The phenomenon - Forest fire risk
A forest fire is a fire that tends to spread in wooded, bushy or arboreal areas, including any anthropogenic structures and infrastructures in the relevant area, or else cultivated or un-cultivated land and pastures adjacent to the areas (art. 2 law n. 353 of 21st November 2000).
A forest fire is a fire spreading and causing damage to vegetation and human settlements. In the latter case, when the fire is near to houses, buildings of places frequented by persons, we talk about interface fires. More appropriately, urban-rural interface is used to define those zones, areas or belts, where there are very close interconnections between anthropogenic structures and natural areas: namely the geographical places where the urban systems meet and interact.
Fires occur in all the Italian regions, be it with various degrees of gravity and at different times of the year. The environmental and climatic conditions of the Italian peninsula help towards developing sources of fire principally in two seasons of the year. In the northern regions of alpine arc – and also in the highest zones of the Apennines – forest fires mainly break out in the winter- spring season, being the driest when vegetation has been dried up by freezing. Whilst the frequent thunder storms in the summer reduce the risk of fire.
On the other hand, in the central-southern peninsular regions with a Mediterranean climate, fires mainly break out in the hot, dry summer season. Some Italian regions experience this phenomenon both in the winter and summer season.
Fires can either be caused by nature or human beings.
Natural fires are very rare and are caused by natural and therefore inevitable events:
• Lightening. These can cause fires during thunderstorms without contemporary rainfall. Fires caused by lightening mainly break out in mountainous areas, where trees are good conductors of electrical discharges. These phenomena are very rare in a Mediterranean type climate like ours.
• Volcanic eruptions. Incandescent lava comes into contact with flammable vegetation.
• Spontaneous combustion. This never occurs in a Mediterranean climate.
Fires originated by human beings can be:
• Culpable (or involuntary). These are caused by the irresponsible and careless behavior of man, often in breach of the law and conduct. Not with the purpose of voluntarily incurring damage as such. Causes may be: Farming and forestry activities. Fire is used to burn stubble, destroy vegetation left over from farming and forestry work processes and to renew pastures and uncultivated land. These operations are often done near to woodlands and wasteland, easy prey to fire, especially in the periods of greater risk.
• Abandoned cigarette stubs and matches. Cigarette ashes and stubs abandoned or thrown away along paths, forestry tracks and railway lines can fall onto dry grass or other dry vegetation and spark off a fire, also by effect of the air being moved by vehicles passing by or wind.
• Recreational and tourist activities (barbecues not put out properly), crackers launched, waste burnt in illegal rubbish dumps, badly maintained long-distance electricity lines.
• Willful (voluntary). Fires are kindled voluntarily, with the intention of incurring damage to woodland and environment. The causes:
• To make a profit. The aim is to use the area destroyed by fire to satisfy interests linked with building speculation, poaching or to extend farmland.
• Protests and revenge. The action is born from resentment toward private parties, public authorities or provisions adopted, such as setting up protected areas. In many cases the intention is to damage a tourist area. In other cases a willful behavior may be attributed to basic problems such as pyromania and mythomania.
In the fire classification there are also fires of unknown origin, whenever it is impossible to identify a precise origin.
The predisposing factors of fires are all the aspects that favor the onset and the spread of fire. They are the elements of reference for developing risk prediction indicators:
• Features of vegetation: the presence of species more or less flammable and combustible, water content, the state of maintenance of the forest
• Climatic conditions: the factors that most influence possible fires are wind, humidity and temperature
• humidity, in the form of water vapor, affects the amount of water present in the plant fuel: the lower the water content in the fuels the easier they will burn
• wind removes moisture from the air and leads to an increase of oxygen, move the heat towards new fuel and can carry firebrands, and create new outbreaks of fire. The most significant characteristics of the wind are direction and speed. The direction determines the form that the fire takes in its evolution; the wind speed, instead, will affect the speed of propagation
• the temperature of the fuel and surrounding air are the key factors that determine the way in which the fire is lit and spreads, impacting directly on the time of flammability of plant materials
• Morphology of the soil: the morphology of the terrain affects fires especially with the slope (the velocity of propagation increases in sloping soils) and exposure (southwest slopes are more exposed to the action of the sun and therefore less humid) .
Depending on how it originates, a fire can be:
- underground fire: it slowly burns herbal substances below ground level (the moss, peat, humus undecomposed). The combustion is slow, but it goes out with difficulty
- surface fire: it burns the top layer of vegetation at ground level (grass, leaves and dead branches)..
It is the most common type of fire in our forests and also the most easily controllable. The fire is quick but intense
- crown fire: a fire that advances from top to top of trees and it is the most difficul to control;
- di barriera: l'incendio di chioma si unisce ad un incendio di superficie. E' estremamente intenso e distruttivo.
The damage caused by the fires affect the vegetation, fauna, soil, air and the landscape. The extent of the damage depends on both the behavior and the characteristics of the flame front (speed, feed, height and length of the flame, the depth of the face), both by the properties of the environment affected by the fire.
The damage caused by the passage of the fire can be measured in terms of time and space: the former can occur immediately or in the longer term, the latter may have an impact within the area covered or the surrounding areas.
From a point of view of time, damage can be classified into:
• damage of first order: it occurs at the time of the event, or immediately after the event. Are the direct result of the combustion process (the damage and death of plants, the fuel consumption, the production of smoke and the heating of the soil)
• damage of second order: it occurs in a much longer period of time, from days to months and even decades after the event (erosive phenomena, the smoke dispersion of the and vegetation succession).