Geoscience Reference
In-Depth Information
comparison to a country such as Spain, where huge efforts have been made to try
and find out the exact origins of the fires so that preventative methods can be taken
to fight against such fires. Only a small number of the remaining 50% of identified
cases were created by arsonists, but it is not sure the behaviour of arsonists is the
same, according to the weather conditions are dangerous or not.
Whether a fire is started on purpose or not, there needs to be a flame or a spark
present. However, flames and sparks only develop into full-scale fires if the
different conditions that are necessary for a fire to start are all present. Factors that
lead to the creation of a forest fire include: the disposal of burning cigarette ends on
the ground; uncontrolled burning; sparks that come from machines on building sites
(for example, the fire in the Adrets in the department of the Var in the south of
France burned 400 hectares on July 16, 2007); the overheating of car engines, which
transfers the forest fire into the neighboring forest (the Mandelieu forest fire
destroyed almost 1,500 hectares on the July 5, 2007), etc. The existence of a
combustible (as far as forest fires are concerned the combustible is vegetation), and
the presence of an environment prone to combustion need to be present. In other
words the flame needs to have to a source that is flammable.
Even with the presence of a combustible, the conditions that influence
inflammation and combustion need to be sufficient so that a fire can develop: this is
not always possible whenever it rains or due to the dew that falls on the ground in
the evening. It is at this point when the surrounding weather conditions influence
what happens, not only current weather conditions but also previous weather
conditions, which govern the water status of the ground.
8.1.2.2.
Susceptibility
Vegetal biomass plays the role as a combustible and can burn rather intensely.
The exact heat given off by the biomass varies, depending on these different
parameters as follows:
- the nature and the quantity of vegetable oils (terpenes) and hydrocarbons
present, both of which burn very easily in their gaseous states. The morphology and
make-up of the vegetation whose surface:volume ratio favors a rapid combustion
means that it is easier for smaller objects to burst into flames. For example, it is
more difficult for a log to burst into flames than hay. Flames will spread much
quicker and it is much easier for them to spread if an area is densely covered with
vegetation and if all of the individual layers of vegetation can be easily reached by
the flames, for example, moss, herbaceous areas, shrubs, and forest areas. This is
why the notion of clearing areas around houses that are threatened by summer forest
fires in the Mediterranean region of France, was made compulsory after the
introduction of the 50 m (sometimes 100 m) law. These laws state that land within a
50 or 100 m radius of houses in the Mediterranean region of France must be cleared
so that the risk of a forest fire starting is reduced. The herbaceous layer is
completely removed and the shrubland layer is strongly reduced, and in some cases
the forest layer is also reduced. If we take the land-use type of this region into
