Crazy or needed? Massive tidal barrier proposed for Christchurch estuary

Thursday, 20 November 2025

Christchurch mayor Phil Mauger wants to resurrect plans to build a tidal barrier across the Ihutai Avon-Heathcote Estuary to reduce flooding.

The decades-old idea has been labelled as “crazy” 20th century thinking by a river advocate, while some Redcliffs locals could welcome the plan if it reduced the impact of sea level rises, the local residents’ association says.

Mauger told The Press this week he wanted Christchurch City Council staff to look into a plan to build a barrier, spanning from the Southshore Spit to near Shag Rock.

The barrier would hold back the advancing tide, allowing the rivers to drain more freely into the estuary, preventing flooding in the river catchments.

An added benefit would be that people could walk or cycle across the barrier, providing direct access between New Brighton and the Coastal Pathway, Mauger said.

The idea has been around for decades, with the first of five reports done on the topic dating back to 1963. It was last considered in 2015 when a pre-feasibility study was completed.

Cost and environmental concerns led the council in 2015 to opt against conducting a full feasibility study, which would have cost $2m. The project was expected to cost $430m at the time.

The pre-feasibility report concluded while a barrier could be constructed to control estuary water levels, a tidal barrier would not resolve Christchurch’s flood risk challenges in isolation.

“What remains unclear from this study is whether a tidal barrier is a favourable option when considered as part of the larger mix of options,” the report conducted by GHD consultants said.

A barrier would impact the estuary’s ecology, water quality, landscape, cultural value and recreation as well as the form and structure of the spit and surrounding beaches, the report said.

However, the level of impact would depend on how often the gate was closed, the report said.

The study looked at two possible locations: one by Shag Rock and the other by Beachville Rd. It deemed Beachville Rd more preferable, even though the barrier was longer at 600m compared to 250m, but the spit was more stable in that spot.

Tidal barriers are used on The Thames and extensively in the Netherlands, Mauger said.

“The Netherlands is the same size as Canterbury but with 17m people in it and a quarter of it is below sea level and they survive. They know about pumping water out of places so we should be able to do something.”

Mauger said he had no idea how much it would cost, but said the council was already spending $1m for every 100 metres of stop banks it was building around the Pages Rd bridge.

“People go on about, ‘Phil what is your legacy project?’ and I go…but this, in my view makes common sense and I don’t think we should run away from something that is common sense. It would fix a lot of flooding in the Heathcote and the Avon.”

When asked if he intended for this project to be his legacy, Mauger said: “No, not really, but it’s something that I think is worth looking at.”

“There were a lot of pictures drawn way back then. I want to have it rekindled and staff are coming back to us.”

Mauger was meeting with staff on Thursday to discuss the project.

“I want to know what the benefits are. I don’t want to know what the cons are, or tell us, but we’ve got to decide the pros and cons.”

He said the gates could even be used to make sure there was always water in the estuary allowing for more water sports.

“And if the estuary ends up with say 50% more fresh water in it than sea water coming in and out, it might get a bit more growth or something going, I don’t know, next thing you know there’s too much sea lettuce in it. I don’t know.”

Ōpāwaho Heathcote River Network chairperson Chrissie Williams said it was a 20th century approach to think the natural environment could be manipulated for human benefit.

'Small changes in the estuary and river environments can have significant and unpredictable effects on the ecosystems and river infrastructure.'

Williams said the money would be better spent mitigating flooding within the river catchment, including in the red zone land, and building further wetlands and ponds in upper Ōpāwaho catchment.

“It’s a crazy idea. It’s not feasible and incredibly expensive. The costs would have gone up tremendously since 2015.”

Redcliff Residents’ Association chairperson Christine Toner said many Redcliffs residents, especially those with homes close to sea level, would probably welcome a move to prevent flooding and rising sea levels, but she was concerned about any possible environmental impact and the costs.

She said she had observed estuary birds using the sand banks to feed during low tide, so wondered where they would get their food if there was no low tide.

“Sounds like an enormous burden on the city of Christchurch and who would pay for it?”

Avon-Heathcote Estuary Ihutai Trust chairman Joe Davies said the trust did not want to comment and would make a submission if the plan was put out to public consultation.