How is risk determined?

There are three essential components in the determination of risk, each of which should be separately quantified:

a) the hazard occurrence probability: the likelihood of experiencing any natural or technological hazard at a location or in a region

b) the elements at risk: identifying and making an inventory of people or buildings or other elements which would be affected by the hazard if it occurred, and where required, estimating their economic value

c) the vulnerability of the elements at risk: how damaged the buildings or injured the people or other elements would be if they experienced some level of hazard.

Figure

There is a variety of methods of presenting the above information to illustrate the data describing risk. These can often be represented on a map. This is an essential tool in evaluating development projects because you can see if a project site is located in an area of high risk.

An example of mapping is the Potential Loss Study. This consists of mapping the effect of expected hazard occurrence probability across a region or country. It shows the location of communities likely to suffer heavy losses. The effect of the hazard of each area is calculated for each of the communities within those areas to identify the “Communities Most at Risk”. This shows, for example, which towns or villages are likely to suffer highest losses, which should be priorities for loss-reduction programs, and which are likely to need most aid or rescue assistance in the event of a major disaster.

The following is an example of potential loss mapping. It presents risk as the levels of losses that would occur if a certain level of hazard were to occur at all the locations simultaneously. In this case the type of loss plotted (Map 4) is urban earthquake casualties in Turkey. Casualties are defined as those people whose houses are liable to be totally destroyed by the largest expected earthquake - a measure used because it has been found in Turkey to correlate closely with the numbers of killed and injured. The potential loss plotted in each location is derived from three other types of geographically varying data, which are shown in Maps 1,2 and 3. (See figure. 8.1)

Figure 8.1 Potential loss study

Map 1 shows the earthquake hazard in terms of the maximum intensity of earthquake which might possible occur there.

1 - HAZARD

Map 2 shows the elements at risk - in this case the total size of the urban population. Larger towns and dries are plotted individually, and are identified by circles whose area represents the population. The population in the smaller towns of 2,000 to 25,000 population is shown in the form of a population density. Other elements at risk could be mapped in a similar way.

2 - ELEMENTS AT RISK (population)

Map 3 shows one aspect of the vulnerability of those elements at risk. The casualties are caused by the collapse of buildings. The vulnerability of a building depends primarily on the type of construction. A useful approximate classification of the building types in Turkey divides them into just three types: rubble and adobe walls, brick and timber walls, and reinforced concrete frame. An estimate has been made about the expected proportions of buildings that will collapse.

3 - VULNERABILITY

Map 4 shows the analysis of the three preceding maps for each location. This is derived by estimating the numbers of people living in each building type, (from Maps 2 and 3) and then estimating the potential proportion of collapsed buildings of each type if the largest earthquake were to occur there. The total potential casualties are obtained by adding those from all three building types.

4 - CASUALTY RISK (potential loss of life)