Following the impacts of Cyclone Gabrielle in February 2023, Council is investigating flood resilience improvements across Tairāwhiti.
This project is to determine the viability of upgrading the existing stopbanks that service the Te Karaka township to the same level of service as the downstream Waipaoa stopbank flood control scheme upgrades (ie to convey a 5625 cumec flood event).
The community expectation is to raise and widen the existing 4.2km Te Karaka flood control scheme which also consists of 1.8km of rock protection. The scope of the project is primarily earthworks, there's also culvert renewals, road realignment, flood mitigation measures, rock armouring, stream realignment, floodgate modifications and other associated works related to the upgrade of the scheme to a higher level of service / higher level of flood protection and flood resilience than existed pre-Cyclone Gabrielle.
The Te Karaka township has a population of around 525. The area being investigated largely focuses on the township but extends out to include a number of small pockets that experienced flooding.
Flood modelling will extend 11.5km to include the Waikohu River as a 10.5km branch downstream by 7km to McMillian Road.
Te Karaka
This shows the extent of flooding in Cyclone Gabrielle. In an area of 187 hectares, 60 dwellings are categorised at FOSAL Category 2.
The existing flood control scheme doesn’t account for climate change and is currently susceptible to significant flooding and therefore a major risk to people and property living and working in the Te Karaka area.
What's happening now?
We're at Stage 1
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 landowners and iwi to support a resource consent application.
The calibrated hydraulic modelling has been completed, with optioneering work underway assessing several improvement options and to understand implications on flood levels with each option.
When the stopbanks are raised there's potential for increased flooding upstream and/or downstream of the township which may raise objections due to increase flooding risks especially if the effects are determined to be 'more than minor' under the Resource Management Act.
Implement necessary flood resilience mitigation measures, steps may include:
- Upgrade existing stopbanks surrounding Te Karaka (4.2km).
- Renew existing pipe/culverts under the stopbank including floodgates and associated assets.
- Raise the state highway at 2 locations that are known scheme low points.
A 12month period for consultation and consenting is considered realistic given the current stage of the project and the existing level of uncertainty.
This timeframe may be ambitious given that the modelling and optioneering work has not been finalised and effects of improvement options fully understood and assessed.
When the stopbanks are raised there's potential for additional flooding upstream and/or downstream of the township which may raise objections due to increase flooding risks especially if the effects are determined to be more than minor. As a result, there may be land purchase/s required which would likely incur delays in the critical path.
Optioneering work is currently assessing several improvement options to understand implications and effects.