Impacts of fuel-break structure on wind-flows and fire propagation simulated with FIRETEC

This study focuses on the effects of fuel structure and especially its spatial heterogeneity in the context of fuel-break design. The coupled atmosphere-wildfire behaviour model HIGRAD/FIRETEC was used to simulate wind fields and fire propagation in a complex landscape including forest-to-break and break-to-forest transitions. Two different Mediterranean ecosystems were used: a Pinus halepensis stand, with a typical medium fuel load, and a dense Pinus pinaster stand, with a very high fuel load. In both ecosystems, two forest zones were separated by a 200 m fuel-break with the same understorey everywhere in the domain.

The study was separated in two parts. First, the break-induced winds were simulated with FIRETEC. The impact of the fuel-break structure (cover fraction, clump size) on the mean wind and turbulence statistics were analyzed. A significant increase of wind velocity was observed when the cover fraction was reduced within the break. In addition, for a given cover fraction, the aggregation of fuel into larger tree clumps was associated with larger wind velocity than when fuel was distributed in smaller clumps.

In the second part of the study, a fire line was ignited in the area upwind to the break and the fire propagation was computed using the precomputed wind fields of the first part of the study. The fire spread in the upwind forest area before crossing the break and propagating in the downwind forest area. A decrease of fire intensity occurred after several meters of propagation in the fuel-break. This intensity decrease was highly significant when the cover fraction was lower or equal to 25 %, but negligible at 50 %. This result suggests the existence of a threshold between 25 and 50 % cover fraction in the conditions of the study. The fire intensity was the lowest without any trees (when the cover fraction was 0 %) and was a little lower than in the 25 % cover fraction case. However, the complete clearing of trees induced a much stronger inclination of the plume than when some trees remained on the fuel-break. It resulted in higher gas temperatures downwind to the fire, due to a stronger advection of hot gases. This suggests that a few remaining trees (between 0 and 25 % cover fraction) might improve firemen security.

In the present study, the size of the tree clumps on the fuel-break did not show significant effects on fire propagation. Damage to tree was a little more significant. But the differences were very strong when we compared the fire behaviour on heterogeneous fuel-breaks to the ones obtained with homogenized fuel (for a same given fuel load). This result suggests that the local fuel structure does not affect significantly the fire behaviour, but that assuming a fully homogeneous structure of fuels when they are highly heterogeneous with low cover fraction (like on fuel-breaks) can significantly affect fire behaviour prediction. This underlines the interest of fire models able to take into account the three-dimensional structure of fuels and fires for studying complex scenarios such as fuel-break impact.