Following the impacts of Cyclone Gabrielle in February 2023, Council is investigating flood resilience improvements across Tairāwhiti.
Within the Te Ārai catchment the city's main water pipeline was damaged causing supply interruptions for 20 days. To date approx $14.1m has been spent repairing watermain bridges as well as $2.25m removing woody debris. There are 10 FOSAL Category 2 dwellings within the catchment that will be lifted if viable.
The Te Ārai catchment area faces an increased risk of flooding resulting from the backwater effects of sea level rise and climate change. To address this, Council is investigating flood resilience improvements in the Te Ārai catchment.
This project focuses on identifying and implementing cost effective solutions to reduce the likelihood of repeat damage from similar weather events in this community.
Te Ārai catchment
Manutuke
The catchment covers an area of 3,131 hectares, with around 300 dwellings at risk of flooding.
This project aims to safeguard lives, protect property, community facilities and evacuation routes.
What's happening now?
We're at Stage 1 and 50% complete.
The initial investigations phase - completing hydraulic models, developing flood hazard maps and a cost/benefit analysis for flood mitigation options. It also includes consultation with the community, iwi and infrastructure owners over the mitigation options. Then the most cost-effective option can then be progressed to consent, detailed design and construction.
Potential mitigation measures include stream clearing/widening, additional attenuation, improved evacuation routes, protection of critical infrastructure and the identification, enhancement and protection of secondary flow paths to minimise the areas prone to flooding. The installation of new stopbanks is unlikely.
Once preferred solutions have been identified, these will be presented for second stage approval. The preferred solution will consider climate change, changes in the river and interventions to mitigate the risk of flooding within the catchment.
Option analysis
A hydraulic model of the Te Ārai catchment is requirement to enable options assessments. The model needs to investigate the backwater effect from the Waipaoa River with recently increased stopbank flood heights.
Once complete, the model will enable assessment of mitigation options and will identify any secondary flows paths that could be enhanced and protected.
Hazard mapping (both current and future) will allow comparison to assess insurability and any need for future land use constraints.
Preferred objectives
The preferred option/s will be confirmed once Stage 1 works are completed. Identification, enhancement and legal protection (preferably with easements) of secondary flows paths, planning controls to prevent such paths from being compromised and community education are likely to be the preferred options.
This is categorised as a complex project - cost benefit analysis will be done prior to Stage 2 approval and before any decision to proceed. Any works would also consider protecting the state highway as well as land.
Programme
The proposed programme for the resilience improvement project - it may change once investigations are complete.
