Assessing Hazards! Filling the Holes in our Risk Assessment Methodology

Tony Wang & Simran Bassan - Woods

The 2023 Auckland Anniversary flood events and ex-tropical Cyclone Gabrielle in mid-February brought with them record rainfalls, slips and subsidence, which caused extensive damage to the three waters, transport, and infrastructure.These events resulted in significant economic losses and even the loss of life, highlighting a need to better address flood-related risks and minimise the impacts of future flood events. It is now more than ever important for communities to assess the risks they face to inform future planning and build long term resilience. 

In the Auckland Region, the flood hazard classification method provided in Auckland Council Stormwater Flood Modelling Specifications is commonly used to assess flood hazards and understand their significance. Flood vulnerability curves and flood hazard definitions provided in the Australian Rainfall Runoff Guidelines (ARR) further define these flood hazards in relation to their effects on people, vehicles, and structures.

Both the ARR and Auckland Council modelling specifications fall short in their methodology as they do not detail how to assess the impacts of the loss of infrastructure and its effects on the local economy. The available knowledge can be supplemented with the implementation of the RiskScape methodology detailed in NIWA’s technical report ‘RiskScape: Flood fragility methodology’, August 2010 (NIWA, 2010). This methodology provides a way to assess household content damages, economic loss, infrastructural loss and identifies locations where to or where not to build any future infrastructure which may be affected by flooding. The RiskScape methodology sets thresholds and assigns classes with different levels of associated expected damage. With relation to future planning and building flood resilience communities, this information can be used to identify the most effective flood protection measures and to inform the development of any flood emergency response plans where necessary. 

Flood fragility curves can be used to define a damage ratio. Damage ratios can predict expected damage for a building and its contents, the functional downtime for a business post flood event and the inundation depth for stormwater, water supply, and sewage pump stations to determine the loss/downtime of the critical infrastructures. Overall, this method provides insights into the vulnerability of communities to flood events, depicting the potential harm or disturbance they may experience.

Currently, no single tool/guidance stands out as assessing the full extent of the consequences that a flood can have on the community. We believe it is in the best interest of the industry to standardise an approach that can be adopted at a national or regional level. The combination of AC modelling specifications, ARR and RiskScape methodologies are examples of methodologies that can be used to understand the damage caused by major flood events and ultimately inform planning for future development.

This paper explores the methodology to evaluate flood impacts and discusses waysin which flood-related risks can be better represented using various tools and methodologies and validated against the January flood event. Understanding the impacts of these hazards will form the basis of future planning to build long term resilience.