Wildland firefighter safety zones: a review of past science and summary of future needs

Abstract

Current wildland firefighter safety zone guidelines are based on studies that assume flat terrain, radiant heating, finite flame width, constant flame temperature and high flame emissivity. Firefighter entrapments and injuries occur across a broad range of vegetation, terrain and atmospheric conditions generally when they are within two flame heights of the fire. Injury is not confined to radiant heating or flat terrain; consequently, convective heating should be considered as a potential heating mode. Current understanding of energy transport in wildland fires is briefly summarised, followed by an analysis of burn injury mechanisms within the context of wildland fire safety zones. Safety zone theoretical and experimental studies are reviewed and a selection of wildland fire entrapments are examined within the context of safe separation distances from fires. Recommendations are made for future studies needed to more fully understand and define wildland firefighter safety zones.

An empirically based approach to defining wildland firefighter safety and survival zone separation distances

“…Several factors could be causing this difference, which include addressing a known underprediction bias in the physical models, determination of the actual probability level that is acceptable for non-fatal injuries and model quality. Underprediction of separation distance in the current set of physically based models is likely related to the failure to include the effects of convective heat transfer on safety zone size (Zárate et al 2008;Butler 2014aButler , 2014b) and the use of burns to bare skin as the only mechanism of injury. A safety zone should be an area of sufficient size to ensure that firefighters remain uninjured from all possible injury mechanisms including excessive inhalation of smoke or hot combustion gases (National Wildfire Coordinating Group 1996).…”

“…Particular emphasis was placed on gathering information related to the key variables identified in previous research (Butler and Cohen 1998a;Zárate et al 2008;Rossi et al 2011;Butler 2014a), such as flame height (or flame length) and separation distance from the flames. An attempt was made to estimate the variables of interest when they were not explicitly described using photographs or other descriptive information.…”

“…The models are generally based on identification of the fire energy source strength, the time-temperature thresholds for burn injury to bare skin and the distance necessary to prevent injury (Butler 2014a). The earliest known guideline was developed by Green and Schimke (1971) in relation to fuel breaks for high-intensity brush fires based on radiant heating.…”

“…Additionally, Zárate et al (2008) and Rossi et al (2011) produced other physically based radiant heat transfer models of safe separation distance using similar approaches with different assumptions regarding flame front temperature and width. Despite the wide use of these models in operational settings, they have known limitations in regards to their assumptions of using only radiant heating, no wind or slope influence, failing to incorporate mechanisms of injury other than burns to bare skin, and the lack of direct validation in the field (Butler 2014a). Recent measurements have shown that convective energy transport can exceed radiative transport in wildland fires (Yedinak et al 2010;Frankman et al 2013).…”

Safety zones and convective heat: numerical simulation of potential burn injury from heat sources influenced by slopes and winds

“…Gettle and Rice 2002). Consequently, current guidelines for safety zone dimensions used in the United States assume flat ground and do not consider wind speed as a factor (Butler andCohen 1998a, 1998b). Recent work, however, suggests that convective heat transfer is more important to fire behaviour than had been thought in the past (Frankman et al 2010, Frankman et al 2012, with potentially significant implications for firefighter safety.…”