To cut or not to cut: Effect of vegetation height and bulk density on wildfire propagation under varying wind and slope conditions

Abstract

The frequency, intensity and span of wildfires have surged in the past decades, mainly driven by global changes in climatic patterns. While grasslands cover nearly 40 % of the Earth’s surface, they account for approximately 80 % of the burned area caused by wildfires. Aiming to limit the Rate of Spread (RoS) and intensity of grassland fires, mowing is typically adopted as a management strategy in different parts of the world. However, recent studies suggest that the RoS may actually increase when grasses are cut, and therefore this strategy may need reconsideration. This paper combines results from previous experimental studies conducted in Australian grasslands with a significant number of three-dimensional field-scale wildfire propagation simulations under different ambient wind velocities, vegetation heights, and terrain slopes to assess whether grass cutting is an effective strategy to mitigate fire propagation in grasslands. Simulations are carried out using the Fire Dynamics Simulator (FDS). Previous investigations on how the vegetation height (𝐻𝑔 ) affects the RoS of the fire have led to contradictory results. In this paper, we have found a positive correlation between 𝐻𝑔 and 𝑅𝑜𝑆∕𝑢10∕𝑀 instead, where 𝑅𝑜𝑆∕𝑢10 is the relative RoS, 𝑢10 is the wind speed 10 m above ground level, and 𝑀 is the fuel moisture content. This was observed across all datasets considered and all simulations conducted, provided that the bulk density of the fuel decreases with increasing 𝐻𝑔 —as is typically observed in nature—and that the fire is in plume-driven propagation mode. For wind-driven propagation, the reverse is observed in simulations: decreasing 𝐻𝑔 (shorter grass, decreasing fuel load, increasing bulk density) leads to increasing 𝑅𝑜𝑆∕𝑢10 for constant 𝑀. Further experimental research is needed to confirm this trend, which appears somewhat counter-intuitive. These findings suggest that the practice of mowing grasses can effectively curb fire propagation, although it may be rendered ineffective and even counter-productive under specific conditions such as areas prone to high winds, as both vegetation characteristics and fire propagation modes significantly affect the fire dynamics.