SENAT
Report n° 117 (2007-2008) by M. Roland COURTEAU, Senator (for the parliament office for the evaluation of scientific and technological choices) - Appendix to the minutes of the 7 December 2007 session
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B. THE TSUNAMI RISK
Tsunami hazard and tsunami risk are different. Tsunami risk represents a potential danger which, if and when it occurs, can have catastrophic results.
1. The risk factors
In environmental terms, risk can be defined as "the possibility that an event occurs which is likely to disrupt the natural equilibrium". Risk results from the combination of three factors: 1) a hazard, 2) stakes and 3) the stakes' vulnerability to the hazard.
a) The hazard
The "hazard" represents the source of danger. To evaluate the risk, it is necessary to determine the hazard's probability of occurrence, as well as its intensity and frequency. However, the risk is not limited to the hazard alone. For instance, a tsunami generating a 3-metre-high wave on a desert island represents a minor risk; however, if the same wave were to strike the beaches at Antibes (French Riviera) on a national holiday in the middle of July, the results would be dramatic. Therefore, the notion of risk is linked to the notion of what is at stake.
b) The stakes
The "stakes" are those persons, property, facilities and/or environments threatened by the hazard and likely to suffer damage should it occur. These stakes can be divided into five separate categories:
- the human stakes;
- the economic and financial stakes which concern commercial, craft, industrial, agricultural and tourist activities;
- the social stakes, which include everything that affects social cohesion and the functioning of society;
- the environmental stakes, which cover the possible damage to ecosystems and biodiversity;
- the heritage stakes, which concern historical and cultural monuments and a region's "public image".
These stakes can suffer varying degrees of damage, depending on the hazard's intensity:
- physical injuries to persons;
- structural damage to the urban fabric, to goods and property and to networks;
- functional damage disrupting day-to-day activities (cut telephone and gas lines, power outages, disrupted modern communication networks such as the Internet);
- environmental damage to the ecosystem;
- damage to the historical and cultural heritage.
c) Vulnerability
The hazard's capacity to damage the stakes varies according to their vulnerability. Faced with a tsunami, a few simple actions can save lives: a strong shaking and a retreating sea are forerunners of a tsunami and should incite people to leave the coast and seek refuge in buildings above the third floor.
The Sumatra tsunami of 26 December 2004 is a case in point: many lives would have been saved if the concerned populations had had a basic understanding of this hazard. We could have spared ourselves these horrible images of the receding sea, the large waves already forming on the horizon and numerous tourists in the process of gathering shells or watching the approach of the oncoming waves. In this case, the population was made all the more vulnerable for its not having been properly informed.
Being vulnerable means being physically exposed to a hazard and presenting a certain fragility to the catastrophe that could occur. Vulnerability can vary over time, since its depends principally on human activity. Today, the world's population is particularly vulnerable to tsunamis due to densely inhabited coastlines.
Indeed, the transportation revolution and a globalized economy have greatly increased international trade flows and pushed industry to the coast, resulting in increased harbour traffic and the creation of vast industrial harbour areas. Likewise, tourist and leisure-activity development is concentrated on the coasts. The rapid development of these activities has led to the massive urbanization of the concerned coastlines.
The following figures allow for a quantification of this "coastalizing" trend.
Today, nearly half of Europe's population lives within 50 kilometres of the continent's 70,000 kilometres of coastline (nearly 40% of the world's population lives within 100 kilometres of the coast). Average population density in France is slightly higher than 100 inhabitants per square kilometre; however, this figure rises to over 250 inhabitants for the coastal districts and is over 600 for the Provence-Alpes-Côte-d'Azur region.
In addition, the mountainous relief of volcanic islands - as much in the Pacific as in the Indian Ocean and the Caribbean - concentrates these islands' populations along the coast.
One must not underestimate the subjective component of vulnerability linked to how the threat is perceived. Risk only exists when the social group or individual considers it or himself as being "fragile" faced with a given natural phenomenon. Different groups react differently to the same event: while some do not recognize the existence of any danger, others do but accept to live with it, while still others refuse to accept it.
In the developed world, our notion of risk has evolved from a fatalistic vision of risk as something divinely determined (and so largely unaffected by human-based protective measures) to the notion of managed risk (and, consequently, a right to protection).
One might think that in the case of natural risks, the question of "responsibility" is irrelevant. However, recent changes in our understanding of risk and responsibility demonstrate that, in fact, this is not the case and increasingly society is looking to protect itself against these "natural" risks. The creation of legal and institutional structures, such as risk-prevention agencies and policies (the establishment of earthquake-construction standards, for example) illustrates the desire of governments to both protect their citizens and limit their liability in the event of a catastrophe.