Digging trenches for anchored tree protection in the Hakatere/Ashburton River. Photo: Supplied / LDR
Environment Canterbury (ECan) is rolling out millions of dollars' worth of flood resilience projects across Mid and Selwyn districts - part of a long-term push to strengthen river defences after the devastating 2021 floods.
And they just got a boost with government funding to do even more.
ECan rivers manager David Aires said the Ashburton flood protection system was upgraded between 2005 and 2009 to withstand a major flood event expected only once every 200 years. Upstream, the North and South Branches are built for smaller, 50-year events.
"The May 2021 event was approximately a one-in-200-year event in most of the waterways, and we saw many breakouts, overflows, scour, erosion and deposition of gravel."
It was a big wakeup call, prompting a rethink of how ECan manages its river systems.
The 30-year infrastructure strategy, signed off in 2024, has a strong focus on improving flood protection and a greater emphasis on ongoing river management.
It all takes time and money to implement.
ECan already spends about $16 million a year maintaining $850m in flood infrastructure but "the scale of the challenge means local ratepayers alone cannot sustain these costs".
A $21.5m government contribution from the Regional Infrastructure Fund will help accelerate projects - support ECan hopes will evolve into a permanent co-funding model.
"We believe that flood resilience must be viewed as a shared, long-term investment," Aires said.
Since 2020, ECan has worked with other regional councils to advocate to the government for a permanent co-investment model he said.
ECan estimates Canterbury's flood protection and drainage schemes prevent around $9 billion in damage and lost income each year.
According to a report commissioned by the River Managers Special Interest Group in 2018, $1 spent on flood protection saves $55 in recovery.
Recovery since the 2021 floods
The immediate focus following the 2021 flood was on recovery.
"Led locally by our river engineers, we worked closely with landowners and contractors to identify urgent repairs and longer-term improvements," Aires said.
By mid-2022, most major structural repairs were complete and erosion protection planting was underway he said.
"Further high flows that winter caused additional damage to newly repaired sites.
"Over the following two years, we completed a programme of more than 400 repair jobs, restoring resilience to what it was before the floods and in some cases adding additional strength."
The region-wide 2021 flood recovery effort was funded through a mix of Environment Canterbury general rate ($13.9m) and National Emergency Management Agency (NEMA) co-funding ($8.3m).
Environment Canterbury rivers manager David Aires. Photo: Supplied / LDR
Gravel removal
To reduce flood risk, ECan is ramping up gravel extraction from the Hakatere/Ashburton River.
Since 2022, nearly 400,000 cubic metres have been removed - enough to fill 160 Olympic pools.
"It is likely that 50,000 cubic metres will be extracted from the North Branch this summer to a stockpile ready for use in the new Ashburton bridge abutments.
"There is still approximately 800,000 cubic metres that could be removed to achieve desired bed levels in the North Branch," Aires said.
Individuals and companies can apply for Gravel Authorisations, with 11 active in the Ashburton North Branch, to extract gravel for things like farm tracks, building foundations, subdivisions, roading, civil construction and concrete he said.
ECan is also working with NZTA, Ashburton District Council, the gravel industry and the community to address gravel build-up in the district, particularly in the North Branch, he said.
Economics of extraction
Costs and access issues for gravel extraction in remote parts of the Ashburton River are slowing efforts to manage rising bed levels.
"Access and proximity both play a part," Aires said.
"Gravel is expensive to transport in large volumes, so normally gravel has been sourced close to easy access points for the end use."
It means in some problematic areas, the riverbed keeps building up with more material - a process called aggradation, he said.
Only around 15 percent of the region's gravel comes from river extraction, and the rest comes from land-based operations he said.
"Taking gravel out of the river for flood protection can be expensive, and access can be a problem. To help with this, we've set aside money to help enable access in the Ashburton catchment."
Gravel extraction and managing riverbed levels are key parts of maintaining flood resilience.
"Every braided river is unique, and every flood is different - gravel build-up isn't the sole reason we see breakouts," the spokesperson said.
"Gravel extraction is an important tool, but it's just one of several measures used for flood protection."
Breakout flows from North Branch Hakatere/Ashburton River, looking upstream along Longs Ford Road. Photo: Supplied / LDR
Prevention over restoration
Flood protection is about helping communities live safely beside their rivers "without worrying every time it rains".
"Our job is to assess and reduce risk where we can within the budget we have, we target our efforts where they'll make the most difference," Aires said.
Over the next few years, ECan plans upgrades across the Ashburton and Selwyn catchments - even more now with the government co-funding.
ECan's 2024-34 long-term plan has money allocated to increase resilience in the North Branch of the Ashburton/Hakatere, including $1.5 million for the stopbank changes and gravel removal through the Blands Reach section of the Ashburton North Branch between 2025 and 2030.
ECan also has a $1.2 million project to relocate stopbanks along the Ashburton North Branch near Walkhams Road.
Aires said they have purchased some land and are negotiating further locations while considering options for the location of the stopbanks.
"We intend to construct these new stopbanks over the next two summers, pending all land negotiation and consenting processes."
A future project will focus on the Carters and Lagmhors Creeks, upstream of Tinwald, and the possible diversion of flood flows to the Hakatere/Ashburton River.
Aires said the recently announced co-funding investment from the government will accelerate some of the works in the Hakatere/Ashburton catchment, which will include gravel extraction, berm strengthening, upgrades to existing infrastructure, and channel capacity improvements to further increase flood resilience and protect the community.
Flood funding
Maintenance works for river resilience infrastructure in the Ashburton catchment and lower Selwyn River are funded 70 percent from targeted rates collected from the directly benefiting properties, 15 percent is sourced from a district-wide rate, and 15 percent from a region-wide general rate.
Aires said that in the wider Selwyn catchment, ECan is trialling a district-wide rating approach to collect funds for river works throughout the district.
"We are initially focusing on clearing out large woody trees and debris from the active river channels in the upper catchment."
LDR is local body journalism co-funded by RNZ and NZ On Air.
