Geoscience Reference
In-Depth Information
consideration, as far as vegetation cover is concerned, and which can be represented
as a raster in a geographic information system (GIS), it becomes clear that this type
of geographical information is a very useful tool so that the spread capacity of the
flames can be estimated. The process involves examining and measuring how
sensitive an area is to fires which might burn through it [CAR 05];
- the water present in leaves and branches also reduces the effects of
combustion. During combustion all of the water present in leaves is evaporated, and
this evaporation means that energy is taken from the leaves (for example, 537
calories are removed per gram of water). This removal of energy means that this
energy can no longer be transformed into heat, and as a result this decrease in
calorific energy means that it becomes more difficult for a fire to spread and the
speed at which it spreads decreases.
Parameters, other than the morphology and the nature of the combustible,
influence the way in which a fire spreads, and also provide information the
“susceptibility” of a particular area to fires. Topography is another parameter that
should be taken into consideration and it acts at two different levels: first,
topography influences wind direction, and all of the different weather factors that
exist, especially in terms of exposure. Second, topography can change the angle at
which the flame comes into contact with the surface of the ground. As far as a
perfectly vertical flame is concerned (in other words there is no wind present), if the
topographic slope is steep, it means that the angle is weak and as a result the energy
of the flame is better transferred to vegetation, which in turn makes it easier for a
fire to spread, as well as speeding up the rate at which it progresses. If this angle is
increasingly closed, there is an increase in the amount of radiation (that is emitted
by the flame), which affects the surface of the ground, and thus, the vegetation. In
addition, there is an increased chance that the convection column will affect the tops
of the plants. Wind also has the same effect, and as there tends to be wind on the
slopes of mountains it can be assumed that slopes, and relief in general, tend to be
areas that are affected by fires. Moreover, the wind is seen as being less a hazard
factor (due to a minor role for fire ignition) than a key factor due to its major role in
helping fires spread from one area to another.
When we consider all of the different factors that influence the way in which a
fire develops, it is necessary to make a distinction between the different time periods
that are used so that it becomes possible to explain the situation occurring at a
particular moment:
- time scales linked to the relief transformations (slopes, etc.) during thousands
of years and this time scale may be considered as being invariable;
- time scales related to human actions that may have been taking place for tens
or hundreds of years, as the evolution of the vegetation cover; and also time scales
of a few years or months, concerned by the creation of new houses, other buildings
or roads, etc;
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