The objective of this study is developing a new wireless sensor network strategy for analogue measurements to monitor and mitigate natural hazard. The manuscript presents a novel solution for performing analogue measurements during natural hazards through a wireless sensor network that supports mesh topologies. It is based on the Open Link State Routing (OLSR) protocol for routing data through a set of routers supporting every version of the IEEE 802.11 standard (WiFi), from a to ax. In order to overcome every limitation in previous wireless systems, the article presents an overview of existing mesh technologies for microcontroller-based architecture (IEEE 802.15.4) and CPU-based architecture (IEEE 802.11), mainly dedicated to seismic or volcanic hazards. We highlight each of the decisive advantages of the presented technology compared to previous hazard monitoring systems. The new technology purposes multiple topologies, modern network performances and original measurement in real case scenarios, with a same generic solution adaptable to several natural hazards. Topology tests and outdoor experiments were performed in open air; the latter provided analogue measurements in the context of a prescribed vegetation fire and river. These results are discussed in terms of the network latency, user mobility, and measurement uncertainties. Finally, the manuscript concludes with an outlook offered by the novel system for a better understanding of mitigating extreme natural events.

We introduce a simple and wide class of multifractal spatial point patterns as Cox processes which intensity is multifractal, i.e., the class of Poisson processes with a stochastic intensity corresponding to a random multifractal measure. We then propose a maximum likelihood approach by means of a standard Expectation–Maximization procedure in order to estimate the distribution of these intensities at all scales. This provides, as validated on various numerical examples, a simple framework to estimate the scaling laws and therefore the multifractal properties for this class of spatial point processes. The wildfire distribution gathered in the Prométhée French Mediterranean wildfire database is investigated within this approach that notably allows us to compute the statistical moments associated with the spatial distribution of annual likelihood of fire event occurrence. We show that for each order q, these moments display a well defined scaling behavior with a non-linear spectrum of scaling exponents ζq. From our study, it thus appears that the spatial distribution of the wildfire ignition annual risk can be described by a non-trivial, multifractal singularity spectrum and that this risk cannot be reduced to providing a number of events per km2. Our analysis is confirmed by a direct spatial correlation estimation of the intensity logarithms whose the peculiar slowly decreasing shape corresponds to the hallmark of multifractal cascades. The multifractal features appear to be constant over time and similar over the three regions that are studied.

This study presents a new wireless measurement system for outdoor fire experiments based on an OCARI wireless sensor network (WSN). The open-source radio communication OCARI stack ('Open Communication Protocol for Ad Hoc Reliable Industrial Instrumentation') allows to overcome the limitations of the previous wireless solutions for fire measurements, especially those based on proprietary systems, such as Zigbee, wherein users cannot control energy consumption and timestamping. This paper presents the design of an Atmel/ARM-based platform compatible with the wireless OCARI stack, adapted to record and communicate data from heat transducers-namely, a K-type thermocouple and heat fluxmeter. A 5-node WSN using heat sensors faces a 10 m 2 natural fire of excelsior (pine wood) in outdoor conditions. Measurements from the OCARI-WSN were compared with the same measurements simultaneously recorded on a datalogger, a standard wired solution in fire science. Thus, six fire experiments were performed for different fuel loads, and the incident heat radiation varies with the fire size under variable wind conditions. The technological breakdown of timestamping identified in previous low-energy consumption and low-cost wireless systems is overcome by the present solution.

Data collection in the field is fundamental in providing relevant information during fire spread across vegetation or in industrial environments. Considering the challenge and costs of obtaining measurements in the presence of a fire at such a large scale, the development of non-intrusive optical methods is a good alternative. As part of an ongoing effort to improve the understanding of wildfire spread and provide useful tools for fire management, an image-based diagnostic system is being developed for the three-dimensional reconstruction of the turbulent flame in the field. The method allows the time-resolved measurement of the geometric flame features (height, surface area and volume). This paper presents the application of two synchronized stereovision systems to static line of fire scenarios. Each stereovision system, with 1 m inter-camera distance, was located at 12 m from the fire and captured complementary images from front and rear views. Dual spectral band (visible-NIR) cameras were used to enhance fire pixel detection. A series of experiments were carried out on 5 m long and 2 m wide beds of excelsior in the open with fuel loads of 2, 4 and 8 kg/m2. The maximum flame heights were in the range of 1.2–2.4 m. Measurement of the heat flux densities was performed at several positions using thermopile-type sensors. Peak heat fluxes of 6.4–15.7 kW/m2 were measured at 3 m from the fire. The thermal radiation impinging ahead of the flame front was characterized according to its geometric properties. The results exhibit not only a linear relationship between the flame volume and radiant heat flux but also high correlations between their temporal fluctuations. The image processing technique enables measurements of flame volume to be a reliable substitute for the usual fire thermal properties.

In modelling the wildfire behaviour, good knowledge of the mechanisms and the kinetic parameters controlling the thermal decomposition of forest fuel is of great importance. The kinetic modelling is based on the mass-loss rate, which defines the mass-source term of combustible gases that supply the flames and influences the propagation of wildland fires. In this work, we investigated the thermal degradation of three different fuels using a multi-scale approach. Lab-scale experimental diagnostics such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), use of the cone calorimeter (CC) or Fire Propagation Apparatus (FPA) led to valuable results for modelling the thermal degradation of vegetal fuels and allowed several upgrades of pyrolysis models. However, this work remains beyond large-scale conditions of a wildland or forest fire. In an effort to elaborate on the kinetic models under realistic natural fire conditions, a mass-loss device specifically designed for the field scale has been developed. The paper presents primary results gained using this new device, during large-scale experiments of controlled fires. The mass-loss records obtained on a field scale highlight the influence of the chemical composition and the structure of plants. Indeed, two species with similar chemical and morphological characteristics exhibit similar mass-loss rates, whereas the third presents different thermal behaviour. The experimental data collected at a field scale led to a new insight about thermal degradation processes of natural fuel when compared to the kinetic laws established in TGA. These new results provide a global description of the kinetics of degradation of Mediterranean forest fuels. The results led to a proposed thermal degradation mechanism that has also been validated on a larger scale.

In modelling the wildfire behaviour, good knowledge of the mechanisms and the kinetic parameters controlling the thermal decomposition of forest fuel is of great importance. The kinetic modelling is based on the massloss rate, which defines the mass-source term of combustible gases that supply the flames and influences the propagation of wildland fires. In this work, we investigated the thermal degradation of three different fuels using a multi-scale approach. Lab-scale experimental diagnostics such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), use of the cone calorimeter (CC) or Fire Propagation Apparatus (FPA) led to valuable results for modelling the thermal degradation of vegetal fuels and allowed several upgrades of pyrolysis models. However, this work remains beyond large-scale conditions of a wildland or forest fire. In an effort to elaborate on the kinetic models under realistic natural fire conditions, a massloss device specifically designed for the field scale has been developed. The paper presents primary results gained using this new device, during large-scale experiments of controlled fires. The mass-loss records obtained on a field scale highlight the influence of the chemical composition and the structure of plants. Indeed, two species with similar chemical and morphological characteristics exhibit similar mass-loss rates, whereas the third presents different thermal behaviour. The experimental data collected at a field scale led to a new insight about thermal degradation processes of natural fuel when compared to the kinetic laws established in TGA. These new results provide a global description of the kinetics of degradation of Mediterranean forest fuels. The results led to a proposed thermal degradation mechanism that has also been validated on a larger scale.

The complex interactions between the inclined terrain and the flow generated by the fire make the slope one of the most influencing factors on fire spread. In order to gain a deeper understanding of the mechanisms involved in wildfires spreading upslope, the investigation of flow dynamics and heat transfers is fundamental. This paper reports a series of fire spread experiments conducted across a porous bed of excelsior in a large-scale facility, under both no-slope and 30° up-slope conditions. The coupling of particle image velocimetry and video imaging allowed characterizing the flow pattern with respect to the fire front. Simultaneous heat flux measurements with high scan rate were also performed at the edge of the fuel bed. From the collected data, the increase of the rate of spread with increasing slope is attributed to a major change in fluid dynamics surrounding the flame. For horizontal fire spread, flame fronts exhibit quasi-vertical plume resulting from the buoyancy forces generated by the fire. These buoyancy effects induce an inward flow of ambient air that is entrained laterally into the fire from both sides. Flame radiation is the dominant fuel preheating mechanism. Under upslope conditions, the fire plume is tilted toward the unburnt vegetation, increasing radiation levels. The air entrainment at the burnt side of the fire strongly influences the downstream flow, which becomes attached to the surface over a characteristic length scale. Ahead of the flame front, the induced wind blows away from the fire rather than toward it, enhancing convective heating. Periodical forward bursts of flame combined with distant fuel ignitions were also observed. The heat flux measurements confirmed the existence of such convective mechanisms.

This study examines the spread of a natural fire over an inclinable bench for several vegetative fuel-load and slope conditions and on an intermediate scale (18 m(2)). A pioneering combination of heat flux measurements and a large-scale particle image velocimetry system is presented. Three different fuel loads and two slopes (0 and 30) are investigated. The results provide new insights into the fluid mechanics of fire spread, elucidating the air entrained flow motion generated by the fire as a function of the fuel load and slope in particular. For horizontal fires, increases in the vertical velocities and the size of the mixing layer, which is formed by the elevation of hot gases in the atmosphere, are observed with increasing fuel load. The air ahead of the fire is aspired far from the fire, suggesting convective cooling of the unburned vegetation. For 30 degrees-upslope fires, the measurements show a convective heat transfer generated by hot gases advancing towards the unburned vegetation, which is supported by the elongation of the fire head. The experimental technique presented in this study is, therefore, effective for extracting valuable data for fire modeling.

Wireless sensor networks (WSNs) may offer the opportunity to eliminate most of the extension cables and wires in digital systems, allowing operation far from any infrastructure. This opportunity coincides with a great increase in cost-effectiveness in an overall fire detection and monitoring system for forests, buildings or industrial sites. Our purpose is to evaluate this opportunity. After presenting the three main technologies for wireless communications to non experts, we retained the Zigbee protocol for this study. We then investigated whether the use of a WSN with this protocol is valuable for measuring heat quantities during a fire spreading over a vegetation fuel bed. Experiments are performed under both lab scale indoor and real outdoor conditions. The method consists of comparing temperatures and radiant heat fluxes gained with the wireless technology with those recorded at the same location through a wired data acquisition system. Delays due to the wireless radio communications are identified and explained. We also observe information loss for measurements performed in the fire front. Finally, we highlight that fires can be detected satisfactorily by WSN equipment in indoor and outdoor conditions. However, we also show that measurement accuracy obtained from wired systems cannot be obtained with the present wireless technology, and we do not recommend their use at the present time for fire monitoring and mitigation.

Slope is among the most influencing factor affecting the spread of wildfires. A contribution to the understanding of the fluid dynamics of a fire spreading in these terrain conditions is provided in the present paper. Coupled optical diagnostics are used to study the slope effects on the flow induced by a fire at laboratory scale. Optical diagnostics consist of particle image velocimetry, for investigating the 2D (vertical) velocity field of the reacting flow and chemiluminescence imaging, for visualizing the region of spontaneous emission of OH radical occurring during gaseous combustion processes. The coupling of these two techniques allows locating accurately the contour of the reaction zone within the computed velocity field. The series of experiments are performed across a bed of vegetative fuel, under both no-slope and 30° upslope conditions. The increase in the rate of fire spread with increasing slope is attributed to a significant change in fluid dynamics surrounding the flame. For horizontal fire spread, flame fronts exhibit quasi-vertical plume resulting in the buoyancy forces generated by the fire. These buoyancy effects induce an influx of ambient fresh air which is entrained laterally into the fire, equitably from both sides. For upward flame spread, the induced flow is strongly influenced by air entrainment on the burnt side of the fire and fire plume is tilted toward unburned vegetation. A particular attention is paid to the induced air flow ahead of the spreading flame. With increasing the slope angle beyond a threshold, highly dangerous conditions arise because this configuration induces wind blows away from the fire rather than toward it, suggesting the presence of convective heat transfers ahead of the fire front.

At the field scale, a strongly burning fire is a turbulent flow (Pitts 1991). In the strictest sense, there is no scientific definition of a turbulent flow regime, only a set of properties. For fire, these properties are described empirically through the following scenario (Santoni et al. 2006): in the gas phase, fire can extend over a large range of space and time scales that may extend up to three decades or more. These scales are organised into a cascade in which large-scale vortices transfer a large amount of mechanical energy to smaller vortices, which dissipate the energy through viscous forces. This cascade scales with a power law, in frequency or wavelength.

A series of nine fire experiments was conducted on a large-scale bench with an inclinable plate to study the effects of a slope on fire spread. The temperature and total and radiant heat flux densities were measured at the top of the fuel bed during the upslope fire spread in still air. Measurements were recorded at five different locations ahead of the fire front. The effects of the slope on the flame topology, fire dynamics, and heat transfers ahead of the fire front were observed. The thermal measurements and image analyses showed a change in the fire spread regime and its related behavior with an increase in the slope. This was observed through an elongation of the flame and its contact surface over the upper part of the fuel bed. When the slope increased, the fire topology changed from a U-shape to a V-shape, and the radiation-dominated thermal environment ahead of the flame front was progressively turned into a mixed convective-radiative one, in which convection finally dominated for steep slope configurations. For fire spreading across a 30 degrees slope, this increase in the curvature of the fire front (V-shape) is associated with the appearance of fire whirls rolling along the flanks of the fire front, which indicates significant changes in the fluid mechanics and heat transfers. The span of the heat transfer increased and a significant amount of energy impinged 1 or 2 m ahead of the flame front because of the flame tilt (radiation) and hot gas flow (convection). The measurements also indicated a greater amount of energy impinging in the center of the fuel bed than at the edge. This study shows the importance of investigating the fluid mechanics of fire in future wildfire researches. (C) 2012 Elsevier Inc. All rights reserved.

In the Mediterranean area, forest fires represent an important and periodic threat to the environment, to socioeconomic activities, and to human beings. In Corsica (France), in order to prevent large-scale fires and to protect forests, low-intensity prescribed burning, or high-intensity thermal pruning, are conducted within increasingly young Corsican pine (Pinus nigra ssp. laricio (Poir.) Maire var. corsicana (Loud.) Hyl.) forests. However, resistance to thermal stress has only been empirically assessed so far through comparisons that do not give any insight into underlying mechanisms. The recommended approach requires the characterization of tolerance levels of pines to experimental fires of various intensities. Experimental prescribed burns with increasing fuel loads (0, 250, 500, 750, and 1000 g m pine needle beds) were performed under young Corsican pine (5 years old) in a departmental nursery at Ajaccio during May 2008. For each fireline intensity reflecting respective fuel loadings, the heat stress was thermodynamically characterized. To this end, temperatures were measured in the soil and in the canopy. The impact of fire was then evaluated on 1) photosynthesis, through measurements of chlorophyll fluorescence (Fv/Fm); 2) lipid peroxidation (malonedialdehyde contents); and 3) morphological parameters (burned foliage, death percentage) before the prescribed burn, followed by measurements 24 hours, 48 hours, and 1 week after burning. Thermal stress was detected 24 hours after burning and increased with fuel load, with a strong alteration of photosynthetic efficiency, an increase in oxidative damage, and a high level of chlorotic foliage (75%) for 750 and 1000 g m. One week after burning, physiological parameters tended to return to pre-burn values, except for the 1000 g m fuel load. We conclude that Corsican pine can be directly affected by heat transfer to needles during burning, and can also be affected by the loss of integrity of xylem vessels (embolism). These results will help to provide recommendations for monitoring of ecosystems through prescribed burning practice.

This study is part of an ongoing effort to improve the understanding of mechanisms that control the spread of fires with a focus on the turbulent flow modified by the flame front. A large-scale PIV system was used to measure the flow field inside and in the vicinity of a flame front spreading across a bed of fuel in an open environment. The vegetative fuel consisted of a 10-m-long and 5-m-wide bed of excelsior (1 kg/m² fuel load) leading to a nearly 1.5-m-high flame front. The velocity field was investigated in a measurement region about 1.5 m high and 2 m long. In such a configuration, a 450-mJ laser source was used to generate the light sheet, and the flow was seeded using zirconium oxide particles (ZrO2). The PIV measurements in the presence of flame were improved by the use of a liquid crystal shutter in front of the PIV camera, allowing very short exposure times and eliminating the flame trace in the tomographic pictures. Despite the variability of the external conditions, leading to a difficult seeding over the whole PIV area, the present study shows the feasibility of the optical method of fluid visualization in the field. The measurements of the velocity fields show some features of the dynamics of fire plumes. This preliminary study demonstrates the feasibility of the method in the open, but some strong efforts to improve the seeding of the flow must be made.

One of the objectives of the present study is to gain a deeper understanding of the heat transfer mechanisms that control the spread of wildfires. Five experimental fires were conducted in the field across plots of living vegetation. This study focussed on characterising heat transfer ahead of the flame front. The temperature and heat flux were measured at the top of the vegetation as the fire spread. The results showed the existence of two different fire spread regimes that were either dominated by radiation or governed by mixed radiant–convective heat transfer. For plume‐dominated fires, the flow strongly responds to the great buoyancy forces generated by the fire; this guides the fire plume upward. For wind‐driven fires, the flow is governed by inertial forces due to the wind, and the fire plume is greatly tilted towards unburned vegetation. The correlations of the temperature (ahead of the flame front) and wind velocity fluctuations change according to the fire regime. The longitudinal distributions of the radiant heat flux ahead of the fire front are also discussed. The data showed that neither the convective Froude number nor the Nelson convection number – used in the literature to predict fire spread regimes – reflect the observed behaviour of wind‐driven fires.

This paper presents a set of four fire spread experiments conducted in the open (i.e. far from the laboratory scale). Plot areas range from 60 to 1250 m(2). Thermal measurements, namely gas temperature and radiant flux densities, have been performed during the spread of large scale fire fronts. The data collected during these experiments are processed and discussed in order to evaluate the contribution to the thermal radiation of both long term and fluctuating parts of the temperature field. This study presents pioneering measurements about the heat transfer governing a wildfire spread in the open. Second, this work attempts to evaluate the incidence of fluctuations in thermal data. Using simple statistical assumptions we show that neglecting fluctuations of the thermal source can lead to underestimate the radiant flux density emitted ahead of the fire front and therefore, the related fire hazard. (C) 2009 Elsevier Ltd. All rights reserved.

A measurement device dedicated to the experimental study of wildland fires is presented in this paper. It consists of a thermocouple and two heat flux gauges (radiant and total) fixed on an insulated support. Some data were obtained in the field during a series of four fire spread experiments conducted across various vegetative fuels, ranging from pine needle bed to shrub. The measurement device was located at the top of the vegetation, in the centre of the plots, and data were collected during fire spread in the preheating, flaming and charring regions. The uncertainties of the temperature and heat fluxes measurements, due to radiation or convection, are evaluated. For this range of experiments and in the limit of one-point measurement, radiation from flames is the dominant heat transfer process in the preheating region.