Around three million tonnes of log are exported from Gisborne Port a year. But around 55 million tonnes of sediment end up in the ocean along the East Coast a year. Photo: Aaron Smale

New Zealand - TAIRĀWHITI'S TRAUMA: Bola and badass gullies: the science of East Coast erosion

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.

* 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.

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