Slipping away

What will it take to heal our eroded land? (12 minute read)

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Every time there’s heavy rain, a little bit more of the land holding up my driveway drops onto the road below. I’ve been watching it for years, and I have a nasty feeling that at some point, perhaps when an earthquake and a major storm coincide, I’m going to have a very expensive problem on my hands. One of the neighbours who shares the driveway got the opinion of an engineer, who suggested that we not worry about it until it happens, so I try to put it out of my mind. But, like many other New Zealanders, I live with the unsettling knowledge that the land I live on is not as solid as it first appears.

Erosion has been on my mind for two reasons lately, aside from the state of my driveway. The first reason is probably obvious – the events of February this year when Cyclone Gabrielle hit the north and east of New Zealand. I was horrified, as we all were, by the massive quantities of sediment and debris which were washed down from the hills. I remembered, too, that this wasn’t the first time that Tairāwhiti had suffered in this way. Cyclone Bola had caused devastation in 1988, although without the debris left behind when forests were logged, because less of the region was under forest in those days.

There’s another reason I’ve become interested in erosion, too. I’ve been trying to understand New Zealand’s water quality problems, and sediment is one of these problems. It destroys the habitat of fish, insects and plants, it increases the risk of flooding and it smothers shellfish beds on our coast. And it’s a waste of a precious resource when soil from our land washes out to sea.

To understand what has happened to our water, we need to understand what happens on land. So, metaphorically speaking, I’m getting out of the water and back onto dry, if not exactly solid, ground.

To help me, and you, understand erosion, I’ve called on the help of Chris Phillips from Manaaki Whenua Landcare Research, who has been studying erosion for decades. He gives me a much broader picture of erosion than just the problems I’ve been thinking about. “Erosion is the wearing down of the earth’s surface. It’s a natural process but it can be exacerbated by human interactions with the landscape. There are a number of different processes that make up or contribute to erosion. There are surface processes which are driven by wind and water, and then there are mass movements which are things like landslides, debris flows, rock avalanches and things like that. Fundamentally those are the two pillars of erosion. There’s also bank erosion, the wearing away of stream and river banks, which can be a mix of the two types.”

I had never thought about types of erosion before, but I should have known that there would be lots of different names and classifications. Classifying and naming things is an essential part of understanding them. It sends me down something of a research rabbit hole, as I try and figure out all the different types of erosion I’ve seen. I’m very familiar with the way rivers and streams eat away at their banks. Landslides, commonly called slips in New Zealand, are what I’m worrying about on my driveway. I’ve encountered wind erosion too. I lived in Christchurch in the 1990s, and remember the hot, gusty nor’wester which would sometimes strip soil from Canterbury farms.

I also have some positive memories of erosion. Last summer, I had a lovely time building sandcastles on the beach with my niece and nephew, and part of the fun was watching the rising tide erode our work away, perhaps helped along by a few extra buckets of water. And one of my happiest memories is of a camping trip that I took in the Nelson Lakes National Park. There, I got to see some of the unique plants found only on scree, which is an unstable rock slope resulting from past rock avalanches.

Scree slope in Nelson Lakes National Park, and two of the native plants which only grow on scree (image credit: Melanie Newfield)

Phillips also talks about the positive side of erosion, when he tells me “We wouldn’t have Kaiteriteri Beach and the golden sands of Golden Bay without erosion.” It has always been a feature of New Zealand, he explains. “New Zealand’s position as a couple of long thin islands in the middle of an ocean, with the resulting weather systems, and high tectonic rates of uplift, means erosion has always been here.”

Big events like major storms have always made an important contribution to erosion. “Things like Gabrielle fundamentally are nothing new, they’ve been around for millennia if not millions of years. What humans have done, since the colonisation of New Zealand largely, is accelerate many of those natural processes.”

We have accelerated erosion, he tells me, by removing the plant cover from the land. “Vegetation, in whatever form it might be, grasses, shrubs, scrub and tall forest, mitigates the intensity of an erosion process. It’s particularly so on the steeper lands.”

But it’s more than just what is happening at the surface which contributes to erosion. The rocks underneath are important too. Despite my fears for my driveway, it turns out Wellington isn’t as bad as I thought. “Because the greywacke¹ is largely pretty stable, you will get landslides, but Wellington and up the coast are not so bad. It’s not the most benign place but it’s not the most erosion-prone.”

Further north is not so lucky. The Wairarapa, Manawatū, upper Whanganui and out through to the East Coast mostly sit on much weaker rock. “It can be a mudstone, a siltstone or a sandstone or a mix of all three, often in layers. Generally these rock types are weaker than other types, and the soils derived from them are similarly weak.”  Because it’s so unstable, there are a lot of slips, but most are minor. “Most of the shallow landslides that we see on pasture land, they look like the little white pock marks on green grass. They’re quite shallow and it’s mostly the topsoil and the regolith, which is the weathered layer between the hard rock and the top soil. It’s really only one or two metres deep at the most.”

Cyclone Gabrielle, however, was not an everyday event. The kinds of landslides seen then were something different. “You get big deep seated landslides that occur in the rock mass itself,” Phillips tells me. “Half a hillside will fall down. We see those on the East Coast but we also see them in places like the northern South Island after the Kaikoura earthquake.”

I haven’t forgotten images of State Highway One covered by a giant slip after the Kaikōura earthquake. Slips on that kind of scale are terrifying. And the slip which covered the main highway was only one of many. Some of them landed on streams and rivers, creating dams. Scientists are monitoring eleven such dams in the Kaikōura region which are at risk of collapsing, and may threaten areas downstream if they do.

Getting back to Cyclone Gabrielle, though, there’s another factor which Phillips mentions as important. “Sometimes it’s the intensity of the rain from a cyclone but sometimes it’s also the antecedent conditions. So that’s how wet things are prior to the big event. And as you know, New Zealand has been wet, wet, wet, for most of this year.”

But, horrifying as it was, Gabrielle was not a unique event. I remember news reports of floods in 2018, when forestry debris poured from the hills and onto farmland and beaches. In fact, tree debris has been a problem during heavy rainfall since at least 2010. Phillips points out that this is a matter of there being more trees in the landscape.

Going back further, I’m old enough to remember Cyclone Bola in 1988. Phillips remembers it too. “I was there. All that land which has caused all the problems for Tolaga Bay was all under farmland prior to Cyclone Bola. There were some farms there that had 50% of the land area affected by landslides. It had exactly the same density of landslides, in the same places, back then as occurred in 2018 and then subsequently in the last couple of events – Hale and Gabrielle. The policy response at the time of Bola, which we all agreed with, was to put trees in those landscapes because that’s what was needed.”

This isn’t news to me, because I’d already been digging into the issue of erosion. After Bola, it was clear that land under native forest or established plantation forest (more than 8 years old) had suffered much less erosion than pasture or newly-planted pine. So, much more of the land was planted in forest. But things were changing in New Zealand during those years. Phillips tells me, “these forests were planted to provide catchment protection and soil conservation value, but it was also in the time when the Crown forests were going through transition, and the rights to manage and harvest the trees were sold.”

This is an important point, and it has a bearing on what we are doing with forests to mitigate climate change today, so I’ll expand on it. Much of the forest planted in the wake of Cyclone Bola was planted to protect the land. The evidence was already clear that harvesting that forest would leave the land extremely vulnerable once again. The intention surely was not for the forest be stripped from the land all at once 30 years later. But that is what happened. “What we saw in 2018, with the first Tolaga slash on beach problem, was a big contiguous area, under multiple ownership, all the same age of tree, all harvested over a few years.”

But, even with the land under forest, it would still be at risk. “Now, hypothetically, if all that was still under forest and hadn’t been harvested, we would not have seen the same degree of impact with the slash and the wood, but we would have seen many landslides.” The problem, Phillips tells me, is not recent. “What we’ve seen with the Hawkes Bay and Gisborne essentially occurred 150 to 160 years ago, when the original forest was removed and it was converted to farmland. We put it back in trees, then we’ve removed the trees and the landscape has responded in a predictable manner.”

Once again, it comes back to the destruction of our forest. But it also comes back to the decision-making in the late 1980s and early 1990s. The amount of woody debris left behind after harvest obviously contributes to the damage, but the problem is much bigger and older than that. The question now is what can be done to improve the situation into the future.

Really, we know the answer – plant the land in trees and, this time, don’t cut them all down. But does it matter which tree? Phillips argues that that’s less important than you might think. “One thing that I have done as part my research is look at tree roots. Fundamentally, trees are like people, they are, by and large, lazy. They don’t go deep, they don’t go far, unless they are limited by resources. What you find is that in most situations, whatever tree we look at has got roots that sit in the top half metre to one metre of soil or regolith. You will find places where there’s an exception to that and occasionally they go a bit deeper, but by and large exotic and native trees are shallow rooted. It’s less about the species in my personal view and more about how many there are, it’s the density. So having closed canopy and dense forests or trees is really important.”

It’s not impossible, Phillips says, that we could get some timber from these protection forests. “I think what we’ve now got is a very strong voice that only talks about protection forests as being a native forest but I think there’s also some other voices that suggest that we could have production forests which serve multiple functions. But that’s only if they are made up of different species [that is, not radiata pine] and managed in different ways.”

I’ll come back to this point in a couple of weeks, because it’s similar to a point made by Yvette Dickinson, a scientist who works on forestry in New Zealand, when I talked to her about pine trees. I will be writing an article based on my interview with Dickinson soon.

The other thing that we need to do, Phillips tells me, is start thinking on a different, much smaller, scale. The decision-making about land use has been too broad. “We’ve got to get a bit more sympathetic with the landscape we’re working in, and that means a much finer scale of resolution for both understanding and management.”

This is also one of the key findings of the inquiry into land use in Tairāwhiti. The inquiry panel called for “a more nuanced vision of a mosaic of sustainable land uses – both protective and productive – that are more appropriate to their place in the landform”.

In creating this new vision, we are going to need a different approach. We can’t keep trying to squeeze a dollar from every bit of land without regard to the impacts. This doesn’t mean the land can’t be profitable. Phillips points out that there are managed forests in Europe where trees are harvested but the cover is maintained because the areas are vulnerable to erosion. Forests can serve more than one purpose. In Europe, there were mistakes in the past, cutting down too much forest, but they’ve learned.

As I think about the coverage in the wake of Cyclone Gabrielle, and in response to Phillips’s comments about how far we are behind Europe in our forest management, I am reminded of a talk I heard a few years ago, by geologist Dan Hikuroa. He spoke of an observation he had made some years previously, when parts of the town of Matatā in the Bay of Plenty were damaged by a flood of mud and debris. There were three marae in the town, and none of them were affected. Coded within a story about a local taniwha, there is guidance about an area to avoid – precisely the area affected by the flood (for more about this I’ve linked here to an excellent article by Kate Evans).

With so many Māori communities among those affected by Cyclone Gabrielle, I find myself reflecting on the fact that their knowledge of these areas dates back to long before the forests were cut down and put into pasture. The science tells us that vulnerable land needs forest – dense, continuous-cover forest – and this crucial information was forgotten in the wake of Cyclone Bola. But Māori knowledge, matauranga Māori, will also have insights into what should go where. We need to make sure our decision-makers are listening, this time.

There will be more from my interview with Chris Phillips in an upcoming article for paid subscribers.

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1. The underlying rock