Estimate of Indice of Risk of Forests Fires from a Data Model of the S.I.G Applied to a Coastal Forest of Bainem (Algeria)

Aichouche Belhadj-Aissa, Mostefa Belhadj-Aissa , Youcef Smara, Boualem Sansal

Houari Boumediene University of Sciences and Technology, Faculty of Electronics and Computer Science - Image Processing and Radiation Laboratory, Algiers (DZ)

In recent years, Remote Sensing and Geographical Information System (GIS) technologies have been the object of considerable interest to all bodies concerned with space and in particular emergency services and disaster management in Algeria, in collaboration with all other bodies responsible for prevention and management of all major risks in Algeria.
Among these risks we can mention the most preoccupying as being the forest fires. Every year, about 30 000 hectares of forests are destroyed by fires in Algeria, fires also take a heavy toll in lives and property and our major preoccupation is to reduce forest fires in the country, thanks to inventory, prevention, management and follow-up.
In the last few years, the data acquired by Earth observation satellites associated with different types of linked information integrated in a Geographical Information System has proved to constitute a viable option for the follow-up of disasters, the identification of areas at risk, and the mapping out of the extent of fire.
The goal of our work is to set up of a GIS integrating remote sensing data for the prevention and management of the fires.
The methodology adopted is as follows:
I) to process and to structure data, to determine and to establish indicators and parameters of the forest fires:
II) to present to decision makers of the sectors concerned, in real time on screen:

In the developed methodology , we use the natural elements of the space of the forest of bainem located near of the town of Algiers on the Mediterranean sea. The figure 1 presents environmental and topographic (relative relief ( 300 m ) ) characteristics of the area of study.
The principal factor of degradation of the Algerian forest is that the fire finds a physical and natural environment favorable to its blossoming and its propagation. The structure and the component of the vegetable formations are also many factors facilitating fires of forests.

Figure 1 - Localisation of area of study

For the design of the data model and the implementation of our methodology, we propose a multilevel diagram which is summarised in the figure 2.

Figure 2 - Block diagram for the prevention and the management of forests fires by GIS

These levels lead us to distinguish various elements of the risk. Each element will be modelled by selecting the necessary components and parameters for the study and a mode of representation. The interpretation and the analysis of these elements starting from the overlapping of the various layers of data of the SIG allow to estimate the indice of the risk, the coding of this indice and the mapping of the risk of forest fire of Bainem. In the global diagram, the definition of scenario and the preparation of plan of forecast and intervention require, in addition to the SIG, the contribution of the institutions concerned with these phenomena. This part is not treated in this article.

Risk modelling
To estimate the risk, it is necessary to model each component of the risk. This stage consists on the selection of the parameters specific to each component (combustibility, slope, etc). The parameters are the environmental and anthropic factors which influence the blossoming, the propagation and the intensity of fire. The principal ones are: vegetation (combustibility, biomass), topography (slope, exposure), climate (moisture of the air, direction of the winds, pluviometry, temperature), human activities (land occupation, not of water...) and history of the area (burned surface, not of blossoming).
These parameters are correlated and their combination influences or causes the blossoming of the fire. Thus we retained, for the modelling, the most relevant elements, the vegetable cover whose descriptive parameters intervene in the calculation of the indice of combustibility IC, the relief represented by the topographic parameters which intervene in the calculation of the topomorphologic indice IM and the anthropic parameter IA. For our application, we used the model of indice of risk IR [JAP. 1998] which we adapted it to the type of risk in Algeria. Its expression is given by:  IR = 5IC+IM+2IA
The characterisation of this indice is based on the space variability of the fire risk from which the determination results from the physical parameters intervening in the model chosen for our application.

Model of the indice of combustibility (IC)
The principal criteria which intervene in a forest fire are the spatial structure of the combustible (biomass, horizontal covering and vertical stratification) and the nature of the dominant species. The IC indice is expressed by the following relation:  IC = 39+0.23BV(E-7.18)
it is an empirical model where BV is calculated from the bio combustible volume, E represents the average combustibility of each type of specie. For our application, we determined these parameters from investigations and observations as well as measurements on the site of Bainem. Then, we added information by spectral analysis of the behaviour of the species from the images of the Landsat satellites Landsat 5 and Landsat 7+ of the topomorphologic area.

Model of the indice (IM)
Three topographic parameters intervene in this model : the slope, the exposure and the altitude which are codified by the topomorphologic characteristics (plains, piémonts and mountains). All these parameters are deduced from the digital terrain model (D.T.M) of the area. This indice is expressed by the following relation: IM = 3p+m*e
Where p represents the slope, m the parameter of topomorphology and e the exposure. We generated the D.T.M from an interferometric study using a couple of images acquired fron the Tandem of satellites ERS1 and ERS2. We refined the D.T.M by another D.T.M using the digitalization of the level lines to the altimetric resolution of 5 m.

Model of the anthropic indice (IA)
The presence of human beings and habitations near the forests constitutes an important parameter which determines the degree of vulnerability of the medium. The anthropic parameter is the principal term in the model of the IA. The index anthropic is expressed by the linear combination of two parameters: IA = IV+D
Where: IV: index of vicinity and D: density. We used, for our site of study, the panchromatic images of SPOT (10 m) and IRS (5,8 m) satellites, the aerial photographs at the 1/40.000 scale and the maps of the city of Algiers at the 1/7500 scale for the realisation of the layers representing the infrastructures, the urban areas as well as the linear structures. We proposed a procedure of estimate of the indice of risk using ENVI and MAPINFO softwares.

References: