Rough terrain movement of slates around Angers (49)

In the strict sense, hazard is the probability of the occurrence at a given point of time of a given event of a given intensity. For a given period, the hazard should be presented as a decreasing continuous curve, indicating a probability of occurrence for each intensity of a given phenomenon. In practice, it is very often impossible to be completely deterministic and to produce this curve at every point in the area to be studied. Intensity defines the extent of the phenomenon, its severity, the influence it can have on an anthropogenic structure. For fontis it is the diameter of the collapsed area that defines the intensity. For settlement, it is rather the angle of slope of the structures tilted to the surface that is considered. The notion of probability of occurrence, which, for its part, reflects the sensitivity of a site to be affected by any of the phenomena analysed, is generally difficult to understand. Whatever the nature of the events feared, the complexity of the mechanisms, the heterogeneous nature of the natural environment, the very partial nature of the information available and the fact that many disorders, sequelae or nuisances, are not repetitive explain that it is generally impossible to reason on the basis of quantitative probabilities (x % risk of developing a disorder). Preference will therefore be given to a qualitative classification characterising a predisposition of the site to be subjected to certain types of disorder or nuisance. It is therefore this notion of predisposition (to the detriment of the probability of occurrence) that will be retained in the remainder of the document. The assessment of this predisposition depends on the combination of a number of factors favourable or unfavourable to initiation and to the development of the anticipated mechanisms whose main known is the supposed depth of the residual voids. It should be noted that polygons defining a hazard class may therefore be the combination of the contribution of several different types of entities.