III. CLASSIFICATION OF CONCEPTS AND TERMS

The series of UNDRO studies of current knowledge on Disaster Prevention and Mitigation use the terms natural hazard risk and damage probability, and define vulnerability (or disaster risk), the product of the values of these two terms. (See for example page 4 of Volume 5 on “Land-Use Aspects of Disaster Prevention and Mitigation”). Similar terms are used in a different sense in seismic studies. Thus Mr. Fournier d’Albe of UNESCO in his paper on “Earthquake Prediction and Risk Management*” uses the term risk to denote the possibility or probability of loss, and defines this as the product of seismic hazard, vulnerability and value, vulnerability in this case being a measure of the proportion of the value which may be expected to be lost as the result of a given earthquake. It is clearly desirable to avoid, if possible, such conflicts of nomenclature and to establish a set of terms for use in disaster studies which will be widely understood and accepted.

* Background paper presented to European Space Agency/Council of Europe Seminar on earthquake prediction, Strasbourg, France, 5 to 7 March 1979. Available from UNESCO’s Earth Sciences Division on request.

The meeting proposed therefore that the following terms and definitions be used:

· NATURAL HAZARD meaning the probability of occurrence, within a specific period of time in a given area, of a potentially damaging natural phenomenon.

· VULNERABILITY meaning the degree of loss to a given element at risk or set of such elements resulting from the occurrence of a natural phenomenon of a given magnitude and expressed on a scale from 0 (no damage) to 1 (total loss).

· ELEMENTS AT RISK meaning the population, buildings and civil engineering works, economic activities, public services, utilities and infrastructure, etc... at risk in a given area.

· SPECIFIC RISK meaning the expected degree of loss due to a particular natural phenomenon and as a function of both natural hazard and vulnerability.

· RISK meaning the expected number of lives lost, persons injured, damage to property and disruption of economic activity due to a particular natural phenomenon, and consequently the product of specific risk and elements at risk.

The above definitions include all the terms used in the UNDRO studies and in UNESCO publications but in several cases the terms used do not correspond. The relationship between the three sets of terms is shown in Table I:

Table I

The proposed definitions appear to be close enough to general usage to have a good chance of wide acceptance. Annex III provides supplementary theoretical discussion of the concept of vulnerability and proposals for the practical evaluation of the risk attached to natural hazards.

An additional concept which may prove of value in practical applications is that of RESISTANCE which controls the level of vulnerability. Resistance depends on numerous factors such as land-use patterns, population and development densities, the quality and implementation of design and the ability to arrest the action of destructive forces in their initial stages thereby avoiding the development of chains of destructive events.

The ESTIMATED LEVEL OF RISK should be calculated as a fundamental element of any physical development planning exercise in the following categories:

- post-disaster settlements reconstruction,
- settlement renewal and modernization,
- expansion of existing settlements,
- building of new settlements,
- development and/or restructuring of the national/regional settlements networks and systems.

The definition of an estimated RISK for alternative site selections and alternative development programmes, should be seen as a major tool in planning and decision-making procedures for preventing or mitigating the consequences of the natural phenomena on the one hand, and to limit development and operational costs on the other.

Policy formulation should encompass the concept of LOCALLY ACCEPTABLE RISK. In order to define LOCALLY ACCEPTABLE RISK, alternative policy and planning options should be formulated and examined. Predictably, each alternative solution will present some internal conflicts between locally acceptable levels of risk and socio-economic goals. Nevertheless, the general notion and philosophy of RISK and of ACCEPTABLE RISK should be applied to all physical planning activities in order to ensure a safer and more appropriate process of urban and regional development. To that end, at the very least a simplified methodology and concept of RISK should be elaborated taking into account availability of local data, planning technology and trends in these fields. It is also imperative to define the reasonable minimum requirements for a meaningfull RISK definition exercise. This challenge could be seen as an important element of UNDRO’s work programme.