3. Important concepts relating to natural hazards

3.1 Natural processes or hazards

Natural phenomena (or hazards) have different physical characteristics, relating to notions of energy and duration, which can be grouped together under the general term of intensity. In a given geographical area, depending on the intensity considered, probabilities of occurrence related to reference durations can be associated with it. A notion of return period of the hazard, with this same intensity, can be defined; or, fixing a particular period, we can define the probabilities of occurrence of the hazard considered for different intensities.

Thus, in the field of lowland flooding, for a particular watercourse and in a well-defined geographical area, the hydrograph or other intensity parameters (water heights, flow velocities)...