From our Chair: Adapting flood protection to climate change

Jenny Hughey, Chair, Environment Canterbury

Jenny Hughey, Chair, Environment Canterbury

There are many ways we can start preparing for the likely effects of climate change on us and on our environment.

Perhaps the most challenging hazard we face in the coming decades is flooding, particularly from our large, braided rivers fed by alpine rain and melting snow.

As nor’west storms in the Southern Alps become more potent, the huge volume of run-off into the Waimakariri, Rakaia, Waitaki, Hurunui, Waiau and Rangitata rivers will potentially cause big trouble for those who live downstream.

Flood protection works in the region

Environment Canterbury manages 59 river control and land drainage schemes that collectively cover over 1,000km of rivers, many smaller streams and over 760km of rural open drains. It also manages a 663km network of stopbanks.

Canterbury’s largest flood protection scheme, the Waimakariri–Eyre–Cust scheme, provides flood protection from the Waimakariri River to the greater Christchurch area, protecting some $118 billion of assets.

Celebrating the Waimakariri Flood Protection Project

It’s important work, which is why I was thrilled that the Waimakariri Flood Protection Project, one of Environment Canterbury’s most significant work programmes of recent years, has been recognised nationally.

In conjunction with our consultant Good Earth Matters, we have won an Association of Consulting and Engineering Gold Award for the 10-year, $40 million project, which together with the existing Waimakariri flood protection infrastructure defends the $118 billion of property and community assets from a damaging flood in the lower reaches of the river.

Frequently, a lot of the work of our expert staff members stays in the background. But not this one. This is a major and obvious project, one that was delivered early and under budget, and how often can you say that of something infrastructural?

Taking action to adapt to climate change

Flood protection is worth remembering how real a threat the Waimakariri River poses for large parts of Christchurch, Belfast and Kaiapoi. That threat will only increase with more frequent, or more intense, storms in the Alps.

In 1868, floodwaters from the river washed through Victoria Square and Cathedral Square in Christchurch, and at one stage were more than a metre deep.

Another major flood in December 1957, the most recent, led to parts of Belfast and Kainga being swamped when the river’s flow topped out at 3990 cumecs (cubic metres per second).

Construction history

The protection scheme was a massive undertaking, involving an upgrade of 35km of primary stop-banks as well as the construction of 25km of new secondary stop-banks, which are especially vital for dealing with climate change and fostering a more resilient region.

Plans were drawn up more than 15 years ago, with building getting under way in 2010 and completion last year. As well as the work on the stop-banks, 8km of rock-armour riverbank protection was also required.

Nearly 700,000 cubic metres of gravel from the river bed were used to build the stop-banks, while 250,000 tonnes of rocks from an Oxford quarry went into the armour.

Anyone who has seen the Waimakariri River in flood will realise what an impressive and frightening sight it can become. The intricate braids in the gravel riverbed disappear as bank-to-bank roiling water roars towards the sea.

The river’s mean flow at the State Highway 1 bridge is 124 cumecs. The primary stop-banks now have a significantly improved design flood capacity of 5500 cumecs, estimated to be required in a flood with a 450-year return period.

The new secondary stop-banks will be able to contain any breakout flows from the primary banks during an even larger flood of 6500 cumecs.

Find out more about the history and construction of the Waimakariri Flood Protection Project.

Newly funded Canterbury projects

Looking elsewhere around the region, Environment Canterbury now has confirmed funding from the Provincial Development Unit ‘shovel ready infrastructure programme’ for flood protection and river improvement works to be carried out throughout the region.

A total works programme of $24.2 million over the next three years has been approved in a co-funding arrangement which has strong multi-purpose outcomes of improving river resilience to flooding and climate change, supporting the Canterbury COVID-19 regional recovery by generating employment for local contractors, and providing environmental benefits for Canterbury’s rivers and our communities.

Find out more about the climate resilience and flood protection funding.

I’d like to say well done to everyone who worked on the Waimakariri Flood Protection Project. We can all be proud of this fantastic, and critical, community asset.