On the intermittent nature of forest fire spread – Part 2

Abstract

Based on analysis of the interaction between a spreading fire and its surrounding environment, in nominally constant and uniform boundary conditions, it is observed that the evolution of the fire front is characterised by fluctuations of its properties, including (in particular) its rate of spread (ROS). Using a database with a wide range of fires with different time-space scales, it is shown that the amplitude of the fluctuation in ROS is proportional to the average value of the ROS and that the frequency of oscillation varies with the type of fire, and for a given fuel, increases with the average ROS. In fast-spreading fires, the large amplitude of ROS increase and sudden decrease promote the intermittent behaviour of the fire. In general, the amplitude and period required for the ROS increase are larger than for its decrease. However, the acceleration and deceleration phases in junction fires do not follow this rule, suggesting the existence of different convective processes of interaction between the flow and fire. This oscillation explains the variability in many fires at all scales and challenges the current interpretation based on the three factors affecting fire spread and the classification of wind or topography-driven fires.

Slope effect on junction fire with two non-symmetric fire fronts

“…The fuel bed for the experiments was composed of dry particles of pine needles of Pinus pinaster with a constant load of 600 g m −2 (dry basis) (Viegas and Pita 2004;Xie et al 2014;Raposo 2016;Raposo et al 2018;Rodrigues et al 2019;Viegas et al 2021Viegas et al , 2022. During the preparation of each test, the air temperature (°C), relative humidity (%) and fuel moisture (m f ) were monitored and the conditions of the fuel load and bulk density were controlled.…”

“…According to Raposo et al (2018), the ROS R A near the intersection point of two fire lines (point A) has a functional dependence: where dots represent the wide set of parameters required to define the fuel bed properties and t is the time that each test lasted after ignition. We consider t as an explicit variable because during the course of the time the fire behaviour changes as it is a dynamic process (Xavier Viegas 2004;Viegas et al 2021Viegas et al , 2022. However, the distance x A travelled by point A and the time t are not independent of each other and the distance x A at a given time t is given as follows:…”

“…The fire spread can be described as a function of either time or space (distance), in accordance with Eqn 4, and the behaviour of fire spread was non-monotonic (cf. Viegas et al 2021Viegas et al , 2022.…”

“…The behaviour of the interaction of two linear fire lines is non-monotonic (Viegas et al 2021(Viegas et al , 2022 and the oscillations tend to increase with the slope angle because the convection process increases and dominates the interaction of these fire fronts. Junction fires are defined by two different phases: acceleration and deceleration phases.…”

“…Wildfire propagation is a complex and dynamic phenomenon that is not fully understood (Pastor et al 2003;Sullivan 2009). Wildfire propagation depends not only on the classic triangle of 'fire factors' -topography, vegetation and meteorology -but also explicitly on time, given the dynamic interaction that it creates with the environment (Viegas 2004(Viegas , 2006Viegas et al 2021Viegas et al , 2022.…”