Scientist Mike Marden has spent decades researching the East Coast's erosion and knows the local soils like few others – and he also knows things have to change. Aaron Smale continues a series on the slow destruction of Tairāwhiti.
Everywhere he looks he can see the risks and vulnerability in the landscape.
If science is observation, Mike Marden has been closely observing and meticulously documenting the changes to the land on the East Coast for decades and is one of the foremost experts on its fragile geology.
Marden lives in a house on a hill where the old Gisborne hospital once stood, looking out over the city. He once lived next to the Waimatā River but got nervous about floods and moved to higher ground. Even now he can’t help but assess locations based on a career as a scientist who spent most of his working life studying the soils and erosion of the East Coast region. He points out a house on the street that is close to a cliff and a slump in the road on the way up.
READ MORE: * Part one: The colonial cul-de-sac: Land loss, land use and the devastation left behind * Part two: Ocean bears brunt of Tairāwhiti erosion
His observations, and the observations of a number of scientists who have studied the unique geology of the region, dovetail seamlessly with the observations of the Destounis brothers and those living on the land. The steep and fragile terrain of the region makes it not only one of the most erosion-prone regions in the country but also globally. Changes to land use over more than a century have activated that proneness in ways that are still playing out – and will continue to play out for another century at least.
Marden did his PhD on the erosion in the Ruahine Ranges and then took up a research role at the Forest Research Institute in Gisborne from 1985 to 1992 and then at Landcare Research from 1992 to his retirement in 2017. That long experience has made him one of a handful of experts on the region’s geology and problems with erosion.
“I was just interested in knowing what the processes were that caused rivers to down-cut and generate sediment and where all that sediment was going to, where it was being stored. You've got a chronic uplift, you've got rivers down-cutting, you’ve got sediment generation, because the hillsides are falling down, putting all the sediment into the river systems. And the sediment then gets transported out to the ocean, and stored on the shelf.”
The lithology, or rock and soil composition, of the East Coast is in geological terms quite young, which explains its behaviour under certain conditions.
“There are two major and distinctly different geological terrains between the coast and for quite a few kilometres inland."
The different lithologies are named by their geological ages and the younger band closest to the coast is referred to as tertiary, while the area further inland along the ranges is older and is referred to as Cretaceous.
“The older inland Cretaceous age lithologies have gone through quite a serious tectonic period of upheaval. So they're crushed, basically fault-crushed, so full of joints, full of cracks. All the lithologies that used to be intact all get broken up. So they're faulted, they're folded. It's a mess.
“Whereas the younger rocks towards the coastal part of the region, the tertiary age ones, they've still got a lot of their structure intact. The main erosion process there is the rivers incise into very deep gorges, or deep, deep river systems.”
These geological features and the rapid destruction of the indigenous forest cover has led to erosion that is some of the worst not only in New Zealand but in the world.
Marden says the recent problems witnessed in the Tairāwhiti are a result of human decisions and action on those fragile soils. In particular, human actions and land-use since colonisation have had drastic impacts on the processes shaping the landscape.
“During the 1880s through to 1920s when the early European settlers started clearing the native forest, it happened really, really quickly across most of the East Coast region within that very short time period. And almost immediately scientists who were visiting this area started to notice wholesale hillside collapse. That started not long after the clearance of the indigenous forest. And there are numerous accounts of hillside collapse filling up the river systems, the transportation of the gravel from the headwater areas downstream, the problems that was creating in terms of river build-up within the Waipaoa river. The landscape in and around the headwaters of the major river systems changed really dramatically.”
Sir James Hector, a Scottish-born geologist who had travelled extensively in Canada and became one of the leading scientific figures in colonial New Zealand, collected evidence of widespread slips and erosion in the 1890s. Despite the damage he and many others observed, for the first 60 years of the 20th Century pastoral farming dominated as a land-use in the East Coast region. This dominance of pastoral farming has contributed to some of the worst erosion in the world.
Scientific measures of drilled core samples have shown a massive increase in siltation at sea but also of river systems themselves that coincide with human settlement and the removal of the forest cover.
A scientific simulation of the sediment discharged from the Waipaoa River catchment over 3000 years “indicated that after Polynesian settlements were established suspended sediment discharge increased by 140 percent, whereas the wholesale land use changes effected by European colonists initially caused suspended sediment discharge to increase by 350 percent, and by 660 percent once the headwaters were deforested.”
But the Waipaoa is only one of three major river catchment systems, including the Hikuwai/Uawa and the Waiapu, that transport massive amounts of sediment into the ocean every year.
“The highest suspended sediment load of any of the three major East Coast rivers is for the Waiapu and it generates 35 million tonnes per annum of suspended sediment. By world standards that's up there for the size of the catchment, because there's only 1700 square kilometre catchment. So that figure is quite impressive,” says Marden.
Impressive for all the wrong reasons. One scientific paper compared the sediment discharged by the Waiapu to the muddy sediment load of California rivers which, excluding the rivers that empty into San Francisco Bay, is 42 million tonnes a year. But this is from a catchment area of nearly 100,000 square kilometres.
But that’s just the Waiapu. There are two other main rivers in the region, the Waipaoa near Gisborne and the Hikuwai/Uawa that discharges in Tolaga Bay, not to mention a number of other smaller rivers. The three main catchments discharge a total of approximately 55 million tonnes of suspended sediment in an average year (Waiapu 35 million tonnes, Waipaoa 14 million tonnes, Hikuwai five million tonnes). The Waiapu and Waipaoa Rivers account for around 42 percent of the sediment yield to the coast from the North Island and 24 percent of the total yield for New Zealand.
The East Coast is also an outlier when compared to other river systems in New Zealand with the exception of South Westland, which has its own distinct characteristics.
While the Waiapu discharges around 35 million tonnes a year from a catchment of 1728 square kilometres, the Waikato River, which has a catchment of 14,490 square km, discharges approximately 0.53 tonnes a year.
But despite a mountain of anecdotal and emerging scientific evidence about the vulnerability of the East Coast – not to mention the social and economic costs – it wasn’t until after World War II that alternatives to sheep were considered. But even then there were conflicting courses of action taken by policy makers.
A massive flood in 1948 caused significant damage in the region, but it took until 1960s for land-use strategies to shift, and even then it only happened gradually.
The NZ Forest Service started a planting programme of around 13,000 hectares of pine around the headwaters of the Mangatu River. Marden says the planting of exotic forestry had obvious results.
“We started to see shortly after the completion of afforestation that the rivers started to change their behaviour. So with less material coming off the hill-slopes and into the river, the river then had a chance to start to rework what was already stored in the riverbed. So that was clear evidence that the trees were doing their job.”
A massive landslip in Tokomaru Bay
Despite this, there was still a stubborn political belief in sheep farming as the country’s economic future. Subsidies were increased not only for sheep farming but for the clearance of Manuka from marginal land that had already been cleared and reverted back to bush.
“There were subsidies involved at the time to basically clear scrub areas in order to boost the sheep numbers for export. In hindsight, a lot of that land is probably reverted again, simply because there's been a lot of subsequent erosion on it. Or it's gone back into forestry. So yeah, that was not a good idea.”
Cyclone Bola and badass gullies
In early March 1988 Cyclone Bola slammed into the East Coast, causing widespread destruction to the region. All the problems that had plagued the Tairāwhiti for the previous 60-plus years came to a head as the hills collapsed in massive slips and landslides. While it was a disaster it was also a chance for scientists like Marden to observe nature’s processes sped up.
“Bola was a five-day event. The first three days it was just fairly constant rain but not intense. During that period the soils were able to absorb a lot of the moisture. But then the latter two days of Bola, the intensity of rainfall increased. And it was during that period that the soils reached total saturation.”
But if subsidies for sheep distorted land-use practices to the detriment of the East Coast up until the mid-1980s, a different industry was about to be given a hand-out that would cause a new kind of destruction. Only the destruction took several decades to manifest.
Marden says the most obvious result was the severe shallow landslides that were widespread across the region. But the saturation also led to another major change.
“After Bola, we started to see more of the deeper-seated failures start to move. I'm referring in particular to some of the earth flows that have been in the landscape for 15,000 years, became wet enough to start to move again.
“So these earth flows are strange things. They've been in the landscape a long time, but they require certain conditions to set them off to reactivate them. And the most common trigger are these storm events, during which you get a lot of rainfall. Because they are deep-seated failures it takes time for the profile to wet up to create mobility. Then they do move, but they just sort of creep. And then they reach another threshold of stability, and they stabilise, and they may stay dormant or stable for decades, until another big event comes along.”
Big events like Cyclone Bola cause even higher volumes of erosion. One scientific estimate calculated that the Waiapu River discharged a total of 73–93 million tonnes – over twice the average annual sediment load – in only a few days during Cyclone Bola. Monitoring sites that existed in Hawkes Bay during Bola showed the rainfall for Gabrielle was more intense over a shorter period of time.
But for all the havoc Bola wrought on the East Coast it gave scientists like Marden clear evidence that trees – whether native or indigenous – gave protection to the fragile land on the East Coast.
“Following Bola we understood the significance of having basically a closed canopy forest in mitigating the incidence of shallow landslides. There was a big difference between the density of landslides on the pasture versus that under a mature closed canopy forest.”
Despite the results being roughly similar for native and exotic forest cover, the decision was made to plant predominantly pines for its fast-growing characteristics and economic potential.
The storm hit at a particular political cross-roads in terms of policy response. The economic slash and burn of Rogernomics in the mid to late 1980s led to Prime Minister of the time David Lange labelling agriculture a sunset industry. For many the label seemed an apt description of the state of the sector in those years – freezing works shuttered around the country as sheep numbers plummeted after Douglas set fire to a whole raft of farming subsidies that had distorted prices, propped up uneconomic farms and led to destructive environmental practices. Many farmers walked off their land before 1988. For many, Bola was the last straw.
But Bola also provoked a government response in the opposite direction. While farming subsidies had been canned for the sake of levelling up the government’s finances and to supposedly create a more efficient industry, the damage that Bola did to the East Coast created a whole new set of subsidies for a different industry.
“The immediate response after Cyclone Bola was the introduction of East Coast Forestry Project, which aimed to replant 60,000 hectares of severely eroding pasture land and establish it in forest. The East Coast Forestry Project was a 28-year government-funded program. From memory they achieved a target of maybe 30-35,000 hectares. It is somewhat short of what they were aiming to target. But a lot of ... mum and dad investment companies purchased a lot of land I presume, quite cheaply, a lot of farmland.
“There were some farms that were so badly damaged that there was no hope of them being economic any longer. There was no thought of subdividing the good bits of the farm that survived the storm and keeping it in farming. Everything was planted. So in total between the mum and dad investors, and the East Coast Forestry Project, there was 100,000 hectares of forest planted post cyclone Bola in largely between 1992 and 1997. That's a lot of forests.”
Along with the forestry planted before Bola, that 100,000 plus hectares of pine trees is what is being harvested now. While Gisborne Port has other products getting shipped off its wharf, the majority of the tonnage is pine logs. And that tonnage has increased exponentially in the past 20 years. According to the port’s own numbers, in 2003/04 that figure stood around 481,836 tonnes. By 2010/11 that had increased threefold to 1,517,040 tonnes and by 2015/16 it was up to 2,302,404 tonnes. The Eastland Wood Council now says it’s over three million tonnes.
But while those three million tonnes of logs a year are being shipped out of Poverty Bay and up the East Coast on their way to China and other markets, those ships are moving through waters that have more than 50 million tonnes of silt coming off the hills those trees once stood on. That is just silt. It doesn’t include the waste from those logs that also end up in the ocean.
The government subsidising of pine under the East Coast Forestry Project only finished in 2020. That is now having knock-on effects on the community, the economy and the environment that will continue for decades. But there is also a political inertia that has meant any decisions to change course are slow, never mind actual action.
But the main driver of pine forestry was to stabilise the land. In a cruel irony, the harvesting of all those trees has now made that very same land extremely exposed and vulnerable to erosion once again. Only this time the slips carry the timber industry’s rubbish with it when they tumble down the hills in cyclone events like Hale and Gabrielle and even in lesser flood events like the half dozen that have hit in the past 18 months.
Mike Marden says it's the landslides that carry the slash into waterways.
“We had storms back in 2013, 14, 15, 17, 18, and then Hale, and then Gabrielle and they're all coincided with the harvesting of these forests where the trees were planted on steep slopes leading down to streams. That's where the bulk of what we're calling slash – waste-wood – was left on the slope because there's no market for it. When you get these major cyclonic storms and you get the landslides on these steep slopes, it's the landslides, not the rain, that transports the woody debris into the streams. And that's why we've had these massive slash problems.”
The cost of the damage caused by Gabrielle runs into billions and a significant chunk of that will be a direct result of flood waters filled with slash damaging roads and bridges. Then there’s the cost of communities cut off and property destroyed, alongside the already mentioned impact on the ocean.
The land becomes even more vulnerable as the roots of the mature trees that have been harvested disintegrate and no longer absorb the water from the hills. Any newly planted trees take a good five to eight years before they create any stability, but even older trees are getting knocked over in slips that are now occurring, as seen behind Tokomaru Bay.
And the landscape is being fundamentally changed at a pace that can’t be slowed down easily or maybe at all.
Researching this story involved wading through a number of scientific reports that read like a PhD thesis on nuclear physics written in Russian and translated into Japanese. But every so often a phrase would pop out that could be understood by a reasonably literate person. In one such paper Marden used the term “badass gullies” that provoked a loud guffaw from the reader in the middle of a Gisborne café.
When the term is mentioned, Marden gives a wry smile. The term might be slightly comical, but what it refers to isn’t very funny when he translates it.
“I didn’t coin the term, by the way, that’s come out of the American literature,” he says. But the Americans would be impressed with the badass gullies in the Tairāwhiti.
“There are really two types of gullies. There's the linear ones that are longer than they are wider, and the amphitheatre-shaped galleries which are wider than they are longer. The linear galleries are very common or most common within the tertiary terrain, we've still got these lithologies that have still got some structure, they've still got some strength. So that the gullies once they incise, they don't tend to widen.
“The amphitheater gullies on the other hand, they're the most common within the Cretaceous terrain. So these are gullies where you get sides that slowly collapse simply because the geology is not strong enough. It's got no structure to it. So it fails by other processes, mainly slumping. Badass gullies is a term that describes those gullies that have basically gotten so big and are so active, that we can't treat them.”
These badass gullies are a major problem in China. The Chinese have tried to mitigate and halt their progress by building step-like dams to prevent them moving down in floodplains and waterways, but at huge cost.
“We can't afford to do that. Economically, we can't. So we just plant pine trees where we can around them. Some of the pine trees don't survive, of course, because you've got this continual collapse of the sidewalls around these gullies and pine trees tend to fall into them. So those are your badass guys.”
Marden says badass gullies are one problem. But the silting up of rivers is also reducing the capacity of those rivers, which means they spill over their banks more regularly or start cutting into productive land on the flats.
“Land productivity is on the decline because we're losing so much topsoil whether it be farmland or whether it be from forestry land. As it has been harvested there's an increase in landslide incidents. Once you lose that topsoil, you're basically down to bedrock and you can't grow much on bedrock. You can't even plant into it. We know from previous studies down in the Wairarapa that once you've lost your soil, your dry matter production – we're talking about pasture here – I think your dry matter production only ever gets back to 80 percent of production.”
That’s if the land doesn’t completely disappear out to sea.
“The Waiapu is a very wide river and after every flood you'll find that it changes its course. So it might be here one flood and then it will be over here another flood. And quite often a channel can be directed towards a bank and it can undercut the bank. The people who've got land bordering these rivers are losing flatland, which is pretty bloody scarce anyway. Up in that area those valleys are quite narrow. The river is undercutting the paddocks closest to the river, so that's another consequence.
“And the other one is that some of the very low-lying flat paddocks that they use for grazing and cropping are being inundated with gravel. It flows over onto the paddocks, low-lying paddocks, and they're lost. They become part of the river system. So that’s a loss of production. When you're talking about somebody's livelihood – somebody's grazing cows, or sheep or growing maize or whatever – that has major implications for the economics of the unit.”
The massive volume of sediment and forestry waste going out into the ocean raises another question. The coastal shelf is only around 20km wide before it drops into what is known as the Hikurangi Trench. The trench is part of the Pacific Ring of Fire and is formed by one tectonic plate diving under another, which makes the land and sea along this seam vulnerable to major earthquakes.
It’s no coincidence that Ngāti Pōrou’s founding story is Paikea riding a whale – the Hikurangi Trench is a marine superhighway that connects to the Kermadec Trench that links all the way up to Tonga in the north and down to Kaikoura in the south. The fault line then turns inland and runs along the Southern Alps.
But the Hikurangi Trench also provides scientists with a case study in the risk of an earthquake causing a slump of the seafloor that could lead to a tsunami. During the Kaikoura earthquake in 2016 a huge of volume of material moved into a section of the trench.
Niwa scientist Joshu Mountjoy says the area along the East Coast drops off sharply into a Canyon system that stretches up into the Pacific
"So typically the edge of the shelf is around kind of 150 to 200 metres' depth. It's really wide at Hawkes Bay and Gisborne it's kind of tens of kilometres off the coast.
"The Kaikoura Canyon where it starts it's about 2000 metres. Once you're off Gisborne it's just a bit over 3000 meters deep. And then as soon as once you get up to the Kermadec trench part of it you get really deep and that's 10 kilometres.
"It's a real distinguishing feature of New Zealand really, because that whole area off the East Coast, all of the submarine canyons down in the south are channelling sediment that's coming from the South Island, Cook Strait, and moving it up to this thing called the Hikurangi Channel, which is like a 2000-kilometre long sediment channel. So it's sort of like a Waikato River underneath on the seafloor. And with the Kaikoura earthquake we had a chance to really see how that thing works. We have evidence for a huge event going down there. And we had definite evidence for it making it as far as Hawkes Bay but looks like it's likely to have gone all the way out to the 2000 kilometre length. So, you know, really large-scale processes happening out there."
He said although there was evidence of significant movement of sediment into the channel from the Kaikoura earthquake, it did not cause a tsunami.
"The fault to the south of Kaikoura Peninsula broke right into the canyon. We've measured three metres of uplift on the edge of the canyon, but there was no really large landslides. And so that's one of the kind of interesting conundrums."
There is ongoing scientific work studying the impact of sedimentation on the seafloor in the area, and the impact on sea life.
"Some of the current work we're doing is just trying to understand the impact of the sedimentation on fisheries habitat."
So was the planting of so many pines in the Tairāwhiti a mistake? Marden says there were certainly mistakes made, particularly around the lack of thinking ahead to what would happen when the pines being planted were harvested 30 years later.
“It had never crossed anybody's mind that this was not the right thing to do. Because clearly we had learned that trees make a hell of a difference in mitigating against these storms. And so we thought we were future proofing the landscape against future storms.
“But we didn't delve into it finely enough to say, well look, during Bola, we had all these landslides in these locations. Hey, maybe we shouldn't put pine trees there because in 30 years’ time, we're going to be harvesting them and there could be issues. It simply didn’t cross anybody’s mind.
“We've been slow to realise the implications of some of the things we've done in the past. And it's only just becoming evident that we may have rushed things in terms of where we plant, what we plant, or how we go about restoring stability to hillsides.
“We're still learning. But things really do need to change now. And I think Hale and Gabrielle have given us the information that we need. And we've actually known about it for quite some time, but we've never quite got it into policy. But now that the damage out there is so extensive we need to be very careful about how we manage the land going forward.”
NEXT: Bought out and sold down the river
