Types of flooding hazard

AEP and ARI

You might see references to an annual exceedance probability (AEP) or an average recurrence interval (ARI). They are different ways to describe the same flood event – but what’s the difference between them?

An AEP shows the probability of a certain-sized flood occurring in a single year.

An ARI is the average time between floods of a certain size (also known as a return period).

For example, a 1% AEP flood has a one per cent, or one-in-100, chance of occurring in any year. While a 100-year ARI flood might happen once every 100 years on average, that same flood still has a one-in-100 chance of occurring in any given year.

A good way to picture an AEP is by rolling a dice. If you roll and get a six, there was a one-in-six chance that would happen. If you pick up the dice and roll it again, the chance of you rolling another six is still one-in-six.

Large, infrequent floods have a low AEP, and smaller, more frequent floods have a higher AEP.

We use digital modelling software to predict flooding hazard, creating what we call flood hazard models. NIWA's High Intensity Rainfall Design System (HIRDS) provides information about the predicted high-intensity rainfall, in the form of tables that input into the models. They provide information based on historical rainfall, predicting rainfall for different AEP events as well as for a range of climate change adjusted scenarios.

Current rainfall flood models are useful to understand current risk, while climate change adjusted rainfall models inform future development and design of infrastructure that is intended to last for many years, and which is likely to be affected by future climate change conditions.

For development and network design, NPDC is currently recommending the High Baseline Emissions Scenario RCP8.5 (2081-2100) climate change adjustment. This means that we have used the high-risk scenario, that assumes greenhouse gas concentrations continue to increase at the current or accelerated rate.

We have set two operational targets for our stormwater system.

Level of service (LOS)

Primary flow path (pipes and open channels) are provided to convey stormwater to an appropriate discharge location using gravity flow  In New Plymouth District, LOS is 20 per cent for residential land and 10 per cent AEP for commercial land. (These are also known as one-in-five year ARI and one-in-10 year ARI, respectively).

Level of protection (LOP)

This relates to the severity of a storm where the runoff can’t be conveyed by the flow capacity of the primary network (pipes) and secondary overland flow paths (the route that stormwater takes when the primary network’s capacity is exceeded or blocked).

In New Plymouth District, the existing level of protection standard is one per cent AEP event, or one-in-100 ARI event. In this type of event, the primary network would be overwhelmed and OLFP must be identified and protected to avoid flooding of habitable buildings.

(‘Habitable floors’ refers to the main living or working areas of buildings. It doesn’t include detached garages, sheds, gazebos, etc.)

When we refer to a property at LOP risk, we mean that there is potential for buildings’ habitable floors to be affected by flooding during a storm event up to and including a one per cent AEP event.