Treat soil better and it will soak up floods

By Rebekah White

One thing most of New Zealand can count on is eventually it will rain, and then rain some more, and then keep going, beyond reason or desire. We even joke about it. (What do you call four straight days of rain? Easter.)

Towns and cities face the highest costs from flooding, and the fact they’re covered in impervious surfaces, such as concrete and asphalt, puts them at risk.

A recent study found expected damage from flooding in New Zealand’s urban areas is estimated at $128.6 million a year.

To prevent this, our towns and cities need all the absorbency they can get – to become ‘sponge cities’, with the capacity to soak up increasing amounts of rainfall.

Although rain gardens and green areas are often mandated for new developments, the construction process destroys the sponginess of areas specifically set aside to absorb water, says Sue Ira, a University of Auckland PhD student and former stormwater manager.

The problem is earthworks compress the soil – and squashed soil can’t soak up anything at all.

“We’re literally compacting the life out of it,” says Ira.

“And so, soils that previously would have been nice and permeable are now suddenly acting like impervious surfaces during large storm events.”

The problem we’re creating in our cities

Ira investigated soils in two areas of Auckland, alongside soil scientists from BioEconomy Sciences and Auckland Council engineers.

They found though they had been mapped as permeable areas, some weren’t permeable at all.

These spaces look like they should absorb water, she says – but they don’t always do this in larger storm events.

“We think it’s a lovely green park, and sometimes I feel like, ‘Well, we might as well have just painted the concrete green, because that’s how it’s working’.”

Soil absorbs water when it’s aerated. Press it flat with a roller or a compactor, and it acts more like pavement, even if it’s later planted out.

“Since the 2000s or so, the way we have developed land has changed,” says Ira.

“We now do mass earthworks to create really flat building platforms. And because of this, we’re not leaving any areas within our new subdivisions as natural green pervious areas.”

What happens instead, says Ira, is the topsoil is removed, the land is earthworked, then a thin layer of topsoil is returned.

This isn’t enough for larger plants to establish, or to counteract all the compaction that has taken place underneath.

“So even in some of our green road berm areas or parks, the soils aren’t working as if they were natural soils.”

Instead, under heavy rainfall, they act more like pavements, where water sheets off rather than soaking in.

“And in those larger storms, that’s exactly what we’re seeing.”

How we could make our green spaces spongier

None of New Zealand’s planning documents provide any protection for subsoil, the layer of soil below the topsoil.

“And those subsoils are really important in terms of their water-holding capacity and to allow for the growth of large vegetation,” says Ira.

Nor does the planning process specify how much topsoil must remain on a site.

“And so, what’s happening is people can strip the soil away, and then they’ll put about 150 or 200 millimetres of topsoil back on – which is really not great for growing anything other than some small flowers or shrubs.

"And so, what we need is some level of protection through our planning documents.”

If the subsoil has been squashed, it can be repaired – perhaps not to its original state, says Ira, but de-compacting it will help it function more naturally.

That involves something humans have been doing to soil for thousands of years: ploughing it.

“That allows the soil to start to aerate.”

Another solution, especially where it’s not feasible to de-compact soil for geotechnical reasons, is to add a mound of aerated soil on top, a bit like a raised garden bed.

This allows trees to become established before their roots hit the compacted soil and helps to prevent stunting.

“Then those roots can get in down below into those compacted subsoils, and then they can naturally start to break up some of that compaction,” says Ira.

“Whereas if you just plant them on 150 millimetres of topsoil, they often never get big enough to be able to do that.”

Finally, Ira suggests reconsidering the extent of earthworks necessary for new developments. Certain areas could be left alone by the rollers and compactors.

“Why do we have to earthwork an entire hectare area when only a portion of that area is actually going to be for houses or for a road?”

Soils should be treated as part of our infrastructure

We should treat soils with the same eye for long-term planning as pipes, drains and other stormwater infrastructure, says Ira.

“If we want to look at what is going to work long term for our cities, we need to think about how our infrastructure is going to function over the next hundred years.

"Because what the Auckland Anniversary floods have taught us is that it’s incredibly expensive if we get it wrong.”

Part of the problem, says Ira, is that green infrastructure – the healthy soils, rain gardens and wetlands that are integral to spongy cities – is often created simply to meet the conditions of a building consent.

That means the focus of creating green infrastructure is simply on ticking the boxes of regulations rather than considering how it functions as part of a wider urban network.

“How it’s located on a site, and how it integrates with the surrounding urban environment, is not really being taken into consideration,” says Ira, “because the consent process is allowing developers and councils to focus purely on meeting a regulatory requirement”.

The process of protecting or fostering heathy, permeable soils isn’t a mystery, says Ira; the problem is that the benefits of ‘green infrastructure’ aren’t easy to quantify in financial terms, whereas their long-term maintenance costs are far more real.

Green infrastructure has more upsides than simply dealing with stormwater.

It can help reduce the urban heat island effect, promote the growth of healthy vegetation, increase biodiversity, lower our infrastructure’s carbon footprint, and improve people’s mental and physical wellbeing.

But these factors aren’t included in decision-making processes, says Ira.

“Research is showing that the value of these co-benefits can match or even exceed the stormwater benefits that are typically being measured.

"But because our decision-making process doesn’t generally account for those, these wider co-benefits are often not considered.”

The world is facing unprecedented environmental challenges. Planetary Solutions, an initiative of the Sustainability Hub at Waipapa Taumata Rau, University of Auckland, and Newsroom, explores these issues – and the practical ways we can all be part of the solution.