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In general we talk of natural risk, and within this category, of geological risk, which isn’t strictly correct because when we refer to a natural phenomenon we should talk of a natural or geological hazard, and when this phenomenon affects humans and their activities, we should talk of risk. Following we will explain these concepts and others in more depth in order to clarify the basic concepts of geological risk.
A natural hazard is a physical event and/or phenomenon, potentially harmful, which could cause death or injury, material damage, an interruption to social and economic activity or environmental deterioration. It can be of geological, hydro-meteorological or biological origin. All natural hazards are characterised by their location or scope, magnitude or intensity, and frequency or probability.
Natural hazards can also be caused anthropically. Human activity can lead to the occurrence of a phenomenon or an increase in its destructive ability.
In other words, all natural phenomena have an inherent danger, which we define as the probability of occurrence of a potentially destructive phenomenon in a specific period of time and in a certain area of terrain. The likelihood of it happening is related to the “frequency” of the phenomenon, and its destructive potential to the “magnitude” of it.
Not all geographic zones are under threat from a certain phenomenon. We understand susceptibility to be the possibility of a natural phenomenon occurring in a geographic area, or the area being affected by it. Susceptible areas include those where a phenomenon has been triggered, the start zone, and those that could be affected by its path and runout zone.
As previously mentioned, when a natural phenomenon interacts with human activity, we are talking of risk. Risk is a social and economic concept that estimates the probability of loss of human life and material losses as a result of a natural phenomenon. Therefore, risk is defined as the combination of the threat of a certain phenomenon, the vulnerability of the elements exposed, and their exposure, which is expressed using the following ratio:
Risk = Threat x Vulnerability x Exposure
For risk to exist the three components must coincide. If there is no threat, or no exposure or no vulnerability, there is no risk, therefore it will be zero.
Vulnerability is defined as the level of expected damage to a structure in the event of it being subjected to the action of a phenomenon. Vulnerability depends on each structure and is independent of the threat of the area. This means that a structure could be vulnerable but not at risk because it is located in an area that is risk-free.
Exposure is the number of people, goods, services and processes that are exposed to the action of a threat. It is expressed quantitatively by the number of elements potentially affected.
Geological hazards
A slide is a mass movement of soil or rock, down a slope, caused by gravity. It is often used as a generic term to refer to a variety of mechanisms such as landslides, flows, slips, lateral expansions and complex movements.
Puigcercós, phenomenon that occurred in 1881
A fall is the detachment of rock or soil, mainly due to gravity, because of certain discontinuities. These phenomena can be found in both natural and man-made slopes.
Montserrat, 4 November 2008
A torrential flow is characterised by the rapid movement of a chaotic amount of large detrital matter after a period of intense and/or persistent rain. The movement can begin on the slope or channel and is canalised by a stream or gully, travelling large distances. They normally stop creating debris cones.
Port-Ainé, 12 September 2008
Collapses are vertical movements of rocky masses, of different sizes, that fall in comparison to their previous level or that of the surrounding masses. Collapses are often connected to the existence or formation of underground caves. In general, collapses can be divided into two types: sinkholes and subsidence. A sinkhole is a very fast collapse. Subsidence is a progressive local or regional collapse of the ground’s surface that is more or less regular, with little or no lateral movement; it is a slow movement.
Joncarets (Súria), 4 December 2000
An avalanche is a mass of loosened snow that suddenly slides down a mountain, often sweeping rocks, rubble and mud with it.
Senet, 20 April 2008
