or wind-dominated. These two wildfire regimes in California reflect differences in seasonal timing, ignition sources, and geographical distributions across the state, which is topographically, climatologically, and ecologically diverse [1
]. Management responses need to consider this diversity to effectively mitigate fire risk. Fuel-dominated fires, also known as plume or convection-dominated fires [2
], are predominantly controlled by anomalously high fuel loads and are common in central and northern California conifer forests during peak lightning season (June–July), coinciding with high air temperatures and low precipitation [6
]. These fires tend to occur in low populated regions. The moderate rate of fire spread allows for easier community evacuation and fire suppression activities as a method for mitigating wildfire risk to human life and structures. Consequently, fuel-dominated fires usually do not result in significant loss of lives or properties. In contrast, wind-dominated fires are mostly caused by ignitions from humans or infrastructure failure, such as downed power lines during extreme downslope wind events. These are typically dry, warm, and gusty downslope windstorms occurring on the lee-side of mountain ranges and often have geographically distinct names, such as the North, Diablo, Sundowner, and Santa Ana winds in California [7
]. Large, fast-moving wildfires associated with these strong winds have long been recognized as costly and difficult to mitigate and control [8
]. As population growth expands the wildland urban interface (WUI) into areas of higher wildfire risk, the chance of ignition also increases [9
]. When ignitions coincide with extreme winds and dry fuels, rapid fire spread poses a major threat to communities. The potential for significant loss of life and property increases because fire suppression efforts have very limited success. One example of a catastrophic wind-dominated fire was the Camp Fire in 2018, which burned through the town of Paradise and is currently the deadliest (85 deaths) and most destructive (over 18,000 structures destroyed) wildfire in California history [12
], with less than three hour evacuation times for many people living in Paradise.
A variety of fuels treatments are used to mitigate the severity of wildfires in fire-prone landscapes of the western United States. Commonly employed fuel-reduction treatments include clearing, thinning, surface and ladder fuel removal (also known as shaded fuel breaks), prescribed burning, planting fire-resistant vegetation, and grazing/pasturing. WUI fuel treatments of a minimum width of 400 m have been proposed to protect private property by creating safe zones for direct attack tactics [13
]. Fuel treatments typically modify fire behavior but do not always stop fire growth. Fuel breaks can play an important role in controlling wildfire size and behavior if they are strategically placed, maintained, and accessible for suppression activities by firefighters [14
]. Alternatively, structural hardening and 30–60 m of defensible space around structures can often increase fire resilience more than broad scale fuel treatments far from the WUI [1
A recent study [1
] questions the efficacy of landscape-level fuel-reduction treatments in controlling the overall size of wind-dominated fires, particularly during drought periods when fuel moisture content is very low. Most structure losses during wind-dominated wildfires are associated with spotting (burning embers lofted and transported ahead of the fire front) [12
]. In addition to spotting, wind-dominated fires often exhibit other extreme fire behavior characteristics. High rate of spread, prolific crowning, and the presence of occasional fire whirls (vertically oriented, intensely rotating columns of gas found in or near fires) often inhibit direct fire control efforts [7
]. For wind-dominated fire risks, a more effective vegetation management strategy involves increasing the fuel moisture content around the WUI [12
], as opposed to traditional vegetation removal techniques away from the WUI. Because there is no consensus on optimal fuel break widths, more research is required to understand how fuel breaks in the WUI interact with severe downslope wind-driven fires.
Wildfire behavior modeling is one important tool in fuels management [21
]. A deterministic wildfire modeling approach is one way to assess the effectiveness of fuel treatments as demonstrated by [14
] and [15
], because actual fire scenarios can be represented by a realization of some deterministic processes. Alternatively, a stochastic wildfire modeling approach is also used to create a more holistic picture of spatial wildfire risks using Monte Carlo simulations [22
]. Semi-empirical fire spread models have several advantages over more sophisticated coupled fire-atmosphere models, such as significantly faster computational times and relatively simple model configurations.
The aim of this study is to explore the effectiveness of fuel breaks placed in the WUI (hereafter referred to as WUI fuel breaks) as a potential fuel treatment option for wind-dominated wildfires. We use the Camp Fire as a retrospective case study because the ignition by power line failure resulted in rapid fire spread and very short evacuation time under strong downslope winds and extremely dry fuel conditions. The main research objectives are to examine: (1) the potential downstream impacts of a WUI fuel break on fire intensity, propagation, and arrival time or evacuation time of the Camp Fire; (2) how the downstream fire behavior is affected by the width and fuel properties of the WUI fuel break; and (3) the impacts of background wind speeds on the WUI fuel break. Note that the WUI fuel break that we study is hypothetical and not an actual fuel break that exists in the study area.
This study investigated the effectiveness of WUI fuel breaks on downslope wind-dominated fire behavior, including fire intensity and spread rate. Both directly impact WUI community evacuation time and wildfire damage potential. We found that increasing the width of a WUI fuel break from 200 m to 400 m more than doubled the evacuation time. In addition, the fuel break changed fire behavior by breaking up the advancing fire front into multiple fire fronts on the downstream edge of the fuel break. However, the overall fire intensity downstream of the fuel break remained well above the suppression threshold intensity. Furthermore, the width and degree of curing in grass fuel comprising the WUI fuel break had noticeable impacts on controlling fire arrival and evacuation times downstream of the WUI fuel break. Finally, we showed that the burned area downstream of the WUI fuel break may be affected by both the presence of the fuel break and magnitude of the extreme wind speeds. The latter controlled the lateral extent of fire spread upstream of the fuel break.
While our model experiments provide evidence for the potential utility of WUI fuel breaks in mitigating wildfire hazards, there are several sources of uncertainty rooted in model assumptions. For example, Prometheus does not represent long-range spotting. We acknowledge that not including the influence of long-range spotting in our simulation runs is a limitation of this work, as 200–400 m spotting distances are common [57
]. From an observational perspective, it is unclear whether long-range spotting contributed to the increased rate of spread or if the head fire overran the spot fire ignitions during the Camp Fire. Storey et al. [58
] found that the contribution of spot fires to the overall rate of spread may depend on topography. In their study, spotting accelerated the spread of the fire front in complex terrains. The Camp Fire region is topographically complex and thus, the spot fires may have contributed to uncertainty in model predictions. Operational two-dimensional fire growth models, including Prometheus and FARSITE, also do not currently account for atmospheric stability, which is known to be a major contributor to long-range spotting and nonlinear fire spread and intensity [59
]. Another source of uncertainty is fuel moisture content in the WUI fuel breaks, which we were unable to directly modify. The effect of the FFMC on fire spread was also examined by using a weather patch function in Prometheus to produce rainfall over the WUI fuel break. The precipitation lowered the FFMC value and resulted in substantial reductions in the burned area (Figure S5
). The combined effects of the FFMC and the degree of curing on the WUI fuel break effectiveness warrant further investigation using a set of similar experiments.
The grass fuel type in the WUI fuel breaks tested in this study can be viewed as the most optimistic scenario, as it may be prohibitively expensive to remove trees in the WUI fuel breaks. In our study, we explored a scenario in which all existing trees were converted to grass without leaving any additional fuels on the ground. In reality, more live and dead fuels may be distributed in patches on the ground after a fuel treatment. Partial tree removal is expected to increase evacuation times relative to no WUI fuel break, but not by as much as a fuel break with no trees.
Even though only grass type WUI fuel breaks were explored in this study, varying the degree of curing and fuel load scenarios may represent alternative designs of WUI fuel breaks with similar fire behavior characteristics of the grass fuel type. Agricultural lands, open space parks and preserves, golf courses, and other recreational green zones (i.e., greenbelts) have been proposed or already implemented to create wildfire resilient communities in some parts of California [60
]. Similar assessments of proposed WUI fuel breaks can be conducted to estimate relative evacuation time gains using operational or more sophisticated fire spread models for different fuel break designs and historical weather scenarios. In practice, the design and implementation of greenbelts or WUI fuel breaks requires collaboration and coordination efforts among cities and counties, landowners, and local agencies, such as fire departments and regional park services [13
]. Furthermore, an often-overlooked requirement for the construction of WUI fuel breaks is that when firefighters deploy, there must be an adequate number of firefighter safety zones established along the fuel break [61
California wildfires forced the evacuation order of over one million residents in California during 2017–2019 [62
]. Increasing population and WUI expansion increase the likelihood of human-caused ignitions [12
]. Thus, community evacuations may become more important than in the past to minimize the impacts of wind-dominated fires. Community evacuations during extreme wind-dominated wildfires may pose considerable challenges and stresses on emergency management agencies, as seen during the Camp Fire. Our results suggest that WUI fuel breaks, if constructed sufficiently wide and green (i.e., degree of curing), are beneficial for delaying fire front arrival and gaining evacuation times by several hours.
Currently, California plans to spend $
1.5 billion on vegetation management in 2021 [63
]. Greenbelts constructed on the fringe of urban areas are one promising way to spend these funds and may also be able to reduce overall firefighting costs. They also have the potential to create fire resilient communities, while promoting positive social, economic, and environmental impacts. The primary goal of this work was to quantitatively understand the effectiveness of hypothetical WUI fuel breaks and their interactions with wind-dominated high intensity crown fires under extreme fire weather conditions. Our study contributed to these goals by quantitatively understanding the effectiveness of hypothetical WUI fuel breaks and their interactions with wind-dominated high intensity crown fires under an extreme fire weather condition. To make these experiments more realistic, a combination of topography, location, land use, environmental constraints, ecological impacts, and implementation costs should be incorporated into fire growth simulations with fuel breaks.