Grassfire propagation on sloped terrains at lower wind velocity, with varied ignition fire line widths

The wind speed and slope of the terrain are among the two major weather and topographical factors that determine the rate of surface fire propagation. Our previous physical modelling studies using Wildland-urban Interface Fire Dynamics Simulator (WFDS), at driving wind velocities of 12.5,6,3,1 and 0.1 m/s on slopes –30 to +30 demonstrated that the average rate of fire spread (RoS) varied from those predicted by empirical models. This subsequent study, analyses simulations with varied ignition lines widths of 40, 30, 20 and 10 m, at different upslopes, at a nominal wind speed of 1 m/s. Two scenarios are analysed:(1) the burnable grass plot width reduced to align with the width of the ignition fire line, and (2) using a fixed 40 m grass plot width. For the first scenario, for all four ignition lines, the contours were found to be pointer for higher upslopes and more pointed for wider ignition lines. In terms of RoS, fire intensity and flame length, the simulation results attained convergence at 30m ignition line width for slopes up to +25. The relative RoS results demonstrated that WFDS quasi-steady RoS values, for up to +20, are closer to the Rothermel model slope function, however, the values were lower for +25 and +30, like the experimental studies of Dupuy et al. We observed that the power-law relationship of Dupuy et al. considerably underpredicted the flame lengths. Comparing the two scenarios, the fire contours generally demonstrated a similar progression pattern; however, for the constant width grass plot cases, the isochrones became elongated as the fire progressed and became slightly wider than the ignition fire line towards the end of the grass plot. Generally, no significant difference noted in the RoS values with the two different scenarios. Overall, the findings from this research provide insight into physical and parametric observations with various fire propagation parameters which can lead to further extensive studies to improve the approach of combined wind and slope effects within operational models.