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A thousand years ago a great Polynesian explorer and sailor named Kupe set out from his home in Polynesia, braved the open seas in his waka hourua, and discovered an uninhabited New Zealand. This ‘waka’ was a large, double-hulled, ocean-going canoe, capable of travelling vast distances, driven across the waves by wind… and by strength of arm.

According to Maori oral tradition, over the next few centuries a total of seven of these grand canoes arrived in New Zealand. Their names were Aotea, Kurahaupō, Mataatua, Tainui, Tokomaru, Tākitimu… and Te Arawa. And these became the names of the Maori tribes (or ‘Iwi’).

The Arawa canoe sailed into what is now called the Bay of Plenty on New Zealand’s North Island, and landed in an estuary. The weather was so beautiful, and the ground was so fertile that they named the place ‘Maketū’ after the huge gardens they kept in their lost homeland of Hawaiki. For the next 600 years the people lived alongside the Maketū Estuary and the Kaituna River that flows into it. Kaituna means “eat eels”, and marine life became an important part of the local diet and culture.

Then, a couple of centuries after the arrival of Europeans, a series of projects were initiated culminating in the Kaituna Cut of 1956. The river was diverted, and local wetlands were drained to provide new farmland. As a result, freshwater flows to the estuary diminished, causing siltation. Marine biodiversity crashed… and it kept crashing. For the sub-tribes (or ‘hapū’) still living near and depending on the estuary, it was a calamity.

Learning from the past

This is a story about renewal, and a project that aims to correct the mistakes of the past. That project is the Kaituna River re-diversion and Maketū Estuary Enhancement. The project was created to remedy the long-standing negative effects of the Kaituna Cut on the health of the estuary and the local community. And this project, although impressive in scope, does not stand in isolation.

Around the world, people are beginning to realise what they have lost to environmental degradation, and managed coastal re-alignment or conservation projects are given increasing importance. Engineering with, rather than against nature. But to correct the mistakes of the past, it is first necessary to understand the past.

Liam Tapsell is an elder of a sub-tribe of Te Arawa, called Ngati Whakaue ki Maketū. He explains the importance of the region to the local people, “All throughout New Zealand, there are rivers. Some very big rivers. And the rivers belong to a particular tribe. Each tribe has their own river, their own mountain, their own river.”

He says that the word in Maori is Oranga Tonutanga, which means sustenance. The river is a source of strength, life, identity, spirit and sustenance for the people. It is a powerful feeling.

Tapsell works as a cultural monitor for projects and works in the region, and is present in areas that are considered sensitive in case any archaeological finds are unearthed. On the Kaituna restoration project for example, workers found stone tools over 300 years old, as well as special greenstone weapons and tools this is a kind of jade that is important to the Maori.  These finds may go on to the university, or be presented to local collections.

Tapsell’s family and his subtribe, his hapū, have lived in this region for hundreds of years. When he was young, the river was full of marine life and a source of food for the locals. When the plans for the 1956 Kaituna Cut were announced, the people tried to stop it.

“Everyone knew that this is going to be a disaster. If they were to go ahead with this project. So they tried, my grandfather and grandmother, they tried and even sent a delegation to Parliament in Wellington, to voice their concerns,” says Tapsell. “The response to our concerns was “don’t bother us, go away”. They diverted the river, and my grandmother and grandfather they died broken. They died very sad because they knew the consequences, and they were right.”

Even the district engineer, a man called Dale Revington left a note in the files saying that he was pretty sure the 1956 project would kill the estuary.

After the cut the water level became very low and the biodiversity collapsed. The life of fishing and sharing the catch with other tribes passed into memory. Decade after decade, the community campaigned for help. And eventually they were answered.

Yesterday’s solution, today’s problem

At NZ$ 17 million, The Kaituna River re-diversion and Maketū Estuary Enhancement to repair the damage of the past is one of the largest projects the Bay of Plenty Regional Council has ever undertaken. A typical initiative for its environmental division might involve spending NZ$ 50,000 over four years, planting hardy vegetation to protect a wetland or a stream. The Kaituna project would involve substantial work.

Pim de Monchy is the Coastal Catchments Manager for the Bay of Plenty Regional Council. He says that the project is the most significant of his career, but it took a long time to get here.

“The local tribes realised in 1979 that the river’s removal had had a negative effect on the estuary. And so they asked for it back then,” says de Monchy. “And central government in 1988 provided some funding to the Department of Conservation, to do a partial restoration.”

The result was the construction of four 1,800mm diameter circular culverts being placed underneath Ford Road, which is built on the barrier that separated the river and estuary. They were flap-gated on the estuary side to prevent backflow. The works put 4% of the river’s flow back through the estuary, but it wasn’t enough to make a difference to the estuary’s hydrodynamics or ecology to improve the shellfish.

“So, in the 2009 Council term, we have three year Council terms here, there was a councillor from the local community at Maketū called Raewyn Bennett, who represents the Ngati Pikiao iwi, and she advocated for this project quite strongly with her fellow councillors, and was successful in getting [a NZ$ 7 million commitment] at that stage.”

Investigations and modelling began to see what kind of options might be hydraulically feasible, which of those options would then meet the criteria for success from the ecological or cultural perspective. They showed that without a major intervention, any effects would be minimal, and so this became a serious earthworks project.

The excavators return to Kaituna

Starting from the upstream side of the project, the team began by creating a one-kilometre-long, 60m-wide, 2m-deep channel from the river, some way upstream near Ford Road. This was to maximise the freshwater portion of the re-diverted flow to fix the ecological health of the estuary.

They had to dredge 23,000 cubic metres of anoxic mud from that part of the channel because it had been a deposition area for a drainage ditch from the surrounding farmland for the last 40 years.

“And then we constructed what we call a salinity block. And that was to block one of the channels that was present before the project, so that salt water couldn’t short circuit its way to the new culverts,” says de Monchy. “Then, we had to construct a jetty and a floating pontoon for the commercial fisherman and the Coast Guard. They were operating in one of the places that we needed water to flow through, so we shifted their facilities gave them new facilities on that salinity block.

“Then we removed the four existing culverts that were built in 1996. And we had to put temporary sheet piling around both sides of them so we could de-water that side.”

They were working in the floodplain the whole time, so on a couple of occasions there was a large tide or a minor flood that rolled over the top of the temporary sheet piling and flooded the whole site, needing to be pumped out again.

“And then inside that temporary sheet piling within poured concrete foundations, and well before we put the concrete foundations we put the permanent sheet piling that went seven metres down below the concrete in the concrete to the in the culverts so they were 12 no. 2.5m by 2.5m box culverts sit out in an array running north south underneath Ford Road. On the on the downstream side of those culverts, we’ve got vertical slide gates of marine grade stainless steel that operated hydraulically with a spindle to drive them up and down on every tidal cycle.”

These slide gates are around 15mm thick. Every time the tide rises on the river side, it creates a differential in water level with the estuary. This is detected by stilling wells on each side of the gate, and when it exceeds 40mm for more than three minutes, the gates start to open, three at a time. They are set to open at a rate of 250mm per minute, purposefully slow to avoid harming a kayaker or any other river user who might be close to the gate. As the tide falls, the reverse process happens. This system can also be controlled if there is a coastal flood, allowing operators to close the gates, holding back the river water to protect the Maketū township from even worse flooding. They only expect to need this kind of intervention once every 10 or 15 years.

“On the downstream side of the gates, we had to widen the pathway into the estuary so that we had quite a good hydraulic cross section as we didn’t have very much head to work with in this division – I think the maximum is about 400 millimetres or so. So in order to get the volume of water that we wanted from the river into the estuary on the rising tide, we needed that 60m-wide channel of 2m depth to get it to flow through at sort of less than 2m/s.

“Then, on the downstream side again of that of that Ford Road, we acquired some land about 30 hectares of land from one of the farmers in the area and that had been wetland up until the 1950s. And it was really trying its best to be wetland again. So in acquiring that land, we were able to kick start the natural process and use some of the excess cut material from widening those channels to create a range of different wetland and tidal dryland habitats.”

These have now been substantially planted to create places suitable for birds, invertebrates and shellfish, all of which have started to return.

“And then the final piece of the puzzle was inside the estuary itself. A number of cores ways had been constructed in the 1960s and 70s. And they blocked tidal flushing between those different parts of the estuary, and we removed those. “

In removing them, the team had to create a 40-metre-long wooden footbridge to give access to a Maori-owned block of land in the middle of the estuary. Once all 12 gates were fully commissioned in February 2021, the project moved into an environmental monitoring phase. Critical to any modern project is understanding of its impacts, but the historical context made this especially true for the Kaituna River and Maketū Estuary.

Good environmental design

Someone who understands the importance of good environmental design is Steph Brown, the Technical Principal for Planning and Environment at WSP New Zealand. She is involved in planning work to make sure all of the necessary approvals for work are in place, and also environmental training for workers from a range of sectors.

Since the completion of the construction phase of the Kaituna project, she has been involved in compliance, ensuring that all of the planning conditions have been met – some 40 pages of conditions.

“which for a restoration project, seems a lot because you kind of think, well, surely it’s a good thing,” says Brown. “But it was about ensuring that what we thought would happen would actually happen [for example] making sure we don’t cause erosion […] and every year, there’s an annual monitoring report. That’s due, and regular reports back to the community and those sorts of things.”

Brown emphasises the importance of working with nature rather than against it, and says that for this, both an understanding of systems, and engaging with the locals, was critical.

“You need to understand the history of something to understand how it got to where it is, and to look at solutions. And that’s partly understanding what nature would want to do. Because effectively, it’s got to find its own new equilibrium. We’ve intervened again, to help in a positive way this time, but it is going to find its own balance in its own time.

“So that in some respects for me, yes there’s some design, yes there’s some engineering in it but it’s kind of the means to the means. It was more about how are we actually going to get the outcomes that we want to get, which was the restoration. How are we going to be able to maximise that?”

The major problem to be addressed was siltation of the estuary. Previously it would take 15 tidal cycles for the estuary to have a full exchange of water because it was 90% sediment and 10% water, now it takes just two and a half tidal cycles. The increased flow of water, particularly during rain events when the river is at higher than normal, will over time mobilise some of this excess sediment.

“The council has a suite of monitoring sites in the estuary. They are looking not just at the water quality, but at the number and density of shellfish, and they are mapping the algae. In some areas where we had 100% cover, it is almost down to zero.”

The principles of good environmental design are easy to summarise according to Steph. It is about “thinking beyond the now”.

“You’ve really got to think about what it will look like in 50 years’ time and 100 years’ time whereas previously a lot of people might be thinking on this 10 to 30 year scale. And when you’re working with nature… nature doesn’t deal in decades. It deals in these massive timescales. And so we need to be thinking intergenerational, as opposed to short term thinking about solutions.

“They are the tougher decisions to make, though, because you’re trying to think, you know, what is this place going to be like, like, or what should it be like, and 30, 50, 100 years’ time. And it’s hard to imagine that.”

The Mauri returns

Tapsell and the iwi are thinking long term. They are replanting and re-flooding wetlands to rejuvenate land far beyond the estuary. They mobilise school children for this and so use it as a learning opportunity. This work will go on for many years, but they connect children to their environment and their history. Utmost in this, and the true test of the project for the Maori, is a concept known as ‘Mauri’.  

“This is a life force; we believe the river has a life force of its own […] with this re diversion, the Kaituna coming back, the fresh water coming back. The spirit of the river will return and the essence of the river and the vitality of the estuary will come back, its Mauri will come back and re-energise the river. And not only the river, but the people as well.”

Tapsell says that the most emotional part for him was when the teams let the waters back into what was previously wetland. He saw the tide come in as it used to be, pre-1956.

“I always used to say [in anticipation of this day] my grandmother and grandfather. They’ll be looking down with a big smile on their faces, to see the return of all this… because the Mauri of the estuary is returning, the vitality of the estuary, certain types of marine life are returning, and the birds eat a lot of that marine life because they’re returning so the birds return. And this is what’s happening. It’s just like a domino effect. And you go down there and you can see the difference. You can see the vitality of the estuary, of the birds. It’s a tremendous.”

Tapsell says that he will not see the full regeneration in his time, nor will his children, but their children might. There is a Greek proverb that says, “A society grows great when old men plant trees in whose shade they shall never sit”. This is something the Maori know intrinsically… they consider themselves temporary stewards, holding in trust today that which has been passed down from the ancestors, on behalf of those who will be born tomorrow. And the Kaituna River project will help fulfil that trust.

For the episode of Engineering Matters this Long Read is based on, click here