Gravitational natural risks - Geological and hydro-meteorological

Gravity phenomena take many forms and regularly affect people and property to varying degrees of intensity.

Before beginning this article, it is necessary to define the terms used below.

The term "phenomenon" designates the manifestation of a natural (or sometimes man-made) agent that brings into play the fundamental laws of earth physics (gravity, thermodynamics, hydraulics, geodynamics, etc.). The phenomena covered here are limited to earth movements, torrential phenomena and avalanches.

Hazard" is a more complex concept, which can be defined as a phenomenon (flood, landslide, avalanche, earthquake, etc.) that can cause damage, and which is characterized by its probability of occurrence (or frequency) in a given period (reference period), as well as by its physical characteristics (type and intensity of the phenomenon).

Stakes" refer to the people, goods, activities, resources and assets likely to be affected by a natural phenomenon. The notion of "stake" is therefore independent of that of "hazard". Vulnerability", on the other hand, is the measure of damage of all kinds (human, material, etc.), which depends on the intensity of the hazard. Vulnerability thus introduces a financial and societal notion.

Risk" is a measure of the probability and magnitude of damage caused by a natural or man-made event affecting a stake.

Risk therefore results from the level of the hazard, and the nature and vulnerability of the stakes exposed. Thus, a hazard affecting an undeveloped area presents no risk. On the other hand, a low-intensity, low-probability hazard impacting a highly vulnerable area can generate a high level of risk.

There are two main ways to reduce the risk:

• by acting on the hazard (e.g. by limiting the probability of the phenomenon being triggered: active protection strategy, or by limiting propagation or effects: passive protection strategy);

• by acting on the issues at stake and reducing their vulnerability (e.g. by regulating development in exposed areas: prevention strategy through regulatory mapping).

Vulnerability can also be reduced by educating people about risks. This takes the form of training for schoolchildren (awareness of major risks is now part of primary and secondary school curricula), and information for the general public (information on the Internet, communications by local authorities, information for buyers and tenants, etc.).

In the following paragraphs, we describe gravity-related phenomena. For each of them, we will present the methods used to analyze the hazard and the risk,...