Floodgate provides unique twist to Waimakariri Flood Protection Project

The final part of the Waimakariri Flood Protection Project – installation of a retractable 1.1-metre-tall and 12-metre-long floodgate – proved to be one of the most complex.

The retractable floodgate was installed in Belfast this month, marking the end of the construction for the project.

The 1.1-metre-tall and 12-metre-long gate was built in Australia and shipped across the Tasman in a container.

The gate can be closed to bridge the 11.5-metre gap in the Waimakariri River secondary stopbank at Dickeys Road near the Western Belfast Bypass in the event of a flood.

A key aspect of the Waimakariri Flood Protection Project was the development of a 25km secondary stopbank. At this location, the secondary stopbank is higher than road level, leaving a gap where floodwaters could escape into Belfast.

Alternatives considered for closing the gap included raising the road or, in the event of a major flood, using sandbags or erecting a temporary floodwall. 

Raising the road would compromise traffic safety, while sandbagging or building a temporary floodwall is labour intensive and could reduce evacuation time for floodplain residents.

The retractable floodgate was chosen for reliability and ease of closure.

A challenging task

This type of flood protection is common overseas, but the only comparable structure in New Zealand is a recently-completed floodgate at Paeroa installed by Waikato Regional Council.

To make this idea work, a team of local engineers were required, specialising in electrical, structural, civil, and flood protection.

Environment Canterbury principal river engineer Ian Heslop said it needed collaboration.

“What’s complicated about this project was not so much the scale or size, but the elements of different people’s skillsets we needed to bring together,” Heslop said.

“It’s about ensuring we have the same design standard here that we have in the rest of the stopbank system.”

Constructing the flood gate

Construction of the floodgate at Dickeys Road in Belfast was a carefully managed part of the Waimakariri Flood Protection Project.

How does it work?

The gate consists of a stainless-steel frame enclosed with aluminium sheeting and weighs 1.3 tonnes. It is powered by an electric motor and winch. Two wheels support the gate, which run along a track in the road to connect with concrete walls at both ends.

If a flood was to hit, the floodgate would close like a rolling door to ensure the secondary stopbank gap was blocked.

The 24V electric motor and low-geared winch can close the gate in 14 minutes. When closed, a synthetic rubber seal would prevent floodwater leakage under or around the sides of the gate.

In the event of a power outage, the gate has back-up batteries, or can be closed manually.

“We wanted something that we can deploy with minimal fuss if we have a flood,” Heslop said.

Community safety

The main purpose of the floodgate is to further reduce flood risk to Belfast and the surrounding areas.

The floodgate would also ensure that the floodplain residents between the secondary stopbank and Waimakariri River have the maximum available time to safely evacuate.

Environment Canterbury chair Steve Lowndes said the community was at the forefront of people’s minds when the project was developed.

“The Waimakariri Flood Protection Project was designed to ensure the people of Canterbury can have confidence that if a flood hits, we have robust systems in place,” Lowndes said.

“This floodgate is a skilled piece of engineering and shows the level of commitment we have to ensure the community will be safeguarded against extreme weather events.”

The Waimakariri Flood Protection Project

The project began in 2010 and has involved the improvement of the 35km primary stopbank, the development of a 25km secondary stopbank, and 8km of rock armour protection work.

The primary system has been designed to withstand a 5,500 cumec flood, while the secondary system can sustain a 6,500 cumec event. The biggest flood in Canterbury was just under 4,000 cumecs in 1957.

The $40 million project, which assisted with local biodiversity projects and the enhancement of recreational assets, will protect parts of Canterbury from flooding, which has the potential to cause more than $8 billion worth of damage.

Find out more about the Waimakariri Flood Protection Project