13 The concept of risk and risk matrices

Consider a (infrastructure, population, ecosystem, etc.) undergoing impacts (or consequences*) like damage, mortality, disruptions, service interruptions, etc., when exposed to a (heat wave, flood, etc.). The magnitude and extent of the consequences will depend on how extreme the hazard is. Thus, a minor flood, which occurs more frequently, will affect a smaller area with lower flood depths and less significant damage, while a flood of greater magnitude, occurring more rarely, will have more significant consequences. So there is a link between the amplitude, the extreme nature of the hazard, and the scale of the consequences.

The concept of risk makes it possible to balance the of a hazard against the magnitude of the consequences on a system exposed to this hazard. The risk (R) is expressed as the product of the likelihood (L) and the consequences (C): \[ R = L \times C \] Thus, a system exposed to a hazard will be more at risk (R ↗) the more the hazard is probable (L ↗) and the more the consequences are significant (C ↗). The likelihood refers to the probability that a hazard will occur in a given area. It can be qualitative (unlikely, likely, very likely, etc.) or quantitative, in the form of a probability of occurrence. In the latter case, this value can be estimated from a statistical analysis of historical data and expressed as an annual probability of occurrence or as a return period (see fact sheets 3 and 5).

A risk matrix is used to evaluate risks. The likelihood of the hazard and its consequences on a given system are classified into various categories. Table 13.1 shows an example of a risk matrix with five categories for likelihood (from “very unlikely” to “almost certain”) and consequences (from “negligible” to “severe”). The placement of risks into categories remains largely subjective, particularly for consequences that can have several dimensions (material damage, loss of use, mortality, degradation of habitats, etc.).

The risk is calculated by multiplying the likelihood and consequence categories (Table 13.1). So if a “very likely” hazard (category 4) leads to “major” consequences (category 4), the associated risk is 16 (4 x 4) and it is qualified as “major.” According to the classification in Table B.5, the risk can range from “negligible” to “extreme.” The risk therefore increases as we move from the lower left corner to the upper right corner of the risk matrix.

Other classifications of likelihood, consequences and risk are possible. The members of the committee responsible for the adaptation process must agree amongst themselves on the classification to be used. Holding a workshop to reflect on the subject, and consulting with experts (climate science, climate change adaptation, regional planning, civil protection, etc.) should be considered. Although the classifications are subjective, it is essential for them to be understood and accepted by all. However, too many or too few classifications should be avoided.

Table 13.1: Five-level risk matrix integrating likelihood and consequences (adapted from Ouranos and MELCCFP (2024)).

			1	2	3	4	5
			Likelihood
			Very unlikely	Unlikely	Likely	Very likely	Almost certain
Consequence	5	Severe	5	10	15	20	25
	4	Major	4	8	12	16	20
	3	Moderate	3	6	9	12	15
	2	Minor	2	4	6	8	10
	1	Negligible	1	2	3	4	5
Risk level			Negligible R ≤ 2	Minor 2 < R ≤ 4	Moderate 4 < R ≤ 9	Major 9 < R ≤ 16	Extreme 16 < R