Methods to determine probability, consequences and risk 1

Risk can be calculated and expressed in different ways. Two groups of methods are described in a simplified way to give you an idea of how risk analysis can be done.

Qualitative approach

Different experts, both scientists and local professionals and citizens, can make an assessment of how probable a certain event is and also what the consequences would roughly be.

A group of experts and stakeholders together, can then determine the value that is given to certain combinations of probability and consequences.

A basic example is given below.

To attach a certain value to a risk a number of situations are discussed. For example:

1. when this area floods the cellars will hold water and in a small number of houses you will have 10 cm of water.
2. when this area floods the water level will be up to 50 centimetre. Most houses will be flooded and the power supply will fail.
3. when this area floods the water will rise up to 2 meters high and some people may drown. There is extensive economic damage and the nature area will not recover within 10 years from the flood.

These situations are sufficiently different in their description of consequences to discuss the seriousness of them. Now although you would prefer than none of these things would even happen you know you can’t prevent everything.

Now for the probability side you take different probabilities of occurrences. This is often done in “return periods”, because many people find it easier to understand. See the table for an example. Note that a return period is a statistical period: once in a lifetime doesn’t literally mean you will one day experience it, nor that once you have experienced it, you will never experience it again.

The next question is how serious the situations, combined with the probability, are thought to be. You can grade these situations from 1 to 10; 10 is the worst. Determining the worst and the least bad situation is not hard. Grading the situations in between is harder. Not every group will give the same grades but you will see that if you do it in different group more or less the same ranking of events will come out of it.

Then you can discuss what risk you can live with. You can rank the situations from bad to worst. Finally you can discuss what risks are unacceptable and have to be diminished. And also how to do this and how much money the people involved are willing to spend to lower the risk.

This method you can apply in class for your own area, with real or theoretical flood situations.