AUCKLAND FLOODS 2023: ASSESSING RISK TO LIFE FROM FLOODING AT THE PROPERTY LEVEL

F. Macdonald (Auckland Council Healthy Waters), N. Brown (Auckland Council Healthy Waters)

R. Donnelly (Richard Donnelly Ltd)

ABSTRACT 

The extreme weather events of early 2023 caused widespread flooding in the Auckland region. In response, Auckland Council Healthy Waters (ACHW) initiated a rapidly evolving programme of work to quantify and understand the magnitude of the events and their impacts on the community, support and shape the wider recovery effort, and to develop the frameworks and tools that would be needed to enable the assessment of affected properties for potential buyout or risk mitigation works.

For the purposes of central government and Auckland Council’s response to the 2023 severe weather events, it was determined that flood-affected properties would be eligible for consideration for buyout or subsidised risk mitigation where there is a high risk to life to vulnerable people in an existing 1% AEP flood event. However, at the time of the 2023 floods, Healthy Waters did not have a defined framework for assessing risk to life from flooding as, historically, flood risk reduction decisions had been based on mitigating flooding of habitable floors.

This paper describes the Property-level Flood Risk Assessment Framework developed by Auckland Council Healthy Waters to enable risk from flooding on individual residential properties in the Auckland region to be assessed in a consistent, transparent, and objective manner in support of the property categorisation process.

A literature review revealed that existing approaches to flood risk assessment in New Zealand and overseas tend to assume widespread fluvial or coastal flooding which results in a uniform level of flood hazard across a wide area. In contrast, Auckland’s small catchments and steep topography tend to produce pluvial flooding that is highly localised and flashy. This has material implications for the risk assessment methodology. Key challenges encountered in the development of the framework included how to account for the spatial variability of flood hazard at the property level and how to assess flood hazard inside and outside the dwelling, how to account for the decision making of people who might be on a property at the time of flooding, how to account for those who are most vulnerable to physical exposure to floodwaters, and how to integrate flood hazard assessments at multiple locations on a property into a single rating that could represent risk to life to vulnerable people in a 1% AEP event. The paper will provide an overview of these and other challenges encountered in the development and application of the Property-level Risk Assessment Framework and outline the steps that ACHW is taking to adapt the framework for more general use in a planning context.