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Flooding can strike at any time Flooding can have devastating effects on a region. Environment Canterbury has taken significant steps to reduce the risk of potential flood damage from the Waimakariri River.

Canterbury’s Waimakariri River is an unpredictable, braided river that has the potential to rise to extreme levels.

The Waimakariri Flood Protection Project is a massive undertaking that began more than 13 years ago when the plans were first mapped out in 2006.

The project will protect part of Canterbury from flooding, which has the potential to cause more than $8 billion worth of damage, through the improvements to the primary stopbanks, and the development of a comprehensive secondary stopbanking system, giving Cantabrians peace of mind.

The history

A man gets his feet wet during the flooding in Kaiapoi in February 1945.

Major flood events have struck Canterbury multiple times during the past 150 years.

Flooding 1957

High river levels breached the Waimakariri River stopbanks and submerged various parts of the region. The most recent major flood was in 1957 at Coutts Island, near Belfast.

Flooding in Christchurch in 1957 breached the banks of the Waimakariri River.

Flood protection works in the region

Construction team building stopbanks on the Waimakariri River in the 1930s.

Several authorities undertook flood protection works in the region leading up to 1990 when the regional council, Environment Canterbury, recognised that further work was needed to adequately protect the developing floodplain area.

A report in the 1980s from Tony Boyle, Environment Canterbury’s principal hazards analyst, pointed to the potential risks.

Following further investigations, Environment Canterbury principal river engineer Ian Heslop was summoned to the office of then-chief executive Bryan Jenkins in 2006.

Jenkins was also a professional engineer and had grasped the magnitude of the task at hand. Heslop was instructed to map out a plan to make the Waimakariri Flood Protection Project happen.

Consents

In less than a year, Heslop had to create a plan to execute the Waimakariri Flood Protection Project and one crucial aspect of that was securing resource consents.

“Engineers are used to preparing designs and cost estimates, but resource consenting adds a lot of stress,” Heslop said.

Good Earth Matters Consulting in Palmerston North were brought on board to help lead the project. Their principal engineer David Bridges would eventually become the lead consultant for the project.

Their firm had substantial experience in consent planning and engineering, which provided the ideal mix for Heslop’s team to work with.

The project required resource consents from Environment Canterbury, Christchurch City Council, Selwyn District Council and Waimakariri District Council.

To streamline the process, all four agencies agreed to appoint two independent commissioners – a rare move – who would adjudicate.

The first consents were lodged in February 2007, all of which were publicly-notified, and following three hearings they were granted in June 2009.

In 2010, the project received an Association of Consulting Engineers New Zealand Gold Award of Excellence for the consenting of the project.

“It showed that we ran a really excellent consenting process,” Heslop said.

The quarry

Approximately 260,000 tonnes of rock was required for the rock armour protection work spread 8km along various parts of the Waimakariri River.

A reliable and high-quality rock supply was needed, and Environment Canterbury decided to upgrade their View Hill quarry at Oxford to achieve this.

Environment Canterbury’s quarry at View Hill has played an important role in the Waimakariri Flood Protection Project.

The quarry previously produced 8,000 tonnes of rock per year, and production was increased to 260,000 tonnes between 2009-2016 for the project works.

Rock protection is an important aspect of flood protection work.

“It’s a huge increase in production, so there was a lot of work put in by geologists, surveyors, and engineers, to work out the best way to develop the quarry,” Heslop said.

At the start of the project the quarry consisted of one small cutting, and now has seven 10m high benches, with associated stormwater and sediment control works. The work is outsourced to a private contractor, and the basalt rock is broken out with an excavator.

Construction

Construction began in the spring of 2010 and had a 10-year window for completion.

The key goals were:

  • To ensure that both the northern (Waimakariri District) and southern (Christchurch City and Selwyn District) sides of the river had a reduced flood risk
  • Upgrading the existing primary stopbanking system
  • Construction of rock bank erosion protection works
  • Retro-fitting a secondary stopbanking system

The rock armour protection work along the banks of the Waimakariri River required more than 260,000 tonnes of rock.

The stopbank work was meticulously planned to take place in stages during the project.

The project has seen the upgrade of 35km of primary stopbank, and the construction of a 25 km secondary stopbank.

Final stages

The finishing touches will be placed in late 2019, ensuring the project will finish ahead of schedule.

That is largely down to the groundwork being laid in the consenting and planning phases, meaning there have been few hitches.

The project included the upgrade of 35km of primary stopbank, construction of a 25 km secondary stopbank, and 8km of rock armour riverbank protection work.

The secondary stopbank system will provide Canterbury with another layer of protection during a flood event.

“I’m exceptionally proud of the team that brought it through,” Environment Canterbury chief executive Bill Bayfield said.

The earthquakes

The Canterbury earthquakes caused damage to some stopbanks in the region.

Every major project that’s taken place in Christchurch in the past decade has its own earthquake story.

The September 2010 quake struck when the construction phase of the Waimakariri Flood Protection Project was in its early stages.

The stopbanks downstream of the Northern Motorway needed substantial repairs, but there was a bigger challenge for the project team, working with people suffering from the effects of the earthquakes.

Heslop said one site, close to houses in Rushmore Drive in Belfast, required stopbank upgrade work. The work needed to be carried out with particular care to avoid shaking of houses and disturbance of residents.

“That was where the main challenge came, working with the locals and our contractors to minimise stress levels,” Heslop said.

Biodiversity

The project has enhanced Canterbury’s biodiversity assets, contributing to the following developments:

Biodiversity projects

Native vegetation protection and enhancement - McLeans Island & Halkett

Rare native herbs, mosses, shrubs, and trees such as Kōwhai were identified and mapped, and stopbank alignments and construction access routes were chosen to avoid these areas. Stock exclusion fencing was built, and additional native plants were planted to further protect and enhance these areas.

McLeans Island and Crossbank carpark and cycleway development

Native grasses, scrubs, and trees were planted around the newly developed carpark, cycleway entrance, and stopbank access ramp.

Sanctuary Wetland and Engelbrechts Rock Groynes

The 45-hectare sanctuary is an established wetland area at Coutts Island, located between the riverbank and primary stopbank. The wetland has substantial native plant and fishery values and has an ongoing programme of weed and animal pest control, native planting, and improvement of public access by cycleway/walkway development. The Waimakariri Flood Protection Project assisted this with rock riverbank erosion protection, water level control, and riparian native planting.

Smiths Creek - Kaiapoi Island

This 1km spring-fed creek is located between the stopbank and riverbank and has high native and sports fish values. The Waimakariri Flood Protection Project enhanced the creek with rock erosion protection and removal of weed willows, which enabled follow-up native riparian planting. The project also constructed a nearby carpark to improve and manage recreational access for the local area.

“It’s a beautiful space. It’s a beautiful river and now we’ve seen through this project we’ve been able to accelerate the development of these areas around the stopbanks and up and down the river and provide access to them and create quite unique recreational assets and biodiversity assets for the people of greater Christchurch,” Bayfield said.

The future

The completed project works will ensure the primary stopbanks have a significantly improved design flood capacity of 5,500 cumecs. The secondary stopbanking system will be able to contain break-out flows from the primary stopbanks during a very large (6,500 cumec) flood.

For context, the Waimakariri River’s largest flood on record was just under 4,000 cumecs in 1957.

“What we know is we’ve created an asset here that is a foundation legacy for the future. If we hadn’t have done this then the next steps would been even bigger and harder,” Bayfield said.

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