Flood risks in the lower Ōtākaro Avon River Catchment will increase over time if no action is taken, a Christchurch City Council study has found.
Councillors were briefed on the findings of the Multi-Hazards Study (Stage 3) at a workshop today. The study focused on the lower Ōtākaro Avon River Catchment, including the Ōtākaro Avon River Corridor (former Residential Red Zone).
The catchment-based study takes a broad approach and does not look at the risks to individual properties. The Multi-Hazards Study started in 2017, to give the Council a better understanding of how different natural hazards may combine to affect flood risk and what that might mean for the city’s flood defences.
Council Head of Three Waters, Gavin Hutchison says work is under way to build flood defences that will provide significant benefits over the next 30 years by reducing flooding.
“This study shows that significant flood risk will remain, even when these defences are in place, and we will need to do more to manage these risks beyond the next 30 years,” Mr Hutchison says.
Hazards that have the potential to reduce the benefits of planned flood defences include rising groundwater, earthquakes, vertical land movement and coastal inundation (through the dunes), amongst others.
“It’s good to have this information so that we can prepare and plan to fund this work in our long-term plans,” he says. “The financial cost of this work is significant, but the cost to the community of not doing it would be a lot more.”
The modelling shows that without flood defences the risk of above floor flooding in an extreme event will be two-and-a-half times greater by about 2100 as it is today. This would mainly be driven by sea-level rise, more rainfall, and ongoing land settlement.
Over the next 10 or so years, Council is investing in replacing the existing flood defences, to help manage the risk over the next 30 years. After that point, further decisions will be required to raise the new defences, said Mr Hutchison.
“In economic terms, the flood defence scheme currently being built will significantly reduce the number of buildings in the catchment exposed to above-floor flooding and prevent damages of around $630 million by 2060. If the defences are raised again in the future then damage avoided will total around $2.2 billion by 2100,” he said.
The study was peer reviewed by a panel of experts to ensure its technical quality and robustness.
