Kaiapoi flood protection $24 million three-year programme of flood protection works kicks off with Kaiapoi rock works

Flooding is one of the most frequent, damaging, and disruptive natural hazards we face in New Zealand. As our climate changes it is expected that increased rainfall and rising sea levels will lead to increased flooding both in coastal areas and on floodplains.

Recent central government funds for flood protection as a part of COVID-19 economic stimulus has, as a result, been keenly welcomed by local and regional councils across New Zealand. The funding offered in a co-investment arrangement will go a significant way to relieving local rate payers of the cost burden related to the building and maintenance costs of infrastructure required to protect homes and businesses from flooding in a changing climate.

Kaiapoi, with its unique position as a coastal town on a floodplain is particularly vulnerable to flooding. It is the first project off the ranks of several work programmes to be undertaken by Environment Canterbury over the next three years.

Improving climate resilience for Kaiapoi

According to a 2019 report from the National Institute of Water and Atmospheric Research (NIWA), the homes of Cantabrians are particularly at risk from serious flooding due to climate change. Their data places 110,000 Canterbury homes – the most of any New Zealand region - within a flood hazard area, with an estimated replacement value of $34 billion (based on 2016 figures).

Project Lead Ian Heslop attests to the unique situation faced by the township of Kaiapoi.

“Flood protection is vital for the economic and social wellbeing of any community on a floodplain – particularly here in Kaiapoi by the lower Waimakariri River with its proximity to the sea,” he said.

“This location is within the tidal range and we’re also dealing with the Waimakariri river that has a catchment high up in the alps, so there are two things that will happen in this location; one is that as the sea level rises tides will get higher, the other thing is that being an alpine fed river the nor’west weather pattern is going to become more predominant with climate change. As a result, we would expect to see larger floods, so it’s a unique combination of factors that lead to increased flood vulnerability in this area.”

The Kaiapoi area is low-lying and has historically been highly vulnerable to regular flooding. Without the stopbank in place the town would be exposed to flooding from the Waimakariri, Ashley, Cam, Cust & Eyre rivers.

Claire McKay is an Environment Canterbury councillor and also a resident of the Waimakariri district.

“I think it’s really important that we look after this area. We need to look at the impact of sea level rise and climate change here. We know that after the earthquakes the land subsided and that thelocation is very sensitive to high tides and floods. It’s a priority to get this flood protection in place to safeguard the area now and well into the future,” she said.

Councillor Grant Edge, also a Waimakariri District resident agrees.

“Part of the problem is the unreliability and erratic nature of future storm events. Hopefully over time people will appreciate that nature’s changing and it’s changing fast, and we all have to realise what the implications of that are.”

Kaiapoi – a history of flooding

Te Kōhaka-a-Kaikai-a-Waro or Kaiapoi pā was established by Tūrākautahi, the son of Tūāhuriri and one of the principal rangatira who led the Ngāi Tūāhuriri migration to Canterbury.

The pā was named Te Kōhaka-a-kaikai-a-waro — later shortened to Kaiapoi. It became a major Ngāi Tahu trading centre and stronghold. The streams, wetlands and forests of the area were mahinga kai (food and other resource-gathering) sites for the inhabitants of the Kaiapoi pā.

In 1849 Alfred Rhodes and four others sailed up to Kaiapoi and commenced the work of surveying the area. His explorations identified Kaiapoi as a feasible point of entry to North Canterbury, facilitating development of the fertile areas north of the Waimakariri. As the 1850s progressed Kaiapoi grew into an important trading point between the port and Lyttelton, facilitating transport of goods by bullock wagons to sheep stations as far north as Hurunui.

However, it was soon apparent why one of the Te Reo Māori meanings of Waimakariri is “river of cold rushing water”.

In the first three years of the Kaiapoi township’s existence in the 1850’s, it was beset with flooding of the Waimakariri River, enduring 16 devastating floods and spending almost all its revenues trying to keep the river at bay.

Farming began with the arrival of European settlers to the area from 1850 onwards, leading to permanent settlement on the rich plains. To enable and protect this settlement, drainage of the wetland areas and containment of the rivers was commenced.

Since 1859, engineers have been developing systems and structures to protect the Canterbury region from flooding of the Waimakariri River. The works developed by Environment Canterbury beginning in 1989 build on a legacy of pioneering engineers and flood protection works.

Building on early pioneering protection works

Major floods in 1865 and 1868 flooded Kaiapoi and Christchurch, with a resulting increase in efforts by engineers to contain the river. These works tended to focus on the southern (Christchurch) side of the river, creating understandable tensions between communities on the north and south sides.

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

The Waimakariri River Improvement Act was passed in 1922 and addressed these tensions by requiring flood and erosion protection along both sides of the river. The comprehensive Hays No. 2 Scheme; consisting of Waimakariri River straightening, stopbanking, and erosion control planting, was completed from the 1930’s to 50’s. Stopbanks were also constructed on the Eyre, Cust, Cam, and Kaiapoi Rivers to further reduce flood risk to Kaiapoi.

The North Canterbury Catchment Board took over scheme maintenance in 1946. Major floods in 1950, and the largest flood on record of 1957, caused breakouts threatening Christchurch and Kaiapoi. This resulted in the 1960 Scheme and upgrade over the period to the 1970s. The works included an increase in Waimakariri River stopbank flood capacity, erosion control measures to reduce riverbed aggradation, and rock bank erosion protection works at critical locations.

Environment Canterbury took over flood protection scheme maintenance from 1989 and although floods had been successfully contained since 1957, a hazards analysis highlighted the need for upgrades to protect the increased population and assets rapidly being established on the floodplain.

This discovery initiated the Waimakariri Flood Protection Project completed in 2019, a ten-year, $40 million infrastructure project that reduces the risk of flooding in Christchurch city and Waimakariri and Selwyn districts. It adds strength and resilience to the flood protection system already in place and significantly lowers the risk of break-out during major flood events.

Major floods during the 1950s threatened Christchurch and Kaiapoi

The Waimakariri flood protection project

The Waimakariri Flood Protection Project was a massive undertaking starting in 2010 that involved upgrading the primary stopbank, as well as the addition of a secondary stopbanking system.

This ambitious works programme included the upgrade of 35km of primary stopbank, construction of a 25km secondary stopbank, and 8km of rock armour riverbank protection works.

The completed works ensure the primary stopbank has a significantly improved design flood capacity of 5000 cumecs with an estimated 300-year return period.

The secondary stopbank, in particular, is vital for climate change adaptation as we look ahead to building a more resilient region. The secondary stopbanking system will be able to contain break-out flows from the primary stopbank during a very large flood of up to 6500 cumecs.

In conjunction with consultant Good Earth Matters, Environment Canterbury recently won an Association of Consulting and Engineering Gold Award for the project.

McIntoshs bend rock works

Work is currently underway to strengthen a 400m length of stopbank and 700m length of riverbank using 19,000 tonnes of rocks from View Hill Quarry near Oxford.

The Waimakariri and Kaiapoi River stopbank protects Kaiapoi township and the surrounding area from flooding. They are part of the Waimakariri-Eyre-Cust flood protection scheme running the 70 kilometre length of the Waimakariri River from the foothills to the sea, providing wider flood protection for parts of Christchurch city, Selwyn district, and Waimakariri district.

In the 1960s, riverbank rock armour was installed to prevent erosion and stopbank breach on this very sharp bend.

Ian Heslop, who was the project lead for the Waimakariri Flood Protection Project, is also helming the project at McIntoshs bend, the popular fishing and recreational area located at the end of Ferry Road in Kaiapoi.

“This is a particularly difficult part of the river due to a sharp bend in an area where the river has also narrowed over the years causing a deep scour hole and increasing erosion vulnerability. This has resulted in an unacceptable level of risk of the river cutting through the riverbank and breaching the stopbank. It’s really important that we cut down that erosion and related flood risk,” he said.

“We need to strengthen the rock armouring along the riverbank and place some rock armouring along the stopbank to cut down the risk of the river breaking out. These new rock works will reduce erosion and flood risk to a level consistent with the remainder of the Waimakariri-Eyre-Cust flood protection system.”

Work is currently underway to strengthen a 400m length of stopbank and 700m length of riverbank using 19,000 tonnes of rocks from View Hill Quarry near Oxford.

Environment Canterbury councillor Grant Edge is a keen supporter of the project.

“I’m really pleased that we have flood protection works being upgraded here for a tricky part of the river that’s under threat from erosion”.

Public access and safety during construction

Public safety during construction is a key priority. Truck movements and construction activities will be separated from the parking, walking, and fishing areas in use to ensure minimal disturbance to the surrounding areas. To do this construction traffic will be directed around the car park, and the stopbank ramp will be fenced and widened.

This will allow continued riverbank fishing access to the whole of the Waimakariri riverbank up to the end of April 2021, with no restrictions during the brown trout (sea run) & salmon fishing seasons.

Some restrictions to the area will be necessary to complete the work:

  • March 2021 - Pegasus cycleway and walkway will remain accessible throughout construction by diverting the track through the adjoining paddock; except for the last week of February and first two weeks of March 2021, during which the oxidation ponds section will be closed. This is necessary in order for us to complete the stopbank rock armouring.
  • Mid-April to July 2021 - fishing access will be restricted to one section of riverbank at a time. One section will close (either section A or section B) whilst the other will remain open to allow the riverbank works to be completed. No construction will take place on weekends or public holidays.

Flood protection funds a boost for biodiversity and recreational values

Partially funded from the Ministry of Business Innovation and Employment’s Provincial Development Unit (PDU) the McIntoshs bend rock works are a flood protection and climate change resilience priority for Kaiapoi and the Waimakariri district.

Funding from the Climate Resilience Programme of Flood Risk Management Projects has significantly reduced local community cost and helped kick-start projects that would have otherwise taken decades to fund through current rating districts.

The cost estimate for this project is $3.2 million, supported by a 64 per cent grant in a co-funding arrangement between the PDU and Environment Canterbury.

These new funds for flood protection support a holistic approach to working in our rivers and encompass works that enhance local biodiversity and the recreational value of this popular area.

Ian Heslop (left) talks with MBIE's Warren Gilbertson.

Ian notes the additional complexities of working in this particular location.

“This project has been a big challenge. We’ve had to get an understanding of the recreational and ecological factors that are very important at this location. It’s a low-lying area and there are important wetlands for us to work around. This is also a very valuable fishing spot and there’s a well-used walkway and cycleway along the stopbank. We had to figure out a programme of works that enabled us to work around all these issues with minimal disruption to people, and in particular to the wetland areas. The key issue, given that we are working on the fringes of these wetlands is not to change their natural character.”

“In fact, as the project progresses, we’ll be gradually removing weed willows where they are encroaching into wetlands and doing some native plantings to improve biodiversity in the area.

We’re really looking for a net gain in terms of biodiversity outcomes with this project, working closely with our internal environmental advisors.”

Councillor Edge reiterated the value of the recreational benefits at McIntoshs bend to the community and the need to support and develop the multiple uses of the area.

“The network of cycleways and walkways from the town through to the coast is really good. The amenity values of an area like this by the river with its fishing, wetland areas and habitat for wildlife is important and the local community genuinely values the location. As a recreational resource it’s really outstanding,” he said.

Plans include improving safe pedestrian access to the riverbank for fishing and permanent toilet facilities plus car parking improvements are being considered.

Councillor John Sunckell (on the right) emphasised the importance of seizing this opportunity to make transformational change for the benefit of our iconic braided rivers.

One of the things that we’ve articulated through our Long-term Plan is the importance of opportunities for transformation, particularly within our braided rivers. This funding from the central government gives us the ability to not just upgrade our flood protection assets but also to look at biodiversity, our wetlands and flora and fauna. The transformation that we are looking for, and that the community are looking for, are really well served by this programme,” said Sunckell.

Braided Rivers are a precious and iconic part of the Canterbury environment. Environment Canterbury is prioritising the protection and restoration of their unique values across the region as part of our Braided River Revival/Whakahaumanu Ngā Awa ā Pākihi work.

The legacy of this project is not just flood protection, but enhancement of numerous values including recreation and biodiversity - making it a real community asset for the people of Kaiapoi.

With funding from the PDU for further work across the region, Environment Canterbury is starting to work in the Ashley River/Rakahuri, and others in Canterbury, to enhance a range of biodiversity values, including natural braided river habitats.