Flood-prone rivers in the spotlight
Lessons learned from devastating floods in May 2021 are helping to shape planning for Canterbury’s rivers.
Environment Canterbury (ECan) is proposing to invest $25 million a year, over the next 10 years, to boost flood protection as part of its draft 2024-34 long-term plan (LTP).
While flooding caused by Cyclone Gabrielle in Hawke's Bay last year attracted headlines, Canterbury has had its own damaging floods in recent years.
In 2019, the Rangitata River bridge was closed for several days following a flood, while the May 2021 flood caused widespread disruption, prompting a rethink of how ECan manages its river systems.
“A Cyclone Gabrielle event hasn’t happened, as far as we know, in Canterbury’s history, but one-in-200 year events seem to be happening more and more,’’ said councillor Ian Mackenzie, who chairs the council’s catchment resilience committee.
The 2021 flood event showed that the old river and drainage rating model was no longer fit for purpose, as rivers need to be looked at as a whole river system, he said.
To ensure the financial burden is spread across the region, the council is seeking to introduce a mix of district-wide and region-wide rating.
Without its flood protection, land drainage and erosion control schemes, staff estimated there would be about “$9 billion per year of combined damages and lost earning potential”.
Canterbury has about half of New Zealand’s river assets, with the regional council having oversight of 58 river schemes.
Flood protection work was urgently needed in Waimakariri District, the council’s operations general manager, Leigh Griffiths, said.
Changes to the landscape caused by the growing urban environment and the 2010 and 2011 earthquakes meant “the level of flood protection is lower than would be expected”, she said.
About $700,000 a year is spent on maintenance in the Ashley-Rakahuri River, with the council proposing to increase that to $800,000 per year and to initiate an eight-year, $15m upgrade programme as part of the LTP.
The work, for which the council will seek central government funding, is likely to be similar to work planned in Selwyn District.
Christchurch, Kaiapoi and part of Selwyn are protected by two levels of flood protection on the Waimakariri River, which has capacity for a one-in-500 year event.
It costs about $5m a year, with $1m coming from a targeted rate of $12 per ratepayer per year. The balance is funded by income from grazing and forestry on council-owned land between the stopbanks.
The capacity in some South Canterbury rivers means they “are expected to overtop their stopbanks” in a one-in-50-year flood event, while Ashburton is protected for a one-in-200-year event.
The proposed Selwyn flood prevention work will be funded by a district-wide targeted rate, and will include clearing out channels and flow paths, weed control and planting, and building resilience.
Exotic weeds, including poplars and willows, and loose shingle posed the biggest challenges, Mackenzie said.
“Bridges are more likely to survive if they’re not being treated as a battering ram [in a flood].
“Exotic weeds are one of the biggest problems. If we stopped doing what we do, these exotic weeds would choke rivers and push them out to the berms.”