DUPUY J.L.

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405
Mediterranean Forest Week of Antalya
Understanding the behaviour
of wildfire on the scale
of landscape: a 3-dimensional
physical approach
by Jean-Luc DUPUY
Introduction
Understanding the behaviour of wildfire can be attempted theoreti-
cally, experimentally or by modelling. The scientific literature teaches
us that the propagation of wildfire and its impact result from mecha-
nisms that even when taken singly are complex but, in fact, act
together coupled in tandem: aerodynamics, heat transfer, thermal
degradation of vegetation (pyrolysis), combustion. Though the theory of
such mechanisms is well established and provides a qualitative under-
standing of each of them, their combination, allied to the highly vari-
able nature of forest fuels and the need to quantify each phenomenon,
necessitates experimentation and modelling. However, experimenting
with forest fires in natural conditions is difficult: weather conditions
and the level of moisture in the vegetation vary daily and are not easy
to predict. Furthermore, carrying out measurements is complicated,
while burning is subject to legal constraints and property rights. For
such reasons, modelling has long been the preferred tool for studying
the mechanisms of forest fires and forecasting their spread and impact.
The models for the propagation of wildfire belong to two broad cate-
gories, empirical and physical (SULLIVAN 2009a, b). Empirical models
are designed using databases of experimental results, frequently
obtained in the field, sometimes in the laboratory, and they establish a
statistical connection between certain characteristics of fire, most often
the speed of its advance, to environmental conditions (wind, slope) and
the type of fuel. By the very nature of these models, the validity of the
forecasts based on them is limited to the conditions which governed
their design –a range of winds, slopes and vegetation- but which do not
permit us to investigate the mechanisms of wildfire. Moreover, up until
forêt méditerranéenne t. XXXI, n° 4, décembre 2010
the 1990s, such models took no account of
aerodynamics and simplified excessively the
processes involved in combustion, generally
confined to data about the characteristics of
a flame. It is only since the end of the ‘90s
that a more exhaustive physical approach
has taken into account combustion and the
aerodynamic effects which result from the
interaction of atmospheric wind, winds
induced by the fire, vegetation and topogra-
phy. This approach requires projecting the
equations for conservation in physics onto a
spatial grid spread over time. The technolog-
ical development in computational capacity
has been largely responsible for this evolu-
tion. The models resulting from this
approach are scarce (three or four in the
world) and here we will focus on the
HIGRAD-FIRETEC model developed jointly
by the Los Alamos National Laboratory
(USA) and INRA- Avignon (France) (LINN et
al. 2005, PIMONT et al. 2010). This model
makes it possible to carry out numerical sim-
ulations for the spread of wildfire including
(i) three dimensions (ii) natural conditions
and (iii) a small-scale landscape (< 2 km),
such functional parameters are not as yet all
present in other models. The spatial resolu-
tion of the model is around 2 m.
Using a few examples, we will show in
what way the model helps to understand the
propagation of forest fire and illustrate its
potential for operational predictions.
Validating the model
The model used for investigating or fore-
casting the behaviour of wildfire must be
validated if realistic conclusions are to be
drawn from numerical simulations. Several
validation procedures have been carried out
for the HIGRAD-FIRETEC model. The three
experimental fires visible in picture 1 were
all ignited at the same time but with lines of
fire of different widths. It is quite clear that
the greater the width, the faster the fire
spreads. The model was able to reproduce
this observation (LINN et al. 2005).
Predictions by the model about the speed of
propagation were also compared to speeds
observed in various conditions of plant cover
and environment. Table 1 shows that predic-
tions were very reasonable though it should
be noted that the fires were in low-level veg-
etation (no trees). A farther-reaching com-
parison with experimental crown fires con-
ducted in Canada is currently under way.
Simulations of wind blowing within a forest
have also been successfully validated on the
basis of accurate data (PIMONT et al. 2009).
This is an important validation because
wind, an essential factor in the behaviour of
wildfire, displays complex behaviour amid
tree cover. The model has also been used in
Galicia (Spain) to simulate experimental
counter-fires and the qualitative conclusions
drawn from the experiments and the model
were similar.
It should be noted that validating a model
is often a difficult procedure because the
modeller must know in precise detail all the
experimental conditions and be capable of
representing them in the model. The proce-
dure can lead to modifications in the model
so as to enhance the forecasting but such
modifications themselves entail yet further
validation.
Importance
of 3-dimensional modelling
Some physical models represent fire only
in two spatial dimensions, for example on a
vertical plane (vertical cross-section of a line
of fire); this offers the advantage of a reduc-
tion in the time needed for calculating a sim-
ulation. But the need to take into account
three dimensions appears in a study applied
to counter-fires. The efficacy of a counter-fire
is assumed to be enhanced by a draught cre-
ated some tens of metres or more in front of
the fire being fought (ARÉVALO 1968, BENOIT
DE COIGNAC 1986). Three-dimensional simu-
lations with the HIGRAD-FIRETEC model
revealed that in numerous situations the
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Mediterranean Forest Week of Antalya
Picture 1:
Experimental fires in
Australian grassland.
The three fires were
started at the same
time but with three
different widths.
© P. Cheney
and A. Sullivan
(Cheney et al. 1993).
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Mediterranean Forest Week of Antalya
reality of such draughts is limited to only a
few metres. In fact, only special topographi-
cal conditions (fire travelling down a slope)
may give rise to such a draught before the
fire at distances having a material effect on
operations. These results corroborate obser-
vations recorded during the course of experi-
mental counter-fires.
To highlight the role of a three-dimen-
sional matrix in aerodynamic phenomena
associated with wildfire, the propagation of
surface fires in various conditions of wind
and vegetation were simulated in both two
and three dimensions using the HIGRAD-
FIRETEC model. The horizontal profiles of
the local wind, in front of the fire as well as
at the height of vegetation, were extracted
from the simulations and compared for the
two positions (cf. Fig. 1). It appears that the
“lee- or up-fire” wind is systematically
underestimated by two-dimensional calcula-
tions and that a draught at a significant dis-
tance from the fire can appear in such simu-
lations. In a three-dimensional calculation,
the prevailing wind can “blow through” the
line of fire at certain points and be deflected
upwards by the strength of the plume at
points nearby whereas in two-dimensional
calculations the prevailing wind can only
“pass” above the flames or “flatten” the
flames and the plume. It can be seen from
such wind profiles that a two-dimensional
calculation will predict conditions more
favourable for igniting a counter-fire than
will a three-dimensional one. Finally, while
draughts can eventually occur locally and
momentarily in front of the fire-line, they are
unstable from one minute to the next or from
one spot to another. This makes the efficacy
of draughts from counter-fires that much
more haphazard. Figure 2 shows why: the
wind “downwind” of the fire acquires a struc-
ture made up of lines of wind in parallel,
with speeds alternately fast and slow; and
close to the line of fire there appear localised
updraughts coexisting along with counter-
vailing air flows in the direction of the pre-
vailing wind. The result shown in Fig.2 was
obtained with the prevailing wind speed
assumed to be constant, which explains the
air flow pattern of parallel lines. A wind with
a varying direction would generate more
complex structures.
These conclusions do not call into question
the principle of counter-fires used to deprive
a wildfire of fuel. They indicate that a line of
counter-fires ignited close to the front of the
Fuel type Fuel height Fuel load FMC (1) Fire width Wind speed Rate of spread (m/s)
(m) (kg/m2) (%) (m) (m/s) FIRETEC Experiment
Shrubland 0.3 0.5 80 10 2.7 at 2 m 0.08 0.07
France (2) 0.4 0.8 140 10 5.6 at 6 m 0.10 0.07
0.4 0.8 70 10 5.7 at 6 m 0.12 0.09
Shrubland
Spain (3) 0.5 2.2 65 25 2.1 at 2 m 0.039 0.043
Grassland 0.7 0.7 5 50 3.0 at 2 m 0.7 0.7-0.8
Australia (4) 0.7 0.7 5 50 6.0 at 2 m 2.8 1.8-2.7
Tab. I:
FIRETEC predicted versus
experimental rates of
spread in shrubland and
grassland fires.
(1) FMC Fuel moisture
content
(2) INRA’s data, Beauchamp
and Trou du rat experiments
(3) CIFL’s data, Edres and
Carballas sites, Galicia
(4) Cheney et al. 1993
Fig. 1: “Down-fire” wind speed as a function of the distance from the fire
line at 0.75 m from the ground (left) and at 2.3 mm from the ground (right).
In black: two-dimensional simulations, in grey: three-dimensional simulations.
In the basic context, the vegetation is a homogeneous garrigue bush cover
with kermès oak and a moderate prevailing wind (20km/hr at 10 m above
ground level). The weak wind corresponds to a 10 km/hr wind, strong wind
to 40 km/hr, with the same vegetation. Dry fuel (bottom)
was also tested (scrubland in summer).
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Mediterranean Forest Week of Antalya
wildfire does not guarantee a positive effect
related to a draught being created whereas
such a line certainly increases the degree of
risk in comparison to fires set off farther
away from the wildfire.
Behaviour and impact
of wildfire in tree stands
The simulation of fires in plant cover rep-
resenting a stand of Aleppo pine thinned out
in its centre made it possible to quantify the
effect of thinning as a function of its severity
and structure (PIMONT et al. 2010). The
degree of severity was defined by stands left
after thinning with 50%, 25% or 0% cover
(an untouched stand having a cover of 75%).
The structure of the thinned area (clumps of
trees) was tested for a stand showing 25%
tree cover, using clumps of 4, 10 and 20m
diametre, as well as for a theoretical case in
which the tree cover was uniform throughout
the thinned area. The low bush cover
remained unchanged in the thinned-out
area, thus likely to represent a fuel break
lacking in upkeep.
Figure 3a shows a clear reduction in the
intensity of the wildfire (strength in kW/m)
with 25% thinning. 50% tree cover led to no
significant reduction from an operational
point of view. With no trees (0%), the
strength was equivalent to the 25% level:
without trees, the total biomass is less (which
reduces the intensity of the fire) but the wind
at the level of the bushes is stronger (increas-
ing the speed of propagation of the fire and
hence its intensity). The model takes into
account these effects. Figure 3b shows that
the clump structure has no impact on the
intensity of the fire. In fact, additional simu-
lations have shown clumps impact on the
intensity of the fire when the trees have a
thick, densely-leaved canopy to an extent not
found in Mediterranean pines.
An unexpected outcome of the study is
the identification of “streets” of tree crowns
less affected by the fire (Fig. 4). Such
streets are linked to a swirling effect of vor-
tices around a longitudinal axis parallel to
the prevailing wind. The swirling has a
strong impact as it rises (upward move-
ment of hot gases) and less as it plunges
(descent of cooler air).
In a simulation of a surface fire spreading
beneath a tree canopy, the HIGRAD-
FIRETEC model predicted the air tempera-
ture within the canopy and notably made it
possible to estimate the height of leaf
scorching on the trees on the basis of a
lethal temperature. The threshold is gener-
ally set equal to the exposure of pine nee-
dles to a temperature of 60° for one minute.
Leaf scorching is often a reliable indicator
of the likely mortality for pines after wild-
fire (FERNANDES et al. 2007). Simulations of
a source of heat with a fixed intensity
beneath an Aleppo pine stand showed that
a limited mortality was only ensured at
very low intensities (<500kW/m). This is
particularly the case with Aleppo pine
because the lowest branch with needles is
close to the ground. The result indicates
that there are only fairly limited slots for
controlled burning. These simulations have
revealed an insignificant impact by the pre-
vailing wind (straight wind or wind high
above the canopy) on leaf scorching when
fire intensity is fixed. But leaf scorching
increases with wind because the intensity of
surface fires is increased by wind. Finally, a
greater leaf scorching density leads to leaf
higher up (when the leafy area is uniformly
spread out horizontally i.e. no gaps between
the trees). The more dense the vegetation,
the more it hinders the movement of air,
thus limiting the mixture of fresh air with
the combustion gases. The temperature
then rises at the same level above the
ground when the leaf cover is denser (effect
observed for an index of leaf surface area
from 2 to 6).
Fig. 2:
Local “down-fire”
winds, seen from
above. The prevailing
wind is blowing from
left to right in direction
x. The coloured con-
tour lines represent gas
isotherms (temperature
in Kelvin). The vectors
represent the
horizontal dimension
of local wind speed.
409
Mediterranean Forest Week of Antalya
Wildfire and topography
In order to simulate the propagation of a
wildfire on a map, existing operational simu-
lators must combine the effects of the local
wind and the slopes in the possible direc-
tions of propagation. Such combinations are
achieved using relatively simple empirical
models that consider the effect to be the
addition or the multiplication of the two fac-
tors. The simulations performed with the
HIGRAD-FIRETEC model do not accept this
approach. Figure 5 shows fire speed simu-
lated as a function of the slope of the terrain
for a strong and for a light wind and for a
narrow and a wide fire (front of 20 or 50 m).
Clearly, in general the effects are neither
cumulative by addition nor multiplication.
Furthermore, such effects are highly depend-
ent on the size of the fire, a factor not taken
into account by existing models.
In furtherance of this study, the simula-
tion of a fire starting out in a canyon high-
lighted a rapid conflagration up the steeper
sides after fairly slow burning in the canyon
bottom. The line of fire at the foot of the
slopes can reach a great length which in turn
favours a very rapid conflagration up the
canyon walls. Such behaviour should be
borne in mind in relation to serious accidents
during operations when a weakly-burning
fire at the bottom of a canyon might lead
people to think there is no particular danger
involved.
Overall, these results can be attributed to
the complex interaction between wind,
topography and fire which together modify
the air flow depending on the situation.
Fig. 3: Intensity of fire (strength in kW/m of width of fire front) as a function
of the distance covered. The fire, starting up in an unthinned Forest, is
spreading from left to right towards a thinned but uncleared stand (Firebreak).
(a) percentage of tree cover in the thinned areas
(75%=unthinned, 0%=no trees).
(b) size of clumps (a: 4 m, b: 10 m, c: 20 m; Hom: homogeneous);
unthinned=75%, mentioned again.
Fig. 4: Damage to trees and crown streets spared by the fire. The images on
the left represent the degree of leaf density remaining in the canopy after
fire (black: low; grey: high), as calculated by HIGRAD-FIRETEC.
The photo above left, taken in the USA, shows that a street can form
a simple pattern. The image below right represents the algebraic value
of the vertical component of the local wind: rising currents (R),
descending (D). The arrows represent the component of the wind
in a vertical plane, enabling the swirling effect – to be seen.
Such local winds are also calculated by the model
3a 3b
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Mediterranean Forest Week of Antalya
Conclusion
A model based on coupling fire and atmos-
phere, such as the HIGRAD-FIRETEC
model, provides the scientist with a very
powerful tool for investigating the complex
interactions that occur during a forest fire.
Both the means for calculating and the level
of expertise involved are at a very high level
such that it is hard to envisage their use by
managers and policy makers. Even so, the
operational scope for numerical simulations
is indeed real and researchers should under-
take to transfer to those involved in opera-
tions new knowledge acquired through such
simulations. Eventually the use of this type
of model by specialist consultancies should
facilitate and accelerate such transfer.
J.-L.D.
Bibliographic
references
Arévalo C. (1968) El contrafuego. Su utilización
para cortar y extinguir los incendios forestales.
Montes, 142, 307-323.
Benoit de Coignac G. (1986) Le contre feu : est-ce
la seule technique efficace d’extinction des
grands incendies ? Forêt méditerranéenne 8(2),
167-172.
Cheney N.P., Gould J.S. and Catchpole W.R.
(1993) The influence of fuel, weather and fire
shape variables on fire-spread in grasslands.
International Journal of Wildland Fire 3,31-44.
Fernandes, P., Vega, J.A., Jimenez, J., and
Rigolot, E. (2007). Pine resistance and survival
to fire: state of the art. FIRE PARADOX, an
innovative approach of integrated wildland fire
management regulating the wildfire problem by
the wise use of fire: solving the Fire Paradox.
Contrat FP6-018505, Commission of the
European Communities, Brussels (BEL),
Deliverable D3.2-1, 39 p.
Linn R.R., and Cunningham P. (2005). Numerical
simulations of grass fires using a coupled atmos-
phere-fire model: basic fire behavior and depend-
ence on wind speed. Journal of Geophysical
Research 110: D13107
Pimont, F., Dupuy, J.-L., Linn, R.R. and Dupont S.
(2009) Validation of FIRETEC wind-flows over a
canopy and a fuel-break. International Journal
of Wildland Fire 18, 775-790.
Pimont, F., Dupuy, J-L., Linn R.R., Dupont S.
(2010) Impacts of canopy structure on wind-flows
and fire propagation simulated with FIRETEC.
Annals of Forest Science, à paraître.
Sharples JJ (2008) Review of formal methodolo-
gies for wind-slope correction of wildfire rate of
spread. International Journal of Wildland Fire
17, 179-193.
Sullivan, A.L. 2009a. Wildland surface fire spread
modelling, 1990-2007. 1: Physical and quasi-
physical models. International Journal of
Wildland Fire 18, 349-368
Sullivan, A.L. 2009b. Wildland surface fire spread
modelling, 1990-2007. 2: Empirical and quasi-
empirical models. International Journal of
Wildland Fire 18, 369-386.
Fig. 5:
Rate of spread as
a function of the slope
angle for two
wind speeds and
two fire widths
Jean-Luc DUPUY
Institut National de la
Recherche
Agronomique (INRA)
UR 629 Ecologie
des Forêts
Méditerranéennes
Site Agroparc,
84914 Avignon
Cedex 9
Email : dupuy@
avignon.inra.fr
forêt méditerranéenne t. XXXI, n° 4, décembre 2010


Understanding the behaviour of wildfire on the scale of landscape : a 3-dimensional physical approach -

Dupuy, J.L.

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