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Ministry for the Environment

In 2010, the Ministry for the Environment produced the report Preparing for future flooding: a guide for local government in New Zealand. This guidance is based on a risk-based approach to flooding, using similar principles to those outlined in this toolbox. In particular, the guide provides an overview of the risk assessment process, including:

  • Rating the level of consequences from a flood (from significant to catastrophic);
  • Rating the likelihood of a specific flood event occurring (rare to almost certain); and
  • Assigning a risk level, given both the consequences and likelihood (low to extreme).

The above approach reflects the risk-based approach presented in this toolbox, in that, the consequences are determined before the likelihood (with the same quantitative descriptors). A risk level is then assigned. Levels of risk are categorised from low to extreme, which for this toolbox is classified on whether a level of risk is acceptable, tolerable or intolerable. In the examples provided in the MfE guidance, the levels of risk (low to extreme) could be converted to acceptable, tolerable and intolerable levels of risk.

An example of a consequence table is presented, which includes social (public safety and community disruption), cultural, economic (local economy and growth; lifelines), and environment. This table has a similar format to the consequence table in this toolbox, with similar consequence categories and quantitative descriptions for public safety/health and safety. The consequence table in this toolbox goes further than the MfE example (Table 1 below) by including quantitative descriptors that allow for a measure of consequence.

An example of a likelihood table (Table 2 below) is provided for a flood occurring within a given time horizon. The results are shown as a percentage, which can then be converted into a likelihood rating (Table 3 below). The advantage of the table is that by providing percentages for a number of flood recurrence intervals across a range of time frames, the final likelihood rating can be more accurately specified.

A risk level is then assigned using a matrix (Table 4 below), using the qualitative descriptors low, moderate, high, extreme.

While this MfE example differs slightly from that presented in this toolbox, it does show a very similar framework and process.

Table 1: An example of consequence ratings
Table 2: Likelihood of the flood occurring within a given time horizon
Table 3: Flood risk likelihood ratings
Table 4: AA risk assignment matrix for setting the level of risk, based on likelihood and consequence