Flood-prone Canterbury rivers in the spotlight
By David Hill, Local Democracy Reporter
Lessons learned from devastating floods in May 2021 are helping 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.
"Nobody is immune to being impacted by flood events."
The 2021 flood event showed 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 the council’s 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 would be felt".
Canterbury has about half of New Zealand’s river assets, with the regional council having oversight of 58 river schemes.
Flood protection work is urgently needed in the 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 the Selwyn district.
Christchurch, Kaiapoi and part of the Selwyn district 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 district 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 poses 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."
■ LDR is local body journalism co-funded by RNZ and NZ On Air.
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Officials discuss ways to protect flood-prone beach settlement
By David Hill, Local Democracy Reporter
‘‘Pragmatic’’ solutions have been suggested for a flood prone North Canterbury beach settlement.
Gore Bay, near Cheviot, is a popular coastal holiday spot, but faces threats from flooding and sea level rise.
The Hurunui District Council is creating a coastal adaptation plan for Gore Bay and is working with Environment Canterbury to plan for flood protection for the Jed River and Buxton Creek.
Deputy mayor Vince Daly said the creek and river were prone to flooding in heavy rain events, leaving the Buxton Campground and the coastal road vulnerable to flooding.
‘‘The creek builds up when it is in flood, and the shingle builds up, preventing it from flowing out to sea.’’
Daly said Environment Canterbury presented some ‘‘pragmatic’’ solutions at a recent meeting with Gore Bay / Port Robinson Reserve Advisory Group.
A Hurunui District Council report presented to the Cheviot Community Committee in February said the flood hazard was increasing due to the formation of a lagoon between the river and the creek mouths.
The intensity of recent rain events was increasing the erosion threat to the campground’s banks and to land around the bridge abutments.
Environment Canterbury regional lead river operations, Cliff Thomas, said options presented to the community had exceeded the budget set for the project.
An engineering consultant is now assessing the options and will provide a design for Environment Canterbury staff to consider.
‘‘A channel could be cut to allow the river to let water flows out to the ocean when certain conditions are met, such as flood risk due to high rainfall,’’ Thomas said.
‘‘However, a consent would be required to do this.’’
Longer term sea level rise posed a threat to the access road and to the Buxton and Gore Bay Camping Grounds, Daly said.
‘‘The elevation of the houses means slips are probably a bigger issue for residents than sea level rise, but it does pose a risk to the road.
‘‘This is one of the best Canterbury beaches and it is a very popular spot.’’
The Hurunui District Council began its ‘‘Coastal Conversations’’ project alongside five beach communities in 2020.
Three coastal adaptation plans have been completed for Amberley Beach, Leithfield Beach and Motunau, while the council is also working with the Gore Bay and Conway Flat/Claverley beach communities.
■ LDR is local body journalism co-funded by RNZ and NZ On Air.
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