Vulnerability evaluation

Vulnerability is the propensity of things to be damaged by a hazard. People's lives and health are at risk directly from the destructive effects of the hazard. Their incomes and livelihood are at risk because of the destruction of the buildings, crops, livestock or equipment which these depend on. Each type of hazard puts a somewhat different set of elements at risk. Most of disaster mitigation work is focused on reducing vulnerability, and in order to act to reduce vulnerability, development planners need an understanding of which elements are most at risk from the principal hazards which have been identified. These are discussed in more detail in the module on Disaster Mitigation and are summarized in table 4.

It is important for development planners to make some effort to quantify the tangible aspects of vulnerability and loss to assist mitigation and preparedness planning. Some methods for doing this are discussed below. But, as explained earlier, the 'intangible' aspects of vulnerability will often be as important as the quantifiable aspects and must not be neglected. Local experience is a good guide to what is vulnerable in a society, and the list of potentially vulnerable elements should be supplemented by a study of written reports and the knowledge (often never recorded) of those who lived through previous disasters.

Table 4
Principal elements vulnerable to specific hazards

Quantifying vulnerability

VULNERABILITY

Vulnerability is defined as the degree of loss to a given element at risk (or set of elements) resulting from a given hazard at a given severity level. (The distinction between this definition and that of risk is important to note. Risk combines the expected losses from all levels of hazard severity, taking account also of their occurrence probability.) The vulnerability of an element is usually expressed as a percentage loss (or as a value between 0 to 1) for a given hazard severity level. The measure of loss used depends on the element at risk, and accordingly may be measured as a ratio of the numbers of killed or injured to the total population, as a repair cost or as the degree of physical damage defined on an appropriate scale. In a large number of elements, like building stock, it may be defined in terms of the proportion of buildings experiencing some particular level of damage.

The vulnerability of a set of buildings to a hurricane of 130 km/hr may be defined as:

"20% of buildings suffering heavy damage or worse, experiencing 130 km/hr winds"

or

"average repair cost ratio of 5%, experiencing 130 km/hr winds"

Vulnerability of human populations may be expressed in terms of mortality or morbidity:

"5% killed and 20% injured in an earthquake of intensity VIII"

Q. What is the difference between risk and vulnerability?

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Table 5
This is an example of a damage probability matrix for landslides

Failure probability for slope of low stability, summer conditions, earthquake shaking of various intensities.¹⁷

As the severity of the hazard increases, the level of damage that the element is likely to suffer will increase. The 20% of buildings suffering heavy damage in a 130 km/hr winds is likely to increase to 50% in a 160 km/hr wind.

For a full definition of vulnerability, the expected damage level at every level of severity of hazard would need to be defined. Vulnerability for a range of events of different severities can be given by means of a damage probability matrix.

Where data permits, a continuous vulnerability function mapping values of damage to values of hazard severity can be defined graphically or mathematically as an equation.

An example of this is the vulnerability relationship for a class of buildings against increasing severities of ground shaking in an earthquake, compiled from a collection of damage statistics (see figure 10). In figure 11, five different vulnerability functions are plotted for unreinforced masonry buildings, one for each of the 5 damage levels ranging from D1 (slight damage) to D5 (total collapse). The D1 curve shows the percentage of buildings in a large sample which would be expected to experience damage level D1 or above at any level of ground shaking indicated on the horizontal scale.