The terrain shape and morphology have great importance both for fire ignition and behaviour. The former through wind regimes, solar exposure, rainfall and air temperature and humidity distribution, which all impact vegetation distribution [1] and moisture contents [2].
Topography is a key aspect of fire spread intensity and behaviour. It includes several aspects of the terrain, also described as the landscape. Here, we introduce some general insights as well as dedicated sections on some aspects that significantly change fire behaviour. In certain conditions, fire spread might be dominated by the effect of topography, also involving specific convective patterns.
In brief, topography may influence several aspects important for fire behaviour, including:
Terrain inclination in relation to the horizontal reference, or slope, is a major parameter characterizing topography. It can be measured as an angle or a percentage of inclination (indicating the number of meters that we climb when we travel horizontally for a given distance). The steeper slopes are well known as a key driver of fire spread and intensity. With slope the flames become closer to the fuel, which due to their smaller angle, favours significantly heat-transfer processes, leading to increased fire progression uphill. However, if the fire front is progressing downhill, these phenomena are not as effective and fire spread is decreased due to increased flame angle. Downslope spreading fires will propagate at a ROS close or smaller than that in horizontal ground. Knowing the role of topography and in particular of a certain landscape feature is crucial to understand fire spread behaviour, and to model and better conduct fire suppression measures.
Slope angles above 20º (36%) to 25º (47%) can have an important effect on fire spread and on fire acceleration. This is particularly important for slope angles above 30º (58%) for which the displacement of the personnel on foot or even on vehicles becomes difficult and dangerous. Inclination angles above 40º (84%) are not common, as the soil does not remain stable at such angles, but it may occur in some regions like in volcanic hills.
South-facing landscapes are normally drier due to increased sunlight and wind exposure. These landscapes are known to have decreased fuel moisture content that leads to increased fire spread when compared to other landscape directions.
In normal conditions, altitude may affect temperature and wind patterns. Also, vegetation typology and growth, and fuel moisture content may be changed accordingly. Overall, these aspects may change fire behaviour due to changes caused by elevation.
The terrain curvature may change fire spread, changing its speed due to flame angle and heat-transfer efficiency to the unburnt fuels. In brief, straight slopes have a more predictable fire behaviour, while concave or convex slopes are normally associated to increased fire spread, nearer to the top or the bottom following the location of the steeper slopes in the landscape, respectively. One relevant case is the canyon shaped terrain in which the concave shape of the terrain enhances the convective flow towards the combustion zone, causing a feedback effect that produces fire acceleration.
The ridges are normally associated with complex wind patterns, namely direction changes due to flow separation and recirculation, and vortex formation. Wind may have additional features when flowing over or around ridges, creating complex fire behaviour patterns that can either accelerate or decelerate its spread. If the fire is wind-driven, it can quickly spread laterally along the ridge and widen its transverse dimension, therefore entering the downhill slope with a much wider front. These areas might be also associated with increased probability of fire whirls formation or horizontal extended fire spread, being both described below as specific features of fire behaviour. On the other hand, in particular conditions, ridges may act as natural barriers inducing the decrease of fire spread due to wind or slope-derived effects.
Complex topography is a term that integrates the presence of several landscape characteristics, which are normally associated with increased slopes and terrain particular features. The complex topography significantly affects fire behaviour and suppression activities since it is harder to predict fire behavior effectively and establish safe strategies. Complex slopes, terrain features, and exposure will lead to multiple features of fuels and wind direction or airflow, resulting in alternative fire spread patterns that are difficult to anticipate and simulate. This increasingly difficult scenario is also generally associated with an increased probability of spotting behaviour.
Complex fire topography features are associated with complex interactions with wind flow and, therefore, potentially difficult to predict fire behaviour. For example, in inclined ridges [3], fire can spread faster downhill than upslope in parts of the terrain.
The presence of canyons embedded in flat slopes must also be considered a factor of increased risk as fire behaviour is completely modified in the slope when it enters the nearby canyon if it is relatively shallow.
When fires are mainly affected by topography, fire spread, and behaviour are increased by terrain-induced patterns, including higher intensity or direction caused by steeper slopes, sunlight exposure, or local wind patterns. Elevation may change local conditions, leading to a change in the spread of fires. Terrain curvatures and ridges may pose additional changes in fire progression associated with slopes, wind patterns, and specific fire behaviour modes, i.e., fire whirls and horizontal vorticity. Complex topography will add multiple features that are difficult to integrate, potentiating fire spread intensity and uncertainty.
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Tymstra, C., Bryce, R.W., Wotton, B.M., Taylor, S.W., & Armitage, O.B. (2010). Development and Structure of Prometheus: The Canadian Wildland Fire Growth Simulation Model. In (p. 102). Edmonton, Alberta: Northern Forestry Centre. ↩︎
Nyman, P., Metzen, D., Noske, P.J., Lane, P.N.J., & Sheridan, G.J. (2015). Quantifying the effects of topographic aspect on water content and temperature in fine surface fuel. International Journal of Wildland Fire, 24, 1129-1142. ↩︎
Abouali, A., Viegas, D.X., & Raposo, J.R. (2021). Analysis of the wind flow and fire spread dynamics over a sloped–ridgeline hill. Combustion and Flame, 234, 111724. ↩︎