Flood Hazard in Aotearoa New Zealand Under Current and Future Climates

Abstract

Flooding is one of the costliest hazards facing Aotearoa New Zealand (ANZ). We present nationally consistent flood maps organised into four datasets corresponding to a range of current and future climate scenarios in Aotearoa New Zealand, along with the methodology used to create them. This was developed through the Endeavour project Mā te haumaru ō te wai: flood resilience for Aotearoa New Zealand (https://niwa.co.nz/hazards/ma-te-haumaru-o-nga-puna-wai-o-rakaihautu-ka-ora-mo-ake-tonu), funded by the Ministry of Business, Innovation & Employment (MBIE). These flood maps were generated by a semi-automatic workflow linking a chain of models. For each catchment, rainfall corresponding to a 1% AEP (Annual Exceedance Probability) design storm event was routed downstream using hydrological modelling in the upper catchment and injected into a hydrodynamic model to produce fluvial-pluvial inundations maps. Tide was imposed on open sea boundaries and levees were enforced. An adaptable grid down to 4 m resolution was used and hazard layers of maximum water depth, maximum water elevation and maximum (depth times velocity) were generated. Flood hazard was first generated for actual climate conditions (or one degree warmer than pre-industrial global climate conditions). Using relations between global temperature increase and local extreme rainfall intensity, future climate flood maps were generated for scenarios corresponding to an increase of 1°C, 2°C and 3°C compared to actual current climate conditions (i.e., two, three and four degrees warmer than pre-industrial global temperature). These national maps can be used to reveal actual and future flood hazards for the country, especially in rural areas where flood hazard has never been assessed. They can also illustrate how flood-prone areas may evolve with climate change. They present a first consistent national flood assessment needed to inform policy. These maps were also used for a first consistent national flood risk assessment and are accessible on the Earth Sciences New Zealand (ESNZ) datahub under an attribution non-commercial licence.